Dynamic Initiation and Propagation of Multiple Cracks in Brittle Materials
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Xiaodan Ren
2013-07-01
Full Text Available Brittle materials such as rock and ceramic usually exhibit apparent increases of strength and toughness when subjected to dynamic loading. The reasons for this phenomenon are not yet well understood, although a number of hypotheses have been proposed. Based on dynamic fracture mechanics, the present work offers an alternate insight into the dynamic behaviors of brittle materials. Firstly, a single crack subjected to stress wave excitations is investigated to obtain the dynamic crack-tip stress field and the dynamic stress intensity factor. Second, based on the analysis of dynamic stress intensity factor, the fracture initiation sizes and crack size distribution under different loading rates are obtained, and the power law with the exponent of −2/3 is derived to describe the fracture initiation size. Third, with the help of the energy balance concept, the dynamic increase of material strength is directly derived based on the proposed multiple crack evolving criterion. Finally, the model prediction is compared with the dynamic impact experiments, and the model results agree well with the experimentally measured dynamic increasing factor (DIF.
Study on subsurface-inclined crack propagation during machining of brittle crystal materials
Guo, Jiawen; Chen, Jianbin; Li, Jia; Fang, Qihong; Liu, Youwen
2016-05-01
There is an immense need to obtain high-quality surface and subsurface on brittle material owing to the advantage of its improved performance. Thus, in this paper, we proposed a mechanical and numerical study of fracture mechanics from the perspective of external loading and indentation geometry in brittle machining. Stress intensity factors are computed to analyze various impacts of external loading and indentation configuration on subsurface crack propagation. Results indicate that the main fracture mode for inclined crack is shear rather than opening and the apex angle of the indentation plays an important role in fracture behavior. As a certain external loading is exerted to the surface of the silicon, a large apex angle of indentation may lead to strong shielding effect on mode II crack propagation. A relationship between critical value of external loading to the crack propagation and the apex angle of the indentation is given in this paper that shows quantitative indication for suppression of crack growth.
Shabir, Z.; Van der Giessen, E.; Duarte, C.A.; Simone, A.
2009-01-01
Two different approaches to intergranular crack propagation in brittle polycrystals are contrasted. Crack paths resulting from a method that allows a detailed description of the stress field within a polycrystal are compared to cracks dictated by topological considerations. In the first approach, a
Yong Li; Hao Zhou; Weishen Zhu; Shucai Li; Jian Liu
2015-01-01
The initiation, propagation, coalescence and failure mode of brittle jointed rock mass influenced by fissure water pressure have always been studied as a hot issue in the society of rock mechanics and engineering. In order to analyze the damage evolution process of jointed rock mass under fracture water pressure, a novel numerical model on the basis of secondary development in fast Lagrangian analysis of continua (FLAC3D) is proposed to simulate the fracture development of jointed rock mass u...
Mantič, V; Blázquez, A; Graciani, E; París, F
2013-01-01
The crack onset and propagation at the fibre-matrix interface in a composite under tensile/compressive remote biaxial transverse loads is studied by a new linear elastic - (perfectly) brittle interface model. In this model the interface is represented by a continuous distribution of springs which simulates the presence of a thin elastic layer. The constitutive law for the continuous distribution of normal and tangential of initially linear elastic springs takes into account possible frictionless elastic contact between fibre and matrix once a portion of the interface is broken. A brittle failure criterion is employed for the distribution of springs, which enables the study of crack onset and propagation. This interface failure criterion takes into account the variation of the interface fracture toughness with the fracture mode mixity. The main advantages of the present interface model are its simplicity, robustness and its computational efficiency when the so-called sequentially linear analysis is applied. Mo...
Protection of brittle film against cracking
Musil, J.; Sklenka, J.; Čerstvý, R.
2016-05-01
This article reports on the protection of the brittle Zrsbnd Sisbnd O film against cracking in bending by the highly elastic top film (over-layer). In experiments the Zrsbnd Sisbnd O films with different elemental composition and structure were used. Both the brittle and highly elastic films were prepared by magnetron sputtering using a dual magnetron. The brittle film easily cracks in bending. On the other hand, the highly elastic film exhibits enhanced resistance to cracking in bending. Main characteristic parameters of both the brittle and highly elastic films are given. Special attention is devoted to the effect of the structure (crystalline, amorphous) of both the brittle and highly elastic top film on the resistance of cracking of the brittle film. It was found that (1) both the X-ray amorphous and crystalline brittle films easily crack in bending, (2) the highly elastic film can have either X-ray amorphous or crystalline structure and (3) both the X-ray amorphous and crystalline, highly elastic top films perfectly protect the brittle films against cracking in bending. The structure, mechanical properties and optical transparency of the brittle and highly elastic sputtered Zrsbnd Sisbnd O films are described in detail. At the end of this article, the principle of the low-temperature formation of the highly elastic films is also explained.
Energy Technology Data Exchange (ETDEWEB)
Aihara, S.; Tsuchida, Y. [Nippon Steel Corp., Tokyo (Japan); Machida, S.; Yoshinari, H. [The University of Tokyo, Tokyo (Japan). Faculty of Engineering
1996-12-31
A proposal was made previously on a model of brittle crack propagation and arrest that considers the effect of crack opening suppression by using unbroken ligaments generated on steel plate surface and the effect that cracks precede in the central part of the plate thickness, based on a local limit stress theory for brittleness fracture. This paper discusses applicability of this model to a mixed type test, and elucidates causes for difference in the arrest tenacity of both types in a double tensile test of the standard size. The brittle crack propagation and arrest model based on the local limit stress theory was found applicable to a simulation of the mixed type test. Experimental crack propagation speed history and behavior of the arrest were reproduced nearly completely by using this model. When load stress is increased, the arrests in the mixed type test may be classified into arrests of both inside the steel plate and near the surface, cracks in the former position or arrest in the latter position, and rush of cracks into both positions. Furthermore, at higher stresses, the propagation speed drops once after cracks rushed into the test plate, but turns to a rise, leading to propagation and piercing. 8 refs., 15 figs., 3 tabs.
Gear Crack Propagation Investigation
1995-01-01
Reduced weight is a major design goal in aircraft power transmissions. Some gear designs incorporate thin rims to help meet this goal. Thin rims, however, may lead to bending fatigue cracks. These cracks may propagate through a gear tooth or into the gear rim. A crack that propagates through a tooth would probably not be catastrophic, and ample warning of a failure could be possible. On the other hand, a crack that propagates through the rim would be catastrophic. Such cracks could lead to disengagement of a rotor or propeller from an engine, loss of an aircraft, and fatalities. To help create and validate tools for the gear designer, the NASA Lewis Research Center performed in-house analytical and experimental studies to investigate the effect of rim thickness on gear-tooth crack propagation. Our goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. In addition, we investigated the effect of rim thickness on crack propagation life. A finite-element-based computer program simulated gear-tooth crack propagation. The analysis used principles of linear elastic fracture mechanics, and quarter-point, triangular elements were used at the crack tip to represent the stress singularity. The program had an automated crack propagation option in which cracks were grown numerically via an automated remeshing scheme. Crack-tip stress-intensity factors were estimated to determine crack-propagation direction. Also, various fatigue crack growth models were used to estimate crack-propagation life. Experiments were performed in Lewis' Spur Gear Fatigue Rig to validate predicted crack propagation results. Gears with various backup ratios were tested to validate crack-path predictions. Also, test gears were installed with special crack-propagation gages in the tooth fillet region to measure bending-fatigue crack growth. From both predictions and tests, gears with backup ratios
Crack propagation modeling using Peridynamic theory
Hafezi, M. H.; Alebrahim, R.; Kundu, T.
2016-04-01
Crack propagation and branching are modeled using nonlocal peridynamic theory. One major advantage of this nonlocal theory based analysis tool is the unifying approach towards material behavior modeling - irrespective of whether the crack is formed in the material or not. No separate damage law is needed for crack initiation and propagation. This theory overcomes the weaknesses of existing continuum mechanics based numerical tools (e.g. FEM, XFEM etc.) for identifying fracture modes and does not require any simplifying assumptions. Cracks grow autonomously and not necessarily along a prescribed path. However, in some special situations such as in case of ductile fracture, the damage evolution and failure depend on parameters characterizing the local stress state instead of peridynamic damage modeling technique developed for brittle fracture. For brittle fracture modeling the bond is simply broken when the failure criterion is satisfied. This simulation helps us to design more reliable modeling tool for crack propagation and branching in both brittle and ductile materials. Peridynamic analysis has been found to be very demanding computationally, particularly for real-world structures (e.g. vehicles, aircrafts, etc.). It also requires a very expensive visualization process. The goal of this paper is to bring awareness to researchers the impact of this cutting-edge simulation tool for a better understanding of the cracked material response. A computer code has been developed to implement the peridynamic theory based modeling tool for two-dimensional analysis. A good agreement between our predictions and previously published results is observed. Some interesting new results that have not been reported earlier by others are also obtained and presented in this paper. The final objective of this investigation is to increase the mechanics knowledge of self-similar and self-affine cracks.
International Nuclear Information System (INIS)
Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity is ∼800 m s−1, which corresponds to ∼0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny-shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of ∼79 nm. Juxtaposition of the crack velocity with this spacing suggests that the crack takes ∼10−10 s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed
Scaling properties of crack branching and brittle fragmentation
Directory of Open Access Journals (Sweden)
Uvarov S.
2011-01-01
Full Text Available The present study is focused on the correlation of scaling properties of crack branching and brittle fragmentation with damage accumulation and a change in the fracture mechanism. The experimental results obtained from the glass fragmentation tests indicate that the size distribution of fragments has a fractal character and is described by a power law.
Yang, Sheng-Qi; Huang, Yan-Hua; Ranjith, P. G.; Jiao, Yu-Yong; Ji, Jian
2015-12-01
Based on experimental results of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code (PFC^{2D}). Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro-parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fissure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.
Barés, Jonathan; Barlet, Marina; Rountree, Cindy; Barbier, Luc; Bonamy, Daniel
2014-11-01
We analyze the intermittent dynamics of cracks in heterogeneous brittle materials and the roughness of the resulting fracture surfaces by investigating theoretically and numerically crack propagation in an elastic solid of spatially-distributed toughness. The crack motion split up into discrete jumps, avalanches, displaying scale-free statistical features characterized by universal exponents. Conversely, the ranges of scales are non-universal and the mean avalanche size and duration depend on the loading microstructure and specimen parameters according to scaling laws which are uncovered. The crack surfaces are found to be logarithmically rough. Their selection by the fracture parameters is formulated in term of scaling laws on the structure functions measured on one-dimensional roughness profiles taken parallel and perpendicular to the direction of crack growth.
Fracture statistics of brittle materials with intergranular cracks
International Nuclear Information System (INIS)
When brittle materials are used for structural purposes, the initial design must take their relatively large dispersion in fracture stress properly into account. This is difficult when failure probabilities must be extremely low, because empirically based statistical theories of fracture, such as that of Weibull, cannot reliably predict the stresses corresponding to failure probabilities much lower than n-1, where n is the number of specimens tested. Recently McClintock proposed a rational method of predicting the size distribution of intergranular cracks. The method assumed that large cracks are random aggregations of cracked grain boundaries. The present paper employs this method to find the size distribution of penny-shaped cracks, and also P(f), the probability of failure of a specimen of volume V subjected to a tensile stress sigma. The present paper is a pioneering effort, which should be applicable to ceramics and related materials
Brittle-tough transitions during crack growth in toughened adhesives
Thoules, Michael
2008-03-01
The use of structural adhesives in automotive applications relies on an effective understanding of their performance under crash conditions. In particular, there is considerable potential for mechanics-based modeling of the interaction between an adhesive layer and the adherends, to replace current empirical approaches to design. Since energy dissipation during a crash, mediated by plastic deformation of the structure, is a primary consideration for automotive applications, traditional approaches of fracture mechanics are not appropriate. Cohesive-zone models that use two fracture parameters - cohesive strength and toughness - have been shown to provide a method for quantitative mechanics analysis. Combined numerical and experimental techniques have been developed to deduce the toughness and strength parameters of adhesive layers, allowing qualitative modeling of the performance of adhesive joints. These techniques have been used to study the failure of joints, formed from a toughened adhesive and sheet metal, over a wide range of loading rates. Two fracture modes are observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth is associated with a toughened mode of failure; the dynamic crack growth is associated with a more brittle mode of failure. The results of the experiments and analyses indicate that the fracture parameters for quasi-static crack growth in this toughened system are essentially rate independent, and that quasi-static crack growth can occur even at the highest crack velocities. Effects of rate appear to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appears to be stochastic in nature, and it does not appear to be associated with the attainment of any critical value for crack velocity or loading rate. Fracture-mechanics models exist in the literature for brittle-ductile transitions in rate-dependent polymers, which rely on rate dependent values of toughness
Crack propagation in prestressed plates
Energy Technology Data Exchange (ETDEWEB)
Farshad, M.; Flueler, P. [EMPA, Duebendorf (Switzerland)
1995-12-31
A second-order theory of initially stressed plates in the plane stress mode was used to find the dynamic stress field in the vicinity of a crack tip. Rapid crack propagation (RCP) behaviour associated with stresses caused by internal pressure and temperature was examined. The flat plate was placed under thermal conditions and was prestressed in such a way as to simulate hoop stress that may be expected in a pipe made of the same material. The presence of the tensile prestress in the thin plate raised the levels of the principal stress values and affected the direction at which the maxima of principal stress occurred. Second-order effects played an important role in the vicinity of the crack tip. Increased crack speed caused increased stress levels. There existed a limiting value at which the stresses at the crack tip became unbounded. The limiting value was affected by prestressing. The concept of simulating RCP testing of polymer pipes by a test on a conditioned plate of the same material, prestressed to simulate hoop stress caused by internal pressure, was judged to be reasonable. 6 refs. 5 figs.
Multiscale modeling of crack initiation and propagation at the nanoscale
Shiari, Behrouz; Miller, Ronald E.
2016-03-01
Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly "converting" the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region. The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.
Propagation and band width of smeared cracks
Slobbe, A.T.
2015-01-01
The crack band approach (in the smeared crack concept) is widely used for the modeling of concrete fracture and is an important analysis technique within advanced engineering. However, the simulations can be impeded by mesh-induced directional bias. Cracks prefer to propagate along continuous mesh l
Crack propagation in rubber-like materials
International Nuclear Information System (INIS)
Crack propagation in rubber-like materials is of great practical importance but still not well understood. We study the contribution to the crack propagation energy (per unit area) G from the viscoelastic deformations of the rubber in front of the propagating crack tip. We show that G takes the standard form G(v,T) = G0[1+f(v,T)] where G0 is associated with the (complex) bond-breaking processes at the crack tip while f(v,T) is determined by the viscoelastic energy dissipation in front of the crack tip. As applications, we discuss the role of crack propagation for adhesion, rolling resistance and sliding friction for smooth surfaces, and for rubber wear. (topical review)
Crack propagation in Hastelloy X
International Nuclear Information System (INIS)
The fatigue and creep crack growth rates of Hastelloy X were examined both in air and impure helium. Creep crack growth rate is higher in air and impure helium at 6500C. Initial creep crack growth from the original sharp fatigue crack is by an intergranular mode of fracture. As the cracking accelerates at higher stress intensities, growth is by a mixed mode of both intergranular and transgranular fracture. Fatigue crack growth rate increases with increasing temperature and decreasing frequency for the range of stress intensities reported in the literature and is lower in impure helium than in air
A study on fatigue crack propagation considering crack tip plasticity
International Nuclear Information System (INIS)
Fatigue crack propagation of materials considering crack tip plasticity was studied. For this, fatigue tests were performed with compact tension (CT) specimens of Inconel 690, Inconel 600, Inconel 718 and Type 304 stainless steel at room temperature. Fatigue test on Inconel 600 was performed to be used as a reference data of Inconel 690. Inconel 718 specimen, which has very high yield strength, was selected to simulate different plasticity at the crack tip in comparison with 304 stainless steel. The effect of specimen thickness on fatigue crack propagation was studied with 304 stainless steel of 3mm-, 6mm- and 25mm-thick specimens. Inconel 690 has been proposed as a substitute material for Inconel 600 in pressurized water reactor (PWR) steam generator tube application. This alloy was developed to improve the stress corrosion cracking resistance of Inconel 600. Now, it is known that Inconel 690 has better intergranular stress corrosion cracking (IGSCC) property than Inconel 600. But, more data of Inconel 690 about mechanical properties are needed in steam generator design. To investigate the effects of heat treatment on yield strength and fatigue crack propagation of Inconel 690, tensile tests and fatigue tests were performed on heat-treated specimen. From the test results, it is believed that chromium carbide precipitates at the grain boundaries reduce fatigue crack growth rate (FCGR) of Inconel 690 by crack tip blunting as far as the fatigue cracking is intergranular fracture mode. To investigate the effect of residual stress on fatigue crack propagation, residual stresses were introduced by induction-heat treatment. And, the distribution of residual stresses was measured with 3mm-thick 304 stainless steel by X-ray diffraction (XRD) measurement. From the tests, it was found that FCGR was increased in tensile residual stress region and decreased in compressive region. From the fatigue tests on 304 stainless steel, it was found that FCGR of thick specimen was faster
DBEM crack propagation for nonlinear fracture problems
Directory of Open Access Journals (Sweden)
R. Citarella
2015-10-01
Full Text Available A three-dimensional crack propagation simulation is performed by the Dual Boundary Element Method (DBEM. The Stress Intensity Factors (SIFs along the front of a semi elliptical crack, initiated from the external surface of a hollow axle, are calculated for bending and press fit loading separately and for a combination of them. In correspondence of the latter loading condition, a crack propagation is also simulated, with the crack growth rates calculated using the NASGRO3 formula, calibrated for the material under analysis (steel ASTM A469. The J-integral and COD approaches are selected for SIFs calculation in DBEM environment, where the crack path is assessed by the minimum strain energy density criterion (MSED. In correspondence of the initial crack scenario, SIFs along the crack front are also calculated by the Finite Element (FE code ZENCRACK, using COD, in order to provide, by a cross comparison with DBEM, an assessment on the level of accuracy obtained. Due to the symmetry of the bending problem a pure mode I crack propagation is realised with no kinking of the propagating crack whereas for press fit loading the crack propagation becomes mixed mode. The crack growth analysis is nonlinear because of normal gap elements used to model the press fit condition with added friction, and is developed in an iterative-incremental procedure. From the analysis of the SIFs results related to the initial cracked configuration, it is possible to assess the impact of the press fit condition when superimposed to the bending load case.
Molecular dynamics simulation of propagating cracks
Mullins, M.
1982-01-01
Steady state crack propagation is investigated numerically using a model consisting of 236 free atoms in two (010) planes of bcc alpha iron. The continuum region is modeled using the finite element method with 175 nodes and 288 elements. The model shows clear (010) plane fracture to the edge of the discrete region at moderate loads. Analysis of the results obtained indicates that models of this type can provide realistic simulation of steady state crack propagation.
Local and non-local approaches to creep crack initiation and propagation
Mikhailov, SE; Namestnikova, IV
2003-01-01
A functional form of local brittle strength conditions for a time- or history-dependent materials is presented. The particular strength condition associated with the Robinson linear damage accumulation rule and the power-type durability diagram is employed to formulation and analysis of creep crack initiation and propagation problem. The problem is reduced to a non-linear integral Volterra equation, which can be transformed to a linear one for the case of a single crack. Analytical solutions ...
Yang, Sheng-Qi; Tian, Wen-Ling; Huang, Yan-Hua; Ranjith, P. G.; Ju, Yang
2016-04-01
To understand the fracture mechanism in all kinds of rock engineering, it is important to investigate the fracture evolution behavior of pre-fissured rock. In this research, we conducted uniaxial compression experiments to evaluate the influence of ligament angle on the strength, deformability, and fracture coalescence behavior of rectangular prismatic specimens (80 × 160 × 30 mm) of brittle sandstone containing two non-coplanar fissures. The experimental results show that the peak strength of sandstone containing two non-coplanar fissures depends on the ligament angle, but the elastic modulus is not closely related to the ligament angle. With the increase of ligament angle, the peak strength decreased at a ligament angle of 60°, before increasing up to our maximum ligament angle of 120°. Crack initiation, propagation, and coalescence were all observed and characterized from the inner and outer tips of pre-existing non-coplanar fissures using photographic monitoring. Based on the results, the sequence of crack evolution in sandstone containing two non-coplanar fissures was analyzed in detail. In order to fully understand the crack evolution mechanism of brittle sandstone, numerical simulations using PFC2D were performed for specimens containing two non-coplanar fissures under uniaxial compression. The results are in good agreement with the experimental results. By analyzing the stress field, the crack evolution mechanism in brittle sandstone containing two non-coplanar fissures under uniaxial compression is revealed. These experimental and numerical results are expected to improve the understanding of the unstable fracture mechanism of fissured rock engineering structures.
Burrowing mechanics: burrow extension by crack propagation.
Dorgan, Kelly M; Jumars, Peter A; Johnson, Bruce; Boudreau, B P; Landis, Eric
2005-02-01
Until now, the analysis of burrowing mechanics has neglected the mechanical properties of impeding, muddy, cohesive sediments, which behave like elastic solids. Here we show that burrowers can progress through such sediments by using a mechanically efficient, previously unsuspected mechanism--crack propagation--in which an alternating 'anchor' system of burrowing serves as a wedge to extend the crack-shaped burrow. The force required to propagate cracks through sediment in this way is relatively small: we find that the force exerted by the annelid worm Nereis virens in making and moving into such a burrow amounts to less than one-tenth of the force it needs to use against rigid aquarium walls. PMID:15690029
Modelization of crack growth in brittle, disordered materials
Tzschichholz, F
1994-01-01
The thesis consists of four main chapters. In Ch.2 we discuss experimental results concerning the scaling behavior and fractality of fracture surfaces. In Ch.3 continuum and discrete models for fracture mechanics are reviewed and partially extended. In Ch.4 we present numerical results for a finite size scaling of the macroscopic fracture stress in the absence of any disorder in the material. We discuss in Ch.5, the main chapter, the technological important problem of hydraulic fracturing of heterogeneous solids. We have performed intensive computer simulations on this problem and discuss the conditions under which the resulting cracks may develop fractal patterns. We also determine the opening volume of the crack and the elastic stress field in the bulk, quantities that are accessible experimentally. postscript file 'ft_phd93.ps.gz' (1.9Mb) only via ftp server 'ftp.gkss.de'.Login as 'ftp', password: 'your-email', cd /pub/doc, binary mode, get ft_phd93.ps.gz, quit, and then decompress with 'gunzip ft_phd93.ps...
Oscillatory instability of crack propagations in quasi-static fracture
Sasa, Shin-ichi; Sekimoto, Ken
1993-01-01
Crack propagations in quasi-static fracture are studied theoretically. The Griffith theory is applied to discuss a crack extension condition and motion of crack tips in straight propagations. Stability of the straight propagations is investigated based on the simple assumption that a curvature near the crack tip is determined by a singular shear stress. It is shown that straight propagations become unstable under ceatain conditions. Combined with boundary effects, this instability leads to an...
Crack propagation studies and bond coat properties in thermal barrier coatings under bending
Indian Academy of Sciences (India)
A K Ray; N Roy; K M Godiwalla
2001-04-01
Ceramic based thermal barrier coatings (TBC) are currently considered as a candidate material for advanced stationary gas turbine components. Crack propagation studies under bending are described that were performed on plasma sprayed ZrO2, bonded by MCrAlY layer to Ni base superalloy. The crack propagation behaviour of the coatings at room temperature in as received and oxidized conditions revealed a linear growth of the cracks on the coating till the yield point of the super alloy was reached. High threshold load at the interface between the ceramic layer and the bond coat was required to propagate the crack further into the bond coat. Once the threshold load was surpassed the crack propagated into the brittle bond coat without an appreciable increase in the load. At temperatures of 800°C the crack propagated only in the TBC (ceramic layer), as the ductile bond coat offered an attractive sink for the stress relaxation. Effects of bond coat oxidation on crack propagation in the interface region have been examined and are discussed.
In situ fatigue-crack-propagation experiment
International Nuclear Information System (INIS)
An in-reactor fatigue experiment was conducted in the Oak Ridge Research Reactor to determine the effects of dynamic irradiation on fatigue crack propagation. Eight 20% cold-worked 316 stainless steel specimens were precracked to various initial crack lengths, linked together to form a chain, and inserted into a specially designed in-reactor fatigue machine. Test conditions included a maximum temperature of 4600C, an environment of sodium, a frequency of 1 cycle/min, and a stress ratio of 0.10. Results indicated that (1) no effects of dynamic irradiation were observed for a fluence of 1.5 x 1021 n/cm2 (E > 0.1 MeV); and (2) crack growth rates in elevated temperature sodium were a factor of 3 to 4 lower than in room temperature air
International Conference on Dynamic Crack Propagation
1973-01-01
The planning meeting for a conference on Dynamic Crack Propagation was held at M.LT. in February 1971 and attended by research workers from several industrial, governmental and academic organizations. It was felt that a more specialized meeting would provide a better opportunity for both U.S. and foreign researchers to exchange their ideas and views on dynamic fracture, a subject which is seldom emphasized in national or international fracture conferences. Dynamic crack propagation has been a concern to specialists in many fields: continuum mechanics, metallurgy, geology, polymer chemistry, orthopedics, applied mathematics, as well as structural design and testing. It impinges on a wide variety of problems such as rock breaking and earthquakes, pressure vessels and line pipes, comminution and the per formance of armament and ordnance, etc. Advances have been numerous, covering theories and experiments from both the microscopic and macro scopic points of view. Hence, the need for comparing the theoretical ...
Extended propagation model for interfacial crack in composite material structure
Institute of Scientific and Technical Information of China (English)
闫相桥; 冯希金
2002-01-01
An interfacial crack is a common damage in a composite material structure . An extended propaga-tion model has been established for an interfacial crack to study the dependence of crack growth on the relativesizes of energy release rates at left and right crack tips and the properties of interfacial material characterize thegrowth of interfacial crack better.
Experiments on buoyancy-driven crack around the brittle-ductile transition
Sumita, Ikuro; Ota, Yukari
2011-04-01
We report the results of laboratory experiments exploring how a buoyancy-driven liquid-filled crack migrates within a viscoelastic medium whose rheology is around the brittle-ductile transition. To model such medium, we use a low concentration agar, which has a small yield stress and a large yield strain (deformation) when it fractures. We find that around the transition, the fluid migrates as a hybrid of a diapir (head) and a dyke (tail). Here the diapir is a bulged crack in which fracturing occurs at its tip and closes at its tail to form a dyke. A small amount of fluid is left along its trail and the fluid decelerates with time. We study how the shape and velocity of a constant volume fluid change as two control parameters are varied; the agar concentration ( C) and the density difference Δρ between the fluid and the agar. Under a fixed Δρ, as C decreases the medium becomes ductile, and the trajectory and shape of the fluid changes from a linearly migrating dyke to a meandering or a bifurcating dyke, and finally to a diapir-dyke hybrid. In this transition, the shape of the crack tip viewed from above, changes from blade-like to a cusped-ellipse. A similar transition is also observed when Δρ increases under a fixed C, which can be interpreted using a force balance between the buoyancy and the yield stress. Our experiments indicate that cracks around the brittle-ductile transition deviates from those in an elastic medium by several ways, such as the relaxation of the crack bulge, slower deceleration rate, and velocity becoming insensitive to medium rheology. Our experiments suggest that the fluid migrates as a diapir-dyke hybrid around the brittle-ductile transition and that fluid migration of various styles can coexist at the same depth, if they have different buoyancy.
Compression Stress Effect on Dislocations Movement and Crack propagation in Cubic Crystal
Suprijadi,; Yusfi, Meiqorry
2012-01-01
Fracture material is seriously problem in daily life, and it has connection with mechanical properties itself. The mechanical properties is belief depend on dislocation movement and crack propagation in the crystal. Information about this is very important to characterize the material. In FCC crystal structure the competition between crack propagation and dislocation wake is very interesting, in a ductile material like copper (Cu) dislocation can be seen in room temperature, but in a brittle material like Si only cracks can be seen observed. Different techniques were applied to material to study the mechanical properties, in this study we did compression test in one direction. Combination of simulation and experimental on cubic material are reported in this paper. We found that the deflection of crack direction in Si caused by vacancy of lattice,while compression stress on Cu cause the atoms displacement in one direction. Some evidence of dislocation wake in Si crystal under compression stress at high tempera...
Cleavage crack propagation and arrest in a nuclear pressure vessel steel
International Nuclear Information System (INIS)
The integrity assessment of Reactor Pressure Vessels, mainly based on crack initiation, can be completed by studying crack propagation and arrest. Whereas engineering approaches do not take into account dynamic effects, these effects are important in unstable cleavage crack propagation, arrest and possible propagation re-initiation events. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture in the framework of local approach to fracture. Experiments were carried out on thin CT 25 specimens made of 16MND5 PWR vessel steel at five temperatures (-150 degrees C, -125 degrees C, -100 degrees C, -75 degrees C, -50 degrees C). Two kinds of crack path, straight or branching path, were observed. Branching cracks appear for the highest critical loadings at initiation, that increase the elastic stored energy and the effect of plasticity. The elastic-viscoplastic behavior of the ferritic steel was studied up to a strain rate of 104 s-1 and taken into account in the numerical simulations. The extended Finite Element Method (X-FEM) was used in CAST3M FE software to model crack propagation. Numerical computations combine a local non linear dynamic approach with a RKR type fracture stress criterion. The different physical micro-mechanisms, involved in cleavage fracture, were examined by the means of SEM fracture surface analyses at different temperatures and strain rates for the two kinds of crack path. The links of the critical fracture stress with both temperature and strain rate for straight crack path as well as analyses of branching crack phenomena were considered by the means of Scanning Electron Microscopy (SEM) fracture surface analyses, 3D quantitative optical microscopy and FE computations in order to aim at a robust physical justification of the propagation model which has already been developed at CEA in the frame of the B. Prabel PhD. (authors)
Detection of subcritical crack propagation for concrete dams
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
Subcritical propagation of cracks is a warning sign of fracture.If such propagation is detected at an early stage,timely maintenance measures can be taken to prevent the failure of structures.To detect the subcritical propagation of a crack,the crack needs to be monitored continuously in a long term, which is not realistic under certain conditions.However,cracks in concrete dams can be monitored continuously by dam monitoring to offer possible detection for subcritical propagation.In this paper, with measured crack openings from dam monitoring,a state equation for characterizing crack development is established based on the grey system theory.The relation between the stability of the equation and the subcritical crack propagation is investigated,then a criterion is proposed for detecting subcritical propagation.An example demonstrates the validity of the criterion and its potential for practical application.
Directory of Open Access Journals (Sweden)
Yongshui Kang
2014-10-01
Full Text Available Water-bearing rocks exposed to freezing temperature can be subjected to freeze–thaw cycles leading to crack initiation and propagation, which are the main causes of frost damage to rocks. Based on the Griffith theory of brittle fracture mechanics, the crack initiation criterion, propagation direction, and crack length under freezing pressure and far-field stress are analyzed. Furthermore, a calculation method is proposed for the stress intensity factor (SIF of the crack tip under non-uniformly distributed freezing pressure. The formulae for the crack/fracture propagation direction and length of the wing crack under freezing pressure are obtained, and the mechanism for coalescence of adjacent cracks is investigated. In addition, the necessary conditions for different coalescence modes of cracks are studied. Using the topology theory, a new algorithm for frost crack propagation is proposed, which has the capability to define the crack growth path and identify and update the cracked elements. A model that incorporates multiple cracks is built by ANSYS and then imported into FLAC3D. The SIFs are then calculated using a FISH procedure, and the growth path of the freezing cracks after several calculation steps is demonstrated using the new algorithm. The proposed method can be applied to rocks containing fillings such as detritus and slurry.
Overview of strength, crack propagation and fracture of nuclear reactor moderator graphite
Energy Technology Data Exchange (ETDEWEB)
Moskovic, R., E-mail: robert.moskovic@magnoxsites.com [Magnox Limited, Oldbury Technical Centre, Oldbury Naite, South Gloucestershire BS35 1RQ (United Kingdom); Heard, P.J. [Interface Analysis Centre, University of Bristol, Bristol BS2 8BS (United Kingdom); Flewitt, P.E.J. [Magnox Limited, Oldbury Technical Centre, Oldbury Naite, South Gloucestershire BS35 1RQ (United Kingdom); Interface Analysis Centre, University of Bristol, Bristol BS2 8BS (United Kingdom); H.H. Wills Laboratory, Department of Physics, University of Bristol, Bristol BS8 1TL (United Kingdom); Wootton, M.R. [Magnox Limited, Oldbury Technical Centre, Oldbury Naite, South Gloucestershire BS35 1RQ (United Kingdom)
2013-10-15
Highlights: • Fracture behaviour. • Cracking initiation and growth. • Different loadings configurations. • Fracture mechanisms. -- Abstract: Nuclear reactor moderator graphite is an aggregate of needle coke filler particles within a matrix of fine coke flour particles mixed with pitch binder. Following extrusion in green condition, impregnation with liquid pitch binder and graphitisation, a polygranular aggregate with orthotropic properties is produced. Its mechanical properties under several different loading conditions and associated cracking behaviour were examined to establish crack initiation and propagation behaviour. Both virgin and radiolytically oxidised material were examined using optical and electron optical microscopy, focused ion beam microscope and digital image correlation. The appearance of force vs. displacement curves varied with type of loading. Mostly linear elastic traces occurred in uniaxial tensile and flexural tests. Large departures from linear elastic behaviour were observed in standard uniaxial and diametral compression testing. Digital image correlation has shown that the initiation of cracking involves formation of a process zone which grows to a critical size of approximately 3–5 mm before a macro-crack is initiated. Cracks straddle a torturous path which zigzags between the filler particles through the matrix consistent with crack propagation along the filler matrix interface. This paper provides an overview of strength, crack propagation and fracture of nuclear reactor moderator graphite. It reviews the physical processes and mathematical approaches that have been adopted to describe the behaviour of brittle materials and then considers if they apply to reactor core graphites.
Hydride precipitation crack propagation in zircaloy cladding during a decreasing temperature history
International Nuclear Information System (INIS)
An assessment of safety, design, and cost tradeoff issues for short (ten to fifty years) and longer (fifty to hundreds of years) interim dry storage of spent nuclear fuel in Zircaloy rods shall address potential failures of the Zircaloy cladding caused by the precipitation response of zirconium hydride platelets. To perform such assessment analyses rigorously and conservatively will be necessarily complex and difficult. For Zircaloy cladding, a model for zirconium hydride induced crack propagation velocity was developed for a decreasing temperature field and for hydrogen, temperature, and stress dependent diffusive transport of hydrogen to a generic hydride platelet at a crack tip. The development of the quasi-steady model is based on extensions of existing models for hydride precipitation kinetics for an isolated hydride platelet at a crack tip. An instability analysis model of hydride-crack growth was developed using existing concepts in a kinematic equation for crack propagation at a constant thermodynamic crack potential subject to brittle fracture conditions. At the time an instability is initiated, the crack propagation is no longer limited by hydride growth rate kinetics, but is then limited by stress rates. The model for slow hydride-crack growth will be further evaluated using existing available data. (authors)
Directional crack propagation of granular water systems.
Mizuguchi, Tsuyoshi; Nishimoto, Akihiro; Kitsunezaki, So; Yamazaki, Yoshihiro; Aoki, Ichio
2005-05-01
Pattern dynamics of directional crack propagation phenomena observed in drying process of starch-water mixture is investigated. To visualize the three-dimensional structure of the drying-fracture process two kinds of experiments are performed, i.e., resin solidification planing method and real-time measurement of water content distribution with MR instruments. A cross section with polygonal structure is visualized in both experiments. The depth dependency of cell size is measured. The phenomenological model for water transportation is also discussed. PMID:16089617
Dislocation mechanism based model for stage II fatigue crack propagation rate
Mazumdar, P. K.
1986-01-01
Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain: (1) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth, (2) the relative resistance of dislocation mechanisms to the crack growth, and (3) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism. The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental results taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.
Fatigue crack propagation in short-fiber reinforced plastics
Directory of Open Access Journals (Sweden)
K. Tanaka
2015-10-01
Full Text Available The influence of fiber orientation on the crack propagation behavior was studied with single edgenotched specimens which were cut from an injection-molded plate of short-fiber reinforced plastics of polyphenylenesulphide (PPS reinforced with 30wt% carbon fibers. Specimens were cut at five fiber angles relative to the molding direction, i.e. = 0° (MD, 22.5°, 45°, 67.5°, 90° (TD. Fracture mechanics parameters derived based on anisotropic elasticity were used as a crack driving force. Macroscopic crack propagation path was nearly perpendicular to the loading axis for the cases of MD and TD. For the other fiber angles, the crack path was inclined because the crack tended to propagate along inclined fibers. For mode I crack propagation in MD and TD, the resistance to crack propagation is improved by fiber reinforcement, when the rate is correlated to the range of stress intensity factor. The crack propagation rate, da/dN, was slowest for MD and fastest for TD. For each material, the crack propagation rate is higher for larger R ratio. The effect of R ratio on da/dN diminished in the relation between da/dN and the range of energy release rate, GI. Difference among MD, TD and matrix resin becomes small when da/dN correlated to a parameter corresponding the crack-tip radius, HGI, where H is compliance parameter. Fatigue cracks propagated under mixed loading of mode I and II for the fiber angles other than 0° and 90°. The data of the crack propagation rate correlated to the range of total energy release rate, Gtotal, lie between the relations obtained for MD and TD. All data of crack propagation tend to merge a single relation when the rate is correlated to the range of total energy release rate divided by Young’s modulus.
Steady-state propagation of interface corner crack
DEFF Research Database (Denmark)
Veluri, Badrinath; Jensen, Henrik Myhre
2013-01-01
Steady-state propagation of interface cracks close to three-dimensional corners has been analyzed. Attention was focused on modeling the shape of the interface crack front and calculating the critical stress for steady-state propagation of the crack. The crack propagation was investigated by...... estimating the fracture mechanics parameters that includes the strain energy release rate, crack front profiles and the three-dimensional mode-mixity along the interface crack front. A numerical approach was then applied for coupling the far field solutions based on the Finite Element Method to the near...... field (crack tip) solutions based on the J-integral. The adopted two-dimensional numerical approach for the calculation of fracture mechanical properties was compared with three-dimensional models for quarter-circular and straight sided crack front shapes. A quantitative approach was formulated based on...
International Nuclear Information System (INIS)
This article presents the proposition of a new type of tests to characterize the crack initiation and crack propagation under thermal shock. The main advantage of this test is to use a simple modified compact test specimen. In a first part, two thermal shocks performed in the laboratory between 400 deg. C and 20 deg C and their numerical analysis are presented. This first study shows the ability of the test to initiate and propagate a crack which can be well predicted by a numerical analysis. In a second part, a preliminary analysis for thermal shock at lower temperatures (between 20 deg C and - 196 deg C) is presented. It shows that this simple modified CT specimen allows also the study of thermal shock effects near the ductile/brittle transition temperature of the material. (authors)
Three dimensional fatigue propagation modeling of a nozzle corner crack
International Nuclear Information System (INIS)
The problem of nozzle corner fatigue crack propagation has been addressed since more than 25 years (Brookhaven, 1975, Kobayashi, 1979). The high stress concentration in a nozzle corner under pressure and thermal shocks, make necessary the check of fatigue crack initiation and propagation risks in such a zone. Most of the integrity studies carried out over 40 years of service do include fatigue crack propagation assessments. However a realistic analysis of the crack extension is a complex three-dimensional problem. The fatigue growth is driven by the Stress Intensity Factor (SIF) values along the crack front. These SIF depend not only on the crack size but also on its shape and the location along the crack front. Therefore the crack shape is likely to change continuously. The current practice for stress intensity factors calculation consists in using influence functions established for a given type of crack front shape (semi-elliptical) and which take into account only in-depth stress gradients. This leads for instance to an overestimation of the crack extension and is not able to take into account the protective role of a thermal sleeve. The aim of the present study is to get through 3 dimensional finite element computations of the SIF, a more realistic estimate of the fatigue crack propagation of nozzle corner cracks. Namely, a reliable description of the crack shape evolution was expected for analyzing the risk of break through and examining the feasibility of non destructive examinations. (authors)
The role of microcracking on the crack growth resistance of brittle solids and composites
International Nuclear Information System (INIS)
A set of numerical analyses of crack growth was preformed to elucidate the influence of microcracking on the fracture behavior of microcracking brittle solids and composites. The random nucleation, orientation and size effects of discrete nucleating microcracks and resulting interactions are fully accounted for in a hybrid finite element model. The results obtained from the finite element analysis are compared with the continuum description of the microcracking. Although continuum description can provide a reasonable estimation of shielding, it fails to resolve the details of micromechanism of toughening resulting from microcracking, since not every shielding event during the course of crack extension corresponds to an increase in the R-curve. Moreover, as seen in the composite cases, the local events leading to toughening behavior may not be associated with the microcracking even in the presence of a large population of microcracks
Axial crack propagation and arrest in pressurized fuselage
Kosai, M.; Shimamoto, A.; Yu, C.-T.; Walker, S. I.; Kobayashi, A. S.; Tan, P.
1994-01-01
The crack arrest capability of a tear strap in a pressurized precracked fuselage was studied through instrumented axial rupture tests of small scale models of an idealized fuselage. Upon pressurization, rapid crack propagation initiated at an axial through crack along the stringer and immediately kinked due to the mixed modes 1 and 2 state caused by the one-sided opening of the crack flap. The diagonally running crack further turned at the tear straps. Dynamic finite element analysis of the rupturing cylinder showed that the crack kinked and also ran straight in the presence of a mixed mode state according to a modified two-parameter crack kinking criterion.
Anomalous crack propagation in reinforced natural rubber
Sotta, Paul; Gabrielle, Brice; Long, Didier; Vanel, Loic; Albouy, Pierre-Antoine; Peditto, Francesca
2009-03-01
In reinforced natural rubber, crack propagation in mode I exhibits rotation of the tear in a direction perpendicular to the usual one. Our objective is, first, to understand the impact of this phenomenon on fracture toughness of the material, and, secondly, to understand how this phenomenon is related to the specific properties of reinforced natural rubber. To this aim, we combine measurements of ultimate properties, measurements of the number and length of tear rotations as a function of loading velocity and temperature, and investigation of material heterogeneities at sub-micrometric scales, originating both from fillers and strain-induced crystallites (strain-induced crystallinity is measured up to failure by X ray diffraction), in natural rubber samples reinforced by nanometric aggregates. Observations suggest that tear rotation is related both to the mechanical anisotropy induced by strain-induced crystallinity and to the dissipative properties of the material at high strain.
Laser-Based Instrument Measures Propagation Of Cracks
Lee, Rupert U.; Cox, Robert B.; Youngquist, Robert C.; Sentz, John T.; Rose, Kenneth A.
1995-01-01
Report describes use of commerical laser displacement meter to measure propagation of cracks in stainless-steel specimens in stress tests in corrosive (salt-spray) environment. Measurements directed toward determining time from beginning of each test until onset of propagation of crack.
The effect of welding residual stresses on brittle fracture in an internal surface cracked pipe
International Nuclear Information System (INIS)
The change in brittle fracture properties due to the presence of welding residual stresses (RS) is studied in a pipe. Welding RS are simulated by using a 3-D finite element (FE) model and experimentally verified. An internal circumferential thumbnail crack is introduced at the weld line. The modified Beremin model is used as local approach method to predict toughness distributions in the presence of welding RS. The model is calibrated using experimental fracture tests at −150 °C consisting of low and high constraint specimens. The results indicate that, in the welded pipe, the fracture toughness will decrease dramatically in comparison with the as-received pipe. For the same fracture probability of 90%, fracture toughness will decrease from 65 to 40 MPa √m. This is due to the influence of tensile welding RS on the crack tip stress state, which result in a decrease in the opening mode stresses at the near crack tip up to 40%. - Highlights: • A Two-pass pipe welding process is simulated and obtained residual stresses (RS) are experimentally verified. • An internal semi-elliptical circumferential cracked pipe is considered and Modified Beremin model is used. • Two sets of specimens with high and low crack-tip constraint are used for model calibration. • Welding RS will cause to change crack-tip stress field and fracture probability, dramatically. • RS cause 40% increase in maximum opening stress and 38% decrease in fracture toughness, for a fracture probability of 90%
Monte Carlo simulation of micro crack propagation behavior for stress corrosion cracking
International Nuclear Information System (INIS)
A calculation code of Monte Carlo simulation for micro crack propagation behavior of stress corrosion cracking has been developed. Improvements for micro crack growth rate treatments and stress distributions have been made. Calculated crack depth distributions were compared with the CBB test results for sensitized stainless steels and low carbon stainless steels with hardened layer. For sensitized stainless steels, the calculated crack depth distribution could well reproduce the CBB test results by taking into account crack growth rates obtained from experiments. For low carbon stainless steels, although considering stress distributions improved the overestimation of crack depths, the calculated crack depth distribution could not well reproduce the CBB test result. The results revealed that the effects of the crack growth rate and the stress distribution on micro crack propagation behaviors. (author)
Propagation of stress corrosion cracks in alpha-brasses
Energy Technology Data Exchange (ETDEWEB)
Beggs, Dennis Vinton
1981-01-01
Transgranular and intergranular stress corrosion cracks were investigated in alpha-brasses in a tarnishing ammoniacal solution. Surface observation indicated that the transgranular cracks propagated discontinuously by the sudden appearance of a fine crack extending several microns ahead of the previous crack tip, often associated with the detection of a discrete acoustic emission (AE). By periodically increasing the deflection, crack front markings were produced on the resulting fracture surfaces, showing that the discontinuous propagation of the crack trace was representative of the subsurface cracking. The intergranular crack trace appeared to propagate continuously at a relatively blunt crack tip and was not associated with discrete AE. Under load pulsing tests with a time between pulses, ..delta..t greater than or equal to 3 s, the transgranular fracture surfaces always exhibited crack front markings which corresponded with the applied pulses. The spacing between crack front markings, ..delta..x, decreased linearly with ..delta..t. With ..delta..t less than or equal to 1.5 s, the crack front markings were in a one-to-one correspondence with applied pulses only at relatively long crack lengths. In this case, ..delta..x = ..delta..x* which approached a limiting value of 1 ..mu..m. No crack front markings were observed on intergranular fracture surfaces produced during these tests. It is concluded that transgranular cracking occurs by discontinuous mechanical fracture of an embrittled region around the crack tip, while intergranular cracking results from a different mechanism with cracking occurring via the film-rupture mechanism.
Dynamic delamination crack propagation in a graphite/epoxy laminate
Grady, J. E.; Sun, C. T.
1991-01-01
Dynamic delamination crack propagation in a (90/0) 5s Graphite/Epoxy laminate with an embedded interfacial crack was investigated experimentally using high speed photography. The dynamic motion was produced by impacting the beamlike laminate specimen with a silicon rubber ball. The threshold impact velocities required to initiate dynamic crack propagation in laminates with varying initial crack positions were determined. The crack propagation speeds were estimated from the photographs. Results show that the through the thickness position of the embedded crack can significantly affect the dominant mechanism and the threshold impact velocity for the onset of crack movement. If the initial delamination is placed near the top of bottom surface of the laminate, local buckling of the delaminated plies may cause instability of the crack. If the initial delamination lies on the midplane, local buckling does not occur and the initiation of crack propagation appears to be dominated by Mode II fracture. The crack propagation and arrest observed was seen to be affected by wave motion within the delamination region.
Crack propagation analysis of rotating shafts by vibration measurement
International Nuclear Information System (INIS)
The valuation of rotor crack damage is done by shaft vibration monitoring followed by an interpretatioin of the results, which are frequently supported by calculations. The connection between crack depth, contour and shape of the crack surfaces of propagating transverse crack and the vibration response of a slender uniform shaft made of a material, which is tough to fracture, is experimentally found and is illustrated with the help of examples. Beach marks are set in the structural surface to mark the correct contour of the cracked cross section at selected points of time. Suggestions are made how cracked rotors can be modelled. (orig.)
International Nuclear Information System (INIS)
Simultaneous time-resolved measurements of photon emission (PE) and fast crack propagation upon bending fracture were conducted in silica glass and soda lime glass. Observation of fracture surfaces revealed that macroscopic crack propagation behavior was similar between the silica glass and soda lime glass when fracture loads for these specimens were comparable and cracks propagated without branching. However, a large difference in the PE characteristics was found between the two glasses. In silica glass, PE (645–655 nm) was observed during the entire crack propagation process, whereas intense PE (430–490 nm and 500–600 nm) was observed during the initial stages of propagation. In contrast, only weak PE was detected in soda lime glass. These results show that there is a large difference in the atomic processes involved in fast crack propagation between these glasses, and that PE can be used to study brittle fracture on the atomic scale
Environmentally Influenced Mixed Mode Fatigue Crack Propagation of Titanium Metal Matrix Composites
Mahulikar, Deepak; Marcus, H. L.
1984-01-01
Effect of humid air environments on the mixed mode fatigue crack propagation behavior of B4C-B and BORSIC reinforced Ti-6A1-4V metal matrix composites was studied. Humid environments enhanced the mixed mode fatigue crack propagation rates in the as-received titanium matrix composites at R = 0.1. The effect was more pronounced in the transverse and 45 deg specimens. A transition in failure modes from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. The transition took place at around 100 Pa water vapor pressure, where a steep rise in fatigue crack propagation rate was noticed. At R = 0.5, however, no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The fiber splitting is suggested as an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields (radial and mode III stresses) on the brittle boron fibers weakened by the humidity.
Small fatigue crack propagation in Y2O3 strengthened steels
International Nuclear Information System (INIS)
This paper is focused on two type of Y2O3 strengthened steels (Fe–14Cr ODS and ODS-EUROFER). Small fatigue crack propagation was experimentally measured using special small cylindrical specimens (diameter 2 and 2.6 mm) with shallow notch grinded in the gauge length. In the middle of this notch, a pre-crack of length of 50 μm was fabricated using a focused ion beam technique. Fatigue crack growth rate was measured for different applied total strain amplitudes and described using plastic part of the J-integral. Obtained results were compared with published data of EUROFER 97. The effect of the oxide dispersion on small fatigue crack propagation was found rather insignificant. Ferritic Fe–14Cr ODS steel shows more brittle behaviour, i.e. for the same cyclic plasticity, characterised by the plastic part of the J-integral, the small cracks grow faster. A new methodology for residual lifetime prediction of structures containing physically small cracks, based on plastic part of the J-integral, is presented
Riesch, J.; Höschen, T.; Linsmeier, Ch; Wurster, S.; You, J.-H.
2014-04-01
Tungsten is a promising candidate for the plasma-facing components of a future fusion reactor, but its use is strongly restricted by its inherent brittleness. An innovative concept to overcome this problem is tungsten fibre-reinforced tungsten composite. In this paper we present the first mechanical test of such a composite material using a sample containing multiple fibres. The in situ fracture experiment was performed in a scanning electron microscope for close observation of the propagating crack. Stable crack propagation accompanied with rising load bearing capacity is observed. The fracture toughness is estimated using the test results and the surface observation.
International Nuclear Information System (INIS)
An electrochemical model for crack propagation in film-covered metals, based on the film- rupture concept in stress corrosion cracking (SCC) of metals and quantitative electrochemical kinetics considerations, is presented and discussed. Using a general expression relating the rate of crack propagation with the electrochemical parameters of metal-environment systems, it is shown that the crack propagation rate depends on the rate constant of the metal dissolution reaction at the film-free crack tip, the ratio between the rate constants of the cathodic and metal dissolution reactions at the outer metal surface as well as on the reversible potentials of the cathodic, metal dissolution and film-forming reactions involved. It is also shown that the model can be applied for calculating the rate of crack propagation and explaining the differences in SCC behaviour of various metal-environment systems as well as for defining electrochemical conditions favourable for SCC. (Original)
CRACK PROPAGATION IN STRUCTURES SUBJECTED TO PERIODIC EXCITATION
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
In the present paper, a simple mechanical model is developed to predict the dynamic response of a cracked structure subjected to periodic excitation, which has been used to identify the physical mechanisms in leading the growth or arrest of cracking. The structure under consideration consists of a beam with a crack along the axis, and thus, the crack may open in Mode Ⅰ and in the axial direction propagate when the beam vibrates. In this paper, the system is modeled as a cantilever beam lying on a partial elastic foundation, where the portion of the beam on the foundation represents the intact portion of the beam. Modal analysis is employed to obtain a closed form solution for the structural response. Crack propagation is studied by allowing the elastic foundation to shorten (mimicking crack growth) if a displacement criterion, based on the material toughness, is met. As the crack propagates, the structural model is updated using the new foundation length and the response continues. From this work, two mechanisms for crack arrest are identified. It is also shown that the crack propagation is strongly influenced by the transient response of the structure.
Experimental study of thermodynamics propagation fatigue crack in metals
International Nuclear Information System (INIS)
This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip
Crack propagation behavior of graphite under cyclic loading condition
International Nuclear Information System (INIS)
The results are described of crack propagation test under cyclic loading condition of graphites used in components of High Temperature Gas Cooled Reactor. The remodelled double cantilever beam specimen of graphite, having a crack range of constant stress intensity factor at the crack edge was used. Three kinds of graphites made in Japan and foreign countries were used; JA (fine grained, isotropic graphite), FA (fine grained, isotropic graphite) and FB (near isotropic graphite). The three had different coke grain sizes and mechanical properties. Following are the results. Critical stress intensity factor K sub( i c) in the graphites are FA JA > FB. The route of crack propagation appears to depend on coke grain size of the graphite. The effects of mean stress intensity factor K sub(m) and maximum stress intensity factor K sub(max) on the crack propagation rate are discussed. (author)
Scaling of crack propagation in rubber sheets
Chen, C. H.; Zhang, H. P.; Niemczura, J.; Ravi-Chandar, K.; Marder, M.
2011-11-01
We have conducted experiments and numerical simulations to investigate supersonic cracks. The experiments are performed at 85 °C to suppress strain-induced crystallites that complicate experiments at lower temperature. Calibration experiments were performed to obtain the parameters needed to compare with a theory including viscous dissipation. We find that both experiments and numerical simulations support supersonic cracks, and we discover a transition from subsonic to supersonic as we plot experimental crack speed curves vs. extension ratio for different sized samples. Both experiments and simulations show two different scaling regimes: the speed of subsonic cracks scales with the elastic energy density while the speed of supersonic cracks scales with the extension ratio. Crack openings have qualitatively different shapes in the two scaling regimes.
Subsurface crack initiation and propagation mechanisms in gigacycle fatigue
International Nuclear Information System (INIS)
In the very high cycle regime (Nf > 107 cycles) cracks can nucleate on inclusions, 'supergrains' and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.
Observations on the nature of micro-cracking in brittle composites
International Nuclear Information System (INIS)
The degree of micro-cracking in BeO-SiC composites due to internal stresses which arise from the mismatch in the coefficients of thermal expansion was monitored by measurements of the thermal diffusivity by the laser-flash technique. The experimental results indicated that micro-cracking was most extensive at approximately 30 and 80 wt % SiC and a minimum at nearly 50 wt % SiC. A theoretical analysis indicated that the magnitude of internal stress increases linearly with SiC content, so that the above observations cannot be attributed to a low internal stress state at approximately 50 wt % SiC. Instead, this effect can be attributed to changes in the statistical variables affecting the brittle fracture as well as the degree of internal stress relaxation. Both these factors are thought to be controlled by the nature of multiaxial stress distribution. At approximately 50 wt % SiC-content, due to anticipated non-hydrostatic triaxial stress distribution, residual stress relaxation is possible in both the components of the composite. However, at low and high fractions of SiC content, such stress relaxation is less likely to occur due to the expected hydrostatic stress distribution in one of the components. (author)
Influence of the residual stresses on crack initiation in brittle materials and structures
International Nuclear Information System (INIS)
Many material assemblies subjected to thermo-mechanical loadings develop thermal residual stresses which modify crack onset conditions. Besides if one of the components has a plastic behaviour, plastic residual deformations may also have a contribution. One of the issues in brittle fracture mechanics is to predict crack onset without any pre-existing defect. Leguillon proposed an onset criterion based on both a Griffth-like energetic condition and a maximum stress criterion. The analysis uses matched asymptotics and the theory of singularity. The good fit between the model and experimental measurements led on homogeneous isotropic materials under pure mechanical loading incited us to take into account residual stresses in the criterion. The comparison between the modified criterion and the experimental measurements carried out on an aluminum/epoxy assembly proves to be satisfying concerning the prediction of failure of the interface between the two components. Besides, it allows, through inversion, identifying the fracture properties of this interface. The modified criterion is also applied to the delamination of the tile/structure interface in the plasma facing components of the Tore Supra tokamak. Indeed thermal and plastic residual stresses appear in the metallic part of these coating tiles. (author)
Surface effects and assessment of crack propagation
International Nuclear Information System (INIS)
A realistic analysis of microcrack interaction at stress concentrations require a three dimensional analysis, and evaluation of fracture parameters along the crack front. Due to its complexity the problem can only be tackled through computational techniques. In this work, we describe the use of the Boundary Element Technique for analysis of surface effects at crack vertex. It is shown that the existence of a vertex singularity at this location poses limitations on actual crack geometries, and hence some common assumptions might not be adequate for a realistic description of surface crack growth. (author). 15 refs, 5 figs
Fatigue Crack Initiation and Propagation of Aluminum Alloy Bearings
Institute of Scientific and Technical Information of China (English)
CHENG Xian-Hua; MA Yan-Yan
2004-01-01
Observation of fatigue crack initiation and propagation during fatigue test in ALSn20Cu bearing has been presented. Journal center orbit, oil film pressure and stress distribution in alloy layer have been calculated and are taken as the basis for theoretically simulating the bearing fatigue process. It is found that the calculated results are in good accordance with the experimental results, which provides a feasible way for investigation of fatigue crack propagation process in the bearing.
Crack Propagation in Plane Strain under Variable Amplitude Loading
DEFF Research Database (Denmark)
Ricardo, Luiz Carlos Hernandes
Crack propagation simulation began with developing of finite element method; the analyses were conducted to obtain a basic understanding of the crack growth and closure processes. Today structural and materials engineers develop structures and materials properties using this technique. In this pa...
Fatigue crack propagation in carburized X-2M steel
Averbach, B. L.; Lou, Bingzhe; Pearson, P. K.; Fairchild, R. E.; Bamberger, E. N.
1985-07-01
The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel (0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on the crack propagation rates. We define an effective stress intensity, Ke = Ka + Ki, where Ka is the applied stress intensity, Ki = σid{i/1/2}, σi is the internal stress, and di is a characteristic distance associated with the depth of the internal stress field. In our work, a value of di = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses.
Suppression of Fatigue Crack Propagation of Duralumin by Cavitation Peening
Directory of Open Access Journals (Sweden)
Hitoshi Soyama
2015-08-01
Full Text Available It was demonstrated in the present paper that cavitation peening which is one of the mechanical surface modification technique can suppress fatigue crack propagation in duralumin. The impacts produced when cavitation bubble collapses can be utilised for the mechanical surface modification technique in the same way as laser peening and shot peening, which is called “cavitation peening”. Cavitation peening employing a cavitating jet in water was used to treat the specimen made of duralumin Japanese Industrial Standards JIS A2017-T3. After introducing a notch, fatigue test was conducted by a load-controlled plate bending fatigue tester, which has been originally developed. The fatigue crack propagation behavior was evaluated and the relationship between the fatigue crack propagation rate versus stress intensity factor range was obtained. From the results, the fatigue crack propagation rate was drastically reduced by cavitation peening and the fatigue life of duralumin plate was extended 4.2 times by cavitation peening. In addition, the fatigue crack propagation can be suppressed by 88% in the stable crack propagation stage by cavitation peening.
Particle Discrete Method Based on Manifold Cover for Crack Propagation of Jointed Rock Mass
Directory of Open Access Journals (Sweden)
Yang Ping
2014-01-01
Full Text Available The rock mass can be assumed to be homogeneous material from a macroscopic view; however, it is the heterogeneous material in mesoscopic scale and its physicomechanical properties are discontinuous in space. The failure of jointed rock mass was usually caused by the initiation, propagation, and coalescence of new wing cracks derived from primary joint. In order to further study the rock fracture instability, we need to study the expansion of rock cracks under external loads from the macro-meso perspective. This paper, based on the manifold cover concept, proposes a new discrete element numerical method, manifold particle discrete (MPD, combined with the particle contact model and the introduced concept of stress boundary. The proposed method can easily simulate the crack generation, propagation, and coalescence of jointed rock mass from the macro-meso perspective. The whole process of rock fragmentation is thereafter reproduced. By analyzing the manifold cover and sphere particle model, this paper constitutes the sphere unit cover function of three-dimensional manifold cover, establishes tetrahedron units, and obtains the equilibrium equation and compatible equation of the MPD model. For rock-like brittle material, crack propagation process can be simulated, and it also verifies the accuracy of the proposed numerical method.
Propagation and arrest of cleavage cracks in a nuclear pressure vessel steel
International Nuclear Information System (INIS)
The safety of nuclear structures is crucial while the service time of nuclear power stations is planned to be extended up to 60 years. Initiation stage of cracks is still considered as a key issue, but more and more component integrity analyses investigate the crack arrest possibility. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture. Experiments using standard measuring techniques and a high-speed framing camera system, as well as Scanning Electron Microscope fracture surface analyses were carried out on thin CT specimens made of 16MND5 PWR vessel steel. The elastic-viscoplastic behavior of the ferritic steel has been studied and taken into account in numerical simulations. The extended Finite Element Method (X-FEM) is used in CAST3M finite element analysis software to model crack propagation. Numerical computations combine a local non-linear dynamic approach and a fracture criterion based on critical cleavage stress, whereas current standards in the nuclear field use a global static approach to fracture to depict crack initiation and arrest. The links of the criterion with temperature and strain rate are considered thanks to experiments, SEM fractographies and 2D computations in order to get a robust physical model which can be effective for model-based predictions of industrial structures. (authors)
FEM Modeling of Crack Propagation in a Model Multiphase Alloy
Institute of Scientific and Technical Information of China (English)
Lihe QIAN; Seishi NISHIDO; Hiroyuki TODA; Tosliro KOBAYASHI
2006-01-01
In this paper, several widely applied fracture criteria were first numerically examined and the crack-tip-region Jintegral criterion was confirmed to be more applicable to predict fracture angle in an elastic-plastic multiphase material. Then, the crack propagation in an idealized dendritic two-phase Al-7%Si alloy was modeled using an elastic-plastic finite element method. The variation of crack growth driving force with crack extension was also demonstrated. It is found that the crack path is significantly influenced by the presence of α-phase near the crack tip, and the crack growth driving force varies drastically from place to place. Lastly, the simulated fracture path in the two-phase model alloy was compared with the experimentally observed fracture path.
Numerical modeling of crack propagation using different criteria
International Nuclear Information System (INIS)
Full text: Modeling the propagation of a crack is of prime importance in fracture mechanics. The finite element method has proved to be very well suited for the study of fracture mechanics. Nevertheless, modeling the propagation of a crack through a finite element mesh turns out to be difficult because of the remeshing stage, which implies modifications of the mesh topology. We suggest here to compute the crack propagation direction using three different crack propagation criteria and then to propagate it with a specific remeshing stage. This technique has been developed in a special version of the FORGE2 code which deals with viscoplastic or elastic-viscoplastic multimaterials structures. The remeshing stage is based on a Delaunay type algorithm and enables to deal with multiple edges and multiple materials. The stage of crack initiation is based on a critical stress or a critical damage parameter. If these values are exceeded, an internal line representing the crack is added to the definition of the domain and a remeshing stage is performed. Once the crack has been initiated, the crack propagation direction is computed using one of the three following criteria: the maximal circumferential stress criterion, the strain energy density fracture criterion and the maximal strain energy release rate criterion. Special attention is paid to the implementation techniques and the comparison of these criteria on elastic or elastic-plastic examples. The Gθ method introduced by Destuynder is used to compute the strain energy release rate. This method turns out to be very accurate and is easily generalized to elastic-plastic or thermo-dependent materials. Several examples of validation are presented and comparisons of crack path predictions by these criteria are detailed. Refs. 3 (author)
Rapid crack propagation in PE-HD pipes
Energy Technology Data Exchange (ETDEWEB)
Farshad, M.; Flueler, P. [EMPA, Duebendorf (Switzerland)
1995-12-31
Rapid crack propagation (RCP) in polymer pipes was reviewed. A theoretical model for crack initiation and propagation was described. Experimental results obtained for RCP in PE-HD pipes were discussed. It was noted that RCP only appeared above a certain `critical pressure`. Critical pressure was dependent on temperature, pipe dimensions, pipe processing, pipe material properties, residual stresses, aging, and service conditions. Further work was recommended in the areas of RCP testing, development of theoretical models, extension of investigations to longer (5 m) pipes, and development of crack arresters. 2 figs., 7 refs.
Shock-induced brittle cracking in HVPE-GaN processed by laser lift-off techniques
International Nuclear Information System (INIS)
A study on brittle cracking in GaN films processed by laser lift-off is presented. Two kinds of cracks were found in the N-polar face of GaN after the laser lift-off process, namely perpendicular cracks along the {1 −1 0 0} planes and lateral cracks along the (0 0 0 −1) plane, respectively. Single-shot laser damage is studied to understand the cracking mechanism. The damage morphology indicates that the GaN material on the edge of the laser ablation area experiences three loading modes: shear stress PS, longitudinal compressive stress PL and transverse tensile stress PT. Under shock PL, lateral cracks likely appear and extend from the illuminated region along the interface in mode I. Furthermore, two different kinds of perpendicular cracks were found, namely shear cracks (PC I) and deflection cracks (PC II). A strong PS gives rise to PC I while a cooperative action of PL and PT results in PC II. In addition, there exist a critical effective spot size dPth and a critical ratio of the laser spot size dL to the effective spot size dP, when cracks occur over them. (paper)
Influence of Re on the propagation of a Ni/Ni3 Al interface crack by molecular dynamics simulation
International Nuclear Information System (INIS)
The influence of Re on the propagation of a (0 1 0)[1 0 1] crack in the Ni/Ni3Al interface, including crack propagation velocity, crack-tip shape, and dislocation emission, is investigated using a molecular dynamics method with a Ni–Al–Re embedded-atom-method potential. The propagation velocity of the crack noticeably decreases at 5 K when 3 or 6 at% Re atoms are added into the Ni matrix. At 1033 K, the crack tip becomes blunter and emission of dislocations becomes easier with Re addition, owing to the larger bond strength between Re and Ni atoms. Furthermore, we calculate the unstable stacking energy (γus), surface energy (γs), and adhesion work (Wad) of the interface. When Re atoms are randomly doped into a Ni matrix, γs/γus increases correspondingly. This means that Re addition decreases brittleness and improves ductility. The calculation also shows that γus is not affected by Re–Ni atomic interaction, and that Re–Re atomic interaction has some effect on γus. In addition, Wad increases with Re addition, and a small increase in Wad results in a larger decrease in crack velocity. This indicates that Re–Ni atomic interaction restrains crack propagation velocity at low temperature. (paper)
Dynamic propagation and cleavage crack arrest in bainitic steel
International Nuclear Information System (INIS)
In complement of the studies of harmfulness of defects, generally realized in term of initiation, the concept of crack arrest could be used as complementary analyses to the studies of safety. The stop occurs when the stress intensity factor becomes lower than crack arrest toughness (KIa) calculated in elasto-statics (KI ≤ KIa). The aim of this thesis is to understand and predict the stop of a crack propagating at high speed in a 18MND5 steel used in the pressure water reactor (PWR). The test chosen to study crack arrest is the disc thermal shock test. The observations under the scanning electron microscope of the fracture surface showed that the crack arrest always occurs in cleavage mode and that the critical microstructural entity with respect to the propagation and crack arrest corresponds to at least the size of the prior austenitic grain. The numerical analyses in elasto-statics confirm the conservatism of the codified curve of the RCC-M with respect to the values of KIa. The dynamic numerical analyses show that the deceleration of the crack measured at the end of the propagation is related to the global dynamic of the structure (vibrations). The transferability to components of crack arrest toughness obtained from tests analysed in static is thus not assured. The disc thermal shock tests were also modelled by considering a criterion of propagation and arrest of the type 'RKR' characterized by a critical stress sc which depends on the temperature. The results obtained account well for the crack jump measured in experiments as well as the shape of the crack arrest front. (author)
Lawn, Brian
1993-01-01
This is an advanced text for higher degree materials science students and researchers concerned with the strength of highly brittle covalent-ionic solids, principally ceramics. It is a reconstructed and greatly expanded edition of a book first published in 1975. The book presents a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective. Particular attention is directed to the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics. These elements hold the key to the future of ceramics as high-technology materials--to make brittle solids strong, we must first understand what makes them weak. The underlying theme of the book is the fundamental Griffith energy-balance concept of crack propagation. The early chapters develop fracture mechanics from the traditional continuum perspective, with attention to linear and nonlinear crack-tip fields, equilibrium and non-equilibrium crack states. It then describes the at...
Propagation and interactions of cracks in Si induced by H supply into He-filled cracks
International Nuclear Information System (INIS)
The phenomena of interaction and propagation of cracks under the contribution of hydrogen were studied in (001) silicon substrate in which an array of scattered over-pressurized He-plates was previously introduced at a given depth. Their propagation under subcritical regime was activated through diffusional supply of H atoms introduced by implantation/annealing. Interactions between the tips of non coplanar cracks take place in a nanometric scale; they can be of plastic-type leading to the formation of extended defects or of elastic-type resulting in deviations of crack-tip propagation. While the planar interactions facilitate the propagation of cracks, those of non coplanar-type stop them. The observations were carried out by transmission electron microscopy and the results were discussed and modelled by using concepts of elasticity and fracture mechanics.
Criterion of cleavage crack propagation and arrest in a nuclear PWR vessel steel
International Nuclear Information System (INIS)
The purpose of this PhD thesis is to understand physical mechanisms of cleavage crack propagation and arrest in the 16MND5 PWR vessel steel and to propose a robust predicting model based on a brittle fracture experimental campaign of finely instrumented laboratory specimens associated with numerical computations. First, experiments were carried out on thin CT25 specimens at five temperatures (-150 C, -125 C, -100 C, -7 C, -50 C). Two kinds of crack path, straight or branching path, have been observed. To characterize crack propagation and to measure crack speed, a high-speed framing camera system was used, combined with the development of an experimental protocol which allowed to observe CT surface without icing inside the thermal chamber and on the specimen. The framing camera (520 000 fps) has allowed to have a very accurate estimation of crack speed on the complete ligament of CT (∼ 25 mm). Besides, to analyse experiments and to study the impact of viscosity on the mechanical response around the crack tip, the elastic-viscoplastic behavior of the ferritic steel has been studied up to a strain rate of 104 s-1 for the tested temperatures.The extended Finite Element Method (X-FEM) was used in CAST3M FE software to model crack propagation. Numerical computations combine a local non linear dynamic approach with a RKR type fracture stress criterion to a characteristic distance. The work carried out has confirmed the form of the criterion proposed by Prabel at -125 C, and has identified the dependencies of the criterion on temperature and strain rate. From numerical analyzes in 2D and 3D, a multi-temperature fracture stress criterion, increasing function of the strain rate, was proposed. Predictive modeling were used to confirm the identified criterion on two specimen geometries (CT and compressive ring) in mode I at different temperatures. SEM observations and 3D analyzes made with optical microscope showed that the fracture mechanism was the cleavage associated
Effect of copper on crack propagation in beryllium single crystals
International Nuclear Information System (INIS)
The effect of copper additives on the fracture energy and the development of cracks parallel to the basal plane was studied in zone-refined single crystalline beryllium. At 770K the cleavage planes are very smooth, so the crack propagation energy, which is independent of copper content (less than 2 at. percent Cu) in the range of measurement accuracy, is only a little higher than the surface energy of the basal plane. At room temperature, due to intense plastic processes taking place in front of the crack tip, the fracture energy is an order of magnitude higher than at low temperatures. The effect of copper on the plastic processes can be divided into two regions. In region I (less than 1.2 at. percent Cu), in which the crack propagation energy increases sharply with increasing copper content, crack propagation is controlled by prism slips. The decrease in crack propagation energy in region II (greater than 1.2 at. percent Cu) can be attributed to a reduction of beryllium twinning energy with increasing copper content. (auth)
Parallel FEM simulation of 3-D crack propagation
International Nuclear Information System (INIS)
Full text: Crack propagation simulation is an important topic in many fields, e.g., aeronautical engineering, material sciences, and geophysics. This type of simulation requires a high computational power, mainly at three-dimensional mesh generation and structural analysis steps. These steps usually spend a large amount of computing time and machine resources. The main objective of this work is to provide a fast and accurate system for crack growth simulation in three-dimensional models. The main idea of the methodology presented is to parallelize mesh generation and structural analysis procedures, and to integrate these procedures into a computational environment able to perform automatic arbitrary crack propagation. A parallel mesh generation algorithm has been developed. This algorithm is capable of generating three-dimensional meshes of tetrahedral elements in arbitrary domains with one or multiple embedded cracks. A finite element method program called FEMOOP has been adapted to implement the parallel features. The parallel strategy to solve the set of linear equations is based on an element-by-element scheme in conjunction with a gradient iterative solution. A program called FRANC3D, which is completely integrated with other components of the system, performs crack propagation and geometry updates. The entire system is described in details and a set of parallel simulations of crack propagation are presented to show the reliability of the system. Refs. 4 (author)
Dynamic crack propagation in rock plates
Energy Technology Data Exchange (ETDEWEB)
Holloway, D.C.; Barker, D.B.; Fourney, W.L.
1980-01-01
Rock plates (nominally 20 x 20 x 1.3 cm) of pink Westerly granite, St. Paul Limestone, and Chambersburg Limestone were explosively loaded and observed by high speed photography. The explosive was located in a central hole and the explosive gases were contained with sealed steel caps. The specimens were photographed during the explosive event by 3 different techniques: rotating drum camera, Cranz-Schardin camera used in a reflection mode, and pulsed ruby laser with holographic interferometry. It was determined that the borehole radial cracks in the granite traveled at approx. 44% of the P wave velocity. The borehole radial cracks in limestone initially traveled at ca 28% of the P wave velocity and then quickly slowed to 12% of the P wave speed. Upon the reflection of the stress waves from the boundaries of the model, several cracks were initiated at flaw sites remote from the borehole. In the limestone, these cracks were traveling at speeds on the order of 14% of the P wave velocity. 11 references.
Crack propagation in a welded polyolefin pipe
Czech Academy of Sciences Publication Activity Database
Ševčík, Martin; Hutař, Pavel; Náhlík, Luboš; Lach, R.; Knésl, Zdeněk; Grellmann, W.
2012-01-01
Roč. 3, č. 2 (2012), s. 148. ISSN 1757-9864 R&D Projects: GA ČR GC101/09/J027 Institutional support: RVO:68081723 Keywords : polymers * pipes * butt weld * crack Subject RIV: JL - Materials Fatigue, Friction Mechanics
Investigation about crack propagation paths in thin rim gears
Directory of Open Access Journals (Sweden)
F. Curà
2014-10-01
Full Text Available Crack propagation in gears is a problem related not only to the life of the components, but also to the concept of failsafe design. Fail safe design means to design a component in order that, if a failure occurs, this may cause a “safe failure”. This aspect is very important above all in aerospace industry. As a matter of fact, in aerospace application, the need of reducing weight brings to produce gears with very thick rim and web. Considering thin rim gears, when a crack is nucleated near the tooth root, it may propagate through the tooth (causing the loss of the entire tooth or a portion of it or the propagation may follow a path across the wheel diameter (causing the projection of big parts of the gear that may break the gearbox and may cause serious damage to the aircraft. The first failure mode is define as “failsafe failure” and the second one as “catastrophic failure” and of course has to be avoided. Designers need to have robust design criteria in order to predict crack propagation paths and to avoid catastrophic failures. In literature, few works are present concerning this topic, in particular related to the effect of geometrical parameters that may affect the crack propagation. In this work a numerical analysis about crack propagation in gears with respect to the backup ratio (ratio between tooth height and rim thickness, initial crack position and shape has been done by means of the Extended FEM (XFEM technique, realizing 3D models. XFEM 3D is a relatively new technique consisting in enriching traditional finite elements with more complex shape functions; in this way it is possible to propagate crack also between mesh nodes and to have mesh independent results. Aim of this paper is to highlight the crack propagation path in order to give to designers an high confident design criterion, related to the gear geometry. In particular, the effect of both rim thickness and orientation of the initial crack have been considered
Growth of a brittle crack (001) in 3D bcc iron crystal with a Cu nano-particle
Czech Academy of Sciences Publication Activity Database
Uhnáková, Alena; Machová, Anna; Hora, Petr; Červená, Olga
2014-01-01
Roč. 83, February (2014), s. 229-234. ISSN 0927-0256 R&D Projects: GA ČR GA101/09/1630 Institutional support: RVO:61388998 Keywords : brittle crack extension * 3D * mode I * bcc iron * Cu nano-particle * molecular dynamics * acoustic emission Subject RIV: JG - Metallurgy Impact factor: 2.131, year: 2014 http://www.sciencedirect.com/science/article/pii/S0927025613006575
Crack Propagation in Honeycomb Cellular Materials: A Computational Approach
Directory of Open Access Journals (Sweden)
Marco Paggi
2012-02-01
Full Text Available Computational models based on the finite element method and linear or nonlinear fracture mechanics are herein proposed to study the mechanical response of functionally designed cellular components. It is demonstrated that, via a suitable tailoring of the properties of interfaces present in the meso- and micro-structures, the tensile strength can be substantially increased as compared to that of a standard polycrystalline material. Moreover, numerical examples regarding the structural response of these components when subjected to loading conditions typical of cutting operations are provided. As a general trend, the occurrence of tortuous crack paths is highly favorable: stable crack propagation can be achieved in case of critical crack growth, whereas an increased fatigue life can be obtained for a sub-critical crack propagation.
Role of the pore fluid in crack propagation in glass
Mallet, Céline; Fortin, Jérôme; Guéguen, Yves; Bouyer, Fréric
2015-05-01
We investigate pore fluid effects due to surface energy variation or due to chemical corrosion in cracked glass. Both effects have been documented through experimental tests on cracked borosilicate glass samples. Creep tests have been performed to investigate the slow crack propagation behavior. We compared the dry case (saturated with argon gas), the nonreactive water saturated case (commercial mineralized water), and the distilled and deionized water saturated case (pure water). Chemical corrosion effects have been observed and evidenced from pH and water composition evolution of the pure water. Then, the comparison of the dry case, the mineral water saturated case, and the corrosion case allow to (i) evidence the mechanical effect of the presence of a pore fluid and (ii) show also the chemical effect of a glass dissolution. Both effects enhance subcritical crack propagation.
Fatigue crack propagation in the samples with anticorrosion bulding up
International Nuclear Information System (INIS)
Peculiarities of fatigue crack nucleation and propagation in the samples with anticorrosion building up from the concentrator placed in the melting zone are studied. A plate of experimental lot of 15Kh2NMFA steel δ=100 mm in thick has been used for sample production. The moment of nucleation is shown to depend on the level of loading and the coefficient of cycle asymmetry, if the stress concentrator is presented in the melting zone and there is a possibility for simultaneous ocurrence of the crack both in a base metal and in build-up metal. The character of crack development with different levels of loading and the coefficient of cycle asymmetry both in the base metal and in build-up one is similar. Approaching the sample surface the crack propagation rate is sharply increased
Assessment of damage localization based on spatial filters using numerical crack propagation models
International Nuclear Information System (INIS)
This paper is concerned with vibration based structural health monitoring with a focus on non-model based damage localization. The type of damage investigated is cracking of concrete structures due to the loss of prestress. In previous works, an automated method based on spatial filtering techniques applied to large dynamic strain sensor networks has been proposed and tested using data from numerical simulations. In the simulations, simplified representations of cracks (such as a reduced Young's modulus) have been used. While this gives the general trend for global properties such as eigen frequencies, the change of more local features, such as strains, is not adequately represented. Instead, crack propagation models should be used. In this study, a first attempt is made in this direction for concrete structures (quasi brittle material with softening laws) using crack-band models implemented in the commercial software DIANA. The strategy consists in performing a non-linear computation which leads to cracking of the concrete, followed by a dynamic analysis. The dynamic response is then used as the input to the previously designed damage localization system in order to assess its performances. The approach is illustrated on a simply supported beam modeled with 2D plane stress elements.
Assessment of damage localization based on spatial filters using numerical crack propagation models
Energy Technology Data Exchange (ETDEWEB)
Deraemaeker, Arnaud, E-mail: aderaema@ulb.ac.be [Universite Libre de Bruxelles, Civil Engineering Department (BATir), 50 av. Franklin Roosevelt, CP 194/02, B-1050 Brussels (Belgium)
2011-07-19
This paper is concerned with vibration based structural health monitoring with a focus on non-model based damage localization. The type of damage investigated is cracking of concrete structures due to the loss of prestress. In previous works, an automated method based on spatial filtering techniques applied to large dynamic strain sensor networks has been proposed and tested using data from numerical simulations. In the simulations, simplified representations of cracks (such as a reduced Young's modulus) have been used. While this gives the general trend for global properties such as eigen frequencies, the change of more local features, such as strains, is not adequately represented. Instead, crack propagation models should be used. In this study, a first attempt is made in this direction for concrete structures (quasi brittle material with softening laws) using crack-band models implemented in the commercial software DIANA. The strategy consists in performing a non-linear computation which leads to cracking of the concrete, followed by a dynamic analysis. The dynamic response is then used as the input to the previously designed damage localization system in order to assess its performances. The approach is illustrated on a simply supported beam modeled with 2D plane stress elements.
Fracture mechanics limitations for short cracks and the non-propagating cracks
International Nuclear Information System (INIS)
Fracture mechanics limitations when the crack length is small can be listed as metallurgical or plastical arguments. It was here defined a ''non propagating cracks'' criterion, based on some critical length 1sub(c) (relying Δσe and ΔKth). It is shown a comparison of theoretical and experimental results about initiation and subsequent micropropagation of short cracks at the notch root of 316 L specimen at 200C and 5500C. It was shown that non-propagating cracks occurence could be mainly related to the closure phenomena on a mechanical point of view and to the ability of emitted dislocations to propagate beyond the grain boundary on the metallurgical point of view
Infrared thermography study of the fatigue crack propagation
Directory of Open Access Journals (Sweden)
O.A. Plekhov
2012-07-01
Full Text Available The work is devoted to the experimental study of heat dissipation process caused by fatigue crack propagation. To investigate a spatial and time temperature evolution at the crack tip set of experiments was carried out using specimens with pre-grown centered fatigue crack. An original mathematical algorithm for experimental data treatment was developed to obtain a power of heat source caused by plastic deformation at crack tip. The algorithm includes spatial-time filtration and relative motion compensation procedures. Based on the results of mathematical data treatment, we proposed a way to estimate the values of J-integral and stress intensity factor for cracks with pronounced the plastic zone.
Calculation of crack propagation in stainless steels in hot water
International Nuclear Information System (INIS)
Stress corrosion cracking (SCC) is a significant age-related degradation mechanism for loaded structural materials such as stainless steel used in the core and in coolant circuits of light water reactors. SCC is a synergetic interaction between a stressed material and its environment. The micro scale of the phenomena, the complex nature of the crack's electrochemistry and the large deformation at the crack tip, make mathematical modelling the most suitable approach to investigate, understand and predict SCC. The crack propagation rate depends, inter alia, on the dissolved oxygen and sulphur content, the temperature, the stress intensity, the crack length, the fluid flow, degree of sensitization and the yield strength of the material. Irradiated Type 304 stainless steel (SS304) is susceptible to SCC in the cooling water of nuclear reactors. The relatively pure, high-temperature water of a boiling water reactor (BWR) can be contaminated by sulphur due to resin intrusion and this can accelerate cracking. Therefore crack growth tests of SS304 in dilute sulphuric acid (H2SO4) solutions are often used to simulate SCC of SS304 in BWRs components. SCK-CEN developed a physico-electrochemical simulation tool based on the Finite Element (FE) method. It calculates the crack's environment based on the mass-transport of species, chemical reactions in environment, electrochemical reactions at the metal surface and the mechano-corrosive interaction at the crack tip. Mathematical modelling of the chemistry and electrochemistry in a crack provides a suitable framework for (1) the prediction of the influence of a range of variables, (2) the isolation of the factors controlling crack chemistry and electrochemistry and (3) linking to models of crack-growth kinetics. This activity is a part of a European Community sponsored integrated project PERFECT
Crack spacing threshold of double cracks propagation for large-module rack
Institute of Scientific and Technical Information of China (English)
赵铁柱; 石端伟; 姚哲皓; 毛宏勇; 程术潇; 彭惠
2015-01-01
Large-module rack of the Three Gorges shiplift is manufactured by casting and machining, which is unable to avoid slag inclusions and surface cracks. To ensure its safety in the future service, studying on crack propagation rule and the residual life estimation method of large-module rack is of great significance. The possible crack distribution forms of the rack in the Three Gorges shiplift were studied. By applying moving load on the model in FRANC3D and ANSYS, quantitative analyses of interference effects on double cracks in both collinear and offset conditions were conducted. The variation rule of the stress intensity factor (SIF) influence factor,RK, of double collinear cracks changing with crack spacing ratio,RS, was researched. The horizontal and vertical crack spacing threshold of double cracks within the design life of the shiplift were obtained, which are 24 and 4 times as large as half of initial crack length,c0, respectively. The crack growth rates along the length and depth directions in the process of coalescence on double collinear cracks were also studied.
Lalpoor, M.; Eskin, D. G.; ten Brink, Gert; Katgerman, L.
2010-01-01
Intergranular brittle fracture has been mainly observed and reported in steel alloys and precipitation hardened At-alloys where intergranular precipitates cover a major fraction of the grain boundary area. 7xxx series aluminum alloys suffer from this problem in the as-cast condition when brittle int
Retardation of fatigue crack propagation by indentation technique
International Nuclear Information System (INIS)
This paper used indentations to retard crack development and thus to prolong the crack growth life. The growth retardation resulting from the indentation-induced strain hardening and the crack closure due to indentation-induced residual stresses were explored. The retardation tests using 3-10 kN indentation loads were performed on different thickness specimens of AISI 4130 low alloy steel and AISI 304 stainless steel. These loads were applied using a hemispherical indenter to both sides of the expected crack path on the specimen surfaces. Loads of 4.5 kN and greater increased the microhardness at and around the indentation position, indicating that at those loads the hardness or the strain hardening contributed to retarding crack growth. In addition, all the loads caused different levels of crack closure. The greater the loads, the stronger were the crack closure effect and the accompanying growth retardation. In the 3.5 mm thick AISI 4130 specimens, a 10 kN load exerted the strongest growth retardation to arrest the post-indentation crack propagation
Crack propagation of Ti alloy via adiabatic shear bands
International Nuclear Information System (INIS)
This study was focused on the characterization of the origin and mechanism of crack propagation as a result of hot induction bending of Ti alloy. Plates of Ti–6Al–4V alloy with 12.5 mm of thickness were submitted to hot induction bending below the beta transus temperature. Optical and scanning electron microscopy analysis showed crack formation in the tensile zone. Microstructural evidence showed that cracks propagate through the adiabatic shear bands by Dimple-Void mechanism. However, voids formation before shear banding also occurred. In both mechanisms adiabatic shear bands are formed via dynamic recrystallization where the alpha–beta interphase works as stress concentrator promoting the formation of dimples and voids
Crack propagation of Ti alloy via adiabatic shear bands
Energy Technology Data Exchange (ETDEWEB)
Mendoza, I., E-mail: ivanmendozabravo@gmail.com [Instituto Tecnológico de Veracruz (Mexico); Villalobos, D. [Instituto Tecnológico de Veracruz (Mexico); Alexandrov, B.T. [The Ohio State University (United States)
2015-10-01
This study was focused on the characterization of the origin and mechanism of crack propagation as a result of hot induction bending of Ti alloy. Plates of Ti–6Al–4V alloy with 12.5 mm of thickness were submitted to hot induction bending below the beta transus temperature. Optical and scanning electron microscopy analysis showed crack formation in the tensile zone. Microstructural evidence showed that cracks propagate through the adiabatic shear bands by Dimple-Void mechanism. However, voids formation before shear banding also occurred. In both mechanisms adiabatic shear bands are formed via dynamic recrystallization where the alpha–beta interphase works as stress concentrator promoting the formation of dimples and voids.
Effect of residual stress on fatigue crack propagation
International Nuclear Information System (INIS)
A mathematical model for fatigue crack propagation under the influence of residual stress has been developed which is remarkably applicable to the prediction of any kind of fatigue behaviors under residual stress fields. Since the model describes the interaction of microstructure, mechanism, and mechanics, fatigue crack propagation can be predicted in terms of readily available mechanical or physical properties of the structural material. It is also shown that an excellent correlation can be found between the present model based on the energy balance concept and experimental data, provided that residual stress distribution, process zone size, and tearing modulus are given. The present model shows that increase in compressive residual stresses ahead of crack tip can profoundly reduces catastrophic failure of the important machine and structural parts of reactor components
Fatigue crack propagation properties on corrosion resistant welded joints
International Nuclear Information System (INIS)
Fatigue crack growth resistance properties are obtained through fatigue crack propagation tests. The results, obtained from a log-log plot presents three regions: region I, where the microstructure, mean stress and environment have a high influence. Region II, that presents a linear behavior and region III where the material reaches the fracture toughness and results in an instable fracture. In this work it is studied the behavior of corrosion resistant USI SAC 50 steel welded joints, using compact tension specimens with notch localized on the base metal, heat affected zone and melted zone. It is obtained stable crack propagation equations type Paris equation for the region II, with 95% confidence limit. It is observed that the heat-affected zone presents a major scatter. (authors)
Arrest of rapid crack propagation in polymer pipes
Energy Technology Data Exchange (ETDEWEB)
Flueler, P.; Farshad, M. [EMPA, Duebendorf (Switzerland)
1995-12-31
The design of rapid crack arresters for polymer pipes was studied. Mechanisms that would inhibit a running crack and strengthen existing pipes against dynamic fracture and to enhance their degree of safety were examined. The crack arresters examined were based on the principle that rapid crack propagation (RCP) could not occur in pipe walls that were less than a `critical thickness`. Sections of pipe whose walls were thinned were reinforced with a reinforcing ring. Another variation was to produce a pipe with partially adhered multilayer walls. A third variation tried was a multi-layer pipe segment with a damping element and reinforcing rings. Experiments were successful in reducing RCP, but these preliminary results were considered exploratory and would require further confirmation. 2 figs., 8 refs.
Acoustic emission method for tracing crack propagation in pressure vessels
International Nuclear Information System (INIS)
A hemispherical model and a pressure vessel model were used as samples. In the former model, artificial notches were fabricated at the top. In the latter, two types of artificial notches were fabricated at the points of origin of the maximum stress (corners of the inner surface of the nozzles) in four nozzles with different shapes which were mounted on the model proper. The AE method was used to investigate the process of crack initiation and propagation from these artificial notches by means of repeated loading with internal pressure. It was possible to obtain from the results of these tests much useful data concerning the properties of AE and the points of origin of AE (positional tracking) when cracks are initiated and propagated in structures having complex shapes such as these samples. Simultaneously with the measurements by the AE method, Smek gages and crack gages mounted on the nozzle corners were used to investigate the crack initiation and propagation behavior. It was established that there is a close connection between them
Finite Element Analysis of the Crack Propagation for Solid Materials
Directory of Open Access Journals (Sweden)
Miloud Souiyah
2009-01-01
Full Text Available Problem statement: The use of fracture mechanics techniques in the assessment of performance and reliability of structure is on increase and the prediction of crack propagation in structure play important part. The finite element method is widely used for the evaluation of SIF for various types of crack configurations. Source code program of two-dimensional finite element model had been developed, to demonstrate the capability and its limitations, in predicting the crack propagation trajectory and the SIF values under linear elastic fracture analysis. Approach: Two different geometries were used on this finite element model in order, to analyze the reliability of this program on the crack propagation in linear and nonlinear elastic fracture mechanics. These geometries were namely; a rectangular plate with crack emanating from square-hole and Double Edge Notched Plate (DENT. Where, both geometries are in tensile loading and under mode I conditions. In addition, the source code program of this model was written by FORTRAN language. Therefore, a Displacement Extrapolation Technique (DET was employed particularly, to predict the crack propagations directions and to, calculate the Stress Intensity Factors (SIFs. Furthermore, the mesh for the finite elements was the unstructured type; generated using the advancing front method. And, the global h-type adaptive mesh was adopted based on the norm stress error estimator. While, the quarter-point singular elements were uniformly generated around the crack tip in the form of a rosette. Moreover, make a comparison between this current study with other relevant and published research study. Results: The application of the source code program of 2-D finite element model showed a significant result on linear elastic fracture mechanics. Based on the findings of the two different geometries from the current study, the result showed a good agreement. And, it seems like very close compare to the other published
Hai-Yang, Song; Yu-Long, Li
2016-02-01
The effects of amorphous lamella on the crack propagation behavior in crystalline/amorphous (C/A) Mg/Mg-Al nanocomposites under tensile loading are investigated using the molecular dynamics simulation method. The sample with an initial crack of orientation [0001] is considered here. For the nano-monocrystal Mg, the crack growth exhibits brittle cleavage. However, for the C/A Mg/Mg-Al nanocomposites, the ‘double hump’ behavior can be observed in all the stress-strain curves regardless of the amorphous lamella thickness. The results indicate that the amorphous lamella plays a critical role in the crack deformation, and it can effectively resist the crack propagation. The above mentioned crack deformation behaviors are also disclosed and analyzed in the present work. The results here provide a strategy for designing the high-performance hexagonal-close-packed metal and alloy materials. Project supported by the National Natural Science Foundation of China (Grant Nos. 11372256 and 11572259), the 111 Project (Grant No. B07050), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-12-1046), and the Program for New Scientific and Technological Star of Shaanxi Province, China (Grant No. 2012KJXX-39).
Wang, Jiandong; Li, Liqun; Tao, Wang
2016-08-01
It is generally believed that cracks in metal matrix composites (MMC) parts manufacturing are crucial to the reliable material properties, especially for the reinforcement particles with high volume fraction. In this paper, WC particles (WCp) reinforced Fe-based metal matrix composites (WCp/Fe) were manufactured by laser melting deposition (LMD) technology to investigate the characteristics of cracks formation. The section morphology of composites were analyzed by optical microscope (OM), and microstructure of WCp, matrix and interface were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), in order to study the crack initiation and propagation behavior under different laser process conditions. The temperature of materials during the laser melting deposition was detected by the infrared thermometer. The results showed that the cracks often appeared after five layers laser deposition in this experiment. The cracks crossed through WC particles rather than the interface, so the strength of interface obtained by the LMD was relatively large. When the thermal stress induced by high temperature gradient during LMD and the coefficient of thermal expansion mismatch between WC and matrix was larger than yield strength of WC, the cracks would initiate inside WC particle. Cracks mostly propagated along the eutectic phases whose brittleness was very large. The obtained thin interface was beneficial to transmitting the stress from particle to matrix. The influence of volume fraction of particles, laser power and scanning speed on cracks were investigated. This paper investigated the influence of WC particles size on cracks systematically, and the smallest size of cracked WC in different laser processing parameters was also researched.
Fatigue crack propagation in carburized high alloy bearing steels
Averbach, B. L.; Lou, Bingzhe; Pearson, P. K.; Fairchild, R. E.; Bamberger, E. N.
1985-07-01
Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates, da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, and da/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, Ki where Ki, = σid{i/1/2} (σi = internal stress, di = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes Ke = Ka + Ki where Ka is the applied stress intensity. Values of Ki were calculated as a function of distance from the surface using experimental measurements of σi and a value of di = 11 mm (0.43 inch). The resultant values of Ke were taken to be equivalent to effective ΔK values, and da/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the
Prediction of crack propagation in layered ceramics with strong interfaces
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Šestáková, L.; Hutař, Pavel; Bermejo, R.
2010-01-01
Roč. 77, č. 11 (2010), s. 2192-2199. ISSN 0013-7944 R&D Projects: GA AV ČR(CZ) KJB200410803; GA ČR GA101/09/1821 Institutional research plan: CEZ:AV0Z20410507 Keywords : Ceramic laminate * Crack propagation direction * Residual stress * Flaw tolerant ceramics * Optimal design Subject RIV: JL - Materials Fatigue, Friction Mechanics Impact factor: 1.571, year: 2010
An improved cohesive zone model for ductile dynamic crack propagation
Sagimon Buch, Marc
2008-01-01
Ductile dynamic crack propagation is a current field of research in aerospace industry. The damage created by an explosion in a flying airplane depends on the fracture behaviour of the fuselage materials. Thus the rate of fracture for aluminium 2024 T3 is being studied. Analytical and empirical calculation methods do not predict correctly the experimental fracture velocity. Numerical simulations using cohesive elements with standard material models do not estimate it correctly ...
Crack propagation in touch ductile materials. Phase II
International Nuclear Information System (INIS)
The thrust of this work was to investigate published J material resistance and stress-strain data applicable to the understanding of crack propagation in tough ductile steels, particularly SA 106 Grade B pipe steel. This data has been assembled from PIFRAC, AECB report INFO-0254-1 and Ontario Hydro sources and has been uniformly formatted and presented to facilitate comparison and assessment. While the data is in many aspects incomplete it has enabled an evaluation of the influence of temperature, specimen thickness and specimen orientation to be made in the context of the experimental J-R curves so determined. Comparisons of the stress-strain data within the Ramburg-Osgood formulation are also considered. A further component of this report addresses the development of the required software to utilize what is referred to as the engineering approach to elasto-plastic analysis to investigate the load carrying capacity of selected cracked pipe geometries which are representative of applied crack propagation studies associated with piping systems in the nuclear industry. Three specific geometries and loading situations, identified as Condition A, B and C have been evaluated; the results are presented and illustrate the variation in applied load as a function of an initial and final crack extension leading to instability
Rinne, Mikael
2008-01-01
Subcritical crack growth (SCG) takes place when a crack is stressed below its short-term strength. This slow fracturing process may lead to an accelerating crack velocity and to a sudden unstable failure event. SCG is thought to play an important role in long-term rock stability at all scales, ranging from laboratory samples to earthquake-generating faults. SCG can be detected as rock loosening or as sudden rock movements around excavations. A time-dependent crack growth model is develop...
Niegel, A.; Gudladt, H.-J.; Gerold, V.
1988-01-01
High cycle fatigue crack propagation in grain boundaries was studied in precipitation hardened Al-Zn-Mg specimens containing a single grain boundary (gb) perpendicular to the load axis. In peak-aged bicrystals tested in wet nitrogen atmospheres, cracks propagated in an intercrystalline manner. Under cyclic loading conditions two different mechanisms contribute to crack propagation which are termed stress corrosion cracking (SCC) and intergranular corrosion fatigue (CF). In both cases, it is w...
INFLUENCE OF ACCUMULATED FATIGUE DAMAGE ON CRACK PROPAGATION IN ALCLAD ALUMINUM ALLOY
Карускевич, М. В.; Національний авіаційний університет; Щепак, С. В.; Національний авіаційний університет; Щур, O. I.; National aviation university; Плащинська, А. В.; Інститут механіки імені С. П. Тимошенка НАН України
2013-01-01
Application of damage tolerance principle at operation of aircrafts assumes ability to predict propagation of fatigue cracks. Conducted experiments have proved that local metal damage accumulated due to cyclic loadings has influence on fatigue crack propagation, especially on the initial stages of crack development. Density of deformation relief at the stress concentrator is being used as damage indicator. It has been proved, that there exists correlation between time of crack propagation and...
Modeling of Propagation of Interacting Cracks Under Hydraulic Pressure Gradient
Energy Technology Data Exchange (ETDEWEB)
Huang, Hai [Idaho National Laboratory; Mattson, Earl Douglas [Idaho National Laboratory; Podgorney, Robert Karl [Idaho National Laboratory
2015-04-01
A robust and reliable numerical model for fracture initiation and propagation, which includes the interactions among propagating fractures and the coupling between deformation, fracturing and fluid flow in fracture apertures and in the permeable rock matrix, would be an important tool for developing a better understanding of fracturing behaviors of crystalline brittle rocks driven by thermal and (or) hydraulic pressure gradients. In this paper, we present a physics-based hydraulic fracturing simulator based on coupling a quasi-static discrete element model (DEM) for deformation and fracturing with conjugate lattice network flow model for fluid flow in both fractures and porous matrix. Fracturing is represented explicitly by removing broken bonds from the network to represent microcracks. Initiation of new microfractures and growth and coalescence of the microcracks leads to the formation of macroscopic fractures when external and/or internal loads are applied. The coupled DEM-network flow model reproduces realistic growth pattern of hydraulic fractures. In particular, simulation results of perforated horizontal wellbore clearly demonstrate that elastic interactions among multiple propagating fractures, fluid viscosity, strong coupling between fluid pressure fluctuations within fractures and fracturing, and lower length scale heterogeneities, collectively lead to complicated fracturing patterns.
Incubation time for sub-critical crack propagation in SiC-SiC composites
Energy Technology Data Exchange (ETDEWEB)
El-Azab, A.; Ghoniem, N.M. [Univ. of California, Los Angeles, CA (United States)
1995-04-01
The objective of this work is to investigate the time for sub-critical crack propagation is SiC-SiC composites at high temperatures. The effects of fiber thermal creep on the relaxation of crack bridging tractions in SiC-SiC ceramic matrix composites (CMCs) is considered in the present work, with the objective of studying the time-to propagation of sub-critical matrix cracks in this material at high temperatures. Under the condition of fiber stress relaxation in the bridiging zone, it is found that the crack opening and the stress intensity factor increase with time for sub-critical matrix cracks. The time elapsed before the stress intensity reaches the critical value for crack propagation is calculated as a function of the initial crack length, applied stress and temperature. Stability domains for matrix cracks are defined, which provide guidelines for conducting high-temperature crack propagation experiments.
International Nuclear Information System (INIS)
This research thesis aims at understanding and analysing some mechanisms involved in the dynamic failure under various loadings which are notably present in industrial environment, and more particularly in some parts of EDF's plants where networks of micro-cracks may steadily grow: heterogeneous zones, defects under coating. The author presents a simplified model based on the de-bonding of a film to understand the mechanisms of a sudden failure under a quasi-static loading. He develops a similar model to explore the influence of a defect on crack propagation under a quasi-static loading, and then under a sudden loading. This model is then generalized to the case of several defects, and more particularly very small defects. Finally, the author gives a numerical interpretation of a sudden propagation under quasi-static loading
Shallow crack effect on brittle fracture of RPV during pressurised thermal shock
International Nuclear Information System (INIS)
This report describes the study on behaviour of postulated shallow surface cracks in embrittled reactor pressure vessel subjected to pressurised thermal shock loading in an emergency core cooling. The study is related to the pressure vessel of a VVER-440 type reactor. Instead of a conventional fracture parameter like stress intensity factor or J integral the maximum principal stress distribution on a crack tip area is used as a fracture criteria. The postulated cracks locate circumferentially at the inner surface of the reactor pressure wall and they penetrate the cladding layer and open to the inner surface. Axisymmetric and semielliptical crack shapes were studied. Load is formed of an internal pressure acting also on crack faces and of a thermal gradient in the pressure vessel wall. Physical properties of material and loading data correspond real conditions in VVER-440 RPV. The study was carried out by making lot of 2D- and 3D- finite element calculations. Analysing principles and computer programs are explained. Except of studying the shallow crack effect, one objective of the study has also been to develop further expertise and the in-house developed computing system to make effectively elastic-plastic fracture mechanical analyses for real structures under complicated loads. Though the study concerns VVER-440 RPV, the results are of more general interest especially related to thermal loads. (orig.) (11 refs.)
Asymptotic analysis of mode Ⅰ propagating crack-tip field in a creeping material
Institute of Scientific and Technical Information of China (English)
WANG Zhen-qing; ZHAO Qi-cheng; LIANG Wen-yan; FU Zhang-jian
2003-01-01
Adopting an elastic-viscoplastic, the asymptotic problem of mode I propagating crack-tip field is investigated. Various asymptotic solutions resulting from the analysis of crack growing programs are presented. The analysis results show that the quasi-statically growing crack solutions are the special case of the dynamic propagating solutions. Therefore these two asymptotic solutions can be unified.
Energy Technology Data Exchange (ETDEWEB)
Venkateswara Rao, K.T.; Ritchie, R.O. (Dept. of Materials Science and Mineral Engineering, Univ. of California at Berkeley (United States))
1992-05-30
An experimental study has been made to investigate the cyclic crack propagation resistance of ductile-particle toughened brittle materials, specifically, ductile TiNb-reinforced {gamma}-TiAl intermetallic composites, as a function of microstructure. Under cyclic loading, cracks are found to grow subcritically at stress intensities of 4-5 MPa m{sup 1/2}, far below their maximum toughness levels. Such behavior is associated with the susceptibility of the ductile TiNb phase to fatigue failure, and consequently to the diminished role of crack tip shielding from crack bridging by unbroken TiNb ligaments, in contrast to observations under monotonic loading. No evidence for bridging is seen under cyclic loading and bridging zone lengths in the wake of the crack tip are limited to less than 150 {mu}m, compared with 4-5 mm under monotonic loading. Moreover, crack growth rates are very sensitive to applied {Delta}K level, with measured exponents for the da/dN-{Delta}K relationship ranging from 10 to 20. The exponent decreases with an increase in volume fraction of the ductile phase, but is independent of particle thickness; fatigue thresholds are, however, less affected. (orig.).
Hydraulic fracture and toughening of a brittle layer bonded to a hydrogel
Lucantonio, Alessandro; Noselli, Giovanni; Trepat, Xavier; Desimone, Antonio; Arroyo, Marino
Brittle materials fracture under tensile or shear stress. When stress attains a critical threshold, crack propagation becomes unstable and proceeds dynamically. In the presence of several precracks, a brittle material always propagates only the weakest crack, leading to catastrophic failure. Here, we show that all these features of brittle fracture are radically modified when the material susceptible to cracking is bonded to a poroelastic medium, such as a hydrogel, a common situation in biological tissues. In particular, we show that the brittle material can fracture in compression and can resist cracking in tension, thanks to the hydraulic coupling with the hydrogel. In the case of multiple cracks, we find that localized fracture occurs when the permeability of the hydrogel is high, whereas decreased permeability leads to toughening by promoting multiple cracking. Our results may contribute to the understanding of fracture in biological tissues and provide inspiration for the design of tough, biomimetic materials.
Description of small fatigue crack propagation in ODS steel
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Kuběna, Ivo; Šmíd, Miroslav; Ševčík, Martin; Kruml, Tomáš; Náhlík, Luboš
Zurich: Trans Tech Publications, 2014 - (Clark, G.; Wang, C.), s. 911-916. (Advanced Materials Research. 891-892). ISBN 978-3-03835-008-8. ISSN 1022-6680. [Fatigue 2014 - International Fatigue Congress /11./. Melbourne (AU), 02.03.2014-07.03.2014] R&D Projects: GA MŠk(CZ) EE2.3.30.0063; GA MŠk(CZ) ED1.1.00/02.0068; GA ČR(CZ) GP13-28685P Institutional support: RVO:68081723 Keywords : oxide dispersion strengthened steel * short fatigue crack propagation * J-integral * EUROFER 97 Subject RIV: JL - Materials Fatigue, Friction Mechanics
Image-based detection and analysis of crack propagation in cementitious composites
DEFF Research Database (Denmark)
Pereira, E.B.; Fischer, Gregor; Barros, J.A.O.
2011-01-01
after the cracking process has occurred. The alternative nondestructive methods are often either not precise enough or experimentally too demanding. In this study, the use of an image analysis procedure to capture the crack initiation and propagation process is described, which utilizes digital images...... small crack openings, allowing a thorough investigation and analysis of the cracking processes in concrete....
Calomino, Anthony Martin
1994-01-01
The subcritical growth of cracks from pre-existing flaws in ceramics can severely affect the structural reliability of a material. The ability to directly observe subcritical crack growth and rigorously analyze its influence on fracture behavior is important for an accurate assessment of material performance. A Mode I fracture specimen and loading method has been developed which permits the observation of stable, subcritical crack extension in monolithic and toughened ceramics. The test specimen and procedure has demonstrated its ability to generate and stably propagate sharp, through-thickness cracks in brittle high modulus materials. Crack growth for an aluminum oxide ceramic was observed to be continuously stable throughout testing. Conversely, the fracture behavior of a silicon nitride ceramic exhibited crack growth as a series of subcritical extensions which are interrupted by dynamic propagation. Dynamic initiation and arrest fracture resistance measurements for the silicon nitride averaged 67 and 48 J/sq m, respectively. The dynamic initiation event was observed to be sudden and explosive. Increments of subcritical crack growth contributed to a 40 percent increase in fracture resistance before dynamic initiation. Subcritical crack growth visibly marked the fracture surface with an increase in surface roughness. Increments of subcritical crack growth loosen ceramic material near the fracture surface and the fracture debris is easily removed by a replication technique. Fracture debris is viewed as evidence that both crack bridging and subsurface microcracking may be some of the mechanisms contributing to the increase in fracture resistance. A Statistical Fracture Mechanics model specifically developed to address subcritical crack growth and fracture reliability is used together with a damaged zone of material at the crack tip to model experimental results. A Monte Carlo simulation of the actual experiments was used to establish a set of modeling input
Branicio, Paulo S; Kalia, Rajiv K; Nakano, Aiichiro; Vashishta, Priya
2006-02-17
Atomistic mechanisms of fracture accompanying structural phase transformation (SPT) in AlN ceramic under hypervelocity impact are investigated using a 209 x 10(6) atom molecular-dynamics simulation. The shock wave generated by the impact splits into an elastic wave and a slower SPT wave that transforms the wurtzite structure into the rocksalt phase. The interaction between the reflected elastic wave and the SPT wave front generates nanovoids and dislocations into the wurtzite phase. Nanovoids coalesce into mode I cracks while dislocations give rise to kink bands and mode II cracking. PMID:16606007
Institute of Scientific and Technical Information of China (English)
GAO Xin; WANG Han-gong; KANG Xing-wu
2008-01-01
Based on the mechanics of anisotropic materials,the dynamic propagation problem of a mode Ⅲ crack in an infinite anisotropic body is investigated.Stress,strain and displacement around the crack tip are expressed as an analytical complex function,which can be represented in power series.Constant coefficients of series are determined by boundary conditions.Expressions of dynamic stress intensity factors for a mode Ⅲ crack are obtained.Components of dynamic stress,dynamic strain and dynamic displacement around the crack tip are derived.Crack propagation characteristics are represented by the mechanical properties of the anisotropic materials,i.e.,crack propagation velocity M and the parameter α.The faster the crack velocity is,the greater the maximums of stress components and dynamic displacement components around the crack tip are.In particular,the parameter α affects stress and dynamic displacement around the crack tip.
Energy Technology Data Exchange (ETDEWEB)
Schramm, Britta
2014-07-01
Components with graded fracture-mechanical properties show a fundamentally different crack propagation behaviour than do homogeneous, isotropic structures. This becomes especially evident in investigations on the influence of fracture-mechanical material grading on the stage of steady fatigue crack propagation and on crack propagation speed da/dN. Theoretical findings based on the so-called TSSR concept, which was developed as part of this dissertation, indicate that it can have either positive or negative effects on crack propagation behaviour, depending on various material characteristics and grading constellations. The dissertation reports on experiments for validating theoretical statements on the influence of different structural conditions on crack propagation behaviour. These statements were largely found to be correct. The study thus contributes to the prediction of crack propagation in fracture-mechanically graded components and structures subject to static or cyclical stress.
Evaiuation of Cracks Propagation in Retroflllings with Cinalloy Amalgam
Directory of Open Access Journals (Sweden)
Zarrabian M
2000-06-01
Full Text Available One of the major goals of preradicular surgery is to create a good apical seal. This can be done by sectioning approximately one third of the apex, preparation of a class I cavity, and filling with a biocompatible material. The purpose of this in vitro study was to compare crack propagation in retro filling with two commercially available amalgams. Thirty-four extracted single rooted teeth were divided into two groups. After instrumentation and filling with Gutta percha by lateral condensation method, three millimeter of apex was resected and retro preparations were done by a low speed hand piece and '/> round bur. Then cavities were filled with cinalloy and luxalloy amalgam in-group "one" and "two" respectively. The surface of resected root ends was examined in two stages, after doing retro preparation and retro filling and the presence of any cracks or structural changes was inspected by stereomicroscope 50x. Regard to number and type of cracks, the result of this study showed that there was no significant difference between cinalloy or luxalloy retro fillings. By considering the conditions of this study, cinalloy amalgam can be used as a retro filling material.
Quasicontinuum simulation of crack propagation in bcc-Fe
International Nuclear Information System (INIS)
Highlights: → Multiscale simulation and literature study of fracture in bcc-Fe with 4 orientations. → Investigation of anisotropic vs. isotropic formulations of boundary conditions. → Systematical study of the influence of T-stress on crack tip mechanisms. - Abstract: We have investigated fracture in bcc-Fe through multiscale simulations. The quasicontinuum (QC) method with an embedded atom method (EAM) interatomic potential is applied. The analyses have been carried out assuming different crystallographic orientations and different T-stress under Mode I loading. Both anisotropic and isotropic formulations of the modified boundary layer (MBL) approach has here been investigated and compared. The results show that the mechanisms at the crack tip and the critical stress intensity factor KIc are sensitive to both the crystallographic orientation and whether or not the formulation of the boundary conditions are isotropic or anisotropic. Mechanisms such as cleavage crack propagation, twinning, and dislocation emission are observed in the analyses. A short literature review on atomistic and multiscale simulations of fracture in bcc-Fe has been performed and evaluated, and also compared with the current results.
Effect of local deformation on the propagation of short fatigue cracks in an Al alloy
International Nuclear Information System (INIS)
Aiming to develop a model describing the propagation of short fatigue cracks in an Al alloy the strain and other deformation quantities were measured ahead the crack tip at different stages of fatigue. The method used includes cyclic loading of tensile specimens within a SEM, taking digital images and image processing. The results reveal an important effect of microstructure on the distribution of strain at the crack tip. Rate and direction of crack propagation are related to this strain distribution. A power law was found between crack propagation rate da/dN and the product of accumulated equivalent strain and strain increment. (orig.)
Burrowing in marine muds by crack propagation: kinematics and forces.
Dorgan, Kelly M; Arwade, Sanjay R; Jumars, Peter A
2007-12-01
The polychaete Nereis virens burrows through muddy sediments by exerting dorsoventral forces against the walls of its tongue-depressor-shaped burrow to extend an oblate hemispheroidal crack. Stress is concentrated at the crack tip, which extends when the stress intensity factor (KI) exceeds the critical stress intensity factor (KIc). Relevant forces were measured in gelatin, an analog for elastic muds, by photoelastic stress analysis, and were 0.015+/-0.001 N (mean +/- s.d.; N=5). Measured elastic moduli (E) for gelatin and sediment were used in finite element models to convert the forces in gelatin to those required in muds to maintain the same body shapes observed in gelatin. The force increases directly with increasing sediment stiffness, and is 0.16 N for measured sediment stiffness of E=2.7 x 10(4) Pa. This measurement of forces exerted by burrowers is the first that explicitly considers the mechanical behavior of the sediment. Calculated stress intensity factors fall within the range of critical values for gelatin and exceed those for sediment, showing that crack propagation is a mechanically feasible mechanism of burrowing. The pharynx extends anteriorly as it everts, extending the crack tip only as far as the anterior of the worm, consistent with wedge-driven fracture and drawing obvious parallels between soft-bodied burrowers and more rigid, wedge-shaped burrowers (i.e. clams). Our results raise questions about the reputed high energetic cost of burrowing and emphasize the need for better understanding of sediment mechanics to quantify external energy expenditure during burrowing. PMID:18025018
Ductile-brittle behavior of (001)[110] nano-cracks in bcc iron
Czech Academy of Sciences Publication Activity Database
Machová, Anna; Beltz, G. E.
2004-01-01
Roč. 387, 389C (2004), s. 414-418. ISSN 0921-5093. [ICSMA-13. Budapest, 25.08.2003-30.08.2003] R&D Projects: GA AV ČR IAA2076201; GA MŠk ME 504 Grant ostatní: NSF(US) 0000142 Institutional research plan: CEZ:AV0Z2076919 Keywords : crack * stability * twins Subject RIV: JG - Metallurgy Impact factor: 1.445, year: 2004
Mechanisms of short crack propagation in austenitic–ferritic duplex steel
Energy Technology Data Exchange (ETDEWEB)
Scharnweber, Michael, E-mail: michael.scharnweber@mailbox.tu-dresden.de; Tirschler, Wolfgang; Oertel, Carl-Georg; Skrotzki, Werner
2014-02-10
For applications requiring both high strength and high corrosion resistance, austenitic–ferritic duplex steels are often the material of choice. In this study, cyclic deformation experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462. By measuring the crack opening and crack sliding displacement in situ in a scanning electron microscope, the characteristics of the different crack propagation mechanisms in the two phases are determined. In the ferritic phase, two different appearances of short cracks can be observed, one exhibiting a very smooth and the other one a rather rough surface crack path. Electron backscatter diffraction measurements on the crack-containing grains in addition with high resolution imaging of the topography of the crack flanks reveal that contrary to common assumptions in the literature, short cracks in ferrite do not propagate via single slip. Instead, two different slip systems with an identical slip direction, but different slip planes, are activated. In this context, the specific appearance of different crack paths can be explained with the orientation of the respective grains. Furthermore, a model for discontinuous crack propagation especially of rough cracks in ferrite is developed. Finally, a correlation between the crack propagation rate and the plastic deformation of the crack tip is revealed and the possibility of determining the barrier effect of grain and phase boundaries via the measurement of the plastic deformation of the crack tip is investigated.
International Nuclear Information System (INIS)
Initiation and propagation of fatigue cracks were studied in CT specimens of 316L stainless steel at room and high temperatures with different notch root radii. The number of cycles to crack initiation and the propagation of the crack length until it reaches a few millimeters were obtained using a potential drop system. It can be shown that, using Neuber's equation for estimating the notch plastic strain range, one can find the low cycle fatigue curve of the material. From the representation of the crack propagation rate versus the stress intensity factor for long cracks, which is calculated when l is greater than Ln, that is the estimated notch stress field, one can find the propagation curve of the material. The advantage of this method employing only one experiment to obtain informations about initiation and propagation of fatigue cracks is demonstrated
Huang, Peiyan; Liu, Guangwan; Guo, Xinyan; Huang, Man
2008-11-01
The experimental research on fatigue crack propagation rate of reinforced concrete (RC) beams strengthened with carbon fiber laminate (CFL) is carried out by MTS system in this paper. The experimental results show that, the main crack propagation on strengthened beam can be summarized into three phases: 1) fast propagation phase; 2) steady propagation and rest phase; 3) unsteady propagation phase. The phase 2-i.e. steady propagation and rest stage makes up about 95% of fatigue life of the strengthened beam. The propagation rate of the main crack, da/dN, in phase 2 can be described by Paris formula, and the constant C and m can be confirmed by the fatigue crack propagation experiments of the RC beams strengthened with CFL under three-point bending loads.
Modelling probabilistic fatigue crack propagation rates for a mild structural steel
Directory of Open Access Journals (Sweden)
J.A.F.O. Correia
2015-01-01
Full Text Available A class of fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and local strain-life damage models have been proposed in literature. The fatigue crack growth is regarded as a process of continuous crack initializations over successive elementary material blocks, which may be governed by smooth strain-life damage data. Some approaches account for the residual stresses developing at the crack tip in the actual crack driving force assessment, allowing mean stresses and loading sequential effects to be modelled. An extension of the fatigue crack propagation model originally proposed by Noroozi et al. (2005 to derive probabilistic fatigue crack propagation data is proposed, in particular concerning the derivation of probabilistic da/dN-ΔK-R fields. The elastic-plastic stresses at the vicinity of the crack tip, computed using simplified formulae, are compared with the stresses computed using an elasticplastic finite element analyses for specimens considered in the experimental program proposed to derive the fatigue crack propagation data. Using probabilistic strain-life data available for the S355 structural mild steel, probabilistic crack propagation fields are generated, for several stress ratios, and compared with experimental fatigue crack propagation data. A satisfactory agreement between the predicted probabilistic fields and experimental data is observed.
Crack propagation behavior of TiN coatings by laser thermal shock experiments
International Nuclear Information System (INIS)
Highlights: ► The crack propagation behavior of TiN coating after laser thermal shock experiment was observed by using FIB and TEM. ► Intercolumnar cracks between TiN columnar grains were predominant cracking mode after laser thermal shock. ► Cracks were propagated from the coating surface to the substrate at low laser pulse energy and cracks were originated at coating-substrate interface at high laser pulse energy. ► The cracks from the interface spread out transversely through the weak region of the columnar grains by repetitive laser shock. - Abstract: The crack propagation behavior of TiN coatings, deposited onto 304 stainless steel substrates by arc ion plating technique, related to a laser thermal shock experiment has been investigated using focused ion beam (FIB) and transmission electron microscopy (TEM). The ablated regions of TiN coatings by laser ablation system have been investigated under various conditions of pulse energies and number of laser pulses. The intercolumnar cracks were predominant cracking mode following laser thermal shock tests and the cracks initiated at coating surface and propagated in a direction perpendicular to the substrate under low loads conditions. Over and above those cracks, the cracks originated from coating-substrate interface began to appear with increasing laser pulse energy. The cracks from the interface also spread out transversely through the weak region of the columnar grains by repetitive laser shock.
FATIGUE CRACK PROPAGATION OF Ni-BASE SUPERALLOYS
Institute of Scientific and Technical Information of China (English)
X.B.Liu; L.Z.Ma; K.M.Chang; E.Barbero
2005-01-01
Time-dependent Fatigue Crack Propagation (FCP) behaviors of five Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions.The new concept of damage zone is defined and employed to evaluate the alloys' resistance to hold-time FCP.A special testing procedure is designed to get the maximum damage zone of the alloys.Udimet 720 and Waspaloy show shorter damage zones than alloys 706 and 718.The fractographical analyses show that the fracture surfaces of the specimens under hold-time fatigue conditions are mixtures with intergranular and transgranular modes.As the extension of holding time per cycle, the portion of intergranular fracture increases.The effects of loading stress intensity, temperature, holding time, alloy chemistry, and alloy microstructure on damage zone and the crack growth behaviors are studied.Hold-time usually increases the alloy's FCP rate, but there are few exemptions.For instance, the steady state hold-time FCP rate of Waspaloy at 760℃ is lower than that without hold-time.The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time.
International Nuclear Information System (INIS)
In this study, criterions are proposed to describe crack initiation in the vicinity of an interface in brittle bi-materials. The purpose is to provide a guide for the elaboration of ceramic multi-layer structures being able to develop damage tolerance by promoting crack deflection along interfaces. Several cracking mechanisms are analyzed, like the competition between the deflection of a primary crack along the interface or its penetration in the second layer. This work is first completed in a general case and is then used to describe the crack deviation at the interface in ceramic matrix composites and nuclear fuels. In this last part, experimental tests are carried out to determine the material fracture properties needed to the deflection criteria. An optimization of the fuel coating can be proposed in order to increase its toughness. (author)
International Nuclear Information System (INIS)
Highlights: • Hydrogen facilitates dislocation emission from crack tip. • There are three mechanisms of hydrogen effect on crack propagation. • Hydrogen can either improve or reduce the ductility of single crystal tungsten. - Abstract: An atomic model of single crystal bcc tungsten containing a pre-existing crack was built and molecular dynamics simulations were performed to investigate the crack propagation behavior with and without hydrogen atoms under uniaxial tensile load. Two kinds of crystal orientation were analyzed to study the effect of hydrogen on different crack propagation patterns. The results show that hydrogen can either improve or reduce the ductility of tungsten. High hydrogen concentration could result in the rearrangement of tungsten atoms ahead of the crack tip and reduce the stress concentration in the neighboring area around the crack tip. This will prevent the crack from propagation temporarily and therefore increase the fracture strain. Besides, hydrogen atoms can also facilitate the dislocation emission from the crack tip, which is accompanied by a larger plastic deformation. Both the mechanisms improve the ductility of tungsten. However, a void could be nucleated in a local hydrogen-rich area under tensile load. Its growth and link-up with the main crack will accelerate the crack propagation and speedup the fracture process, which diminishes the ductility of tungsten
In-situ observations on crack propagation along polymer/glass interfaces.
Vellinga, W.P; Timmerman, R.; van Tijum, R.; de Hosson, J.T.M.; Buchheit, TE; Minor, AM; Spolenak, R; Takashima, K
2005-01-01
The propagation of crack fronts along a PET-glass interface is illustrated. The experimental set-up consists of an Asymmetric Double Cantilever Beam in an optical microscope. Image processing techniques used to isolate the crack fronts are discussed in some detail. The fronts are found to propagate
Analysis of crack propagation in concrete structures with structural information entropy
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
The propagation of cracks in concrete structures causes energy dissipation and release, and also causes energy redistribution in the structures. Entropy can characterize the energy redistribution. To investigate the relation between the propagation of cracks and the entropy in concrete structures, cracked concrete structures are treated as dissipative structures. Structural information entropy is defined for concrete structures. A compact tension test is conducted. Meanwhile, numerical simulations are also carried out. Both the test and numerical simulation results show that the structural information entropy in the structures can characterize the propagation of cracks in concrete structures.
Modeling of crack propagation in weak snowpack layers using the discrete element method
Directory of Open Access Journals (Sweden)
J. Gaume
2015-01-01
Full Text Available Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1 failure initiation in a weak snow layer underlying a cohesive slab, (2 crack propagation within the weak layer and (3 tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e. to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. For instance, it is not uncommon to perform field measurements with widespread crack propagation on one day, while a few days later, with very little changes to the snowpack, crack propagation does not occur anymore. Thus far, there is no clear theoretical framework to interpret such observations, and it is not clear how and which snowpack properties affect dynamic crack propagation. To shed more light on this issue, we performed numerical propagation saw test (PST experiments applying the discrete element (DE method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, the relation between mechanical parameters of the snowpack was taken into account so as to compare numerical and experimental results, which were in good agreement, suggesting that the simulations can reproduce crack propagation in PSTs. Finally, an in-depth analysis of the
In situ observations of transgranular crack propagation in high-manganese steel
International Nuclear Information System (INIS)
Crack propagation in high-Mn steel was investigated using in situ transmission electron microscopy. Preferential slips developed in the early stages of deformation on {1 1 1}, followed by the formation of a crack, which propagated along the pre-developed slip traces. A sharp corner was observed at the crack tip along two adjoining {1 1 1} planes. Surface ripples developed when the tip of the crack crossed pre-existing twins on the crack path, which was frequently observed in the surface during the final stage of fracture
Time-dependent corrosion fatique crack propagation in 7000 series aluminum alloys. M.S. Thesis
Mason, Mark E.
1995-01-01
The goal of this research is to characterize environmentally assisted subcritical crack growth for the susceptible short-longitudinal orientation of aluminum alloy 7075-T651, immersed in acidified and inhibited NaCl solution. This work is necessary in order to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA-FLAGRO (NASGRO). This effort concentrates on determining relevant inputs to a superposition model in order to more accurately model environmental fatigue crack propagation.
FATIGUE CRACK INITIATION AND PROPAGATION OF A TiNi SHAPE MEMORY ALLOY
Gloanec, Anne-Lise; Cerrachio, Priscillia; Reynier, Bertrand; Van Herpen, Alain; Riberty, Patrice
2010-01-01
Fatigue crack initiation and propagation stages of a TiNi shape memory alloy are examined thanks to a low cycle fatigue interrupted test. Submitted to fatigue cyclic loading, the response of the alloy presents a classical pseudoelastic response. Two potential initiation crack areas are highlighted: at the phase interfaces or at the grain boundaries. Then, propagation results from the coalescence of many microscopic cracks. These two stages are detectable at the last 20% of the total fatigue l...
Effect of tire rubber particles on crack propagation in cement paste
Directory of Open Access Journals (Sweden)
Nadia Segre
2006-09-01
Full Text Available Tire rubber particles (NaOH-treated and untreated were investigated as possible crack stabilizer and toughness enhancer when added to cement paste through in situ crack propagation measurements using two different types of cement, type I/II and an Interground polypropylene Fiber Cement (IFC. Crack deflection and crack bridging were observed in specimens with untreated rubber in cement type I/II. Crack tip mechanisms associated with crack pinning and acrack arrest were present in type I/II cement and IFC with treated rubber particles. Crack tip mechanisms in IFC with treated rubber lead to the increase in CMOD at the ultimate load level. Crack wake mechanisms in IFC with untreated or treated rubber lead to strain hardening and strain softening behavior. Crack wake bridging mechanisms were replaced by multiple cracking mechanisms in the IFC specimens with treated rubber. The IFC specimens with untreated rubber inclusions provided the best results with respect to toughness enhancement.
Energy Technology Data Exchange (ETDEWEB)
Scharnweber, Michael; Oertel, Carl-Georg; Tirschler, Wolfgang; Skrotzki, Werner [Institut fuer Strukturphysik, TU Dresden, 01062 Dresden (Germany); Mikulich, Vladimir [Fraunhofer-Institut fuer Werkzeugmaschinen und Umformtechnik, 09126 Chemnitz (Germany)
2011-07-01
Austenitic stainless steel (X2 CrNiMo 18 14 3) was cyclically deformed at RT in air under plastic strain control with amplitudes of 5 x 10{sup -4} and 2 x 10{sup -3}. Every 30.000 and 3.000 cycles, respectively, the samples were investigated in the scanning electron microscope in order to determine the propagation rate of the existing microstructurally short cracks as well as the corresponding distance between the crack tip and the opposing grain boundary. The results will be discussed with regard to the barrier effect of grain boundaries to crack propagation as well as the correlation between crack length and propagation rate at different strain amplitudes.
Effects of gear crack propagation paths on vibration responses of the perforated gear system
Ma, Hui; Pang, Xu; Zeng, Jin; Wang, Qibin; Wen, Bangchun
2015-10-01
This paper investigates the dynamic behaviors of a perforated gear system considering effects of the gear crack propagation paths and this study focuses on the effects of a crack propagating through the rim on the time-varying mesh stiffness (TVMS) and vibration responses. Considering the effects of the extended tooth contact, a finite element (FE) model of a gear pair is established based on ANSYS software. TVMS of the perforated gear with crack propagating through tooth and rim are calculated by using the FE model. Furthermore, a lumped mass model is adopted to investigate the vibration responses of the perforated gear system. The results show that there exist three periods related to slots of the gear body in a rotating period of the perforated gear. Gear cracks propagating through tooth and rim both reduce the gear body stiffness and lead to reduction of TVMS besides the crack tooth contact moment, and the TVMS weakening for the former is less than that for the latter. Moreover, the results also show that the gear crack propagating through the rim (CPR) has a greater effect on vibration responses than the gear crack propagating through the tooth (CPT) under the same crack level. Vibration level increases with the increasing crack depth, especially for the gear with CPR.
Fatigue and Creep Crack Propagation behaviour of Alloy 617 in the Annealed and Aged Conditions
Energy Technology Data Exchange (ETDEWEB)
Julian K. Benz; Richard N. Wright
2013-10-01
The crack propagation behaviour of Alloy 617 was studied under various conditions. Elevated temperature fatigue and creep-fatigue crack growth experiments were conducted at 650 and 800 degrees C under constant stress intensity (triangle K) conditions and triangular or trapezoidal waveforms at various frequencies on as-received, aged, and carburized material. Environmental conditions included both laboratory air and characteristic VHTR impure helium. As-received Alloy 617 displayed an increase in the crack growth rate (da/dN) as the frequency was decreased in air which indicated a time-dependent contribution component in fatigue crack propagation. Material aged at 650°C did not display any influence on the fatigue crack growth rates nor the increasing trend of crack growth rate with decreasing frequency even though significant microstructural evolution, including y’ (Ni3Al) after short times, occurred during aging. In contrast, carburized Alloy 617 showed an increase in crack growth rates at all frequencies tested compared to the material in the standard annealed condition. Crack growth studies under quasi-constant K (i.e. creep) conditions were also completed at 650 degrees C and a stress intensity of K = 40 MPa9 (square root)m. The results indicate that crack growth is primarily intergranular and increased creep crack growth rates exist in the impure helium environment when compared to the results in laboratory air. Furthermore, the propagation rates (da/dt) continually increased for the duration of the creep crack growth either due to material aging or evolution of a crack tip creep zone. Finally, fatigue crack propagation tests at 800 degrees C on annealed Alloy 617 indicated that crack propagation rates were higher in air than impure helium at the largest frequencies and lowest stress intensities. The rates in helium, however, eventually surpass the rates in air as the frequency is reduced and the stress intensity is decreased which was not observed at 650
Fatigue and creep crack propagation behaviour of Alloy 617 in the annealed and aged conditions
International Nuclear Information System (INIS)
The crack propagation behaviour of Alloy 617 was studied under various conditions. Elevated temperature fatigue and creep-fatigue crack growth experiments were conducted at 650 and 800 deg. C under constant stress intensity (ΔK) conditions and triangular or trapezoidal waveforms at various frequencies on as-received, aged, and carburized material. Environmental conditions included both laboratory air and characteristic VHTR impure helium. As-received Alloy 617 displayed an increase in the crack growth rate (da/dN) as the frequency was decreased in air which indicated a time-dependent contribution component in fatigue crack propagation. Material aged at 650 deg. C did not display any influence on the fatigue crack growth rates nor the increasing trend of crack growth rate with decreasing frequency even though significant microstructural evolution, including γ' (Ni3Al) after short times, occurred during ageing. In contrast, carburized Alloy 617 showed an increase in crack growth rates at all frequencies tested compared to the material in the standard annealed condition. Crack growth studies under quasi-constant K (i.e. creep) conditions were also completed at 650 deg. C and a stress intensity of K = 40 MPa√(m). The results indicate that crack growth is primarily intergranular and increased creep crack growth rates exist in the impure helium environment when compared to the results in laboratory air. Furthermore, the propagation rates (da/dt) continually increased for the duration of the creep crack growth either due to material ageing or evolution of a crack tip creep zone. Finally, fatigue crack propagation tests at 800 deg. C on annealed Alloy 617 indicated that crack propagation rates were higher in air than impure helium at the largest frequencies and lowest stress intensities. The rates in helium, however, eventually surpass the rates in air as the frequency is reduced and the stress intensity is decreased which was not observed at 650 deg. C. (authors)
Crack propagation in transition zone of NE Krsko steam generator tubes
International Nuclear Information System (INIS)
Stress corrosion cracks in tube expansion transition region are recognized as one of the major tube degradation problems in operating steam generators. Possibility to predict stable crack propagation during an operational cycle enables adequate maintenance activities. Present report introduces the statistical approach for crack propagation modelling as a possible way to determine the crack propagation model. A NE Krsko specific model has been developed based on nondestructive measurement data and by comparison with large Belgian crack length data base, published in literature. Also, the classification of nondestructive measurement indications is performed, based on the known tube history. Such classification is aimed to help in the determination of most possible time of crack initiation. (author)
International Nuclear Information System (INIS)
In order to improve the mechanical behaviour of materials used as neutron absorbers in nuclear reactors, we have developed CERCER or CERMET composites with boron and hafnium. Thus a new composite B4C/HfB2 has been especially studied. We have identified three kinds of degradation under irradiation (thermal gradient, swelling due to fission products and accidental corrosion) that induce imposed deformations cracking phenomena. Mechanical behaviour and crack propagation resistance have been studied by ball-on-three-balls and double torsion tests. A special device was developed to enable crack propagation and associated stress intensity factor measurements. Effects of structure and of a second phase are underline. First results show that these materials present crack initiation and propagation resistance much higher than pure boron carbide or hafnium diboride. We observe R-Curves effects, crack bridging or branching, crack arrests, and toughness increases that we can relate respectively to the composite structures. (author)
Energy Technology Data Exchange (ETDEWEB)
Bucak, O.; Mang, F.; Herion, S. [Univ. Karlsruhe (Germany). Versuchsanstalt fuer Stahl, Holz und Steine
1994-12-31
The stress distribution in a hollow section joint depends on the geometric parameters (e.g. width ratio, wall thickness ratio). The different forms of stress distributions cannot be described by the basic methods of building mechanics. The peak stresses occurring at particular spots of the joint are different, which causes differences in the development and propagation of cracks. The crack propagation pattern changes when the tests are carried out under variable load amplitude. When calculating the fatigue life the authors therefore have to take account of the influence of the crack size. This paper presents different forms of cracks (in dependence of the geometric parameters) and the results of crack propagation measurements. Furthermore, the authors make a proposal for considering the influence of low load steps which also considers that the fatigue fife depends on the size of the crack. Uniplanar and multiplanar joints are compared. A further point of discussion is the consideration of multiplanar joints in the calculation rules.
Fatigue Crack Propagation Behavior of TiNi50.6Shape Memory Alloy
Institute of Scientific and Technical Information of China (English)
无
2001-01-01
The fatigue crack propagation behavior of TiNi50.6 shape memory alloy was studied. The experiment results showed that the crack propagation properties of this alloy display difference and similarity in comparison with common metallic materials. Because of the stress concentration there was stress induced martensite transformation (SIMT) near the crack tip though the nominal stress was lower than the threshold stress of SIMT. The position and the amount of SIMT was in situ observed by a quester remote measurement system (QRMS). The observation results showed that the position of SIMT was beside the crack tip and was not in the plastic zone of common metallic materials (in front of the crack tip). The SIMT zone at an angle of about 45° to the direction of the crack propagation, like a butterfly, appeared in the loading process, disappeared in the unloading process and grew larger with the increase of △K. The crack propagation rate(da/dN) followed the linear law in Ig-lg plot. Observation of the crack surface showed fatigue striation clearly. The relationship between the site and the size of the plastic zone and the SIMT zone is discussed and a model is given to explain both the similarity and the difference of the crack propagation property of TiNi50.6 to common metal materials.
Modelling of fatigue crack propagation assisted by gaseous hydrogen in metallic materials
International Nuclear Information System (INIS)
Experimental studies in a hydrogenous environment indicate that hydrogen created by surface reactions, then drained into the plastic zone, leads to a modification of deformation and damage mechanisms at the fatigue crack tip in metals, resulting in a significant decrease of crack propagation resistance. This study aims at building a model of these complex phenomena in the framework of damage mechanics, and to confront it with the results of fatigue crack propagation tests in high pressure hydrogen on a 15-5PH martensitic stainless steel. To do so, a cohesive zone model was implemented in the finite element code ABAQUS. A specific traction-separation law was developed, which is suitable for cyclic loadings, and whose parameters depend on local hydrogen concentration. Furthermore, hydrogen diffusion in the bulk material takes into account the influence of hydrostatic stress and trapping. The mechanical behaviour of the bulk material is elastic-plastic. It is shown that the model can qualitatively predict crack propagation in hydrogen under monotonous loadings; then, the model with the developed traction-separation law is tested under fatigue loading. In particular, the simulated crack propagation curves without hydrogen are compared to the experimental crack propagation curves for the 15-5PH steel in air. Finally, simulated fatigue crack propagation rates in hydrogen are compared to experimental measurements. The model's ability to assess the respective contributions of the different damage mechanisms (HELP, HEDE) in the degradation of the crack resistance of the 15-5PH steel is discussed. (author)
Blunt-crack band propagation in finite-element analysis for concrete structures
International Nuclear Information System (INIS)
The knowledge of concrete fracture is needed in nuclear reactor safety. The question of safety arises from the potential of concrete to crack under thermal loading. It has been postulated that structural concrete could be exposed to very high temperature, which may result from hot reactor coolant or even core debris coming in direct contact with the concrete. The utilization of the blunt crack approach for simulating concrete cracking in a general-purpose code is explored. The difficulties encountered in establishing the proper direction of crack propagation in an arbitrary discretization are described. Crack propagation is considered within the context of two types of solution techniques: (1) implicit solution of the static crack advance, and (2) explicit time integration using a dynamic relaxation technique to simulate the static crack advance. Also, in both solution techniques an elastic model is used to characterize the concrete
Crack initiation and propagation in thermal shock fatigue of stainless steel
International Nuclear Information System (INIS)
Fatigue crack growth tests were performed on austenitic stainless steel subjected to repeated thermal shocks using a specially designed test system for thermal shock fatigue, in which liquid air is sprayed onto the center of a disk-shaped specimen heated to about 200 deg C. The fatigue crack growth was found to occur while producing the striation on the fracture surface each time a thermal shock is applied. The crack growth characteristics in thermal shock fatigue can be well explained by the results of analysis of the thermal stress intensity factor (SIF). The crack propagation rate da/dN achieves its maximum at the same crack length as in the curve giving the relationship between SIF and the crack length. The results of the observation have also been described on the initiation and propagation of small fatigue cracks at the notch root. (author)
Blunt-crack band propagation in finite-element analysis for concrete structures. [LMFBR
Energy Technology Data Exchange (ETDEWEB)
Pfeiffer, P.A.; Bazant, Z.P.; Marchertas, A.H.
1983-01-01
The knowledge of concrete fracture is needed in nuclear reactor safety. The question of safety arises from the potential of concrete to crack under thermal loading. It has been postulated that structural concrete could be exposed to very high temperature, which may result from hot reactor coolant or even core debris coming in direct contact with the concrete. The utilization of the blunt crack approach for simulating concrete cracking in a general-purpose code is explored. The difficulties encountered in establishing the proper direction of crack propagation in an arbitrary discretization are described. Crack propagation is considered within the context of two types of solution techniques: (1) implicit solution of the static crack advance, and (2) explicit time integration using a dynamic relaxation technique to simulate the static crack advance. Also, in both solution techniques an elastic model is used to characterize the concrete.
A criterion for crack initiation and propagation in intact rocks under creep condition
Institute of Scientific and Technical Information of China (English)
CHEN You-liang(陈有亮); SUN Jun(孙钧)
2003-01-01
This paper presents a study on the criterion of crack initiation and propagation under creep conditions, which were investigated using specimens collected from outcropped sandstone rock formations. Beam specimens under three points bending were used in this investigation. The experimental result shows that due to creep deformation, rock crack will inevitably initiate and propagate after a time interval of sustained loading under a load of KI, which is less than fracture toughness KIC but not less than a constant marked as KIC2. KIC2 indicates the ability of rock to resist crack initiation and propagation under creep conditions and it is defined as creep fracture toughness.
Observations on the sub-critical growth and healing of microcracks in brittle ceramics
International Nuclear Information System (INIS)
Micro-cracking in brittle composites was monitored by measuring the temperature dependence of thermal diffusivity by the laser flash method. Depending upon the material system, micro-cracks can exhibit a time dependent growth or healing or a combination of both. A theoretical basis for these observations was established by analyzing the stability and nature of crack propagation of precursor micro-cracks in a spherical inclusion contained in an infinite matrix with different elastic properties
Institute of Scientific and Technical Information of China (English)
Amin Manouchehrian; Mohammad Fatehi Marji
2012-01-01
In many situations rocks are subjected to biaxial loading and the failure process is controlled by the lateral confinement stresses.The importance of confinement stresses has been recognized in the literature by many researchers,in particular,its influence on strength and on the angle of fracture,but still there is not a clear description for the influence of confining stress on the crack propagation mechanism of rocks.This paper presents a numerical procedure for the analysis of crack propagation in rock-like materials under compressive biaxial loads.Several numerical simulations of biaxial tests on the rock specimen have been carried out by a bonded particle model (BPM) and the influence of confinement on the mechanism of crack propagation from a single flaw in rock specimens is studied.For this purpose,several biaxial compressive tests on rectangular specimens under different confinement stresses were modeled in (2 dimensional particle flow code) PFC2D.The results show that wing cracks initiate perpendicular to the flaw and trend toward the direction of major stress,however,when the lateral stresses increase,this initiation angle gets wider.Also it is concluded that in addition to the material type,the initiation direction of the secondary cracks depends on confinement stresses,too.Besides,it is understood that secondary cracks may be produced from both tensile and shear mechanisms.
Brittle and semibrittle creep in a low porosity carbonate rock
Nicolas, Aurélien; Fortin, Jérôme; Regnet, Jean-Baptiste; Dimanov, Alexandre; Guéguen, Yves
2016-04-01
The mechanical behavior of limestones at room temperature is brittle at low confining pressure and becomes semi-brittle with the increase of the confining pressure. The brittle behavior is characterized by a macroscopic dilatancy due to crack propagation, leading to a stress drop when cracks coalesce at failure. The semi-brittle behavior is characterized by diffuse deformation due to intra-crystalline plasticity (dislocation movements and twinning) and microcracking. The aim of this work is to examine the influence of pore fluid and time on the mechanical behavior. Constant strain rate triaxial deformation experiments and stress-stepping creep experiments were performed on white Tavel limestone (porosity 14.7%). Elastic wave velocity evolutions were recorded during each experiment and inverted to crack densities. Constant strain rate triaxial experiments were performed for confining pressure in the range of 5-90 MPa. For Pc≤55 MPa our results show that the behavior is brittle. In this regime, water-saturation decreases the differential stress at the onset of crack propagation and enhances macroscopic dilatancy. For Pc≥70 MPa, the behavior is semi-brittle. Inelastic compaction is due to intra-crystalline plasticity and micro-cracking. However, in this regime, our results show that water-saturation has no clear effect at the onset of inelastic compaction. Stress stepping creep experiments were performed in a range of confining pressures crossing the brittle-ductile transition. In the brittle regime, the time-dependent axial deformation is coupled with dilatancy and a decrease of elastic wave velocities, which is characteristic of crack propagation and/or nucleation. In the semi-brittle regime, the first steps are inelastic compactant because of plastic pore collapse. But, following stress steps are dilatant because of crack nucleation and/or propagation. However, our results show that the axial strain rate is always controlled by plastic phenomena, until the last
Monitoring of solidification crack propagation mechanism in pulsed laser welding of 6082 aluminum
von Witzendorff, P.; Kaierle, S.; Suttmann, O.; Overmeyer, L.
2016-03-01
Pulsed laser sources with pulse durations in the millisecond regime can be used for spot welding and seam welding of aluminum. Seam welds are generally produced with several overlapping spot welds. Hot cracking has its origin in the solidification process of individual spot welds which determines the cracking morphology along the seam welding. This study used a monitoring unit to capture the crack geometry within individual spot welds during seam welding to investigate the conditions for initiation, propagation and healing (re-melting) of solidification cracking within overlapping pulsed laser welds. The results suggest that small crack radii and high crack angles with respect to welding direction are favorable conditions for crack healing which leads to crack-free seam welds. Optimized pulse shapes were used to produce butt welds of 0.5 mm thick 6082 aluminum alloys. Tensile tests were performed to investigate the mechanical strength in the as-welded condition.
Fatigue-crack propagation in advanced aerospace materials: Aluminum-lithium alloys
Energy Technology Data Exchange (ETDEWEB)
Venkateswara Rao, K.T.; Ritchie, R.O.
1988-10-01
Characteristics of fatigue-crack propagation behavior are reviewed for recently developed commercial aluminum-lithium alloys, with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage-tolerant design. Specifically, crack-growth kinetics in Alcoa 2090-T8E41, Alcan 8090 and 8091, and Pechiney 2091 alloys, and in certain powder-metallurgy alloys, are examined as a function of microstructure, plate orientation, temperature, crack size, load ratio and loading sequence. In general, it is found that growth rates for long (> 10 mm) cracks are nearly 2--3 orders of magnitude slower than in traditional 2000 and 7000 series alloys at comparable stress-intensity levels. In additions, Al-Li alloys shown enhanced crack-growth retardations following the application of tensile overloads and retain superior fatigue properties even after prolonged exposure at overaging temperatures; however, they are less impressive in the presence of compression overloads and further show accelerated crack-growth behavior for microstructurally-small (2--1000 {mu}m) cracks (some three orders of magnitude faster than long cracks). These contrasting observations are attributed to a very prominent role of crack-tip shielding during fatigue-crack growth in Al-Li alloys, promoted largely by the tortuous and zig-zag nature of the crack-path morphologies. Such crack paths result in locally reduced crack-tip stress intensities, due to crack deflection and consequent crack wedging from fracture-surface asperities (roughness-induced crack closure); however, such mechanisms are far less potent in the presence of compressive loads, which act to crush the asperities, and for small cracks, where the limited crack wake severely restricts the shielding effect. 50 refs., 21 figs.
Fatigue Crack Propagation Under Variable Amplitude Loading Analyses Based on Plastic Energy Approach
Directory of Open Access Journals (Sweden)
Sofiane Maachou
2014-04-01
Full Text Available Plasticity effects at the crack tip had been recognized as “motor” of crack propagation, the growth of cracks is related to the existence of a crack tip plastic zone, whose formation and intensification is accompanied by energy dissipation. In the actual state of knowledge fatigue crack propagation is modeled using crack closure concept. The fatigue crack growth behavior under constant amplitude and variable amplitude loading of the aluminum alloy 2024 T351 are analyzed using in terms energy parameters. In the case of VAL (variable amplitude loading tests, the evolution of the hysteretic energy dissipated per block is shown similar with that observed under constant amplitude loading. A linear relationship between the crack growth rate and the hysteretic energy dissipated per block is obtained at high growth rates. For lower growth rates values, the relationship between crack growth rate and hysteretic energy dissipated per block can represented by a power law. In this paper, an analysis of fatigue crack propagation under variable amplitude loading based on energetic approach is proposed.
International Nuclear Information System (INIS)
In a series of thermal loading tests at the HDR reactor pressure vessel - thermal stratification, cyclic thermal shock and pressurized thermal shock - the methods applied in safety analysis had to become qualified by a continuous intercomparison of calculated results and experimental data. Above all the complex boundary conditions of the HDR-tests offer a close approximation to the original components, so that they provide a real assessment of the transferability. The results of the thermal mixing tests indicated that during cold water inflow into the RPV longitudinal strains build up in the cylindrical wall which dominate over that in circumferential direction. During the cyclic thermal fatigue tests incipient crack formation in the cladding as well as the behaviour of crack propagation in the cladding and in the base material was analyzed. In the pressurized thermal shock tests, the nozzle region and the cylinder wall in the incipient crack condition were loaded by long cooling streaks. Even in the aggravated loading condition as the result of a routed cold water streak no remarkable indications of crack growth were noticed. In both cases, cyclic and pressurized thermal shock loading, the expected crack propagation was overpredicted by the fracture mechanical methods used. The non-destructive examination methods used were able to locate all of the cracks but they mostly overpredicted the actual crack depth. (orig.)
Crack propagation tests of HIPed DSCu/SS joints for plasma facing components
International Nuclear Information System (INIS)
Aluminum oxide dispersion strengthened copper (DSCu), used as the heat sink, and AISI 316L(N) type stainless steel (SS), used for cooling pipes, were metallurgically joined to be used as first wall of plasma facing components. Hot isostatic pressing (HIP) was proposed as the joining fabrication technique in the first-wall/blanket components. In this study, fracture toughness and fatigue crack growth rate tests were carried out for the fracture strength evaluation of HIPed joints. Permissible crack lengths during fabrication of first-wall components were evaluated from the results of the fracture toughness tests. In crack growth rate tests, the crack prepared in the DSCu propagated, turned at the HIPed interface, and propagated along it. Therefore, it was found that the crack provided in the DSCu heat sink did not propagate through the SS cooling pipe
Analysis on features for propagation of Zircaloy-4 crack growth
International Nuclear Information System (INIS)
In this paper, the fractal properties and entropy properties of the low-cycle fatigue fracture of Zircaloy-4 are primarily investigated. By analyzing the trend of fractal dimension and entropy in the growth direction, obvious segmented zone are discovered in crack growth period. Along the growth direction of crack, the complexity falls gradually in low frequency and vertical high frequency. However, level high frequency and diagonally high frequency are the most complicated in the metaphase of crack growth period. Moreover, in despite of discrepancy of physical meanings and solving methods, fractal dimension and entropy have the accordant rules in characteristic curves in crack growth direction. It means that the fractal dimension and entropy are correlative. Finally, the crack growth period could be plotted out into some stages based on variate curve of the low frequency direction. The subsections in the other directions are analyzed separately as singular point of crack growth . (authors)
Crack propagation under conditions of low cycle fatigue
International Nuclear Information System (INIS)
A literature review is given of convenient concepts describing the mechanical behaviour of a cracked body under cyclic loading. Only the range of high growth rates is considered. However, caused by large scale yielding in this range, the application of linear elastic fracture mechanics is no longer possible. Mechanical parameters which control fatigue crack growth are a modified stress intensity factor, the J-integral, the crack tip opening displacement and a suitable strain amplitude. (orig.) With 20 figs
Influence of local stress concentrations on the crack propagation in complex welded components
Directory of Open Access Journals (Sweden)
C. Fischer
2015-10-01
Full Text Available While a long stable crack propagation phase was observed during experiments of complex welded components, very conservative assessments of the fatigue life were achieved in the past. The difference was explained by the stress gradient occurring over the plate thickness. This paper deals with numerical crack propagation simulations which were performed for different geometrical variants. The variants differ related to global geometry, boundary conditions and weld shape. The analyses aim to investigate how the crack propagation is altered if the structural configuration gets more complex. In conclusion, the stress gradient over the plate thickness, the apparent plate thickness and the notch effect slows down the crack propagation rate if the same stress value being effective for fatigue appears at the weld toe. Thereby, the load-carrying grade of the weld, the weld flank angle and the geometrical configuration have an impact on both the notch effect and the local stress concentration.
Study of the initiation and the propagation of cracks under 3D thermal cyclic loading
International Nuclear Information System (INIS)
The incident which has occurred on the Civaux power plant has shown the noxiousness of thermal loading and the difficulty to take it into account at design level. The objective of this report is to study the initiation and the propagation of crack under thermal loading. In this aim the CEA has developed a new experiment named FAT3D. The various experiments carried out showed the harmfulness of a thermal loading, which makes it possible to rapidly initiate a network of cracks and to propagate one (or some) cracks through the totally thickness of the component under certain conditions. These experimental results associated with a mechanical analysis put at fault the usual criteria of damage based on the variations of the equivalent strain. In addition, the study of the propagation stage shows the importance of the plasticity which, in the case of a thermal loading, slows down the propagation of the crack. (author)
Energy Technology Data Exchange (ETDEWEB)
Le Grognec, P.; Hariri, S. [Ecole des Mines de Douai, 59 (France); Afzali, M.; Jaffal, H. [Centre Technique des Industries Mecaniques, 60 - Senlis (France)
2008-11-15
The aim of this work is to determine the evolution of the degree of noxiousness of a defect in a pressure equipment during its propagation. The estimation of the degree of noxiousness involves the calculation of stress intensity factors at each advance of the crack front. The cracks considered are semi-elliptic. The geometries and loads can be complex in order to cover the main industrial cases. Numerical modelling by finite element method is based on the creation of a crack-block, representing the optimized mesh near the discontinuity. The Paris law allows to describe the fatigue behaviour under cyclic load. A specific program (Python), having the advantages of the calculation codes Castem and Abaqus, allows to compute the propagation approach and makes easier the estimation of the residual lifetime of a structure under cracked pressure. (O.M.)
Fatigue crack propagation in turbine disks of EI698 superalloy
A.A. Shanyavskiy
2013-01-01
In-service fatigue cracking of turbine disks of EI698 superalloy is discussed based on crack growth analyses. In the bolt joint for disks to shaft connecting there is high level of stress-state, which directed to earlier in-disks fatigue crack origination in low-cycle-fatigue regime. Fracture surface pattern such as fatigue striations were used for their spacing measurement and crack growth duration estimating. Developed disk tests on a special bench by the equivalent program to in-service cy...
Overcoming brittleness through bioinspiration and -microarchitecture
Barthelat, Francois; Mirkhalaf, Mohammad; Dastjerdi, Ahmad
2014-01-01
The fracture of highly mineralized natural materials such as bone, teeth, or seashells is largely controlled by the interfaces they contain. These interfaces, relatively weak, deflect and guide cracks into configurations which eventually impede their propagation. As a result, weaker interfaces turn brittle minerals into tough materials which can deform and absorb energy from impacts. To explore these concepts in synthetic materials, we used a 3D laser-engraver to carve arrays of microcracks w...
A method for the 3-D quantification of bridging ligaments during crack propagation
International Nuclear Information System (INIS)
This letter shows how a hole-closing algorithm can be used to identify and quantify crack-bridging ligaments from a sequence of X-ray tomography images of intergranular stress corrosion cracking. This allows automatic quantification of the evolution of bridging ligaments through the crack propagation sequence providing fracture mechanics insight previously unobtainable from fractography. The method may also be applied to other three-dimensional materials science problems, such as closing walls in foams.
Estimation of stepwise crack propagation in ceramic laminates with strong interfaces
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Štegnerová, Kateřina; Hutař, Pavel
2015-01-01
Roč. 9, č. 34 (2015), s. 116-124. ISSN 1971-8993. [International Conference on Crack Paths /5./. Ferrara, 16.09.2015-18.09.2015] R&D Projects: GA ČR(CZ) GA15-09347S Institutional support: RVO:68081723 Keywords : Ceramic laminates * Crack behaviour * Residual stresses * Strain energy density factor * Crack propagation direction Subject RIV: JL - Materials Fatigue, Friction Mechanics http://www.fracturae.com/index.php/fis/article/view/IGF-ESIS.34.12
IN-SITU OBSERVATION OF CRACK PROPAGATION IN PST CRYSTALS OF Ti-49% Al ALLOY
Institute of Scientific and Technical Information of China (English)
无
1999-01-01
The effects of loading axis to the lamellae on the fracture behavior of PST crystals have been investigated by SEM in-situ testing. The results show that the fracture behavior of PST crystals of γ- TiAl base alloys depend on not only the angles between cracks and lamellae, but also the angles of loading axis to the lamellae, and the cracks propagating along different paths show different toughening mechanisms, crack resistance, corresponding fracture behavior and fractography.
International Nuclear Information System (INIS)
This study aims to figure out the crack growth phenomenon by thermal fatigue induced by thermal gradient through thickness of specimen. Firstly, an experimental facility has been developed: a rectangular parallelepiped specimen is subjected to thermal cycling between 350 C and 100 C; the specimen is freed to expand and contract. Two semi-circular notches (0,1 mm depth and 4 mm length) have been machined on the surface of the specimen. A series of interrupted tests has been carried out to characterize and quantify the crack growth in depth and surface of the pre-existing crack. Next, a three-dimensional crack growth simulation has been implemented in ABAQUS. Automation using Python was used to simulate the propagation of a crack under thermal cycling, with re-meshing at crack front after each calculation step. No assumption has been taken on the crack front during the crack propagation. A comparison with test results showed very good agreement on the evolution of crack front shape and on the kinetics of propagation on the edge and the heart of pre-existing crack. An analytical approach was also developed based on the calculation of stress intensity factors (SIC). A two-dimensional approach was first introduced enabling us to better understand the influence of various thermal and geometric parameters. Finally, a three dimensional approach, with an elliptical assumption crack shape during the propagation, leading to a prediction of crack growth on the surface and in depth which is very similar to that obtained numerically, but with computational time much lower. (author)
Energy Technology Data Exchange (ETDEWEB)
Ma, Longzhou
2012-11-30
The nickel-based superalloy INCONEL 617 is a candidate material for heat exchanger applications in the next-generation nuclear plant (NGNP) system. This project will study the crack propagation process of alloy 617 at temperatures of 650°C-950°C in air under static/cyclic loading conditions. The goal is to identify the environmental and mechanical damage components and to understand in-depth the failure mechanism. Researchers will measure the fatigue crack propagation (FCP) rate (da/dn) under cyclic and hold-time fatigue conditions, and sustained crack growth rates (da/dt) at elevated temperatures. The independent FCP process will be identified and the rate-controlled sustained loading crack process will be correlated with the thermal activation equation to estimate the oxygen thermal activation energy. The FCP-dependent model indicates that if the sustained loading crack growth rate, da/dt, can be correlated with the FCP rate, da/dn, at the full time dependent stage, researchers can confirm stress-accelerated grain-boundary oxygen embrittlement (SAGBOE) as a predominate effect. Following the crack propagation tests, the research team will examine the fracture surface of materials in various cracking stages using a scanning electron microscope (SEM) and an optical microscope. In particular, the microstructure of the crack tip region will be analyzed in depth using high resolution transmission electron microscopy (TEM) and electron energy loss spectrum (EELS) mapping techniques to identify oxygen penetration along the grain boundary and to examine the diffused oxygen distribution profile around the crack tip. The cracked sample will be prepared by focused ion beam nanofabrication technology, allowing researchers to accurately fabricate the TEM samples from the crack tip while minimizing artifacts. Researchers will use these microscopic and spectroscopic results to interpret the crack propagation process, as well as distinguish and understand the environment or
Coexistence of ductile and brittle fracture in metals
International Nuclear Information System (INIS)
It is well known that semibrittle body-centered cubic (bcc) metals fail at low temperatures by cleavage that is preceded by crack tip deformation. Sinclair and Finnis proposed a mechanism by which crack tip deformation may be combined with brittle crack extension. In this model, edge dislocations are emitted from a crack tip on an inclined plane under pure mode I loading conditions. The authors propose a new mechanism of brittle fracture of semibrittle metals preceded by crack tip deformation by extending the model of Sinclair and Finnis and by incorporating experimental evidence on mixed mode crack propagation observed by transmission electron microscopy (TEM). They have shown experimentally that, even when the orientation of the dislocations in the plastic zone indicated pure mode III crack tip deformation, the crack opening displacement determined from the relative displacement of the crack flanks showed the presence of an additional mode I component. They have also shown that zigzag crack propagation observed in many metals can occur only if mode I cleavage is superimposed to mode II crack tip deformation
International Nuclear Information System (INIS)
Microcrack generation and propagation play an essential role to predict the long-term behavior of crystalline rock. We developed a new relaxation testing equipment that enables us to observe a granite specimen under the state of relaxation. Series of relaxation tests have been performed under constant temperature. In this paper, we show a few patterns of microcrack propagation and the relationship between pre-existed crack and crack development under the state of relaxation. (author)
The anti-symmetry principle for quasi-static crack propagation in Mode III
Oleaga Apadula, Gerardo Enrique
2007-01-01
In this note we study a basic propagation criterion for quasi-static crack evolution in Mode III. Using classical techniques of complex analysis, the assumption of stable growth is expressed in terms of the parameters defining the elastic field around the tip. We explore the consequences of the local condition obtained and analyse its role as a crack propagation law. In particular, we herein extend to bounded domains a number of results previously obtained for the whole plane.
Fatigue Crack Propagation in Steel A131 Under Ice Loading of Crushing, Bending and Buckling
Institute of Scientific and Technical Information of China (English)
DUAN Menglan(段梦兰); SONG Lisong(宋立崧); FAN Xiaodong(樊晓东); James C.M.LId; FANG Huacan(方华灿)
2001-01-01
Three types of ice loading, which are most commonly present when ice acts on structures,are chosen and simulated for use of fatigue crack propagation tests on offshore structural steel Al31. The three types of ice categorized in accordance with the failure modes when acting on structures called crushing ice, bending ice, and buckling ice,respectively. This paper presents an experimental investigation on the fatigue crack propagation behavior of widely used high strength steel A 131 for offshore jackets in the loading environment of ice crushing, bending, and buckling. The test results of fatigue crack propagation in steel A 13 l under these simulated ice loading at temperature 292K are presented and analyzed in detail in this paper. The amplitude root mean square stress intensity factor is optimized to be the fundamental parameter of fatigue crack propagation for all types of ice loading histories. The results are also compared with constant amplitude fatigue crack propagation conclusions as in wave load mode, and a joint investigation on the results from ice forces, ice-induced vibrations, and ice-induced fatigue crack propagation is conducted, Conclusions are drawn for reference in structural design and material selection for offshore structures in ice environments.
Fatigue crack propagation in turbine disks of EI698 superalloy
Directory of Open Access Journals (Sweden)
A.A. Shanyavskiy
2013-04-01
Full Text Available In-service fatigue cracking of turbine disks of EI698 superalloy is discussed based on crack growth analyses. In the bolt joint for disks to shaft connecting there is high level of stress-state, which directed to earlier in-disks fatigue crack origination in low-cycle-fatigue regime. Fracture surface pattern such as fatigue striations were used for their spacing measurement and crack growth duration estimating. Developed disk tests on a special bench by the equivalent program to in-service cyclic loads have allowed discovering one-to-one correlation between fatigue striation spacing and crack increment in one flight. Number of fatigue striations and beach-marks calculations permitted to estimate crack growth period for the different stages of in-service disks cracking. Equivalent stress level for in-service cracked disks was calculated and compared with stress-level in-tested disks under stress equivalent program to in-service operated cyclic loads. Based on this result non-destructive inspection intervals were discussed and recommended for in-service disks in dependence on number of their flights at the moment of developed inspection to exclude in-flight disks fast fracture.
3D characterization of crack propagation in building stones
Fusi, N.; Martinez-Martinez, J.; Crosta, G. B.
2012-04-01
Opening of fractures can strongly modify mechanical characteristics of natural stones and thus significantly decrease stability of historical and modern buildings. It is commonly thought that fractures origin from pre-existing structures of the rocks, such as pores, veins, stylolythes (Meng and Pan, 2007; Yang et al., 2008). The aim of this study is to define relationships between crack formation and textural characteristics in massive carbonate lithologies and to follow the evolution of fractures with loading. Four well known Spanish building limestones and dolostones have been analysed: Amarillo Triana (AT): a yellow dolomitic marble, with fissures filled up by calcite and Fe oxides or hydroxides; Blanco Tranco (BT): a homogeneous white calcitic marble with pore clusters orientated parallel to metamorphic foliation; Crema Valencia (CV): a pinkish limestone (mudstone), characterized by abundant stilolythes, filled mainly by quartz (80%) and kaolin (11%); Rojo Cehegin (RC): a red fossiliferous limestone (packstone) with white veins, made up exclusively by calcite in crystals up to 300 micron. All lithotypes are characterized by homogeneous mineralogical composition (calcitic or dolomitic) and low porosity (<10%). Three cores 20 mm in diameter have been obtained for each lithotype. Uniaxial compressive tests have been carried out in order to induce sample fracturing by a series of successive steps with application of a progressive normal stress. Crack propagation has been checked after each stress level application by microCT-RX following Hg impregnation of the sample (in a Hg porosimeter). Combination of both tests (microCT-RX and Hg porosimeter) guarantees a better characterization of small defects and their progressive propagation inside low-porous rocks than by employing solely microCT-RX (Fusi et al., 2009). Due to the reduced dimensions of sample holder (dilatometers) in porosimeter, cores have been cut with a non standard h/d = 1.5. Several cycles of: a) Hg
The effect of residual stress on creep crack propagation in type 316 stainless steel weld metal
International Nuclear Information System (INIS)
Residual stresses are found to cause creep cracks to propagate at 538 deg C in laboratory specimens made from type 316 weld metal. These stresses were simulated by fixed displacement tests. Cracking occurred in the interior of specimens where triaxial stresses were developed at the crack tip, and was more extensive in thick specimens. A 25-fold increase in elastic follow-up led to a proportionate increase in the amount of crack growth. A method for generalising the results is proposed, whereby the main features which control cracking are all incorporated in a cracking severity index. Crack extension rate under relaxing load was as little as one tenth of that achieved if the minimum load had been held steady from the outset; this is attributed to a change in the balance between strain hardening and recovery rates. (author)
Crack propagation in Aluminum-Nomex panels: Experimental and numerical approach
International Nuclear Information System (INIS)
This paper analyses the damage tolerance behavior of an aluminum-Nomex sandwich panel subjected to shear load. The panel was tested inside a dedicated picture frame shear test fixture in order to apply a pulsating fully shear load. An artificial crack was created at the center of the panel on one of the skins. In order to acquire the propagation data, two crack propagation gauges (CPG) were applied at the apex of this artificial crack. Two finite element (FE) models of the panel were created in order to analyze the stress intensity factor (SIF) parameter of fracture mechanics, during propagation, using two different numerical approaches to schematize the crack propagation. The analytical approach is based on a classic Fe method, with a complete 3D model and manual propagation of the crack, and a dedicated program with automatic propagation on a 2D model (with a simple numerical correction to take 3D behavior into account). Comparison of the experimental data and the analytical results with existing propagation data (NASGRO) for the 2024 aluminum alloy of the skin, showed good agreement. Thus, EF models may also be used to perform damage tolerance evaluation for complex sandwich panel constructions. (author)
Wing crack propagation model under high hydraulic pressure in compressive-shear stress state
Institute of Scientific and Technical Information of China (English)
ZHAO Yan-lin; WANG Wen-jun
2011-01-01
A new wing crack model subjected to hydraulic pressure and far-field stresses was proposed considering the effect of hydraulic pressure in wing crack and the connected part of the main crack on the stress intensity factor at the wing crack tip.With the equivalent crack length leq of the wing crack introduced, the stress intensity factor K1 at the wing crack tip was assumed to the sum of two terms: on one hand a component K(1)1 for a single isolated straight wing crack of length 2l, and subjected to hydraulic pressure in the wing crack and far-field stresses; on the other hand a component K(2)1 due to the effective shear stress induced by the presence of the equivalent main crack. The lateral tensile stress and hydraulic high pressure are the key factors that induce crack propagation unsteadily. The new wing crack theoretical model proposed can supply references for the study on hydraulic fracture in fractured masses, hydraulic fracturing in rock masses.
Reliability analysis of crack propagation behavior of reactor components
International Nuclear Information System (INIS)
A reliability analysis was carried out on a circumferential weld in the main coolant loop of a PWR with the aim of estimating the probability of a leak or break occurring in the planned life cycle of the plant. To get a basis for the reliability analysis the following influence factors were more closely examined: initial crack extent, load spectrum including the emergency 'earthquake' situation and crack growth characteristics. For the actual reliability analysis a computer program was developed, which took the individual input data, in accordance with their statistical parameter, into account in a simulation calculation in line with the Monte Carlo Method. The Forman Formula was used to estimate the fatigue crack growth caused by the sequence of load events. The result was, that the fatigue crack growth, even in the case of large initial cracks, was negligibly small. The probability, that, in the case of very deep initial cracks, one-off high quasi-static load, e.g. during an earthquake, could cause a locally limited crack-through, was estimated to be about 5x10-6 in forty years. (orig./HP)
Modelling of Debond and Crack Propagation in Sandwich Structures Using Fracture and Damage Mechanics
DEFF Research Database (Denmark)
Berggreen, C.; Simonsen, Bo Cerup; Toernqvist, Rikard
Skin-core de-bonding or core crack propagation will often be dominating mechanisms in the collapse modes of sandwich structures. This paper presents two different methods for prediction of crack propagation in a sandwich structure: a fracture mechanics approach, where a new mode-mix method is...... presented, and a local damage mechanics approach. The paper presents a real-life application example, where the superstructure in a vessel pulls the skin off the sandwich deck. The calculations show almost unstable crack growth initially followed by a stabilization, and a nearly linear relation between...
Brittle versus ductile behaviour of nanotwinned copper: A molecular dynamics study
International Nuclear Information System (INIS)
Nanotwinned copper (Cu) exhibits an unusual combination of ultra-high yield strength and high ductility. A brittle-to-ductile transition was previously experimentally observed in nanotwinned Cu despite Cu being an intrinsically ductile metal. However, the atomic mechanisms responsible for brittle fracture and ductile fracture in nanotwinned Cu are still not clear. In this study, molecular dynamics (MD) simulations at different temperatures have been performed to investigate the fracture behaviour of a nanotwinned Cu specimen with a single-edge-notched crack whose surface coincides with a twin boundary. Three temperature ranges are identified, indicative of distinct fracture regimes, under tensile straining perpendicular to the twin boundary. Below 1.1 K, the crack propagates in a brittle fashion. Between 2 K and 30 K a dynamic brittle-to-ductile transition is observed. Above 40 K the crack propagates in a ductile mode. A detailed analysis has been carried out to understand the atomic fracture mechanism in each fracture regime
Modeling of crack propagation in weak snowpack layers using the discrete element method
Gaume, J.; van Herwijnen, A.; Chambon, G.; Birkeland, K. W.; Schweizer, J.
2015-10-01
Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e., to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, in order to compare the numerical and experimental results, the slab mechanical properties (Young's modulus and strength) which are not measured in the field were derived from density. The simulations nicely reproduced the process of crack propagation observed in field PSTs. Finally, the mechanical processes at play were analyzed in depth which led to suggestions for minimum column length in field PSTs.
Fatigue crack propagation in Hastelloy X weld metal
International Nuclear Information System (INIS)
The fatigue crack growth rate of Hastelloy X weld metal increased with stress intensity, temperature, and inverse frequency. The results were correlated with the equation da/dN = (ΔK)/sup n/, for constant frequency or constant temperature. The values of A and n were computed with a linear regression algorithm. With decreasing frequency at constant ΔK and constant temperature (5380C) fatigue crack growth rates approach an upper limit. Fatigue crack growth rate of the weld metal was lower than that reported for base metal at 5380C and lower at 6490C for a frequency of 1 Hz
Propagation of cracks and damage in non aging linear viscoelastic media
International Nuclear Information System (INIS)
Most of France's energy is nuclear. The reactor building comprises a internal and external containment. The internal containment is prestressed to limit the flow of leakage in the internal-external space. The prestress decreases during time by the creep of concrete. It may propagate the cracks by the accidental internal pressure. So we define two research problems: propagation of macro-cracks in viscoelastic structure; effective behavior of micro-cracked viscoelastic material. Firstly, we develop a Burger viscoelastic model of concrete with two approaches: numerical and analytical. Then we solve the problem of single cracks in developing thermodynamically the concept of energy release rate. In the third part we develop a viscoelastic model to study the effective behavior of micro-cracked materials in the case without propagation. The problem of propagation of microcracks is then studied by a numerical approach based on the 'representative pattern morphology'. These studies are finally applied to solve the problems of crack propagation and damage of containment under accidental internal pressure. (authors)
Effect of random microstructure on crack propagation in cortical bone tissue under dynamic loading
Gao, X.; Li, S.; Adel-Wahab, A.; Silberschmidt, V.
2013-07-01
A fracture process in a cortical bone tissue depends on various factors, such as bone loss, heterogeneous microstructure, variation of its material properties and accumulation of microcracks. Therefore, it is crucial to comprehend and describe the effect of microstructure and material properties of the components of cortical bone on crack propagation in a dynamic loading regime. At the microscale level, osteonal bone demonstrates a random distribution of osteons imbedded in an interstitial matrix and surrounded by a thin layer known as cement line. Such a distribution of osteons can lead to localization of deformation processes. The global mechanical behavior of bone and the crack-propagation process are affected by such localization under external loads. Hence, the random distribution of microstructural features plays a key role in the fracture process of cortical bone. The purpose of this study is two-fold: firstly, to develop two-dimensional microstructured numerical models of cortical bone tissue in order to examine the interaction between the propagating crack and bone microstructure using an extended finite-element method under both quasi-static and dynamic loading conditions; secondly, to investigate the effect of randomly distributed microstructural constituents on the crack propagation processes and crack paths. The obtained results of numerical simulations showed the influence of random microstructure on the global response of bone tissue at macroscale and on the crack-propagation process for quasi-static and dynamic loading conditions.
Effect of random microstructure on crack propagation in cortical bone tissue under dynamic loading
International Nuclear Information System (INIS)
A fracture process in a cortical bone tissue depends on various factors, such as bone loss, heterogeneous microstructure, variation of its material properties and accumulation of microcracks. Therefore, it is crucial to comprehend and describe the effect of microstructure and material properties of the components of cortical bone on crack propagation in a dynamic loading regime. At the microscale level, osteonal bone demonstrates a random distribution of osteons imbedded in an interstitial matrix and surrounded by a thin layer known as cement line. Such a distribution of osteons can lead to localization of deformation processes. The global mechanical behavior of bone and the crack-propagation process are affected by such localization under external loads. Hence, the random distribution of microstructural features plays a key role in the fracture process of cortical bone. The purpose of this study is two-fold: firstly, to develop two-dimensional microstructured numerical models of cortical bone tissue in order to examine the interaction between the propagating crack and bone microstructure using an extended finite-element method under both quasi-static and dynamic loading conditions; secondly, to investigate the effect of randomly distributed microstructural constituents on the crack propagation processes and crack paths. The obtained results of numerical simulations showed the influence of random microstructure on the global response of bone tissue at macroscale and on the crack-propagation process for quasi-static and dynamic loading conditions
Fatigue crack initiation and propagation in stainless steels subjected to thermal cycling conditions
International Nuclear Information System (INIS)
The thermal fatigue crack initiation and propagation promoted by thermal quenches of AISI 316 and 304 stainless steels have been studied to correlate with their known behaviour in isothermal strain cycling fatigue at elevated temperatures. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 250-5000C and symmetrically water-quenched on the narrow faces to give equivalent surface strain ranges from 2.8 x 10-3 to 5.4 x 10-3. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8 x 10-3 equivalent to a thermal amplitude of 1500C, no cracking being produced in 500000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain-controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrically quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. The results indicated a good correlation with the fracture behaviour observed from isothermal strain cycling fatigue behaviour in an air environment. (author)
International Nuclear Information System (INIS)
Highlights: → A nanostructured and layered steel exhibits high strength and large ductility. → The excellent combination originates from a multiple interlaminar cracking. → The initiation and propagation of cracks are controlled by three aspects. → The cracks are deflected by interface and arrested by compressive residual stress. → Finally, the cracks are blunted by the graded grain size distribution. - Abstract: A layered and nanostructured (LN) 304 SS sheet was produced by combination of surface mechanical attrition treatment (SMAT) with warm co-rolling. The microstructure of LN sheet is characterized by a periodic distribution of nanocrystalline layers and micron-grained layers with a graded transition of grain size. Tensile test results show that exceptional properties of high yield strength and large elongation to fracture are achieved. A multiple interlaminar cracking was observed by scanning electron microscopy, which is induced by repeated crack initiation and propagation. The toughening mechanisms of the LN sheet are proposed to be controlling the crack propagation path by several strategies. The main cracks initiating at interface defects are arrested by large compressive residual stress, deflected by weak interface bonding and blunted by the graded grain size distribution.
Energy Technology Data Exchange (ETDEWEB)
Chen, A.Y. [School of Material Science and Engineering, University of Shanghai for Science and Technology, 200093 (China); Department of Mechanical Engineering, The Hong Kong Polytechnic University (Hong Kong); Li, D.F. [Department of Mechanical Engineering, Hong Kong Polytechnic University (Hong Kong); Zhang, J.B. [Baosteel Technology Centre, Shanghai 201900 (China); Liu, F.; Liu, X.R. [School of Material Science and Engineering, University of Shanghai for Science and Technology, 200093 (China); Lu, J., E-mail: jianlu@cityu.edu.hk [Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong (Hong Kong)
2011-11-15
Highlights: {yields} A nanostructured and layered steel exhibits high strength and large ductility. {yields} The excellent combination originates from a multiple interlaminar cracking. {yields} The initiation and propagation of cracks are controlled by three aspects. {yields} The cracks are deflected by interface and arrested by compressive residual stress. {yields} Finally, the cracks are blunted by the graded grain size distribution. - Abstract: A layered and nanostructured (LN) 304 SS sheet was produced by combination of surface mechanical attrition treatment (SMAT) with warm co-rolling. The microstructure of LN sheet is characterized by a periodic distribution of nanocrystalline layers and micron-grained layers with a graded transition of grain size. Tensile test results show that exceptional properties of high yield strength and large elongation to fracture are achieved. A multiple interlaminar cracking was observed by scanning electron microscopy, which is induced by repeated crack initiation and propagation. The toughening mechanisms of the LN sheet are proposed to be controlling the crack propagation path by several strategies. The main cracks initiating at interface defects are arrested by large compressive residual stress, deflected by weak interface bonding and blunted by the graded grain size distribution.
International Nuclear Information System (INIS)
Crack propagation tests have been conducting under the conditions of pure cyclic thermal stress or combined cyclic thermal stress and constant primary load for the two types of structural models with and without structural discontinuity in order to establish a rational structural design procedure. In this paper, the outline of the whole plan of tests is described and an example of the test results is shown. In the experiments many types of measuring devices were applied to obtain detailed data of crack propagation behavior. Simplified crack propagation analysis was made using these detailed data and the analytical results are compared with the experimental results. The estimation results were in good agreement with experimental ones when crack opening ratio was taken into consideration in the evaluation. (authors)
Analytical Model for Fictitious Crack Propagation in Concrete Beams
DEFF Research Database (Denmark)
Ulfkjær, J. P.; Krenk, S.; Brincker, Rune
the elastic layer the deformations are modelled by the Timoshenko beam theory. The state of stress in the elastic layer is assumed to depend bi-lineary on local elongation corresponding to a linear softening relation for the fictitious crack. For different beam size results from the analytical model......An analytical model for load-displacement curves of unreinforced notched and un-notched concrete beams is presented. The load displacement-curve is obtained by combining two simple models. The fracture is modelled by a fictitious crack in an elastic layer around the mid-section of the beam. Outside...... the load-displacement curve where the fictitious crack starts to develope, and the point where the real crack starts to grow will always correspond to the same bending moment. Closed from solutions for the maximum size of the fracture zone and the minimum slope on the load-displacement curve is given...
Propagation of Slepyan's crack in a non-uniform elastic lattice
Nieves, Michael; Jones, Ian; Mishuris, Gennady
2012-01-01
We model and derive the solution for the problem of a Mode I semi-infinite crack propagating in a discrete triangular lattice with bonds having a contrast in stiffness in the principal lattice directions. The corresponding Green's kernel is found and from this wave dispersion dependencies are obtained in explicit form. An equation of the Wiener-Hopf type is also derived and solved along the crack face, in order to compute the stress intensity factor for the semi-infinite crack. The crack stability is analysed via the evaluation of the energy release rate for different contrasts in stiffness of the bonds.
International Nuclear Information System (INIS)
In cyclic loading tests carried out on eight 34 CrMo 4 pressure vessels the formation of fatigue cracks originating from starter notches could best be determined by the DC potential drop method and the measurement of crack opening displacement. As the cycles to fatigue crack initiation could be calculated only roughly when applying the constants for a similar material known from the literature, more appropriate constants were derived from the tests. A comparatively plain crack propagation program based on fracture mechanics relationships was found to give too low an estimate of the number of load cycles required for the propagation of semi-elliptical surface flaws in vessels if the stress limits during cyclic loading were kept constant or the minimum stress was raised respectively. In case the maximum stress was reduced, the underestimate was greater by far as the reduction in crack growth rate produced by the compressive residual stress of the high stress was not taken into account. (orig.)
High temperature initiation and propagation of cracks in 12%Cr-steel turbine disks
Directory of Open Access Journals (Sweden)
S. Foletti
2013-10-01
Full Text Available This work aims to study the crack propagation in 12%Cr steel for turbine disks. Creep Crack Growth (CCG tests on CT specimens have been performed to define the proper fracture mechanics which describes the initiation of the crack propagation and the crack growth behaviour for the material at high temperature. Results have been used to study the occurrence of crack initiation on a turbine disk at the extreme working temperature and stress level experienced during service, and validate the use of C* integral in correlating creep growth rate on the disk component, in case C* is numerically calculated through FEM analysis or calculated by the use of reference stress concept.
Czech Academy of Sciences Publication Activity Database
Veselý, V.; Sobek, J.; Frantík, P.; Štafa, M.; Šestáková, L.; Seitl, Stanislav
Zurich : Trans Tech Publications, 2014 - (Šandera, P.), s. 262-265 ISBN 978-3-03785-934-6. ISSN 1013-9826. - (Key Engineering Materials . 592-593). [MSMF 7 - International Conference on Materials Structure & Micromechanics of Fracture /7./. Brno (CZ), 01.07.2013-03.07.2013] Grant ostatní: VUT(CZ) GA13-09518S Institutional support: RVO:68081723 Keywords : Quasi-brittle material * fracture process zone width * crack tip constraint * multi-parameter fracture mechanics * wedge-splitting * three-point bending Subject RIV: JL - Materials Fatigue, Friction Mechanics
International Nuclear Information System (INIS)
The crack growth behaviour of materials for application in turbines at temperatures of 500- 750 C has been investigated. The creep and fatigue service loadings of a real turbine disc were simulated by introducing hold-times. The materials tested were the superalloy PM N18, Inconel 617 and the intermetallic phase β-NiAl of nominally stoichiometric composition. The crack growth tests were conducted in air and in vacuum (10-5 mbar) to assess the influence of the test atmosphere. One of the main objectives was to develop a marker method and its application, as support for the crack growth tests carried out. The width of the marker required for the marker bands could be chosen through the number of stress cycles or the crack growth increment in the marker-cycle. At 500 C, the crack surfaces of the CT specimens of Inconel 617 and PM N18 exhibited mixed fractures with trans- and intercrystalline regions. The fracture development could be divided into three, classical parts. At his temperature for both alloys the KI concept for the evaluation of the crack growth may be used. The RCT specimens of the intermetallic phase β-NiAl fractured in a completely brittle manner with no measurable time to failure. At 500 C, Inconel 617 and especially PM N18 were well suited to the use of the marker method. Measurements of the distances between the marker bands gave a good estimate of the crack growth rates. At the higher test temperature of 750 C, the crack growth rates and the proportion of intercrystalline fracture increased for Inconel 617 and PM N18. In all three materials, the formation of pores and dimpled fracture was observed, especially at high ΔKI values, and the coarse-grained β-NiAl exhibited higher crack growth rates than the fine-grained material. For this temperature the evaluation of the crack growth experiments should be by the KI concept for PM N18 and for Inconel 617 the C* concept is recommended. At the higher test temperature, the increased plasticity of
Estimation of stepwise crack propagation in ceramic laminates with strong interfaces
Directory of Open Access Journals (Sweden)
K. Štegnerová
2015-10-01
Full Text Available During the last years many researchers put so much effort to design layered structures combining different materials in order to improve low fracture toughness and mechanical reliability of the ceramics. It has been proven, that an effective way is to create layered ceramics with strongly bonded interfaces. After the cooling process from the sintering temperature, due to the different coefficients of thermal expansion of individual constituents of the composite, significant internal residual stresses are developed within the layers. These stresses can change the crack behaviour. This results to the higher value of so-called apparent fracture toughness, i.e. higher resistance of the ceramic laminate to the crack propagation. The contribution deals with a description of the specific crack behaviour in the layered alumina-zirconia ceramic laminate. The main aim is to clarify crack behaviour in the compressive layer and provide computational tools for estimation of crack behaviour in the field of strong residual stresses. The crack propagation was investigated on the basis of linear elastic fracture mechanics. Fracture parameters were computed numerically and by author’s routines. Finite element models were developed in order to obtain a stress distribution in the laminate containing a crack and to simulate crack propagation. The sharp change of the crack propagation direction was estimated using Sih’s criterion based on the strain energy density factor. Estimated crack behaviour is qualitatively in a good agreement with experimental observations. Presented approach contributes to the better understanding of the toughening mechanism of ceramic laminates and can be advantageously used for design of new layered ceramic composites and for better prediction of their failure.
Applicability assessment of plug weld to ITER vacuum vessel by crack propagation analysis
International Nuclear Information System (INIS)
In order to improve the fabricability of the vacuum vessel (VV) of International Thermonuclear Experimental Reactor (ITER), applicability of plug weld between VV outer shell and stiffening ribs/blanket support housings has been assessed using crack propagation analysis for the plug weld. The ITER VV is a double-wall structure of inner and outer shells with ribs and housings between the shells. For the fabrication of VV, ribs and housings are welded to outer shell after welding to inner shell. A lot of weld grooves should be adjusted for welding outer shell. The plug weld is that outer shells with slit at the weld region are set on ribs/housings then outer shells are welded to them by filling the slits with weld metal. The plug weld can allow larger tolerance of weld groove gap than ordinary butt weld. However, un-welded lengths parallel to outer sell surface remain in the plug weld region. It is necessary to evaluate the allowable un-welded length to apply the plug weld to ITER VV fabrication. For the assessment, the allowable un-welded lengths have been calculated by crack propagation analyses for load conditions, conservatively assuming the un-welded region is a crack. In the analyses, firstly allowable crack lengths are calculated from the stresses of the weld region. Then assuming initial crack length, crack propagation is calculated during operation period. Allowable initial crack lengths are determined on the condition that the propagated cracks should not exceed the allowable crack lengths. The analyses have been carried out for typical inboard straight region and inboard upper curved region with the maximum housing stress. The allowable initial cracks of ribs are estimated to be 8.8mm and 38mm for the rib and the housing, respectively, considering inspection error of 4.4mm. Plug weld between outer shell and ribs/housings could be applicable. (author)
A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum
Yamakov, V.; Saether, E.; Glaessgen, E.
2009-01-01
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [111] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.
Fatigue crack propagation in welded joint of austenitic steel for nuclear power engineering
International Nuclear Information System (INIS)
The crack propagation characteristics were obtained for Cr-Ni type austenitic steel 08Kh18N10T under variable stress in the individual zones of a welded joint on a pipe. Measurements of the threshold deviation of the stress intensity factor, ΔKp, showed that the root zone of the pipe welded joint was the weakest point as concerns crack propagation. The threshold values obtained for the filler metal on the pipe outer surface were considerably greater than those for the root zone of the welded joint and slightly greater than those for the base material and for the transition between the joint and the base material. The measured propagation response showed that the rate of fatigue crack propagation was for the base material higher by up to one order for low ΔK than for the filler joint and the root zone of the joint. (J.B.). 5 figs., 3 tabs., 6 refs
Prediction of three-dimensional crack propagation paths taking high cycle fatigue into account
Directory of Open Access Journals (Sweden)
Guido Dhondt
2016-01-01
Full Text Available Engine components are usually subject to complex loading patterns such as mixed-mode Low Cycle Fatigue Loading due to maneuvering. In practice, this LCF Loading has to be superimposed by High Cyclic Fatigue Loading caused by vibrations. The changes brought along by HCF are twofold: first, the vibrational cycles which are superposed on the LCF mission increase the maximum loading of the mission and may alter the principal stress planes. Secondly, the HCF cycles themselves have to be evaluated on their own, assuring that no crack propagation occurs. Indeed, the vibrational frequency is usually so high that propagation leads to immediate failure. In the present paper it is explained how these two effects can be taken care of in a standard LCF crack propagation procedure. The method is illustrated by applying the Finite Element based crack propagation software CRACKTRACER3D on an engine blade.
Analytical Model for Fictitious Crack Propagation in Reinforced Concrete Beams without Debonding
DEFF Research Database (Denmark)
Ulfkjær, J. P.; Brincker, Rune
, the crack growth is further simplified by introducing a continuous layer of springs at the midsection mainly representing a simplified material response around the fracture zone. In the reinforcement the strain condition is assumed to be equal to the strain condition in the concrete. the important......The non-linear fracture mechanical problem of combined crack growth and reinforcement action is modelled by adopting a simplified fictitious crack model for concrete and a linear elastic-plastic action for the reinforcement. The softening relation of the concrete is assumed to be linear, however...... question of de-bonding between the reinforcement and the concrete is therefore not considered. The model gives closed form solutions for the whole load displacement curve. Further, the model can describe important effects qualitatively correct. For instance the change of behaviour from brittle to ductile...
Brittle and semi-brittle behaviours of a carbonate rock: influence of water and temperature
Nicolas, A.; Fortin, J.; Regnet, J. B.; Dimanov, A.; Guéguen, Y.
2016-07-01
Inelastic deformation can either occur with dilatancy or compaction, implying differences in porosity changes, failure and petrophysical properties. In this study, the roles of water as a pore fluid, and of temperature, on the deformation and failure of a micritic limestone (white Tavel limestone, porosity 14.7 per cent) were investigated under triaxial stresses. For each sample, a hydrostatic load was applied up to the desired confining pressure (from 0 up to 85 MPa) at either room temperature or at 70 °C. Two pore fluid conditions were investigated at room temperature: dry and water saturated. The samples were deformed up to failure at a constant strain rate of ˜10-5 s-1. The experiments were coupled with ultrasonic wave velocity surveys to monitor crack densities. The linear trend between the axial crack density and the relative volumetric strain beyond the onset of dilatancy suggests that cracks propagate at constant aspect ratio. The decrease of ultrasonic wave velocities beyond the onset of inelastic compaction in the semi-brittle regime indicates the ongoing interplay of shear-enhanced compaction and crack development. Water has a weakening effect on the onset of dilatancy in the brittle regime, but no measurable influence on the peak strength. Temperature lowers the confining pressure at which the brittle-semi-brittle transition is observed but does not change the stress states at the onset of inelastic compaction and at the post-yield onset of dilatancy.
An adaptive finite element procedure for crack propagation analysis
Institute of Scientific and Technical Information of China (English)
ALSHOAIBI Abdulnaser M.; HADI M.S.A.; ARIFFIN A.K.
2007-01-01
This paper presents the adaptive mesh finite element estimation method for analyzing 2D linear elastic fracture problems. The mesh is generated by the advancing front method and the norm stress error is taken as a posteriori error estimator for the h-type adaptive refinement. The stress intensity factors are estimated by a displacement extrapolation technique. The near crack tip displacements used are obtained from specific nodes of natural six-noded quarter-point elements which are generated around the crack tip defined by the user. The crack growth and its direction are determined by the calculated stress intensity factors.The maximum circumference theory is used for the latter. In evaluating the accuracy of the estimated stress intensity factors, four cases are tested consisting of compact tension specimen, three-point bending specimen, central cracked plate and double edge notched plate. These were carried out and compared to the results from other studies. The crack trajectories of these specimen tests are also illustrated.
Shen, J. L.; Zhou, L.; Rowshandel, H.; Nicholson, G. L.; Davis, C. L.
2015-11-01
Alternating current field measurement (ACFM) probes are used to detect and size cracks in a range of engineering components. Crack sizing for this, and other electromagnetic (EM) based NDT systems, relies on relating the signal obtained to the actual crack length. For cracks that do not propagate vertically, such as rolling contact fatigue cracks in rails, predicting the crack depth, which determines the rail depth to be removed by grinding, requires an assumed propagation angle into the material as no method to determine crack vertical angle from the EM signals has been reported. This paper discusses the relationship between ACFM signals and propagation angles for surface-breaking cracks using a COMSOL model. The Bx signal accurately predicts the crack pocket length when the vertical angle is 30-90° but underestimates pocket length for shallower angles, e.g. a 50% underestimate is seen for a 3.2 mm pocket length crack propagating at a vertical angle of 10°. A new measure, the Bz trough-peak ratio, is proposed to determine the crack vertical angle. These are verified by experimental measurements using a commercial ACFM pencil probe for cracks with a range of vertical angles between 10° and 90°.
Analytical Model for Fictitious Crack Propagation in Concrete Beams
DEFF Research Database (Denmark)
Ulfkjær, J. P.; Krenk, Steen; Brincker, Rune
1995-01-01
are modeled by beam theory. The state of stress in the elastic layer is assumed to depend bilinearly on local elongation corresponding to a linear softening relation for the fictitious crack. Results from the analytical model are compared with results from a more detailed model based on numerical......An analytical model for load-displacement curves of concrete beams is presented. The load-displacement curve is obtained by combining two simple models. The fracture is modeled by a fictitious crack in an elastic layer around the midsection of the beam. Outside the elastic layer the deformations...... methods for different beam sizes. The analytical model is shown to be in agreement with the numerical results if the thickness of the elastic layer is taken as half the beam depth. It is shown that the point on the load-displacement curve where the fictitious crack starts to develop and the point where...
Hydrogen-Assisted Crack Propagation in Austenitic Stainless Steel Fusion Welds
Somerday, B. P.; Dadfarnia, M.; Balch, D. K.; Nibur, K. A.; Cadden, C. H.; Sofronis, P.
2009-10-01
The objective of this study was to characterize hydrogen-assisted crack propagation in gas-tungsten arc (GTA) welds of the nitrogen-strengthened, austenitic stainless steel 21Cr-6Ni-9Mn (21-6-9), using fracture mechanics methods. The fracture initiation toughness and crack growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 230 wppm (1.3 at. pct) hydrogen. The fracture initiation toughness and slope of the crack growth resistance curve for the hydrogen-precharged weld were reduced by as much as 60 and 90 pct, respectively, relative to the noncharged weld. A physical model for hydrogen-assisted crack propagation in the welds was formulated from microscopy evidence and finite-element modeling. Hydrogen-assisted crack propagation proceeded by a sequence of microcrack formation at the weld ferrite, intense shear deformation in the ligaments separating microcracks, and then fracture of the ligaments. One salient role of hydrogen in the crack propagation process was promoting microcrack formation at austenite/ferrite interfaces and within the ferrite. In addition, hydrogen may have facilitated intense shear deformation in the ligaments separating microcracks. The intense shear deformation could be related to the development of a nonuniform distribution of hydrogen trapped at dislocations between microcracks, which in turn created a gradient in the local flow stress.
Butt, Ali
Crack propagation in a solid rocket motor environment is difficult to measure directly. This experimental and analytical study evaluated the viability of real-time radiography for detecting bore regression and propellant crack propagation speed. The scope included the quantitative interpretation of crack tip velocity from simulated radiographic images of a burning, center-perforated grain and actual real-time radiographs taken on a rapid-prototyped model that dynamically produced the surface movements modeled in the simulation. The simplified motor simulation portrayed a bore crack that propagated radially at a speed that was 10 times the burning rate of the bore. Comparing the experimental image interpretation with the calibrated surface inputs, measurement accuracies were quantified. The average measurements of the bore radius were within 3% of the calibrated values with a maximum error of 7%. The crack tip speed could be characterized with image processing algorithms, but not with the dynamic calibration data. The laboratory data revealed that noise in the transmitted X-Ray intensity makes sensing the crack tip propagation using changes in the centerline transmitted intensity level impractical using the algorithms employed.
CRACK PROPAGATION BEHAVIOR AND LIFETIME PREDICTION IN ALUMINA AND ZIRCONIA
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
The lifetime prediction of ceramics is discussed on the basis of the relationship between stress intensity factor KI and crack velocity v. The effects of water environment, the cyclic loading and microstructure of material on KI-v characteristics are studied by carrying out the crack growth tests by the double torsion (DT) method under the static and cyclic loading in both environments of air and water for alumina and zirconia. KI-v characteristics determined by the double torsion method are used to predict time-to-failure under the cyclic loading of alumina and zirconia ceramics. The predictions agree qualitatively with the experimental results.
International Nuclear Information System (INIS)
Highlights: • Steady state crack velocity of delayed hydride cracking in Zircaloy-2 was analyzed. • A large stress peak is induced at an end of hydride by volume expansion of hydride. • Hydrogen diffuses to the stress peak, thereby accelerating steady hydride growth. • Crack velocity was estimated from the calculated hydrogen flux into the stress peak. • There was good agreement between calculation results and experimental data. -- Abstract: Delayed hydride cracking (DHC) of Zircaloy-2 is one possible mechanism for the failure of boiling water reactor fuel rods in ramp tests at high burnup. Analyses were made for hydrogen diffusion around a crack tip to estimate the crack velocity of DHC in zirconium alloys, placing importance on effects of precipitation of δ-hydride. The stress distribution around the crack tip is significantly altered by precipitation of hydride, which was strictly analyzed using a finite element computer code. Then, stress-driven hydrogen diffusion under the altered stress distribution was analyzed by a differential method. Overlapping of external stress and hydride precipitation at a crack tip induces two stress peaks; one at a crack tip and the other at the front end of the hydride precipitate. Since the latter is larger than the former, more hydrogen diffuses to the front end of the hydride precipitate, thereby accelerating hydride growth compared with that in the absence of the hydride. These results indicated that, after hydride was formed in front of the crack tip, it grew almost steadily accompanying the interaction of hydrogen diffusion, hydride growth and the stress alteration by hydride precipitation. Finally, crack velocity was estimated from the calculated hydrogen flux into the crack tip as a function of temperature, stress intensity factor and material strength. There was qualitatively good agreement between calculation results and experimental data
Micro mechanical analysis of the coupling between damage and permeability of brittle rocks
International Nuclear Information System (INIS)
This paper presents a coupled model for anisotropic damage and permeability variation in brittle rocks by micro-macro approach. The material damage is represented by space distribution of microcracks (crack orientation, crack length and crack aperture). The evolution of damage is determined from a crack propagation criterion. Effective properties of damaged material are studied using micro mechanical considerations. By using Darcy law for macroscopic fluid flow and assuming laminar flow inside microcracks, the total permeability of the cracked material is obtained by a volume averaging procedure taking into account crack aperture distribution in each orientation in the space. (authors)
Institute of Scientific and Technical Information of China (English)
ZENG Yanping; ZHANG Maicang; DONG Jianxin; ZHANG Lina; XIE Xishan
2005-01-01
In situ fatigue tests in special designed SEM were conducted to trace the whole process of crack initiation and propagation till to fracture in nickel-base P/M superalloy seeded inclusions. The experimental results show that non-metallic inclusions can induce crack initiation. When the inclusion size is larger than the critical one, the crack can propagate as the main crack that induces the specimen to fracture. As a result, the LCF life of the specimen decreases.
Laser cutting silicon-glass double layer wafer with laser induced thermal-crack propagation
Cai, Yecheng; Yang, Lijun; Zhang, Hongzhi; Wang, Yang
2016-07-01
This study was aimed at introducing the laser induced thermal-crack propagation (LITP) technology to solve the silicon-glass double layer wafer dicing problems in the packaging procedure of silicon-glass device packaged by WLCSP technology, investigating the feasibility of this idea, and studying the crack propagation process of LITP cutting double layer wafer. In this paper, the physical process of the 1064 nm laser beam interact with the double layer wafer during the cutting process was studied theoretically. A mathematical model consists the volumetric heating source and the surface heating source has been established. The temperature and stress distribution was simulated by using finite element method (FEM) analysis software ABAQUS. The extended finite element method (XFEM) was added to the simulation as the supplementary features to simulate the crack propagation process and the crack propagation profile. The silicon-glass double layer wafer cutting verification experiment under typical parameters was conducted by using the 1064 nm semiconductor laser. The crack propagation profile on the fracture surface was examined by optical microscope and explained from the stress distribution and XFEM status. It was concluded that the quality of the finished fracture surface has been greatly improved, and the experiment results were well supported by the numerical simulation results.
Corrosion fatigue behavior of fastening hole structure and virtual crack propagation tests
Institute of Scientific and Technical Information of China (English)
Youhong Zhang; Xinlong Chang; Guozhi Lv; Hui Wang; Zhong Li; Yueliang Cheng
2008-01-01
The fatigue crack propagation behavior of the LY12CZ aluminum alloy fastener involving a central hole in air or in 3.5wt% NaC1 solution was investigated. The experimental results indicated that the corrosion fatigue crack growth rate decreased with the increasing loading frequency, and in a corrosive environment, the crack growth rate was slightly larger than that in air.Based on the experimental results, the virtual corrosion fatigue crack propagation tests were investigated and the stochastic process method and the AFGROW simulation method were presented. The normal process and lognormal process were considered for the stochastic process method based on the numerically fitted Paris equation. The distribution of crack size and the corresponding prob-abilistic model of crack length distribution for a given number of cycles can be found by integrating the stochastic process over time.Using the AFGROW software, the virtual simulation was carried out to analyze the corrosion fatigue crack growth behavior and the predicted crack growth curve was in good agreement with the experimental results.
Gangloff, Richard P.; Kim, Sang-Shik
1993-01-01
This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.
International Nuclear Information System (INIS)
Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Dependence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc) in the crack tip vicinity. (orig.)
Shear crack formation and propagation in reinforced Engineered Cementitious Composites
DEFF Research Database (Denmark)
Paegle, Ieva; Fischer, Gregor
2011-01-01
capacity of beams loaded primarily in shear. The experimental program consists of ECC with short randomly distributed polyvinyl alcohol (PVA) fiber beams with different stirrup arrangements and conventional reinforced concrete (R/C) counterparts for comparison. The shear crack formation mechanism of ECC is...
Extreme stress gradient effects on microstructural fatigue crack propagation rates in Ni microbeams
Energy Technology Data Exchange (ETDEWEB)
Sadeghi-Tohidi, F.; Pierron, O. N., E-mail: olivier.pierron@me.gatech.edu [G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405 (United States)
2015-05-18
The fatigue crack propagation behavior of microstructurally small cracks growing under extreme stress gradients was investigated in Ni microbeams under fully reversed cyclic loading. A technique to calculate the crack growth rates in microbeams with two different normalized stress gradients (17% and 50% μm{sup −1}) is developed and validated. Decreasing crack propagation rates are observed over the first 2 μm, and the rates are more than 1 order of magnitude slower for the devices with 50% μm{sup −1} stress gradients. This fundamental knowledge is critical to predict the fatigue reliability of advanced metallic microcomponents under bending such as in microelectromechanical systems or flexible/stretchable electronics.
Effect of defect length on rolling contact fatigue crack propagation in high strength steel
Directory of Open Access Journals (Sweden)
T. Makino
2015-10-01
Full Text Available The objective of the present paper is to clarify the effect of defect length in depth direction on rolling contact fatigue (RCF crack propagation in high strength steel. RCF test and synchrotron radiation micro computed tomography (SR micro CT imaging were conducted. In the case of the defect with the 15 m diameter, flaking life decreased with increasing defect length. In a comparison of the CT image and the SEM view, the shapes of defects and the locations of the horizontal cracks were almost the same respectively. The mechanism of RCF crack propagation was discussed by finite element (FE analysis. Defects led to higher tensile residual stress than that without defects in the region where the defect exists. The shear stress range at 0.1 mm in depth on the middle line of the defect and the range of mode II stress intensity factor at the bottom of a vertical crack increased with increasing defect length.
Fatigue crack propagation behaviors of zirconium and Ti-5Ta alloy in boiling nitric acid
International Nuclear Information System (INIS)
Fatigue crack propagation behaviors of zirconium (Zr) and titanium-5%tantalum (Ti-5Ta) alloy in boiling nitric acid were investigated by fatigue tests. The crack propagation rates (CPR) of them were measured as a function of the stress intensity factor range, ΔK. CPR of Zr in boiling nitric acid was markedly enhanced as compared with that in air at room temperature. On the other hand, any environmental acceleration effect and any fractographic difference were not observed in CPR tests of Ti-5Ta alloy. The fractograph of Zr tested in the nitric acid showed a river pattern type crack. The enhancement effect on CPR of Zr was interpreted with high susceptibility to stress corrosion cracking (SCC) in boiling nitric acid. (author)
3D dynamic simulation of crack propagation in extracorporeal shock wave lithotripsy
International Nuclear Information System (INIS)
Some experimental observations of Shock Wave Lithotripsy(SWL), which include 3D dynamic crack propagation, are simulated with the aim of reproducing fragmentation of kidney stones with SWL. Extracorporeal shock wave lithotripsy (ESWL) is the fragmentation of kidney stones by focusing an ultrasonic pressure pulse onto the stones. 3D models with fine discretization are used to accurately capture the high amplitude shear shock waves. For solving the resulting large scale dynamic crack propagation problem, PDS-FEM is used; it provides numerically efficient failure treatments. With a distributed memory parallel code of PDS-FEM, experimentally observed 3D photoelastic images of transient stress waves and crack patterns in cylindrical samples are successfully reproduced. The numerical crack patterns are in good agreement with the experimental ones, quantitatively. The results shows that the high amplitude shear waves induced in solid, by the lithotriptor generated shock wave, play a dominant role in stone fragmentation.
Energy Technology Data Exchange (ETDEWEB)
Wire, G. L.; Mills, W. J.
2002-08-01
Fatigue crack propagation (FCP) rates for 304 stainless steel (304SS) were determined in 24 degree C and 288 degree C air and 288 degree C water using double-edged notch (DEN) specimens of 304 stainless steel (304 SS). Test performed at matched loading conditions in air and water at 288 degree C with 20-6- cc h[sub]2/kg h[sub]2O provided a direct comparison of the relative crack growth rates in air and water over a wide range of crack growth rates. The DEN crack extension ranged from short cracks (0.03-0.25 mm) to long cracks up to 4.06 mm, which are consistent with conventional deep crack tests. Crack growth rates of 304 SS in water were about 12 times the air rate. This 12X environmental enhancement persisted to crack extensions up to 4.06 mm, far outside the range associated with short crack effects. The large environmental degradation for 304 SS crack growth is consistent with the strong reduction of fatigue life in high hydrogen water. Further, very similar environmental effects w ere reported in fatigue crack growth tests in hydrogen water chemistry (HWC). Most literature data in high hydrogen water show only a mild environmental effect for 304 SS, of order 2.5 times air or less, but the tests were predominantly performed at high cyclic stress intensity or equivalently, high air rates. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, T[sub]r, as recently reported for austenitic stainless steel in BWR water. Fractography was performed for both tests in air and water. At 288 degree C in water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets on the fracture surfaces suggest that hydrogen embrittlement is the primary cause of accelerated cracking.
Hassanifard, S.; Bonab, M. A. Mohtadi; Jabbari, Gh.
2013-01-01
In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.
International Nuclear Information System (INIS)
Ductile crack initiation and propagation within a naturally aged aluminium alloy sheet has been observed in situ via synchrotron radiation-computed laminography, a technique specifically adapted to three-dimensional imaging of thin objects that are laterally extended. Voids and intermetallic particles, and their subsequent evolution during ductile crack extension at different associated levels of stress triaxiality, were clearly observed within fracture coupons of a reasonable engineering length-scale, overcoming the conventional sample size limitation of computed tomography at high resolutions.
Image-based Cohesive Element Modelling of Low Temperature Crack Propagation in Alloy 82 Weld Metal
Klimaytys G, Jivkov AP, Engelberg DL
2014-01-01
Exposure of Alloy 82 welds to hydrogen containing, de-oxygenated aqueous environments at temperatures below 150°C can result in embrittlement, manifested by a significant reduction of its resistance to cracking. The embrittlement is brought about by nano-scale niobium and titanium rich carbonitrides at grain boundaries which act as hydrogen traps. The presence of stresses may then result in low temperature crack propagation (LTCP).The work reported in this paper provides a better understandi...
Directory of Open Access Journals (Sweden)
Reza H. Talemi
2016-03-01
Full Text Available In the past several numerical studies have addressed the ductile mode of fracture propagation. However, the brittle mode of pipeline failure has not received as much attention yet. The main objective of this study is to predict brittle fracture behaviour of API X70 pipeline steel by means of a numerical approach. To this end, the eXtended Finite Element Method (XFEM-based cohesive segment technique is used to model Drop Weight Tear Test (DWTT of X70 pipeline steel at -100°C. In this model the dynamic stress intensity factor and crack velocity are calculated at the crack tip at each step of crack propagation.
International Nuclear Information System (INIS)
Full text: In this paper, the computational simulations of crack propagation in three-dimensional structures are presented using the concept of local approach of fracture in continuum damage mechanics. An isotropic ductile damage model based on the elasto-viscoplastic rule is used for the damage process of a ductile material with large deformations. The crack initiation and propagation are achieved by using a discrete element removing technique when the damage variable in an element reaches a critical damage value. As a softening behavior due to stiffness degradation happens in this study, a modified Riks' continuation technique is adopted for the smooth search of solutions. The crack propagation in this finite element analysis depends on the mesh size around a crack tip because an element with critical damage variable is removed. If very fine meshes are generated around a crack tip, it is possible that more accurate crack propagation can be shown. There-fore, a large number of degree of freedom is needed around a critical point, and supercomputing power should be utilized to solve large-scale nonlinear problems. The main idea of this study is that the damage and crack propagation in three-dimensional structures can be simulated accurately by using large amounts of degree of freedom, and an efficient parallel computing algorithm for nonlinear problems is developed for this purpose. The parallel computation is based on the parallel multifrontal solver and Internet supercomputing. Internet supercomputing is a new supercomputing methodology, which uses numerous general-purpose personal computers connected by the Internet as a parallel computing resource. To test the parallel performance, 0.5 million DOF problem was analyzed in the parallel computing system, which is composed of 64 processors, with 512 MB of main memory. The speedups for 8, 16, 32 processors were 5.9, 15.4, 22.1, respectively. From the results, it can be seen that the developed parallel algorithm
Influence of crack retardation on fatigue crack propagation in steels for railway axles
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Pokorný, Pavel; Hutař, Pavel
Zurich : Trans Tech Publications, 2014 - (Clark, G.; Wang , C.), s. 351-356 ISBN 978-3-03835-008-8. ISSN 1022-6680. - (Advanced Materials Research. 891-892). [Fatigue 2014 - International Fatigue Congress /11./. Melbourne (AU), 02.03.2014-07.03.2014] R&D Projects: GA MŠk(CZ) EE2.3.20.0214 Institutional support: RVO:68081723 Keywords : railway axle * fatigue crack * generalized Willenborg model * crack retardation Subject RIV: JL - Materials Fatigue, Friction Mechanics
Simulation of crack propagation in fiber-reinforced concrete by fracture mechanics
International Nuclear Information System (INIS)
Mode I crack propagation in fiber-reinforced concrete (FRC) is simulated by a fracture mechanics approach. A superposition method is applied to calculate the crack tip stress intensity factor. The model relies on the fracture toughness of hardened cement paste (KIC) and the crack bridging law, so-called stress-crack width (σ-δ) relationship of the material, as the fundamental material parameters for model input. As two examples, experimental data from steel FRC beams under three-point bending load are analyzed with the present fracture mechanics model. A good agreement has been found between model predictions and experimental results in terms of flexural stress-crack mouth opening displacement (CMOD) diagrams. These analyses and comparisons confirm that the structural performance of concrete and FRC elements, such as beams in bending, can be predicted by the simple fracture mechanics model as long as the related material properties, KIC and (σ-δ) relationship, are known
International Nuclear Information System (INIS)
A purely phenomenological study of stress corrosion cracking was performed using the couple Z2CN 18.10 (304L) austenitic stainless steel/boiling MgCl2 aqueous solution. The exploitation of the morphological information (shape of the cracks and size distribution) available after constant elongation rate tests led to the proposal of an analytical expression of the crack initiation and growth rates. This representation allowed to quantitatively characterize the influence of the applied strain rate as well as the effect of corrosion inhibitors on the crack initiation and propagation phases. It can be used in the search for the stress corrosion cracking mechanisms as a 'riddle' for the determination of the rate controlling steps. As a matter of fact, no mechanistic hypothesis has been used for its development. (author)
Peak Stress Intensity Factor Governs Crack Propagation Velocity In Crosslinked UHMWPE
Sirimamilla, P. Abhiram; Furmanski, Jevan; Rimnac, Clare
2012-01-01
Ultra high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint replacement components. However, these bearing materials can fail as a result of in vivo static and cyclic loads. Crack propagation behavior in this material has been considered using the Paris relationship which relates fatigue crack growth rate, da/dN (mm/cycle) versus the stress intensity factor range, ΔK (Kmax-Kmin, MPa√m). However, recent work suggests that the crack propagat...
Finite Element Simulations on Erosion and Crack Propagation in Thermal Barrier Coatings
Ma, Z. S.; Fu, L. H.; Yang, L.; Zhou, Y. C.; Lu, C.
2015-07-01
Erosion of thermal barrier coatings occurs when atmospheric or carbon particles from the combustion chamber are ingested into aviation turbine engines. To understand the influence of erosion on the service life of thermal barrier coatings, we introduce the erosion and crack propagation models, and then by using finite element simulations, determine the relationship between the penetrating depth, the maximum principle stress and impingement variables such as velocity and angle. It is shown that cracks nucleate and extend during the erosion process and the length of a crack increases with the increase of the particle velocity and impact angle.
EXPERIMENTAL INVESTIGATION ON FATIGUE CRACK PROPAGATION UNDER TENSION-TORSION LOADING
Institute of Scientific and Technical Information of China (English)
WANGYing-yu; YAOWei-xing
2004-01-01
Various proportional and nonproportional tension-torsion fatigue tests are conducted on aeronautical material-LY12CZ aluminum alloy. The stress and strain states under tension-torsion loading are analyzed by an elastic-plastic finite element method. The relation between the orientation of crack propagation and each stress and strain component is investigated. Analytical results are compared with experimental data. Results demonstrate that the fatigue cracks tend to be propagated perpendicular to the direction of the largest principle strains under proportional loading, and grow alone one of the maximum shear strain planes under 45° and 90° out-of-phase loadings.
Prediction of fatigue crack propagation life in notched members under variable amplitude loading
Khan, Z.; Rauf, A.; Younas, M.
1997-06-01
One of the interesting phenomenon in the study of fatigue crack propagation under variable amplitude load cycling is the crack growth retardation that normally occurs due to the application of a periodic overload. Fatigue crack growth rate under simple variable amplitude loading sequence incorporating period overloads is studied using single edge notched specimens of AISI304 stainless steel. Load interaction effects due to single and multiple overload have been addressed. Substantial retardation of fatigue crack growth rate is observed due to the introduction of periodic tensile overloads. Estimates of fatigue life have been obtained employing Wheeler model (using Paris and modified Paris equations) and Elber’s model. Analytical predictions are compared with experimental results. Results of these analytical fatigue life predictions show good agreement with the experimental fatigue life data. Fatigue crack propagation rates also have been evaluated from the fractographic study of fatigue striations seen on the fracture surface. Good agreement was found between the experimentally observed crack growth rates and the fatigue crack growth rates determined by the fractographic studies.
Fatigue of Self-Healing Nanofiber-based Composites: Static Test and Subcritical Crack Propagation.
Lee, Min Wook; Sett, Soumyadip; Yoon, Sam S; Yarin, Alexander L
2016-07-20
Here, we studied the self-healing of composite materials filled with epoxy-containing nanofibers. An initial incision in the middle of a composite sample stretched in a static fatigue test can result in either crack propagation or healing. In this study, crack evolution was observed in real time. A binary epoxy, which acted as a self-healing agent, was encapsulated in two separate types of interwoven nano/microfibers formed by dual-solution blowing, with the core containing either epoxy or hardener and the shell being formed from poly(vinylidene fluoride)/ poly(ethylene oxide) mixture. The core-shell fibers were encased in a poly(dimethylsiloxane) matrix. When the fibers were damaged by a growing crack in this fiber-reinforced composite material because of static stretching in the fatigue test, they broke and released the healing agent into the crack area. The epoxy used in this study was cured and solidified for approximately an hour at room temperature, which then conglutinated and healed the damaged location. The observations were made for at least several hours and in some cases up to several days. It was revealed that the presence of the healing agent (the epoxy) in the fibers successfully prevented the propagation of cracks in stretched samples subjected to the fatigue test. A theoretical analysis of subcritical cracks was performed, and it revealed a jumplike growth of subcritical cracks, which was in qualitative agreement with the experimental results. PMID:27332924
International Nuclear Information System (INIS)
Document available in extended abstract form only. This subject is devoted to numerical analysis of crack initiation and propagation in concrete structures due to hydro-mechanical coupling processes. When the structures subjected to the variation in hydraulic conditions, fractures occur as a consequence of coalescence of diffuse damage. Consequently, the mechanical behaviour of concrete is described by an isotropic damage model. Once the damage reaches a critical value, a macroscopic crack is initiated. In the framework of extended Finite Element Method (XFEM), the propagation of localized crack is studied in this paper. Each crack is then considered as a discontinuity surface of displacement. According to the determination of crack propagation orientations, a tensile stress-based criterion is used. Furthermore, spatial variations of mechanical properties of concrete are also taken into account using the Weibull distribution function. Finally, the proposed model is applied to numerical analysis of a concrete liner in the context of feasibility studies for geological storage of radioactive wastes. The numerical results show that the proposed approach is capable to reproduce correctly the initiation and propagation crack process until the complete failure of concrete structures during hydro-mechanical loading. The concrete is most widely used construction material in many engineering applications. It is generally submitted to various environmental loading: such as the mechanical loading, the variation of relative humidity and the exposure to chemical risk, etc. In order to evaluate the safety and durability of concrete structures, it is necessary to get a good knowledge on the influence of loading path on the concrete behaviour. The objective of this paper is to study numerically the crack propagation in concrete structure under hydro-mechanical loading,.i.e. the mechanical behaviour of concrete subjected to drying process. The drying process leads to desiccation
Fatigue crack propagation in steels for railway axles
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Pokorný, Pavel; Hutař, Pavel; Matušek, P.
Zurich: Trans Tech Publications, 2014 - (Šandera, P.), s. 254-257. (Key Engineering Materials. 592-593). ISBN 978-3-03785-934-6. ISSN 1013-9826. [MSMF 7 - International Conference on Materials Structure & Micromechanics of Fracture /7./. Brno (CZ), 01.07.2013-03.07.2013] R&D Projects: GA MŠk(CZ) EE2.3.20.0214 Grant ostatní: VUT(CZ) FSIJ- 13-2046 Institutional support: RVO:68081723 Keywords : railway axle * crack retardation * variable loading amplitude * generalized Willenborg model Subject RIV: JL - Materials Fatigue, Friction Mechanics
Damage description with related crack initiation and propagation conditions
International Nuclear Information System (INIS)
The damage accumulation condition expressed in terms of traction components on a physical plane is considered for both monotonic and cyclic loading conditions. The crack initiation is assumed to correspond to a critical value of damage parameter on a maximum damage plane. A non-local condition is formulated for singular stress or strain regimes. The model is applied to predict damage distribution within the element for cyclic loading condition; in particular for combined torsion and bending. The damage tensors are introduced to describe the predicted damage distribution. (orig.)
Crack propagation in tough ductile materials. Phase I
International Nuclear Information System (INIS)
The report describes and presents the J-resistance curves obtained as a function of crack extension for two representative tough ductile materials namely ASTM516 grade 70 plate steel and SA106 grade B pipe steel. The results were obtained using the ASTM standard method for determining J-R curves, E24.08, 12th Draft, 25th July, 1985. Both compact tension and three point bend tests were employed for the plate steel tests; only compact tension specimens were used to evaluate the pipe steel. All tests were carried out under load control conditions using specimens of different thickness and cut from known orientations within the parent material
Effect of micromorphology of cortical bone tissue on crack propagation under dynamic loading
Wang, Mayao; Gao, Xing; Abdel-Wahab, Adel; Li, Simin; Zimmermann, Elizabeth A.; Riedel, Christoph; Busse, Björn; Silberschmidt, Vadim V.
2015-09-01
Structural integrity of bone tissue plays an important role in daily activities of humans. However, traumatic incidents such as sports injuries, collisions and falls can cause bone fracture, servere pain and mobility loss. In addition, ageing and degenerative bone diseases such as osteoporosis can increase the risk of fracture [1]. As a composite-like material, a cortical bone tissue is capable of tolerating moderate fracture/cracks without complete failure. The key to this is its heterogeneously distributed microstructural constituents providing both intrinsic and extrinsic toughening mechanisms. At micro-scale level, cortical bone can be considered as a four-phase composite material consisting of osteons, Haversian canals, cement lines and interstitial matrix. These microstructural constituents can directly affect local distributions of stresses and strains, and, hence, crack initiation and propagation. Therefore, understanding the effect of micromorphology of cortical bone on crack initiation and propagation, especially under dynamic loading regimes is of great importance for fracture risk evaluation. In this study, random microstructures of a cortical bone tissue were modelled with finite elements for four groups: healthy (control), young age, osteoporosis and bisphosphonate-treated, based on osteonal morphometric parameters measured from microscopic images for these groups. The developed models were loaded under the same dynamic loading conditions, representing a direct impact incident, resulting in progressive crack propagation. An extended finite-element method (X-FEM) was implemented to realize solution-dependent crack propagation within the microstructured cortical bone tissues. The obtained simulation results demonstrate significant differences due to micromorphology of cortical bone, in terms of crack propagation characteristics for different groups, with the young group showing highest fracture resistance and the senior group the lowest.
In-situ observation of crack propagation in SiC/SiC by HVEM
International Nuclear Information System (INIS)
Full text of publication follows: SiC/SiC composites are candidate materials for fusion applications due to their potential to retain strength and exhibit tough behavior at elevated temperatures. The irradiation stability of monolithic beta-SiC has been well studied and the mechanical property evaluation of SiC/SiC composites has been started under the standard test method of Continuous Fiber reinforced Ceramic Composites (CFCC's). It is important to know the initiation, coalescence and growth of crack in SiC/SiC composites. However there are no good tests for measuring the crack propagation at fracture of SiC/SiC composites than before. After mechanical testing, microstructure analysis of fracture region in SiC/SiC composites by Focused Ion Beam (FIB) was done, though the artifact during TEM specimen preparation would be introduced sometime. Recently we successfully developed a piezo driven nano indenting equipment for observation of crack propagation in SiC/SiC under the irradiation by High Voltage Electron Microscope (HVEM). Preliminary result of in-situ observation of shear crack propagation at the interface between SiC fiber and SiC matrix by HVEM shows a good agreement with the result of out situ experiment by using the cross section TEM specimen at the shear fracture interface between SiC fiber and SiC matrix prepared by FIB after the fiber pushing out testing by the nano indenter. The shear crack initiated and propagated at the interface between SiC matrix and carbon coated layer on the SiC fiber. Results of in-situ observation of crack propagation at W and SiC joining interface will be also presented. (authors)
Crack Propagation in a Toughened Epoxy Adhesive under Repeated Impacts
Directory of Open Access Journals (Sweden)
Ian A. Ashcroft
2011-01-01
Full Text Available Adhesives are being increasingly used in structural applications, especially in aerospace, automotive and naval structures, making their structural integrity an important issue. In-service loading histories of such structures usually contain low-energy impacts, repetition of which can significantly affect their performance. This paper deals with the behaviour of the toughened epoxy adhesive FM73 under repeated impacts, known as impact fatigue. Izod impact fatigue tests were performed on FM73 specimens in order to study the evolution of damage and to characterise this via measurable parameters, such as the maximum force and the contact time. A finite element model was developed to simulate the impact tests and this was used to calculate the dynamic strain energy release rate, which was compared with that determined using a simple analytical method. A relationship between the maximum dynamic strain energy release rate and impact fatigue crack growth rate was established that was used as the basis of an impact fatigue crack growth law.
International Nuclear Information System (INIS)
This PhD thesis presents numerical methods is dedicated to three-dimensional crack propagation in the framework of fast explicit structural dynamics using EUROPLEXUS software (currently abbreviated EPX, co-owned by CEA and EC/JRC). An approach based on the well-known XFEM method is proposed, representing the crack through level set functions. Special care is given to the update of the level set functions from the propagation velocity expressed on the crack edge, since the most widely used method based on the solution of Hamilton-Jacobi equations lacks robustness for fast transient crack propagations, even when level-sets are computed on an auxiliary regular finite difference grid. It is therefore chosen instead to implement a 3D approximated geometric method to update both level-sets. As far as failure mechanics is concerned, a local stress criterion on the edge of the crack, first developed by Haboussa et al., gives characteristic parameters of the material fracture. Mechanical equivalent quantities (strain, deformation) around the crack front are weighted by a Gaussian function, which gives more importance to Gauss integration points located near the crack tip. The maximum of the equivalent stress tensor near the crack tip gives the direction of the crack, and the Kanninen equation gives the crack velocity. Besides, because of the discontinuous displacement field, the numerical integration for elements cut by the crack yields performance issues. Increasing the number of quadrature points is CPU time consuming and quite hard to handle if it is chosen to change the number of points only for elements in the vicinity of the crack. Another approach tested here consists in keeping constant the number and position of quadrature points and modifying their weights in cut elements to obtain an accurate integration of several reference discontinuous fields. The proposed methods are tested and validated on significant examples, both two-dimensional, to ensure the backward
Delayed hydrogen cracking test design for pressure tubes
International Nuclear Information System (INIS)
CANDU nuclear power stations pressure tubes of alloy Zr-2,5 % Nb present a cracking phenomenon known as delayed hydrogen cracking (DHC). This is a brittle fracture of zirconium hydrides that are developed by hydrogen due to aqueous corrosion on the metal surface. This hydrogen diffuses to the crack tip where brittle zirconium hydrides develops and promotes the crack propagation. A direct current potential decay (DCPD) technique has been developed to measure crack propagation rates on compact test (CT) samples machined from a non irradiated pressure tube. Those test samples were hydrogen charged by cathodic polarization in an acid solution and then pre cracked in a fatigue machine. This technique proved to be useful to measure crack propagation rates with at least 1% accuracy for DHC in pressure tubes. (author)
International Nuclear Information System (INIS)
This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage
Czech Academy of Sciences Publication Activity Database
Nohava, Jiří; Haušild, P.; Bompard, P.
Vol. A. Praha : CVUT, 2002, s. 592-593. [Workshop 2002. Praha (CZ), 11.02.2002-13.02.2002] Institutional research plan: CEZ:AV0Z2043910 Keywords : electron backscattering difraction, bainitic steel , crack propagation Subject RIV: JK - Corrosion ; Surface Treatment of Materials
Fatigue crack propagation at high temperature (5500C) in stainless steel type 316 L
International Nuclear Information System (INIS)
The effect of different parameters such as temperature, stress ratio R, frequency f, have been investigated for fatigue crack propagation in stainless steel type 316 L. At high frequency (20 Hz), increasing temperature from room temperature to 5500C, produced an increase in crack propagation rate. Fractographic analysis of failed specimens indicated no change in failure mode which was transgranular with fatigue striations indicating a ductile failure process. Transmission electron microscopy analysis showed a change in the plastic deformation mode which was consisted, at room temperature, of twinning and martensitic transformation, characteristic of a low SFE material, and at high temperature, of a disoriented cellular structure characteristic of a high SFE material. At high temperature, raising stress ratio from 0.1 to 0.5, increased crack propagation rate with no change in the failure mode which was transgranular. At high temperature, decreased frequency from 1200 cpm to 4 cpm increased crack propagation rate. Combined effect of temperature and low frequency led to corrosion, and intergranular failure
Effect of segregations on mechanical properties and crack propagation in spring steel
Directory of Open Access Journals (Sweden)
B. Žužek
2015-10-01
Full Text Available Considerable efforts have been made over the last decades to improve performance of spring steels, which would increase the service time of springs and also allow vehicles weight reduction. There are different possibilities of improving properties of spring steels, from modifying the chemical composition of steels to optimizing the deformation process and changing the heat treatment parameters. Another way of improving steel properties is through refining the microstructure and reducing amount of inclusions. Therefore, the focus of the current investigation was to determine the effect of more uniform and cleaner microstructure obtained through electro-slag remelting (ESR of steel on the mechanical and dynamic properties of spring steel, with special focus on the resistance to fatigue crack propagation. Effect of the microstructure refinement was evaluated in terms of tensile strength, elongation, fracture and impact toughness, and fatigue resistance under bending and tensile loading. After the mechanical tests the fracture surfaces of samples were analyzed using scanning electron microscope (SEM and the influence of microstructure properties on the crack propagation and crack propagation resistance was studied. Investigation was performed on hot rolled, soft annealed and vacuum heat treated 51CrV4 spring steel produced by conventional continuous casting and compared with steel additional refined through ESR. Results shows that elimination of segregations and microstructure refinement using additional ESR process gives some improvement in terms of better repeatability and reduced scattering, but on the other hand it has negative effect on crack propagation resistance and fatigue properties of the spring steel.
Modelling 3D crack propagation in ageing graphite bricks of Advanced Gas-Cooled Reactor Power Plant
International Nuclear Information System (INIS)
In this paper, crack propagation in Advanced Gas-cooled Reactor (AGR) graphite bricks with ageing properties is studied using the eXtended Finite Element Method (X-FEM). A parametric study for crack propagation, including the influence of different initial crack shapes and propagation criteria, is conducted. The results obtained in the benchmark study show that the crack paths from X-FEM are similar to the experimental ones. The accuracy of the strain energy release rate computation in a heterogeneous material is also evaluated using a finite difference approach. Planar and non-planar 3D crack growth simulations are presented to demonstrate the robustness and the versatility of the method utilized. Finally, this work contributes to the better understanding of crack propagation behaviour in AGR graphite bricks and so contributes to the extension of the AGR plant lifetimes in the UK by reducing uncertainties. (author)
Hao, Wenfeng; Ma, Liting; Chen, Xinwen; Yuan, Yanan; Ma, Yinji
2016-02-01
The fatigue crack propagation behavior of two different forms of PMMA was studied using two-stage zone model. First, the fatigue crack length and fatigue crack propagation velocities of different specimens were obtained experimentally. Then the effect of material forms and specimen types on the fatigue crack propagation velocities was analyzed. Finally, the data scatter of da/ dN-Δ K curves in different forms and different types of specimens was analyzed. The results show that the expressions of fatigue crack propagation velocities of middle crack tension (MT) specimens and compact tension (CT) specimens in the same form PMMA are similar. And the scatter of MT specimens is larger than CT specimens in two forms of PMMA.
Directory of Open Access Journals (Sweden)
R.S. Tripathi
2001-04-01
Full Text Available In this paper, two grades of steel, viz., plain carbon steel and low alloy steel used in naval ships have been selected for studies on high cycle fatigue, crack propagation, stress intensity and crack opening displacement (COD. Specimen for high cycle fatigue was prepared as per IS: 1608. High cycle fatigue was carried out up to 50,000 cycles at 1000 kgfto 2000 kgfloads. Up to 2000 kgfloads, both the materials were observed within elastic zones. A number of paran1eters, including stress, strain and strain range, which indicate elastic behaviour of steels, have been considered. Low alloy steel specimen was prepared as per ASTM standard: E-399 and subjected to 5,00,000 cycles. Crack propagation, COD, stress intensity, load-cycle variations, load-COD relation, and other related paran1eters have been studied using a modem universal testing machine with state-of-the-art technology
Institute of Scientific and Technical Information of China (English)
Zhenzhong CHEN; Ping HE; Liqing CHEN
2007-01-01
Fatigue crack propagation (FCP) behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high AK region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent.Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.
Fatigue crack propagation of new aluminum lithium alloy bonded with titanium alloy strap
Institute of Scientific and Technical Information of China (English)
Sun Zhenqi; Huang Minghui
2013-01-01
A new type of aluminum lithium alloy (A1-Li alloy) Al-Li-S-4 was investigated by test in this paper.Alloy plate of 400 mm × 140 mm × 6 mm with single edge notch was made into samples bonded with Ti-6Al-4V alloy (Ti alloy) strap by FM 94 film adhesive after the surface was treated.Fatigue crack growth of samples was investigated under cyclic loading with stress ratio (R) of 0.1 and load amplitude constant.The results show that Al-Li alloy plate bonded with Ti alloy strap could retard fatigue crack propagation.Retardation effect is related with width and thickness of strap.Flaws have an observable effect on crack propagation direction.
Mode I Cohesive Law Characterization of Through-Crack Propagation in a Multidirectional Laminate
Bergan, Andrew C.; Davila, Carlos G.; Leone, Frank A.; Awerbuch, Jonathan; Tan, Tein-Min
2014-01-01
A method is proposed and assessed for the experimental characterization of through-the-thickness crack propagation in multidirectional composite laminates with a cohesive law. The fracture toughness and crack opening displacement are measured and used to determine a cohesive law. Two methods of computing fracture toughness are assessed and compared. While previously proposed cohesive characterizations based on the R-curve exhibit size effects, the proposed approach results in a cohesive law that is a material property. The compact tension specimen configuration is used to propagate damage while load and full-field displacements are recorded. These measurements are used to compute the fracture toughness and crack opening displacement from which the cohesive law is characterized. The experimental results show that a steady-state fracture toughness is not reached. However, the proposed method extrapolates to steady-state and is demonstrated capable of predicting the structural behavior of geometrically-scaled specimens.
Research on a Lamb Wave and Particle Filter-Based On-Line Crack Propagation Prognosis Method.
Chen, Jian; Yuan, Shenfang; Qiu, Lei; Cai, Jian; Yang, Weibo
2016-01-01
Prognostics and health management techniques have drawn widespread attention due to their ability to facilitate maintenance activities based on need. On-line prognosis of fatigue crack propagation can offer information for optimizing operation and maintenance strategies in real-time. This paper proposes a Lamb wave-particle filter (LW-PF)-based method for on-line prognosis of fatigue crack propagation which takes advantages of the possibility of on-line monitoring to evaluate the actual crack length and uses a particle filter to deal with the crack evolution and monitoring uncertainties. The piezoelectric transducers (PZTs)-based active Lamb wave method is adopted for on-line crack monitoring. The state space model relating to crack propagation is established by the data-driven and finite element methods. Fatigue experiments performed on hole-edge crack specimens have validated the advantages of the proposed method. PMID:26950130
International Nuclear Information System (INIS)
The welding process introduces defects on the welded joints, as lack of fusion and penetration, porosity, between others. These defects can compromise the structures or components, relative to the crack propagation. This engagement can be studied by fatigue crack propagation tests. The efficiency of the structure, when submitted to a cyclic loading can be evaluated by these tests. The aim of this work is to study the behavior of welded joints containing defects as lack of penetration at the root or between welding passes relative to crack propagation resistance properties, and to compare these properties with the properties of the welded joints without defects. This study was accomplished from fatigue crack propagation test results, in specimens containing lack of penetration between welding passes. With the obtained results, the Paris equation coefficients and exponents that relate the crack propagation rate with the stress intensity cyclic factor for welded joints with and without defects were obtained. (author)
Guian Qian; Chengen Zhou; Youshi Hong
2013-01-01
The influence of environmental medias on crack propagation of a structural steel at high and very-high-cycle fatigue (VHCF) regimes is investigated based on the fatigue tests performed in air, water and 3.5% NaCl aqueous solution. Crack propagation mechanisms due to different crack driving forces are investigated in terms of fracture mechanics. A model is proposed to study the relationship between fatigue life, applied stress and material property in different environmental medias, which refl...
Ultrasonic Analysis of Cracking Propagation Morphology in the Fusion Zone of High Strength Steel
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
Cracking morphology in the fusion zone of HQ130 high strength steel was researched by "the y-slit test" and "three-point bend test", ultrasonic test and microscope. HQ130 and Q J63 high strength steel was welded by Ar+CO2 gasshielded arc welding under the condition without preheating. Experimental results indicated that welding cracks wereproduced in the partially melted zone of the weld root of HQ130 steel side and propagated parallel to the fusionzone. The cracks were developed alternatively between the weld and the partially melted zone, and are not strictlyruptured at W/F (weld metal/fusion zone) boundary surface. Controlling weld heat input (E) about 16 k J/cm couldmake the cracking rate lowest and satisfy the performance requirement of welded joint zone.
Cracks propagation by stress corrosion cracking in conditions of Boiling Water Reactor (BWR)
International Nuclear Information System (INIS)
This work presents the results of the assays carried out in the Laboratory of Hot Cells of the National Institute of Nuclear Research (ININ) to a type test tube Compact Tension (CT), built in steel austenitic stainless type 304L, simulating those conditions those that it operates a Boiling Water Reactor (BWR), at temperature 288 C and pressure of 8 MPa, to determine the speed to which the cracks spread in this material that is of the one that different components of a reactor are made, among those that it highlights the reactor core vessel. The application of the Hydrogen Chemistry of the Water is presented (HWC) that is one alternative to diminish the corrosion effect low stress in the component, this is gets controlling the quantity of oxygen and of hydrogen as well as the conductivity of the water. The rehearsal is made following the principles of the Mechanics of Elastic Lineal Fracture (LEFM) that considers a crack of defined size with little plastic deformation in the tip of this; the measurement of crack advance is continued with the technique of potential drop of direct current of alternating signal, this is contained inside the standard Astm E-647 (Method of Test Standard for the Measurement of Speed of Growth of Crack by fatigue) that is the one that indicates us as carrying out this test. The specifications that should complete the test tubes that are rehearsed as for their dimensions, it forms, finish and determination of mechanical properties (tenacity to the fracture mainly) they are contained inside the norm Astm E-399, the one which it is also based on the principles of the fracture mechanics. The obtained results were part of a database to be compared with those of other rehearsals under different conditions, Normal Chemistry of the Water (NWC) and it dilutes with high content of O2; to determine the conditions that slow more the phenomena of stress corrosion cracking, as well as the effectiveness of the used chemistry and of the method of
Czech Academy of Sciences Publication Activity Database
Caine, J.; Choudhuri, M.; Bose, N.; Mukherjee, S.; Misra, A.A.; Mathew, G.; Salvi, D.; Toro, B.; Pratt, B.R.; Dasgupta, S.; Nováková, Lucie
Amsterdam: Elsevier, 2015 - (Mukherjee, S.), s. 79-106 ISBN 978-0-12-420152-1 Institutional support: RVO:67985891 Keywords : brittle shear zone * brittle tectonics * conjugate faults * faults * kinematic indicators Subject RIV: DC - Siesmology, Volcanology, Earth Structure
Iqbal, AKM Asif; Arai, Yoshio
2016-02-01
The fatigue crack propagation behaviour of a cast hybrid metal matrix composite (MMC) was investigated and compared with the crack propagation behaviour of MMC with Al2O3 and Al alloy in this article. Three dimensional (3D) surface analysis is carried out to analyze the crack propagation mechanism. All three materials clearly show near threshold and stable crack growth regions, but the rapid crack growth region is not clearly understood. The crack propagation resistance is found higher in hybrid MMC than that of MMC with Al2O3 whisker and the Al alloy in the low ΔK region. The crack propagation in the hybrid MMC in the near-threshold region is directed by the debonding of reinforcement-matrix followed by void nucleation in the Al alloy matrix. Besides, the crack propagation in the stable- or midcrack-growth region is controlled by the debonding of particle-matrix and whisker-matrix interface caused by the cycle-by-cycle crack growth along the interface. The transgranular fracture of the reinforcement and void formation are also observed. Due to presence of large volume of inclusions and the microstructural inhomogeneity, the area of striation formation is reduced in the hybrid MMC, caused the unstable fracture.
International Nuclear Information System (INIS)
In the present thesis work the propagation of cracks due to the delayed hydride cracking (DHC) mechanism in Zr-2,5 % Nb pressure tubes is analyzed. For this purpose two different type of tubes of different origin were used: CANDU type (Canada) and RBMK type (Russia). The analyzed figurative parameters were: critical temperature Tc (highest temperature at which DHC phenomenon could occur) and crack propagation velocity by DHC, Vp, in the axial direction. The influence of the memory effect (phenomenon proper of hydride precipitation) was studied, as well as the type of cracks (fatigue or DHC) on Tc. However, no influence of these effects was found. Instead, it was found that Tc varies with the hydrogen content of the specimen, in agreement with previous works. Samples obtained from tubes with different microstructures and similar amounts of hydrogen presented similar Tc values. It was also shown that DHC propagation could occur without precipitated hydrides in the volume. Besides, Vp determinations were performed in temperature ranges and hydrogen amounts of technological importance. Two techniques were set up in order to determine Vp at different temperatures in a single specimen, thus saving time and material. An Arrhenius type variation was found for Vp vs. temperature, for temperatures lower than that corresponding to precipitation. For higher temperatures, but lower than the critical one, velocity decreases with temperature. Determination of Vp vs. temperature was performed for the two above-mentioned materials, whose microstructure and hardness were previously characterized. For RBMK material, which presents a spheroidal β phase, the velocity was lower than the corresponding to CANDU material, in which β phase is formed by continuous plates. In addition, yield stress σY is lower in RBMK material, which presents lower Vp. However, it is considered that the effect of microstructure is more important on Vp since it highly affects diffusion of hydrogen from the
International Nuclear Information System (INIS)
In this paper, the case of Single Edge Notch (SEN) specimens subject to opening/compressive loading was analyzed; The loads are applied in several ratios to evaluate the influence of the specimen geometry, and the Stress Intensity Factor (SIF) K1 values on the directional stability of crack propagation. The main purpose of this work is to evaluate the behaviour of the fracture propagation, when modifying the geometry of the SEN specimen and different relationships of load tension/compression are applied. Additionally, the precision of the numerical and experimental analysis is evaluated to determine its reliability when solving this type of problems. The specimens are subjected to biaxial opening/compression loading; both results (numerical and experimental) are compared in order to evaluate the condition of directional stability on crack propagation. Finally, an apparent transition point related to the length of specimens was identified, in which the behaviour of values of SIF changes for different loading ratios.
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RodrIguez-MartInez, R; Urriolagoitia-Calderon, G; Urriolagoitia-Sosa, G; Hernandez-Gomez, L H [Instituto Politecnico Nacional Seccion de Estudios de Posgrado e Investigacion (SEPI), Escuela Superior de IngenierIa Mecanica y Electrica (ESIME), Edificio 5. 2do Piso, Unidad Profesional Adolfo Lopez Mateos ' Zacatenco' Col. Lindavista, C.P. 07738, Mexico, D.F. (Mexico); Merchan-Cruz, E A; RodrIguez-Canizo, R G; Sandoval-Pineda, J M, E-mail: rrodriguezm@ipn.m, E-mail: urrio332@hotmail.co, E-mail: guiurri@hotmail.co, E-mail: luishector56@hotmail.co, E-mail: eamerchan@gmail.co, E-mail: ricname@hotmail.co, E-mail: jsandovalp@ipn.m [Instituto Politecnico Nacional Seccion de Estudios de Posgrado e Investigacion (SEPI), Escuela Superior de IngenierIa Mecanica y Electrica (ESIME). Unidad profesional, AZCAPOTZALCO, Av. de las Granjas No. 682, Col. Sta. Catarina Azcapotzalco, C.P. 02550, Mexico D.F. (Mexico)
2009-08-01
In this paper, the case of Single Edge Notch (SEN) specimens subject to opening/compressive loading was analyzed; The loads are applied in several ratios to evaluate the influence of the specimen geometry, and the Stress Intensity Factor (SIF) K{sub 1} values on the directional stability of crack propagation. The main purpose of this work is to evaluate the behaviour of the fracture propagation, when modifying the geometry of the SEN specimen and different relationships of load tension/compression are applied. Additionally, the precision of the numerical and experimental analysis is evaluated to determine its reliability when solving this type of problems. The specimens are subjected to biaxial opening/compression loading; both results (numerical and experimental) are compared in order to evaluate the condition of directional stability on crack propagation. Finally, an apparent transition point related to the length of specimens was identified, in which the behaviour of values of SIF changes for different loading ratios.
Dynamic crack propagation in elastic-perfectly plastic solids under plane stress conditions
Deng, Xiaomin; Rosakis, Ares J.
THE phenomenon of steady-state dynamic crack propagation in elastic-perfectly plastic solids under mode I plane stress, small-scale yielding conditions is investigated numerically. An Eulerian finite element scheme is employed. The materials are assumed to obey the von Mises yield criterion and the associated flow rule. The ratio of the crack tip plastic zone size to that of the element nearest to the crack tip is of the order of 1.6 × 10 4. Two subjects of general interest are discussed. These are the asymptotic structure of the crack tip stress and deformation fields, and the appropriateness of a crack growth fracture criterion based on the far-field dynamic stress intensity factor. The crack-line solution by ACHENBACH and LI (Report NU-SML-TR-No. 84-1, Dept. of Civil Engineering, Northwestern University, Evanston, IL 60201, 1984a; in Fundamentals of Deformation and Fracture (edited by B.A. Brilby et al.). Cambridge University Press, 1984b) is discussed and compared to the numerical solution. The results of this study strongly indicate that the crack tip strain and velocity fields possess logarithmic singularities, which is consistent with the assumptions in the asymptotic analysis by Gao ( Int. J. Fracture34, 111, 1987). However, it is revealed that the crack tip field variations in Gao's solution present features often contrary to the numerical findings. To this end, a preliminary asymptotic analysis is performed in an effort to resolve certain issues. Finally, the critical plastic strain criterion ( MCCLINTOCK and IRWIN, in Fracture Toughness Testing and Its Applications, ASTM STP 381, p. 84, 1964) is adopted to obtain theoretical relations between the critical dynamic stress intensity factor and the crack propagation speed. These relations are found to agree well with experimental measurements by Rosakis et al. ( J. Mech. Phys. Solids32, 443, 1984) and by ZEHNDER and ROSAKIS ( Int. J. Fracture, to appear 1990), performed on thin 4340 steel plates whose
Interfacial sliding in fibrous brittle-matrix composites
Miles, Herbert Frederick, II
Ceramic materials have desirable characteristics for use in high temperature applications, but due to their brittle nature they were avoided until the recent advent of ceramic matrix composites (CMCs) in which ceramic fibers are inserted into a ceramic matrix to toughen the material by retarding crack growth. This work investigates the role of sliding at interfaces in making brittle matrix composites (BMCs) more crack resistant. A two-dimensional study investigates the effects of roughness, toughness, and friction on the fracture behavior of BMCs. This study was then expanded to an axisymmetric study of a fiber engulfed by a crack. The results indicate that there are significant interaction effects between friction and the other parameters. To achieve 'long' sliding lengths, the magnitude of the interfacial critical energy release rate must be significantly less than the magnitude required to ensure crack deflection. The study then investigates the three-dimensional nature of a crack as it flows past a fiber. A computational analysis is performed to determine the crack propagation angle at a frictional interface. The computational results show good agreement with a novel experimental analysis using modified DCDC specimens. The experiments show, in real time, the propagation of a crack which is perpendicular to and intersects a frictional interface.
International Nuclear Information System (INIS)
This paper discusses the possibility of in-situ quantitative evaluation of crack propagation using Eddy Current Monitoring (ECM) system in a Boiling Water Reactor (BWR) environment. For the purpose, ECM experiment was conducted at a System Safety Benchmark Facility (SSBF). The target for the monitoring was a large-scale pipe specimen made of austenitic stainless steels 304L, 316L, and carbon steel STS410, connected by weld joints of nickel based alloy 182 within the SSBF. Slits were prepared at the Heat Affected Zones (HAZs) and weld lines from the inner surface of the pipe specimen. Eddy Current Testing (ECT) probes were arranged on the outer surface of the pipe specimen, and observed crack propagation of slits. In order to discuss the crack propagation quantitatively, numerical simulation of eddy current testing signals was carried out. The numerical signals are consistent with the experimental signals, which indicate that in-situ eddy current monitoring system has the capability to monitor the crack growth quantitatively. (author)
Crack propagation in stainless steel AISI 304L in Hydrogen Chemistry conditions (HWC)
International Nuclear Information System (INIS)
Velocities of crack growth in samples type CT pre cracking of stainless steel AISI 304l solder and sensitized thermally its were obtained by the Rising Displacement method or of growing displacement. It was used a recirculation circuit that simulates the operation conditions of a BWR type reactor (temperature of 280 C and a pressure of 8 MPa) with the chemistry modified by the addition of hydrogen with and without the addition of impurities of a powerful oxidizer like the Cu+ ion. In each essay stayed a displacement velocity was constant of 1x10-9 m/s, making a continuous pursuit of the advance of the crack by the electric potential drop technique. Contrary to the idea of mitigation of the crack propagation velocity by effect of the addition of the hydrogen in the system, the values of the growth velocities obtained by this methodology went similar to the opposing ones under normal operation conditions. To the finish of the rehearsal one carries out the fractographic analysis of the propagation surfaces, which showed cracks growth in trans and intergranular way, evidencing the complexity of the regulator mechanisms of the IGSCC like in mitigation conditions as the alternative Hydrogen Chemistry. (Author)
Fatique crack propagation in bimetallic welds influence of residual stresses and metallurgical look
International Nuclear Information System (INIS)
Generally, in nuclear power plants, many components made of austenitic stainless steels are very often replaced by low alloyed steels cladded with stainless steels, mainly for economical reasons. Due to cracks existing at the limit of the two kinds of steel, it is interesting to try to understand how they appear. Residual stresses are generally identified as one of the factors which act to produce these cracks. Measurements of such residual stresses have been performed, using the hole drilling method (drilling of a hole at the center of a gauge roset stuck at the surface of the material). Owing to the obtained results, it is possible to explain the decrease in the crack propagation rate observed, on fatigue crack growth test performed on specimens taken in the transition ferritic/austenitic zone. The stress intensity factor due to the residual stresses is valued by weight function method. It is possible to explain qualitatively the phenomena observed under cyclic loading when using the obtained value of this stress intensity factor. A more quantitative approach based on the use of an efficient stress intensity factor, allow to better describe the effect of residual stresses on the fatigue crack propagation in bimetallic welds
Liaw, Peter K.; Hudak, S. J.; Donald, J. Keith
1982-09-01
The influence of hydrogen environment (448 kPa) on near-threshold fatigue crack propagation rates was examined in a 779 MPa yield strength NiCrMoV steel at 93 °C. An automatically decreasing and increasing stress intensity technique was employed to generate crack growth rates at three load ratios (R = 0.1, 0.5, and 0.8). Results show that the crack propagation rates in hydrogen are slower than those in air for levels of stress intensity range, ΔK, below about 12 MPa√m. The crack closure concept does not explain the slower crack growth rates in hydrogen than in air. Near-threshold growth rates appear to be controlled by the levels of residual moisture in the environments. In argon and air, the fracture morphology is transgranular, while in H2 the amount of intergranularity varies with ΔK and achieves a maximum when the cyclic plastic zone is approximately equal to the prior austenite grain size.
International Nuclear Information System (INIS)
Fatigue crack propagation behavior of ultrafine-grained copper processed by equal channel angular pressing, ECAP, was investigated by electron backscatter diffraction, EBSD, technique as well as atomic force microscopy, AFM. The results show that the crack propagation rate, da/dN, was smaller in ECAP-processed specimen than the coarse-grained counterpart at higher stress intensity factor range, δK. The decrease in the da/dN in spite of the small grain size is attributed to the grain coarsening at the crack tip during fatigue which results in the increase in the roughness induced crack closure. In the range below ΔK p. In the range above ΔK > 5 MPa√m, Region II, small-scale grain coarsening in many grains is dominant due to large ΔK and small Np. The grain coarsening was introduced as a result of the migration of high angle grain boundary. The large initial transgranular strain and unstable grain boundaries are considered to be the causes of the train-induced grain boundary migration. The X-ray stress at the crack tip was found to be small than σ0.2 due to the strain relaxation and grain coarsening. (author)
System for nucleation and propagation of fatigue cracks on SE(B) specimens
International Nuclear Information System (INIS)
The degree of safety that a structural component has against catastrophic fracture in service can be obtained from fracture mechanics parameters. The master curve could be used for integrity evaluation in pressure vessel of nuclear power plant. The pre-crack specimens are used in this evaluation. The tests based on ASTM E 8M and ASTM E 647 standards to determination of material properties related to fracture mechanics, most often performed in a servo-hydraulic drive equipment, are time consuming and costly. This paper presents the development of a system for nucleation and propagation of fatigue cracks on SE(B) specimens. The operating principle consists of a cyclic loading, concentrated in the center of the specimen, transmitted and controlled by an eccentric mechanism. The main contribution of this work is the low-cost technology in the production of fatigue pre-crack, and the possibility of performing the nucleation and propagation of the pre-crack required for obtaining the JIC and CTOD parameters. The experimental results satisfied expectations with respect to the plastic deformation in the crack tip and met the requirements of the standards. (author)
Cracks in high-manganese cast steel
Directory of Open Access Journals (Sweden)
A. Chojecki
2009-10-01
Full Text Available The reasons which account for the formation of in service cracks in castings made from Hadfield steel were discussed. To explain the source of existence of the nuclei of brittle fractures, the properties of cast steel were examined within the range of solidification temperatures, remembering that feeding of this material is specially difficult, causing microporosity in hot spots. This creates conditions promoting the formation of microcracks which tend to propagate during service conditions involving high dynamic stresses, and explains why the cracks are mainly characterized by a brittle nature. The reason for crack formation in service are micro-porosities formed during casting solidification.
Directory of Open Access Journals (Sweden)
Om Prakash Tenduwe
2015-05-01
Full Text Available The fluctuation of stress intensity factor distribution and fatigue crack propagation in HSLA steel were investigated, for this purpose fatigue crack growth test were carried out on five mutually similar configured standard 1CT specimens with reduced thickness using constant amplitude loading cycles under mode-I, with 0.3 stress ratio and maximum load held 11.8 kN. The fluctuation of stress intensity factor distribution were studied experimentally as a function of crack length, elapsed fatigue life cycle and compliance, along with the behavior of fatigue crack propagation in HSLA steel. The fracture morphology was observed by field emission scanning microscopy. ∆K in starting not increased significantly as increasing crack length, number of cycle and compliance, but after reaching the region-II, it is increasing very significantly and slow fatigue crack propagation behavior were observed by the material.
Czech Academy of Sciences Publication Activity Database
Pokorný, Pavel; Náhlík, Luboš; Hutař, Pavel
Zürich: Trans Tech publications, 2015 - (Alfaiate, J.; Aliabadi, M.), s. 469-472. (Key Engineering Materials. 627). ISBN 978-3-03835-235-8. ISSN 1013-9826. [FDM 2014 International Conference on Fracture and Damage Mechanics /13./. São Miguel Island, Azores (PT), 23.09.2014-25.09.2014] R&D Projects: GA MŠk(CZ) EE2.3.30.0063 Institutional support: RVO:68081723 Keywords : residual fatigue lifetime * fatigue crack * fatigue crack propagation * railway axle Subject RIV: JL - Materials Fatigue, Friction Mechanics
Crack propagation in X38CrMoV5 (AISI H11) tool steel
Shah, Masood; Mabru, Catherine; Boher, Christine; Le Roux, Sabine; Rezaï-Aria, Farhad
2009-01-01
A method is proposed for the evaluation of surface fatigue damage of hot forming tools that undergo severe thermo mechanical loading. Fatigue crack propagation in a hot work tool steel X38CrMoV5-47HRC is investigated using single-edge cracked tension specimens (SET). The effect of thickness (ranging from 2,5mm to 0.10mm) and R values is investigated. Numerical simulation is performed by ABAQUS® Standard to evaluate J integral and stress intensity factor KI. The Paris curves are established. S...
Thermal fatigue crack propagation behaviour of F82H ferritic steel
Energy Technology Data Exchange (ETDEWEB)
Kudo, Yusuke E-mail: kudou@fusion.naka.jaeri.go.jp; Kikuchi, Kouichi; Saito, Masakatsu
2002-12-01
This paper presents an issue obtained from thermal fatigue research, which attempts to examine the fusion reactor first wall by fracture mechanics. The research is organised with two different approaches: 1. Studies of the thermal fatigue crack propagation behaviour on notched 5-mm thick plate specimens of ferritic steel F82H (9Cr-1W), compared with 9Cr-1Mo ferritic steel and type 316 stainless steel; 2. Numerical simulations of the stress field caused by thermal loads including fracture mechanics. It is concluded that the stress intensity factor {delta}K{sub I} is substantial for crack growth while cyclic thermal loading.
Energy Technology Data Exchange (ETDEWEB)
Ueda, Y.; Murakawa, H. [Osaka Univ., Osaka (Japan). Welding Research Inst.; Tanigawa, M. [Hitachi Zosen Corp., Osaka (Japan)
1996-12-31
In order to investigate an effect of the plastic deformation, which was generated on ship side outer platings subjected to collision load before crack initiation, on the crack propagation behavior, crack propagation experiments using pre-strained specimens and simulation analysis by means of FEM method were carried out, to discuss about the practical simulation analysis method. As a result of the crack propagation experiments using pre-strained center notched plate specimens, a phenomenon where the crack is apt to propagate due to the pre-strains was confirmed, and measured data of crack tip opening angles were obtained. A method was proposed, in which the critical crack tip opening angle values are corrected by considering the difference between the crack shapes obtained from the FEM analysis model and actually measured, and its effectiveness was confirmed. The finite element size effect was also examined. A method using an equivalent plastic strain as the crack propagation condition was shown to determine the relationship between the element size and the critical value of equivalent plastic strain. 5 refs., 21 figs., 4 tabs.
Modeling time-dependent corrosion fatigue crack propagation in 7000 series aluminum alloys
Mason, Mark E.; Gangloff, Richard P.
1994-01-01
Stress corrosion cracking and corrosion fatigue experiments were conducted with the susceptible S-L orientation of AA7075-T651, immersed in acidified and inhibited NaCl solution, to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA FLAGRO. This environment enhances da/dN by five to ten-fold compared to fatigue in moist air. Time-based crack growth rates from quasi-static load experiments are an order of magnitude too small for accurate linear superposition prediction of da/dN for loading frequencies above 0.001 Hz. Alternate methods of establishing da/dt, based on rising-load or ripple-load-enhanced crack tip strain rate, do not increase da/dt and do not improve linear superposition. Corrosion fatigue is characterized by two regimes of frequency dependence; da/dN is proportional to f(exp -1) below 0.001 Hz and to F(exp 0) to F(exp -0.1) for higher frequencies. Da/dN increases mildly both with increasing hold-time at K(sub max) and with increasing rise-time for a range of loading waveforms. The mild time-dependence is due to cycle-time-dependent corrosion fatigue growth. This behavior is identical for S-L nd L-T crack orientations. The frequency response of environmental fatigue in several 7000 series alloys is variable and depends on undefined compositional or microstructural variables. Speculative explanations are based on the effect of Mg on occluded crack chemistry and embritting hydrogen uptake, or on variable hydrogen diffusion in the crack tip process zone. Cracking in the 7075/NaCl system is adequately described for life prediction by linear superposition for prolonged load-cycle periods, and by a time-dependent upper bound relationship between da/dN and delta K for moderate loading times.
International Nuclear Information System (INIS)
In this paper, the effect of cohesive laws on the finite element analysis of crack propagation using cohesive elements is investigated through three-point bending and double cantilever beam problems. The cohesive elements are implemented into ABAQUS/Standard user subroutines(UEL), and the shape of cohesive law is varied by changing parameters in polynomial functions of cohesive traction-separation relations. In particular, crack propagation behaviors are studied by comparing load-displacement curves of the analysis models which have different shapes of cohesive laws with the same values of fracture energy and cohesive strength. Furthermore, the influence of the element size on crack propagation is discussed in this study
Alam, Mohammad Shah
2005-11-01
Structural integrity is the science and technology of the margin between safety and disaster. Proper evaluation of the structural integrity and fatigue life of any structure (aircraft, ship, railways, bridges, gas and oil transmission pipelines, etc.) is important to ensure the public safety, environmental protection, and economical consideration. Catastrophic failure of any structure can be avoided if structural integrity is assessed and necessary precaution is taken appropriately. Structural integrity includes tasks in many areas, such as structural analysis, failure analysis, nondestructive testing, corrosion, fatigue and creep analysis, metallurgy and materials, fracture mechanics, fatigue life assessment, welding metallurgy, development of repairing technologies, structural monitoring and instrumentation etc. In this research fatigue life assessment of welded and weld-repaired joints is studied both in numerically and experimentally. A new approach for the simulation of fatigue crack growth in two elastic materials has been developed and specifically, the concept has been applied to butt-welded joint in a straight plate and in tubular joints. In the proposed method, the formation of new surface is represented by an interface element based on the interface potential energy. This method overcomes the limitation of crack growth at an artificial rate of one element length per cycle. In this method the crack propagates only when the applied load reaches the critical bonding strength. The predicted results compares well with experimental results. The Gas Metal Arc welding processes has been simulated to predict post-weld distortion, residual stresses and development of restraining forces in a butt-welded joint. The effect of welding defects and bi-axial interaction of a circular porosity and a solidification crack on fatigue crack propagation life of butt-welded joints has also been investigated. After a weld has been repaired, the specimen was tested in a universal
Gupta, Vipul; Hochhalter, Jacob; Yamakov, Vesselin; Scott, Willard; Spear, Ashley; Smith, Stephen; Glaessgen, Edward
2013-01-01
A systematic study of crack tip interaction with grain boundaries is critical for improvement of multiscale modeling of microstructurally-sensitive fatigue crack propagation and for the computationally-assisted design of more durable materials. In this study, single, bi- and large-grain multi-crystal specimens of an aluminum-copper alloy are fabricated, characterized using electron backscattered diffraction (EBSD), and deformed under tensile loading and nano-indentation. 2D image correlation (IC) in an environmental scanning electron microscope (ESEM) is used to measure displacements near crack tips, grain boundaries and within grain interiors. The role of grain boundaries on slip transfer is examined using nano-indentation in combination with high-resolution EBSD. The use of detailed IC and EBSD-based experiments are discussed as they relate to crystal-plasticity finite element (CPFE) model calibration and validation.
Environmentally influenced mixed mode fatigue crack propagation of Titanium metal matrix composites
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Mahulikar, D.; Marcus, H.L.
1984-01-01
The effect of humid air environments on behavior of B/sub 4/C-B and BORSIC reinforced Ti-6A1-4V metal matrix composites was studied. Humid environments enhanced crack propagation rates at R = 0.1. The effect was more pronounced in transverse and 45 deg specimens. A transition from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. At R = 0.5, however, no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The splitting is an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields on the fibers weakened by humidity.
Environmentally influenced mixed mode fatigue crack propagation of Titanium metal matrix composites
International Nuclear Information System (INIS)
The effect of humid air environments on behavior of B4C-B and BORSIC reinforced Ti-6A1-4V metal matrix composites was studied. Humid environments enhanced crack propagation rates at R = 0.1. The effect was more pronounced in transverse and 45 deg specimens. A transition from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. At R = 0.5, however, no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The splitting is an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields on the fibers weakened by humidity
Quenched versus thermal disorder in crack propagation: size (and scales) matter.
Lengliné, Olivier; Schmittbuhl, Jean; Cochard, Alain; Jørgen Måløy, Knut; Toussaint, Renaud
2013-04-01
The slow propagation of crack in heterogeneous material is of fundamental importance for the failure of engineering structure and of natural system, such as seismic faults. Owing to the many interacting processes at play, it however still remains a challenge to describe the precise mechanical formulation that governs the dynamics of such systems. Previous studies dedicated to this issue have mostly been restricted to the zero temperature limit, giving rise to extremal dynamics, or to systems with short range interactions. Here we incorporate in a numerical model of slow crack growth the effect of temperature and long range elastic interactions. This approach provides a more realistic model of crack propagation in heterogeneous media under natural conditions. We adopt the configuration of an interfacial crack system, similar to a designed experimental setup. We recover both at the macroscopic and at the microscopic scales all the reported experimental observations. Namely we are able to observe a similar macroscopic crack evolution, a similar morphology of the crack front line and a similar distribution of local speeds: a self affine morphology with roughness exponent around 0.5 at small scale, and a lower effective roughness at larger scale for the front morphology [1], and a non Gaussian power law velocity distribution, with a fat tail P(v) v-2.6 at large speeds [2,3]. We also evidenced the competition between temperature and disorders, influencing the crack dynamics and modifying the crack pattern. We present analytical derivations that independently recover our numerical and experimental findings of two regimes dominated at small [4] and large scales [5] by quenched and annealed disorders respectively. We demonstrate that the cross-over length between these two regimes varies with the inverse of the temperature. We also show that the distribution of local speeds in our system is controlled by a parameter which depend both on temperature and disorder fluctuations
International Nuclear Information System (INIS)
Computerized simulation is nowadays an integrating part of design and validation processes of mechanical structures. Simulation tools are more and more performing allowing a very acute description of the phenomena. Moreover, these tools are not limited to linear mechanics but are developed to describe more difficult behaviours as for instance structures damage which interests the safety domain. A dynamic or static load can thus lead to a damage, a crack and then a rupture of the structure. The fast dynamics allows to simulate 'fast' phenomena such as explosions, shocks and impacts on structure. The application domain is various. It concerns for instance the study of the lifetime and the accidents scenario of the nuclear reactor vessel. It is then very interesting, for fast dynamics codes, to be able to anticipate in a robust and stable way such phenomena: the assessment of damage in the structure and the simulation of crack propagation form an essential stake. The extended finite element method has the advantage to break away from mesh generation and from fields projection during the crack propagation. Effectively, crack is described kinematically by an appropriate strategy of enrichment of supplementary freedom degrees. Difficulties connecting the spatial discretization of this method with the temporal discretization of an explicit calculation scheme has then been revealed; these difficulties are the diagonal writing of the mass matrix and the associated stability time step. Here are presented two methods of mass matrix diagonalization based on the kinetic energy conservation, and studies of critical time steps for various enriched finite elements. The interest revealed here is that the time step is not more penalizing than those of the standard finite elements problem. Comparisons with numerical simulations on another code allow to validate the theoretical works. A crack propagation test in mixed mode has been exploited in order to verify the simulation
Probabilistic damage model for the crack propagation in Si3N4 ceramics
Czech Academy of Sciences Publication Activity Database
Chlup, Zdeněk; Tatarko, P.; Hadraba, Hynek; Petráš, Roman; Kozák, Vladislav; Dlouhý, Ivo
Toledo: ECERS, 2015. ISBN 978-84-606-9257-7. [ECERS 14 - International Conference of the European Ceramic Society /14./. 21.06.2015-25.06.2015, Toledo] R&D Projects: GA MŠk(CZ) ED1.1.00/02.0068 Institutional support: RVO:68081723 Keywords : damage model * crack propagation * silicon nitride Subject RIV: JH - Ceramics , Fire-Resistant Materials and Glass
Effect of constraint on fatigue crack propagation near threshold in medium carbon steel
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Seitl, Stanislav; Knésl, Zdeněk
Indie : IWCMM14, 2004 - (Mukaherjee, A.; Schmauder, S.; Das, G.). s. 128 [International Workshop on Computational Mechanics of Materials /14./. 23.09.2004-25.09.2004, GOA ] R&D Projects: GA AV ČR KSK1010104; GA ČR GA202/01/0668 Keywords : Fatigue crack propagation rate, constraint * two parameter fracture mechanics * finite elements Subject RIV: JL - Materials Fatigue, Friction Mechanics
Crack propagation in the vicinity of the interface between two elastic materials
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Šestáková, Lucie; Hutař, Pavel
Algérie: The Algerian Association for the Technology Transfer , 2008, s. 1-11. ISBN N. [Damage and Fracture Mechanics. Algérie (DZ), 01.06.2008-05.06.2008] R&D Projects: GA AV ČR(CZ) KJB200410803; GA ČR GP106/06/P239 Institutional research plan: CEZ:AV0Z20410507 Keywords : bimaterial interface * crack propagation direction * layered composites Subject RIV: JL - Materials Fatigue, Friction Mechanics
Effect of constraint on fatigue crack propagation near threshold in medium carbon steel
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Seitl, Stanislav; Knésl, Zdeněk
2006-01-01
Roč. 37, 1-2 (2006), s. 51-57. ISSN 0927-0256 R&D Projects: GA ČR GA101/03/0331; GA ČR GP101/04/P001 Institutional research plan: CEZ:AV0Z20410507 Keywords : Fatigue crack propagation rate * Constraint * Two-parameter fracture mechanics Subject RIV: JL - Materials Fatigue, Friction Mechanics Impact factor: 1.104, year: 2006
Study of crack propagation velocity in steel tanks of PWR type reactor
International Nuclear Information System (INIS)
Description and results of a serie of tests carried out on crack propagation velocity of steels in PWR environment (pressurized high temperature water), in order to examine the effects of metallurgical parameters such as chemical composition of steel, especially sulfur and carbon content, and steel type (laminate or forged steels), effects of mechanical parameters such as loading ratio, cycle form, frequency and application mode of loads and of chemical parameters (anodal dissolution or fatigue with hydrogen)
Particulate Composite Damage: Numerical Estimation of Micro-Crack Propagation Direction
Czech Academy of Sciences Publication Activity Database
Majer, Zdeněk; Marcián, P.; Náhlík, Luboš; Hutař, Pavel; Knésl, Zdeněk
Zurich : Trans Tech Publications, 2014 - (Šandera, P.), s. 445-448 ISBN 978-3-03785-934-6. ISSN 1013-9826. - (Key Engineering Materials. 592-593). [MSMF 7 - International Conference on Materials Structure & Micromechanics of Fracture /7./. Brno (CZ), 01.07.2013-03.07.2013] Institutional support: RVO:68081723 Keywords : particle reinforced composites * interphase * fracture mechanism * finite element method * micro-crack propagation Subject RIV: JL - Materials Fatigue, Friction Mechanics
Propagation of cracks by stress corrosion in conditions of BWR type reactor
International Nuclear Information System (INIS)
In this work, the obtained results when applying the Hydrogen Chemistry to a test tube type Compact Tension (CT), built in austenitic stainless steel 304l, simulating the conditions to those that it operates a Boiling Water Reactor (BWR), temperature 288 C and pressure of 8 MPa are presented. With the application of this water chemistry, seeks to be proven the diminution of the crack propagation speed. (Author)
Crack propagation in the vicinity of the interface between two elastic materials
Czech Academy of Sciences Publication Activity Database
Náhlík, Luboš; Šestáková, Lucie; Hutař, Pavel
Milton Keynes : Springer Science + Business Media, 2009 - (Boukharouba, T.; Elboujdaini, M.; Pluvinage, G.), s. 255-263 ISBN 978-90-481-2668-2 R&D Projects: GA AV ČR(CZ) KJB200410803; GA ČR GP106/06/P239 Institutional research plan: CEZ:AV0Z20410507 Keywords : bimaterial interface * crack propagation direction * layered composites Subject RIV: JL - Materials Fatigue, Friction Mechanics
Effect of medium on the resistance of CSN 41 6445 steel to crack propagation
International Nuclear Information System (INIS)
The following facts were found for the CSN 41 6445 steel during the study of its resistance to stress corrosion in water at 295 degC. The kinetics of fatigue crack propagation in secondary circuit water (pH 9, oxygen content 1/2 then is in very good agreement with the value obtained under the same conditions for the A533 steel. (P.A.). 7 figs., 2 tabs., 6 refs
Predicting fatigue properties of cast aluminum by characterizing small-crack propagation behavior
Caton, Michael John
2001-07-01
The increased use of cast aluminum in structural components requires a deeper understanding of the mechanisms controlling fatigue properties in order to enable improved predictive capabilities. Of particular interest is the ability to model the influence of processing variables on the fatigue performance of alloys used in automotive applications such as engine blocks and cylinder heads. This thesis describes the results of a study conducted on cast W319 aluminum, a commercial Al-Si-Cu alloy used in automotive engine components, and presents a model that effectively predicts fatigue properties in this alloy as a function of material condition. The very high cycle fatigue regime (˜109 cycles) was examined using ultrasonic testing equipment (20 kHz) and distinct endurance limits were observed. The initiation and propagation of small fatigue cracks (˜5 mum to 2 mm) were monitored by a standard replication technique. It was observed that cracks initiate almost exclusively from microshrinkage pores and that the initiation life is negligible even at stresses below the endurance limit. The endurance limits result from the arrest of small cracks. Small crack growth rates were determined for a variety of material conditions where the influence of solidification time, heat treatment, and grain refinement were investigated. In addition, the influences of applied stress amplitude, stress ratio, and loading frequency on small crack growth were examined. A significant small crack effect was identified in this alloy and standard correlating parameters such as DeltaK and DeltaJ do not adequately characterize small crack growth. A correlating parameter written as [(epsilonmax sigmaa/sigma yield)s a] is proposed and shown to effectively characterize small crack growth for a wide range of stresses and a wide range of solidification conditions. In this parameter, epsilonmax is the total strain corresponding to the maximum applied stress, sigmaa is the stress amplitude, sigma yield is
Grain by grain study of the mechanisms of crack propagation during iodine SCC of Zry-4
International Nuclear Information System (INIS)
This paper describes the tests conducted to determine the conditions leading to cracking of a specified grain of metal, focussing on the crystallographic orientation of crack paths, the critical stress conditions and the significance of the fractographic features encountered. In order to get orientable cracking, a technique was developed to produce iodine SCC, by means of pressurizing tubes of a specially heat treated Zry-4 having very large grains, shaped as discs of a few millimeters in diameter and grown up to the wall thickness. Careful orientation of fractured grains, performed by means of a back-reflection Laue technique with a precision better than one degree, has proved that transgranular cracking occurs only along basal planes. The effect of anisotropy, plasticity, triaxiality and residual stresses originated in thermal contraction, has to be considered to account for the influence of the stress state . A grain by grain calculation led to the conclusion that transgranular cracking always occurs on those bearing the maximum resolved tensile stress on basal planes. There are clear indications of the need of a triaxial stress state for the process to occur. Fracture modes other than pseudo-cleavage have been encountered, including intergranular separation, ductile tearing produced by prismatic slip and propagation along twin boundaries. In each case the fractographic features have been identified, and associations have been made with fractographs obtained in normal fuel cladding. (Author)
Lengliné, Olivier; Schmittbuhl, Jean; Elkhoury, Jean; Toussaint, Renaud; Daniel, Guillaume; Maloy, Knut Jurgen
2010-05-01
Observations of aseismic transients in several tectonic context suggest that they might be linked to seismicity. However a clear observation and description of these phenomena and their interaction is lacking. This owes to the difficulty of characterizing with a sufficient resolution processes taking place at depth. Here we aim to study these interactions between aseismic and seismic slip taking advantage of an unique experimental setup. We conducted a series of mode I crack propagation experiments on transparent materials (PMMA). The crack advance is trapped in a weakness plane which is the interface between two previously sandblasted and annealed plexiglass plates. A fast video camera taking up to 500 frames per second ensures the tracking of the front rupture. The acoustic system is composed of a maximum of 44 channels continuously recording at 5 MHz for a few tens of seconds. Piezo-electric sensors are composed of a 32 elements linear array and individual sensors surrounding the crack front. An automatic detection and localization procedure allows us to obtain the position of acoustic emission (A.E.) that occurred during the crack advance. Crack front image processing reveals an intermittent opening which might be linked to the time and space clustering of the AE. An analogy between the mode I (opening) and the mode III (antiplane slip) allows us to interpret our results in term of slip on faults. Our experiment thus helps to reveal the interplay between seismic and aseismic slip on faults.
The behavior of fatigue crack initiation and propagation in AA2524-T34 alloy
International Nuclear Information System (INIS)
Research highlights: → We employed four-point-bend apparatus to obtain fatigue data in a 2524-T34 alloy. → The fatigue strength was found to be 70 pct of their yield strength. → Fatigue cracks always initiated from the second phase particles. → Grain orientation may be the key factor controlling the micro-crack deflection. - Abstract: With the help of a four-point-bend fatigue rig, high-cycle fatigue tests were carried out on an AA2524 Al alloy at room temperature, 15 Hz and R = -0.1 in ambient air. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to capture a detailed view of the fatigue crack initiation and propagation of the samples as well. The fatigue strength is 70 pct of their yield strength. Fatigue cracks were found to be always initiated from the second phase particles or the interface between the second phase particles and matrix. And grain orientation may be the key factor controlling the micro-crack deflection.
Institute of Scientific and Technical Information of China (English)
XU Tao; LI Feng; CHEN Hua; YU Cui-yan
2005-01-01
The formation and growth of thermal fatigue crack in low-chromium semi-steel were investigated by means of optical microscope and scanning electron microscope, and the function of RE in low-chromium semi-steel was analyzed. The results show that the thermal fatigue cracks are mainly generated at eutectic carbides, and the cracks not only grow and spread but also join each other. RE can improve the eutectic carbide′s morphology, inhibit the generation and propagation of thermal fatigue cracks, and therefore promote the activation energy for the crack′s propagation, which is especially more noticeable in case of the RE modification in combination with heat treatment. The mathematical model of the crack propagation is put forward.
Energy Technology Data Exchange (ETDEWEB)
Chen, Y.Q.; Pan, S.P. [School of Materials Science and Engineering, Central South University, Changsha 410083 (China); Zhou, M.Z. [Bichamp Cutting Technology Co., Ltd., Wangcheng Economy Development Area, Changsha 410200 (China); Yi, D.Q., E-mail: yioffice@csu.edu.cn [School of Materials Science and Engineering, Central South University, Changsha 410083 (China); Xu, D.Z. [Thermal Processing Technology Center, Illinois Institute of Technology, Chicago, IL 60616 (United States); Xu, Y.F. [School of Mechanical Engineering, Xiangtan University, Xiangtan 411105 (China)
2013-09-15
Microstructural aspects have fundamental influences on the fatigue crack characteristics of materials. In this paper, effects of inclusions, grain boundaries (GBs) and grain orientations on the fatigue crack initiation and propagation behavior in a 2524-T3 aluminum alloy have been investigated using in-situ scanning electron microscope (SEM) fatigue testing and electron back scattering diffraction (EBSD). The results show that, potential fatigue cracks tend to nucleate along coarse and closely spaced inclusion particles or high-angle GBs. Coarse inclusion particles drastically accelerate local crack growth rates. A model of series crack growing stages is given based on the observation of initiation and growth of cracks at the inclusion region. GBs serve to impede the crack tip from propagation and cause large angle crack deflections, which greatly affects local crack propagation behaviors. In addition, fatigue crack shows a strong tendency to propagate transgranularly grains with high Schmid factors (SFs) and avoid grains with low SFs.
International Nuclear Information System (INIS)
Microstructural aspects have fundamental influences on the fatigue crack characteristics of materials. In this paper, effects of inclusions, grain boundaries (GBs) and grain orientations on the fatigue crack initiation and propagation behavior in a 2524-T3 aluminum alloy have been investigated using in-situ scanning electron microscope (SEM) fatigue testing and electron back scattering diffraction (EBSD). The results show that, potential fatigue cracks tend to nucleate along coarse and closely spaced inclusion particles or high-angle GBs. Coarse inclusion particles drastically accelerate local crack growth rates. A model of series crack growing stages is given based on the observation of initiation and growth of cracks at the inclusion region. GBs serve to impede the crack tip from propagation and cause large angle crack deflections, which greatly affects local crack propagation behaviors. In addition, fatigue crack shows a strong tendency to propagate transgranularly grains with high Schmid factors (SFs) and avoid grains with low SFs
Kroon, Martin
2012-01-01
In the present study, a computational framework for studying high-speed crack growth in rubber-like solids under conditions of plane stress and steady-state is proposed. Effects of inertia, viscoelasticity and finite strains are included. The main purpose of the study is to examine the contribution of viscoelastic dissipation to the total work of fracture required to propagate a crack in a rubber-like solid. The computational framework builds upon a previous work by the present author (Kroon in Int J Fract 169:49-60, 2011). The model was fully able to predict experimental results in terms of the local surface energy at the crack tip and the total energy release rate at different crack speeds. The predicted distributions of stress and dissipation around the propagating crack tip are presented. The predicted crack tip profiles also agree qualitatively with experimental findings.
Om Prakash Tenduwe; Pyare Lal Khunte
2015-01-01
The fluctuation of stress intensity factor distribution and fatigue crack propagation in HSLA steel were investigated, for this purpose fatigue crack growth test were carried out on five mutually similar configured standard 1CT specimens with reduced thickness using constant amplitude loading cycles under mode-I, with 0.3 stress ratio and maximum load held 11.8 kN. The fluctuation of stress intensity factor distribution were studied experimentally as a function of crack length, el...
International Nuclear Information System (INIS)
The quasistatic dissipation of thermically induced cracks in brittle multi-components material with plane boundary areas is studied. The distribution of Eigentension, which is causing the dissipation of cracks, is produced by cooling the composite material from the production temperature to room temperature. Tension distributions, respectively of the fracture-mechanical coefficients were determined by solving of the boundary value problems of the theory of plane thermoelasticity, a based on existence of a plane distortion state, respectively of a plane state of tension. Because of the complicated shape of the free surface one adopted a numerical solution, the finite-element method, to solve the corresponding mixed boundary value problems. (orig.)
Czech Academy of Sciences Publication Activity Database
Malíková, L.; Veselý, V.; Seitl, Stanislav
2015-01-01
Roč. 9, č. 33 (2015), s. 25-32. ISSN 1971-8993 Institutional support: RVO:68081723 Keywords : Near-crack-tip fields * Williams expansion * Crack propagation direction * Multi-parameter fracture criteria * Finite element analysis Subject RIV: JL - Materials Fatigue, Friction Mechanics
Propagation of fatigue crack in amorphous Fe40Ni40B20 at high-frequency loading
Ocelik, V.; Palcek, P.; Diko, P.; Miskuf, J.; Duhaj, P.
1990-01-01
This work studies the fatigue crack growth in Fe40Ni40B20 metallic glass using the stressing by longitudinal ultrasonic waves of frequency 20.2 kHz. A relation between the rate of fatigue crack propagation and stress intensity factor is derived. An extremely low threshold value of the stress intensi
Xiao, Si; Wang, He-Ling; Liu, Bin; Hwang, Keh-Chih
2015-11-01
The J-integral based criterion is widely used in elastic-plastic fracture mechanics. However, it is not rigorously applicable when plastic unloading appears during crack propagation. One difficulty is that the energy density with plastic unloading in the J-integral cannot be defined unambiguously. In this paper, we alternatively start from the analysis on the power balance, and propose a surface-forming energy release rate (ERR), which represents the energy available for separating the crack surfaces during the crack propagation and excludes the loading-mode-dependent plastic dissipation. Therefore the surface-forming ERR based fracture criterion has wider applicability, including elastic-plastic crack propagation problems. Several formulae are derived for calculating the surface-forming ERR. From the most concise formula, it is interesting to note that the surface-forming ERR can be computed using only the stress and deformation of the current moment, and the definition of the energy density or work density is avoided. When an infinitesimal contour is chosen, the expression can be further simplified. For any fracture behaviors, the surface-forming ERR is proven to be path-independent, and the path-independence of its constituent term, so-called Js-integral, is also investigated. The physical meanings and applicability of the proposed surface-forming ERR, traditional ERR, Js-integral and J-integral are compared and discussed. Besides, we give an interpretation of Rice paradox by comparing the cohesive fracture model and the surface-forming ERR based fracture criterion.
International Nuclear Information System (INIS)
It is known that theoretical considerations of fracture under loading by a spherical indenter are based on the concept of pre-existing cracks. However, nucleation and growth of the critical crack could occur during indentation, as happens during microcracking. The goal of the presented research is to develop a new concept of fracture under spherical indentation in a brittle elastic material taking into account the possibility of critical crack nucleation and growth during loading. La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) perovskite has been chosen as a polycrystalline elastic low fracture toughness ceramic to perform indentation using a tungsten carbide spherical indenter. Experimental measurements of ring crack radii for well-polished LSGM cannot be explained within the framework of the pre-existing crack hypothesis. The local risk calculated using the concept of pre-existing cracks gives a most probable range of ring crack radii that does not match the radii measured experimentally. However, the local risk calculated using the assumption of critical crack nucleation and formation during spherical indentation results in a most probable range of ring crack radii which exhibits good agreement with the experimental data.
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In this study, the fatigue crack propagation behavior of Inconel 718 turbine disc with different service times from 0 to 4229 h was investigated at 738 and 823 K. No notable change in microstructural features, other than the increase in grain size, was observed with increasing service time. With increasing service time from 0 to 4229 h, the fatigue crack propagation rates tended to increase, while the ΔKth value decreased, in low ΔK regime and lower Paris' regime at both testing temperatures. The fractographic observation using a scanning electron microscope suggested that the elevated temperature fatigue crack propagation mechanism of Inconel 718 changed from crystallographic cleavage mechanism to striation mechanism in the low ΔK regime, depending on the grain size. The fatigue crack propagation mechanism is proposed for the crack propagating through small and large grains in the low ΔK regime, and the fatigue crack propagation behavior of Inconel 718 with different service times at elevated temperatures is discussed. - Highlights: • The specimens were prepared from the Inconel 718 turbine disc used for 0 to 4229 h. • FCP rates were measured at 738 and 823 K. • The ΔKth values decreased with increasing service time. • The FCP behavior showed a strong correlation with the grain size of used turbine disc
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The effect of elevated temperature aqueous environments upon the initiation and propagation of fatigue cracks in low-alloy steels is discussed in terms of the several parameters which influence such behavior. These parameters include water chemistry, impurities within the steels themselves, as well as factors such as the water flow rate, loading waveform and loading rates. Some of these parameters have similar effects upon both crack initiation and propagation, while others exhibit different effects in the two stages of cracking. In the case of environmentally-assisted crack (EAC) growth, the most important impurities within the steel are metallurgical sulfide inclusions which dissolve upon contact with the water. A ''critical'' concentration of sulfide ions at the crack tip can then induce environmentally-assisted cracking which proceeds at significantly increased crack growth rates over those observed in air. The occurrence, or non-occurrence, of EAC is governed by the mass-transport of sulfide ions to and from the crack-tip region, and the mass-transport is discussed in terms of diffusion, ion migration, and convection induced within the crack enclave. Examples are given of convective mass-transport within the crack enclave resulting from external free stream flow. The initiation of fatigue cracks in elevated temperature aqueous environments, as measured by the S-N fatigue lifetimes, is also strongly influenced by the parameters identified above. The influence of sulfide inclusions does not appear to be as strong on the crack initiation process as it is on crack propagation. The oxygen content of the environment appears to be the dominant factor, although loading frequency (strain rate) and temperature are also important factors
Holroyd, N. J. Henry; Scamans, G. M.
2011-12-01
Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al2CuMg where E a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E a is independent of copper content for stress intensity factors below ~14 MNm-3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm-3/2, with E a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested
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Up to 90% of the life time of cyclically loaded components is determined by short crack initiation and propagation. This stage of the fatigue damage process is strongly influenced by microstructural features, e.g. grain boundaries and crystallographic grain orientation. Therefore LEFM can not be applied in a reasonable manner explaining the demand for a mechanism-related modelling method. The present study deals with mechanical testing and microstructural examinations applied to the relatively new β-titanium alloy LCB. The results are used as data base to develop a new short crack model that is based on the model of Navarro and de los Rios. By using various techniques such as electron back-scattered diffraction and finite-element calculations the origin of crack initiation is revealed and the characteristics of crack propagation is determined. (orig.)
International Nuclear Information System (INIS)
The mechanisms of crack propagation in metallic films on polymeric substrates have been studied through in situ atomic force microscopy observations of thin films under tensile stresses and finite element stress calculations. Two series of films - ones deposited with ion beam assistance, the others without - have been investigated. The observations and stress calculations show that ion beam assistance can change drastically the propagation of cracks in coated materials: by improving the adhesion film/substrate, it slows down the delamination process, but in the same time enhances the cracks growth in the thickness of the material
Interplay between polarization rotation and crack propagation in PMN-PT relaxor single crystals
Directory of Open Access Journals (Sweden)
Fei Fang
2014-01-01
Full Text Available Investigations on the interconnection between the polarization rotation and crack propagation are performed for [110]-oriented 74Pb(Mg1/3Nb2/3O3-26PbTiO3 relaxor ferroelectric single crystal under electric loadings along [001] direction. The crystal is of predominantly monoclinic MA phase with scatter distributed rhombohedral (R phase under a moderate poling field of 900 V/mm in [001] direction. With magnitude of 800 V/mm, a through thickness crack is initiated near the electrode by electric cycling. Static electric loadings is then imposed to the single crystal. As the applied static electric field increases, domain switching in the monoclinic MA phase and phase transition from MA to R phase occur near the crack. The results indicate that the crack features a conducting one. Whether domain switching or phase transition occurs depends on the intensity of the electric field component that is perpendicular to the applied electric field.
Telesman, J.; Antolovich, S. D.
1985-01-01
The important metallurgical factors that influence both constant amplitude and spectrum crack growth behavior in aluminum alloys were investigated. The effect of microstructural features such as grain size, inclusions, and dispersoids was evaluated. It was shown that a lower stress intensities, the I/M 7050 alloy showed better fatigue crack propagation (FCP) resistance than P/M 7091 alloy for both constant amplitude and spectrum testing. It was suggested that the most important microstructural variable accounting for superior FCP resistance of 7050 alloy is its large grain size. It was further postulated that the inhomogenous planar slip and large grain size of 7050 limit dislocation interactions and thus increase slip reversibility which improves FCP performance. The hypothesis was supported by establishing that the cyclic strain hardening exponent for the 7091 alloy is higher than that of 7050.
Experimental and numerical modelling of ductile crack propagation in large-scale shell structures
DEFF Research Database (Denmark)
Simonsen, Bo Cerup; Törnquist, R.
2004-01-01
This paper presents a combined experimental-numerical procedure for development and calibration of macroscopic crack propagation criteria in large-scale shell structures. A novel experimental set-up is described in which a mode-I crack can be driven 400 mm through a 20(+) mm thick plate under fully...... plastic and controlled conditions. The test specimen can be deformed either in combined in-plane bending and extension or in pure extension. Experimental results are described for 5 and 10 mm thick aluminium and steel plates. By performing an inverse finite-element analysis of the experimental results...... general good and it is believed that the presented results and experimental-numerical calibration procedure can be of use in practical finite-element simulations of collision and grounding events with the use of shell elements. As discussed, the paper provides a clean framework for further development of...
Uribe, David; Steeb, Holger
2016-04-01
The use of imaged based methods to determine properties of geological materials is becoming an alternative to laboratory experiments. Furthermore, the combination of laboratory experiments and image based methods using micro computer tomography have advanced the understanding of geophysical and geochemical processes. Within the scope of the "Shynergie" project, two special topics have been studied using such combination: a) the generation and propagation of cracks in rocks (specially wing cracks) and b) the time dependence of transport properties of rocks due to chemical weathering. In this publication, we describe the design considerations of our micro CT scanner to manipulate rock samples that have been subjected to the experiments to determine the above mentioned phenomena. Additionally, we discuss the preliminary experimental results and the initial interpretations we have gathered from the observations of the digitized rock samples.
Effect of frequency and environment on fatigue-crack propagation of SA533B-1 steel
International Nuclear Information System (INIS)
The effects of a decrease in the frequency of cyclic loading on the fatigue-crack propagation characteristics of SA533B-1 steel in various environments were investigated. Frequency levels of 10 Hz,1.0 Hz and 0.1 Hz were employed in laboratory air, distilled water and a 3.5% NaCl solution. As the loading frequency was decreased, statistically significant increases in the fatigue-crack growth rates for the distilled water and salt water environments, as compared to those measured in laboratory air, were observed. These increases in growth rates were limited to certain ranges of stress intensity range values depending upon the frequency level being tested. A hydrogen embrittlement mechanism is proposed to explain the increase in growth rates based upon a fractographic analysis. (author)
International Nuclear Information System (INIS)
Highlights: • The alloy shows lower crack growth rate under IP-TMFCG loading. • Larger plastic zone size at the crack tip was observed under IP-TMFCG loading. • Primary crack tends to form branches when it enters prismatic slip oriented grain. - Abstract: Fatigue crack growth (FCG) mechanisms have been studied in light of the interaction of a propagating crack with local crystallographic orientations of primary alpha (αP) and secondary alpha (αS) colonies in a near α Timetal 834 Ti-alloy under thermomechanical fatigue (TMF) loading using electron backscattered diffraction (EBSD). FCG testing at in-phase (IP) and out-of-phase (OP) TMF loading have been carried out at two temperature intervals 300 °C ↔ 450 °C and 450 °C ↔ 600 °C. EBSD analysis and microhardness measurements have confirmed that larger cyclic plastic zone size at the crack tip leads to lower crack propagation under IP-TMFCG loading as compared to OP-TMFCG loading. EBSD analysis has also confirmed that crack dissipates more energy to propagate when it passes from a soft grain oriented with its c-axis normal to the loading direction and encounters a hard grain with its c-axis parallel with the loading direction
Rudraraju, Siva Shankar; Garikipati, Krishna; Waas, Anthony M.; Bednarcyk, Brett A.
2013-01-01
The phenomenon of crack propagation is among the predominant modes of failure in many natural and engineering structures, often leading to severe loss of structural integrity and catastrophic failure. Thus, the ability to understand and a priori simulate the evolution of this failure mode has been one of the cornerstones of applied mechanics and structural engineering and is broadly referred to as "fracture mechanics." The work reported herein focuses on extending this understanding, in the context of through-thickness crack propagation in cohesive materials, through the development of a continuum-level multiscale numerical framework, which represents cracks as displacement discontinuities across a surface of zero measure. This report presents the relevant theory, mathematical framework, numerical modeling, and experimental investigations of through-thickness crack propagation in fiber-reinforced composites using the Variational Multiscale Cohesive Method (VMCM) developed by the authors.
Lipovsky, B.; Dunham, E. M.
2012-12-01
Crack waves are guided waves along fluid-filled cracks that propagate with phase velocity less than the sound wave speed. Chouet (JGR, 1986) and Ferrazzini and Aki (JGR, 1977) have shown that such waves could explain volcanic tremor in terms of the resonant modes of a finite length magma-filled crack. Based on an idealized lumped-parameter model, Julian (JGR, 1994) further proposed that the steady flow of a viscous magma in a volcanic conduit is unstable to perturbations, leading to self-excited oscillations of the conduit walls and radiation of seismic waves. Our objective is to evaluate the possibility of self-excited oscillations within a rigorous, continuum framework. Our specific focus has been on basaltic fissure eruptions. In a typical basaltic fissure system, the magnitudes of the wave restoring forces, fluid compressibility and wall elasticity, are highly depth dependent. Because of the elevated fluid compressibility from gas exsolution at shallow depths, fluid pressure perturbations in this regime propagate as acoustic waves with effectively rigid conduit walls. Below the exsolution depth, the conduit walls are more compliant relative to the magma compressibility and perturbations propagate as dispersive crack waves. Viscous magma flow through such a fissure will evolve to a fully developed state characterized by a parabolic velocity profile in several to tens of seconds. This time scale is greater than harmonic tremor periods, typically 0.1 to 1 second. A rigorous treatment of the wave response to pressure perturbations therefore requires a general analysis of conduit flow that is not in a fully developed state. We present a linearized analysis of the coupled fluid and elastic response to general flow perturbations. We assume that deformation of the wall is linear elastic. As our focus is on wavelengths greatly exceeding the crack width, fluid flow is described by a quasi-one dimensional, or width-averaged, model. We account for conservation of magma
International Nuclear Information System (INIS)
The process of I-mode crack propagations in α-Fe for uniaxial tension experiments are simulated by molecular dynamics (MD) methods. The formation process of dislocation and fracture mechanisms in the crack growing under various temperatures were studied. The results show that the crack propagation is a process of successive emission of dislocation. The dislocation-free zone and the stacking faults were initially formed at crack tip. When the stress KI increased into 0. 566 MPam1/2, one layer of atoms near crack tip would be separated into two layers which produced a dislocation. The first dislocation was emitted when stress KI reached 0.669 MPam1/2. With the temperature increasing, the critical stress intensity factor decreased gradually and the dislocation emission correspondingly became faster as well. (authors)
Energy Technology Data Exchange (ETDEWEB)
Min, Zhang [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China); Xi-ping, Song, E-mail: xpsong@skl.ustb.edu.cn [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China); Long, Yu; Hong-liang, Li [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China); Ze-hui, Jiao; Hui-chen, Yu [National Key Laboratory of Science and Technology on Advanced High Temperature Structural Materials, Beijing Institute of Aeronautical Materials, Beijing 100095 (China)
2015-01-12
In this paper, the fatigue crack initiation and propagation behavior of a high-Nb TiAl alloy with nearly lamellar microstructure was studied by in situ scanning electron microscope observation at 750 °C. Dog-bone shaped specimens with a single-edge notch were used in the test. The results showed that the fatigue crack initiated first at the central portion of the notch, and then shifted to the edge portion. As the cycle numbers went on increasing, these cracks joined together and formed a main fatigue crack, which could propagate along the surface of the specimen. During the fatigue crack propagation two or three propagation stages were found depending on the microstructure of the crack tip. When the fatigue crack was parallel to the lamellar laths, it exhibited the rapid, steady and accelerated propagation stages successively, while when the fatigue crack was perpendicular to the lamellar laths, it exhibited only the steady and accelerated propagation stages, with no rapid propagation stage being found. In these different propagation stages the fatigue crack propagation rates were different and depended intensively on the lamellar laths orientation, lamellar colony size, equiaxed gamma grains and peak stress intensity factor K{sub max}. Based on the experimental data it was concluded that the fatigue crack initiation lifetime was much longer than the propagation lifetime for the single-edge notched specimens at 750 °C.
International Nuclear Information System (INIS)
In this paper, the fatigue crack initiation and propagation behavior of a high-Nb TiAl alloy with nearly lamellar microstructure was studied by in situ scanning electron microscope observation at 750 °C. Dog-bone shaped specimens with a single-edge notch were used in the test. The results showed that the fatigue crack initiated first at the central portion of the notch, and then shifted to the edge portion. As the cycle numbers went on increasing, these cracks joined together and formed a main fatigue crack, which could propagate along the surface of the specimen. During the fatigue crack propagation two or three propagation stages were found depending on the microstructure of the crack tip. When the fatigue crack was parallel to the lamellar laths, it exhibited the rapid, steady and accelerated propagation stages successively, while when the fatigue crack was perpendicular to the lamellar laths, it exhibited only the steady and accelerated propagation stages, with no rapid propagation stage being found. In these different propagation stages the fatigue crack propagation rates were different and depended intensively on the lamellar laths orientation, lamellar colony size, equiaxed gamma grains and peak stress intensity factor Kmax. Based on the experimental data it was concluded that the fatigue crack initiation lifetime was much longer than the propagation lifetime for the single-edge notched specimens at 750 °C
International Nuclear Information System (INIS)
Research highlights: → Composite coatings on the aluminized steel were prepared by the plasma electrolytic oxidation (PEO) technique, which comprised of Fe-Al layer, Al layer and Al2O3 layer. → The evaluation method of the crack critical opening displacement δc was introduced to describe quantitatively the resistance of Al layer to the propagation behavior of cracks and evaluate the fracture behavior of composite coatings. → The crack propagating model was established. - Abstract: This paper investigates the in situ tensile cracks propagating behavior of composite coatings on the aluminized steel generated using the plasma electrolytic oxidation (PEO) technique. Cross-sectional micrographs and elemental compositions were investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The composite coatings were shown to consist of Fe-Al, Al and Al2O3 layers. The cracks propagating behavior was observed in real-time in situ SEM tensile test. In tensile process, the cracks were temporarily stopped when cracks propagated from Fe-Al layer to Al layer. The critical crack opening displacement δc was introduced to quantitatively describe the resistance of the Al layer. There was a functional relation among the thickness ratio tAl/tAl2O3, the δc of composite coatings and tensile cracks' spacing. The δc increased with the increasing of the thickness ratio (tAl/tAl2O3). The high δc value means high fracture resistance. Therefore, a control of the thickness ratio tAl/tAl2O3 was concerned as a key to improve the toughness and strength of the aluminized steel.
Oscillatory instability in slow crack propagation in rubber under large deformation
Endo, Daiki; Sato, Katsuhiko; Hayakawa, Yoshinori
2012-07-01
We performed experiments to investigate slow fracture in thin rubber films under uniaxial tension using high-viscosity oils. In this system we observed an oscillating instability in slowly propagating cracks for small applied strains. The transition between oscillatory and straight patterns occurred near the characteristic strain at which rubber exhibits a nonlinear stress-strain relation. This suggests that nonlinear elasticity plays an important role in the formation of the observed pattern. This was confirmed by numerical simulation for neo-Hookean and linear elasticity models.
Diffuse interface approach to brittle fracture
International Nuclear Information System (INIS)
We present a continuum model for the propagation of cracks and fractures in brittle materials. The components of the strain tensor ε are the fundamental variables. The evolution equations are based on a free energy that reduces to that of linear elasticity for small ε, and accounts for cracks through energy saturation at large values of ε. We regularize the model by including terms dependent on gradients of ε in the free energy. No additional fields are introduced, and then the whole dynamics is perfectly defined. We show that the model is able to reproduce basic facts in fracture physics, like the Griffith's dependence of the critical stress as a minus one half power of the crack length. In addition, regularization makes the results insensitive to the numerical mesh used, something not at all trivial in crack modeling. We present and example of the application of the model to predict the growth and curving of cracks in a non-trivial geometrical configuration. (author)
Energy Technology Data Exchange (ETDEWEB)
Li, Lifei [Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191 (China); Zhang, Zheng, E-mail: zhangzh@buaa.edu.cn [Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191 (China); Shen, Gongtian [China Special Equipment Inspection and Research Institute, Beijing 100013 (China)
2015-06-11
The influence of grain size on fatigue and corresponding acoustic emission (AE) features in commercial-purity zirconium were investigated. Fatigue crack propagation tests were conducted at room temperature with AE monitored simultaneously. The fatigue properties and AE sources were discussed combined with the microstructural and fractographic observations. The results showed that the increased grain size affected the fatigue crack stable propagation rate slightly, but it resulted in a significant increase of the AE counts rate. During the crack stable propagation, for the specimens with small-sized grains and medium-sized grains, the relationships between AE counts rate and fatigue stress intensity factor range were both generally according with the Pairs law, except some local fluctuations due to the regional occurrence of deformation twins. Especially, for the specimen with large grains, higher AE counts rate presented persistently, which were caused by twins appearing continuously at the edge of the crack. These results suggest that as the grain size increased in commercial-purity zirconium, twin became more frequent and made a more important contribution to the fatigue process, and the AE technique was sensitive to the crack propagation and the twin incidents during fatigue crack growth.
International Nuclear Information System (INIS)
The influence of grain size on fatigue and corresponding acoustic emission (AE) features in commercial-purity zirconium were investigated. Fatigue crack propagation tests were conducted at room temperature with AE monitored simultaneously. The fatigue properties and AE sources were discussed combined with the microstructural and fractographic observations. The results showed that the increased grain size affected the fatigue crack stable propagation rate slightly, but it resulted in a significant increase of the AE counts rate. During the crack stable propagation, for the specimens with small-sized grains and medium-sized grains, the relationships between AE counts rate and fatigue stress intensity factor range were both generally according with the Pairs law, except some local fluctuations due to the regional occurrence of deformation twins. Especially, for the specimen with large grains, higher AE counts rate presented persistently, which were caused by twins appearing continuously at the edge of the crack. These results suggest that as the grain size increased in commercial-purity zirconium, twin became more frequent and made a more important contribution to the fatigue process, and the AE technique was sensitive to the crack propagation and the twin incidents during fatigue crack growth
International Nuclear Information System (INIS)
Laser peening without protective coating (LPwC) treatment is one of surface enhancement techniques using an impact wave of high pressure plasma induced by laser pulse irradiation. High compressive residual stress was induced by the LPwC treatment on the surface of low-carbon type austenitic stainless steel SUS316L. The affected depth reached about 1mm from the surface. High cycle fatigue tests with four-points rotating bending loading were carried out to confirm the effects of the LPwC treatment on fatigue strength and surface fatigue crack propagation behaviors. The fatigue strength was remarkably improved by the LPwC treatment over the whole regime of fatigue life up to 108 cycles. Specimens with a pre-crack from a small artificial hole due to fatigue loading were used for the quantitative study on the effect of the LPwC treatment. The fracture mechanics investigation on the pre-cracked specimens showed that the LPwC treatment restrained the further propagation of the pre-crack if the stress intensity factor range ΔK on the crack tip was less than 7.6 MPa√m. Surface cracks preferentially propagated into the depth direction as predicted through ΔK analysis on the crack by taking account of the compressive residual stresses due to the LPwC treatment. (author)
3D numerical analysis of crack propagation of heterogeneous notched rock under uniaxial tension
Wang, S. Y.; Sloan, S. W.; Sheng, D. C.; Tang, C. A.
2016-05-01
Macroscopic notches play an important role in evaluating the fracture process zone (FPZ) and the strengths of a heterogeneous rock mass. Crack initiation, propagation and coalescence for unnotched, single-notched and double-notched rock specimens are numerically simulated in a 3-D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. For the unnotched case, special attention is given to the complete stress-strain curve and the corresponding AE events for the failure process of rock specimen. By comparing with published experimental results, the simulation results from RFPA3D are found to be satisfactory. For the single-notched case, the effect of the length and the depth of the single notch and the thickness of the specimen on the failure mode and peak stress are evaluated. The 3D FPZ is very different from that in two dimensions. For the double-notched case, the effects of the separation distance and overlap distance of the double notches, as well as influence of the homogeneity index (m) are also investigated. As the overlap distance increases, the direction of the principal tensile stress at each notch-end changes from a perpendicular direction (tensile stress field) to a nearly parallel direction (compressive stress field), which affects the evolution of the cracks from the two notches.
Fatigue-crack propagation response of two nickel-base alloys in a liquid sodium environment
International Nuclear Information System (INIS)
The elevated temperature fatigue-crack propagation response of Inconel 600 and Inconel 718 was characterized within a linear-elastic fracture mechanics framework in air and low-oxygen liquid sodium environments. The crack growth rates of both nickel-base alloys tested in liquid sodium were found to be considerably lower than those obtained in air. This enhanced fatigue resistance in sodium was attributed to the very low oxygen content in the inert sodium environment. Electron fractographic examination of the Inconel 600 and Inconel 718 fatigue fracture surfaces revealed that operative crack growth mechanisms were dependent on the prevailing stress intensity level. Under low growth rate conditions, Inconel 600 and Inconel 718 fracture surfaces exhibited a faceted, crystallographic morphology in both air and sodium environments. In the higher growth rate regime, fatigue striations were observed; however, striations formed in sodium were rather ill-defined. These indistinct striations were attributed to the absence of oxygen in the liquid sodium environment. Striation spacing measurements were found to be in excellent agreement with macroscopic growth rates in both environments
Directory of Open Access Journals (Sweden)
G. M. Domínguez Almaraz
2015-10-01
Full Text Available Crack initiation and propagation have been investigated on the polymeric material ABS (Acrylonitrile Butadiene Styrene, under ultrasonic fatigue testing. Three controlled actions were implemented in order to carry out fatigue tests at very high frequency on this material of low thermal conductivity, they are: a The applying load was low to limit heat dissipation at the specimen neck section, b The dimensions of testing specimen were small (but fitting the resonance condition, in order to restraint the temperature gradient at the specimen narrow section, c Temperature at the specimen neck section was restrained by immersion in water or oil during ultrasonic fatigue testing. Experimental results are discussed on the basis of thermo-mechanical behaviour: the tail phenomenon at the initial stage of fatigue, initial shear yielding deformation, crazed development on the later stage, plastic strain on the fracture surface and the transition from low to high crack growth rate. In addition, a numerical analysis is developed to evaluate the J integral of energy dissipation and the stress intensity factor K, with the crack length
International Nuclear Information System (INIS)
Fatigue (high- and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and SEM to improve methods of monitoring of damage accumulation during fatigue test and to verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue regime using the Luong method [1] by “in-situ” infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both high cycle fatigue and gigacycle fatigue regimes. Fracture surface analysis for plane and cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometer-profiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ∼300 μm has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest fracture surface and considered as the trace of the fatigue crack path corresponding to the Paris kinetics
Alternating brittle and ductile response of coherent twin boundaries in nanotwinned metals
International Nuclear Information System (INIS)
Nanotwinned metals have opened exciting avenues for the design of high strength and high ductility materials. In this work, we investigate crack propagation along coherent twin boundaries in nanotwinned metals using molecular dynamics. Our simulations reveal that alternating twin boundaries exhibit intrinsic brittleness and ductility owing to the opposite crystallographic orientations of the adjoining twins. This is a startling consequence of the directional anisotropy of an atomically sharp crack along a twin boundary that favors cleavage in one direction and dislocation emission from the crack tip in the opposite direction. We further find that a blunt crack exhibits ductility in all cases albeit with very distinct deformation mechanisms and yield strength associated with intrinsically brittle and ductile coherent twin boundaries
International Nuclear Information System (INIS)
Full text: In performing residual life assessment of a high temperature power plant component, prediction of propagation behavior of crack like defect is sometimes required. When evaluating crack propagation rate of a given crack subjected to creep-fatigue loading, J-integral type of fracture mechanics parameters, elastic-plastic J-integral for fatigue and C(t) for creep, are needed to estimate. Although these parameters may be estimated by detailed finite element analysis (FEA) modeling inelastic deformation behavior, simplified method without costly computation is usually preferred. Simplified defect assessment procedures, which are mostly based on the failure assessment diagram approach, are being standardized in many countries. The author too presented defect assessment procedures for elevated temperature power plant components in where several simplified approaches including the reference stress approach were proposed. When an analyst performs defect assessment with the failure assessment diagram or the reference stress approach, the analyst needs solutions of stress intensity factor and limit load for the cracked component to evaluate. These solutions may be obtained by systematically performed finite element analyses and can form fracture mechanics database. Elastic follow-up factors for displacement-controlled loading also may be obtained by FEA and stored in database. The fracture mechanics database used for the software tools described here includes these FEA solutions related to elastic/inelastic response of cracked geometries. This paper describes computer software tools enabling users to perform easily defect assessment of high temperature power plant component. These tools are web-based and can be used remotely. FEA computation is also possible by using a Java tool for preparing FEA data and sending the data to a cgi at the computation site. If a user needs a new solution of stress intensity factor, limit load or elastic follow-up factor, he can
Study of interfacial crack propagation in flip chip assemblies with nano-filled underfill materials
Mahalingam, Sakethraman
No-flow underfill materials that cure during the solder reflow process is a relatively new technology. Although there are several advantages in terms of cost, time and processing ease, there are several reliability challenges associated with no-flow underfill materials. When mum-sized filler particles are introduced in no-flow underfills to enhance the solder bump reliability, such filler particles could prevent the solder bumps making reliable electrical contacts with the substrate pads during solder reflow, and therefore, the assembly yield would be adversely affected. The use of nano-sized filler particles can potentially improve assembly yield while offering the advantages associated with filled underfill materials. The objective of this thesis is to study the thermo-mechanical reliability of nano-filled epoxy underfills (NFU) through experiments and theoretical modeling. In this work, the thermo-mechanical properties of NFU's with 20-nm filler particles have been measured. An innovative residual stress test method has been developed to measure the interfacial fracture toughness. Using the developed residual stress method and the single-leg bending test, the mode-mixity-dependent fracture toughness for NFU-SiN interface has been determined. In addition to such monotonic interfacial fracture characterization, the interface crack propagation under thermo-mechanical fatigue loading has been experimentally characterized, and a model for fatigue interface crack propagation has been developed. A test vehicle comprising of several flip chips was assembled using the NFU material and the reliability of the flip-chip assemblies was assessed under thermal shock cycles between -40°C and 125°C. The NFU-SiN interfacial delamination propagation and the solder bump reliability were monitored. In parallel, numerical models were developed to study the interfacial delamination propagation in the flip chip assembly using conventional interfacial fracture mechanics as well as
Energy Technology Data Exchange (ETDEWEB)
Andrier, B. [EDF/GDL, 2 Rue Ampere, 93206 Saint-Denis Cedex (France); Garbay, E. [EDF/GDL, 2 Rue Ampere, 93206 Saint-Denis Cedex (France); Hasnaoui, F. [EDF/DRD, 1 Avenue du General de Gaulle, 92141 Clamart (France); Massin, P. [CNRS-EDF, UMR 2832, 1 Avenue du General de Gaulle, 92141 Clamart (France)]. E-mail: patrick.massin@edf.fr; Verrier, P. [EDF/DRD, 1 Avenue du General de Gaulle, 92141 Clamart (France)
2006-02-15
It is known from international feedback that the rotor shafts of the turbo-generators with disk shrunk technology may have transverse cracks located near the keys which maintain the bond between the core of the shaft and the surrounding disks in case of over speed. It was understood that the cracks were initiated by fretting between the keys and the shaft and that they propagated due to a fatigue mechanism generated by the rotational flexion of the shafts under gravity. The destructive observation now correlated to the service history of the shaft shows different mixed modes propagation phases and a stopped circumferential crack evolution during the last months of service of the shaft. Mechanical studies based on the determination of the stress intensity factors provide the evolution of the stress intensity factors during the crack propagation. They give access to information not available otherwise to explain the observed crack profiles. Finally, experimental investigations are needed to obtain the kinetics as a function of the stress intensity factors. The information provided is helpful in determining the possible crack profiles to be detected by the most suitable vibratory surveillance systems before failure in service of the shaft line.
International Nuclear Information System (INIS)
It is known from international feedback that the rotor shafts of the turbo-generators with disk shrunk technology may have transverse cracks located near the keys which maintain the bond between the core of the shaft and the surrounding disks in case of over speed. It was understood that the cracks were initiated by fretting between the keys and the shaft and that they propagated due to a fatigue mechanism generated by the rotational flexion of the shafts under gravity. The destructive observation now correlated to the service history of the shaft shows different mixed modes propagation phases and a stopped circumferential crack evolution during the last months of service of the shaft. Mechanical studies based on the determination of the stress intensity factors provide the evolution of the stress intensity factors during the crack propagation. They give access to information not available otherwise to explain the observed crack profiles. Finally, experimental investigations are needed to obtain the kinetics as a function of the stress intensity factors. The information provided is helpful in determining the possible crack profiles to be detected by the most suitable vibratory surveillance systems before failure in service of the shaft line
Energy Technology Data Exchange (ETDEWEB)
Gong, Xing, E-mail: gongxingzfl@hotmail.com [SCK-CEN (Belgian Nuclear Research Centre), Boeretang 200, B-2400 Mol (Belgium); KU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Box 2450, B-3001 Heverlee (Belgium); Marmy, Pierre, E-mail: pierre.marmy@sckcen.be [SCK-CEN (Belgian Nuclear Research Centre), Boeretang 200, B-2400 Mol (Belgium); Qin, Ling, E-mail: Ling.Qin@mtm.kuleuven.be [KU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Box 2450, B-3001 Heverlee (Belgium); Verlinden, Bert, E-mail: Bert.Verlinden@mtm.kuleuven.be [KU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Box 2450, B-3001 Heverlee (Belgium); Wevers, Martine, E-mail: Martine.Wevers@mtm.kuleuven.be [KU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Box 2450, B-3001 Heverlee (Belgium); Seefeldt, Marc, E-mail: Marc.Seefeldt@mtm.kuleuven.be [KU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, Box 2450, B-3001 Heverlee (Belgium)
2014-11-17
The low cycle fatigue properties of a modified 9Cr–1Mo ferritic–martensitic steel (T91) have been tested in stagnant liquid lead–bismuth eutectic (LBE) with oxygen concentrations ranging from 1.16×10{sup −6} to 6.0×10{sup −10} wt% at 350 °C. The effect of liquid metal embrittlement (LME) on fatigue endurance, fatigue crack propagation modes and secondary cracking has been studied. The results showed that the fatigue lives of T91 steel in a low oxygen concentration LBE were drastically reduced compared to those in vacuum due to the presence of LME. The microstructural observations on the fatigue crack propagation modes revealed that fatigue cracks in LBE mainly propagate across prior-austenite grain boundaries and then cut through martensitic lath boundaries, simultaneously leaving a few plastic flow traces and characteristic brittle features. Intergranular and interlath cracking occurred occasionally and their occurrence depended on the orientation of the boundaries relative to the stress axis. The complexity of the LME-induced fracture features can be attributed to a mixture of the multiple failure modes. No obvious plastic shear strain localization was present around the crack tips when LME occurred. However, using a high resolution electron backscatter diffraction (EBSD) technique, highly localized plastic shear strain was observed in the vicinity of the crack tips in vacuum, manifested by the presence of very fine subgrains along the crack walls. A qualitative mechanism was proposed to account for the LME phenomenon in the T91/LBE system. In addition, the secondary cracking at fatigue striations was different in the presence of LBE compared to vacuum. This phenomenon was elucidated by taking into account the influence of the LME on the fatigue crack propagation rate.
International Nuclear Information System (INIS)
The low cycle fatigue properties of a modified 9Cr–1Mo ferritic–martensitic steel (T91) have been tested in stagnant liquid lead–bismuth eutectic (LBE) with oxygen concentrations ranging from 1.16×10−6 to 6.0×10−10 wt% at 350 °C. The effect of liquid metal embrittlement (LME) on fatigue endurance, fatigue crack propagation modes and secondary cracking has been studied. The results showed that the fatigue lives of T91 steel in a low oxygen concentration LBE were drastically reduced compared to those in vacuum due to the presence of LME. The microstructural observations on the fatigue crack propagation modes revealed that fatigue cracks in LBE mainly propagate across prior-austenite grain boundaries and then cut through martensitic lath boundaries, simultaneously leaving a few plastic flow traces and characteristic brittle features. Intergranular and interlath cracking occurred occasionally and their occurrence depended on the orientation of the boundaries relative to the stress axis. The complexity of the LME-induced fracture features can be attributed to a mixture of the multiple failure modes. No obvious plastic shear strain localization was present around the crack tips when LME occurred. However, using a high resolution electron backscatter diffraction (EBSD) technique, highly localized plastic shear strain was observed in the vicinity of the crack tips in vacuum, manifested by the presence of very fine subgrains along the crack walls. A qualitative mechanism was proposed to account for the LME phenomenon in the T91/LBE system. In addition, the secondary cracking at fatigue striations was different in the presence of LBE compared to vacuum. This phenomenon was elucidated by taking into account the influence of the LME on the fatigue crack propagation rate
Crack propagation in SiCf/SiC ceramic matrix composite under static and cyclic loading conditions
International Nuclear Information System (INIS)
SiCf/SiC ceramic matrix composite material is of high interest for potential application as a structural and barrier material in fusion systems. It possesses reasonable fracture toughness over a range of temperatures and, due to the low atomic number of its constituents, is appealing for low activation reasons. This study examines the mechanical durability of a Nicalon fiber-SiC composite which has been tested at temperatures up to 1400 C to determine its resistance to crack propagation under static and cyclic loading conditions. The crack growth characteristics are governed by the fiber and interface failure modes. These, in turn are affected by loading parameters, temperature and environmental effects. The material shows R-curve behavior, due to fiber bridging of the crack wake. The material also shows time dependent crack growth at elevated temperature, but not at room temperature. However, cyclic loading does induce crack extension at room temperature. ((orig.))
Crack Propagation Property of 316 Stainless Steel%316不锈钢的裂纹扩展性能
Institute of Scientific and Technical Information of China (English)
阮於珍; 陈金陵
2000-01-01
The thermal fatigue test was performed on 316 SS to understand the crack propagation behavior underthe cycling temperature environment. The crack propagation rate got from the test is low, so the loop with crackcould be re-used in regular monitering condition.%从实用出发对316不锈钢管材进行了热循环下的裂纹扩展试验。试验表明该材料在交变热应力下的裂纹扩展速率缓慢，带裂纹的管道在监督下可以继续运行。
Ohmura, Etsuji; Kawahito, Yuta; Fukumitsu, Kenshi; Okuma, Junji; Morita, Hideki
2011-02-01
Stealth dicing (SD) is an innovative dicing method developed by Hamamatsu Photonics K.K. In the SD method, a permeable nanosecond laser is focused inside a silicon wafer and scanned horizontally. A thermal shock wave propagates every pulse toward the side to which the laser is irradiated, then a high dislocation density layer is formed inside the wafer after the thermal shock wave propagation. In our previous study, it was concluded that an internal crack whose initiation is a dislocation is propagated when the thermal shock wave by the next pulse overlaps with this layer partially. In the experimental result, the trace that a crack is progressed gradually step by step was observed. In this study, the possibility of internal crack propagation by laser pulses was investigated. A two-dimensional thermal stress analysis based on the linear fracture mechanics was conducted using the stress distribution obtained by the axisymmetric thermal stress analysis. As a result, the validity of the hypothesis based on a heat transfer analysis result previously presented was supported. Also it was concluded that the internal crack is propagated by at least two pulses.
International Nuclear Information System (INIS)
Synchrotron X-ray tomography was performed during in situ fatigue crack propagation in two small-size specimens made of nodular graphite cast iron. While direct image analysis allows us to retrieve the successive positions of the crack front, and to detect local crack retardation, volume correlation allows for the measurement of displacement fields in the bulk of the specimen. The stress intensity factors (SIFs), which are extracted from the measured displacement fields and the corresponding local crack growth rate all along the front, are in good agreement with published results. In particular, it is possible to link the non-propagation of a crack with crack closure in the crack opening displacement maps or with a local value of the measured SIF range. It is shown that a non-uniform closure process along the crack front induces an asymmetric arrest/growth of the crack.
Microstructural effects on the creep and crack propagation behaviors of γ-Ti aluminide alloy
International Nuclear Information System (INIS)
Gamma titanium aluminides class of materials possess several unique physical and mechanical properties. These characteristics can be attractive for specific industrial applications. By applying different heat treatment schedules one can change the microstructural features of this class of materials. In the present investigation, two heat treatment schedules were used to produce two different microstructures, duplex (D) and nearly lamellar (NL) in the cast and HIP'ed Ti-47Al-2W-0.5Si alloy. The tensile strength and creep behavior, in the 700--850 C temperature range, of this alloy have been determined and correlated to the corresponding microstructures. In addition, the fatigue crack propagation behavior in this alloy has been studied at different temperatures. The results on the creep behavior showed that the alloy with nearly lamellar microstructure has a strongly improved creep strength as compared with that of the duplex microstructure
Richey, Edward, III
1995-01-01
This research aims to develop the methods and understanding needed to incorporate time and loading variable dependent environmental effects on fatigue crack propagation (FCP) into computerized fatigue life prediction codes such as NASA FLAGRO (NASGRO). In particular, the effect of loading frequency on FCP rates in alpha + beta titanium alloys exposed to an aqueous chloride solution is investigated. The approach couples empirical modeling of environmental FCP with corrosion fatigue experiments. Three different computer models have been developed and incorporated in the DOS executable program. UVAFAS. A multiple power law model is available, and can fit a set of fatigue data to a multiple power law equation. A model has also been developed which implements the Wei and Landes linear superposition model, as well as an interpolative model which can be utilized to interpolate trends in fatigue behavior based on changes in loading characteristics (stress ratio, frequency, and hold times).
Capsule design for irradiation of fatigue crack propagation specimens in ORR
International Nuclear Information System (INIS)
The capsule has been designed to allow the irradiation of fatigue crack propagation specimens with the 0.5T compact tension (0.5T CT) geometry at elevated temperatures in the high-flux region of the ORR core. Temperature control will be achieved by a balance between gamma heat generation in the specimens and heat loss to the reactor cooling water through an externally controlled variable gas gap. The design of the ORR capsule has been derived from NRL capsules used to irradiate reactor pressure vessel steels in a number of research reactors, with modifications to the geometry to allow for the high gamma heating rate encountered in the core of ORR. The first assembly of this design will be irradiated as experiment MFE-3, scheduled for insertion in ORR by the end of FY 78
Low temperature crack propagation of nickel-based weld metals in hydrogenated PWR primary water
International Nuclear Information System (INIS)
The effect of hydrogenated PWR primary water on the Low Temperature Crack Propagation (LTCP) susceptibility of nickel-based weld metals Alloy 182, 82, 152 and 52 was studied performing J-R tests at a slow displacement rate in simulated low temperature PWR primary water. When tested in an environment with high hydrogen content (100 cm3 H2/kg H2O), all the studied materials showed a remarkable decrease in the fracture toughness (JIC or JQ) values compared with the air test results. Alloy 182 showed the lowest average fracture toughness values in each test environment. The results obtained at a lower hydrogen content (∼ 30 cm3 H2/kg H2O) suggest, that Alloy 182 is the most susceptible nickel-based weld metal to LTCP, especially at low hydrogen contents. Intergranular cracking was predominant when the JIC value was low. Test results of pure weld metal Alloys 182 and 52 were also compared with the results of dissimilar metal weld (DMW) specimens of Alloy 182 and 52. The pure weld metals were substantially more susceptible to LTCP than the DMW specimens. Pre-exposure to high temperature hydrogenated water did not affect the fracture toughness of any of the test materials. The degradation in toughness is assumed to be caused by a hydrogen-induced intergranular cracking mechanism, where the precipitates in the weld metals acting as hydrogen trapping sites play an important role. The LTCP susceptibility of the studied alloys is discussed based on the present hydrogen embrittlement mechanisms. (authors)
Directory of Open Access Journals (Sweden)
Venkatesh
2015-08-01
Full Text Available n th e paper an experime n tal i n ve s tigation was carri ed out to study the Mixed Mode Crack Parameters of High Performance Concrete beams subjected to three point bending . The cubes and cylinders will be tested on Universal Testing Machine to find out the Compressive strength and split tensile strength. For High Performance Concrete, the partial replacement of cement with Grou nd Granulated Blast Furnace Slag (GGBS and sand with the ROBO sand (crusher dust will be taken. In this investigation the study of Mixed Mode Crack Propagation in High Performance Concrete b eams with eccentrically placed notch at a distance (L/4 from mi d span of the beam under a three point bending test i.e., with a central point load will be done. In High Performance Concrete , the Variation of volume fraction s of Ground Granulated Blast furnace Slag (GGBS and ROBO sand in the casting of beams will carr ied out to find the Mixed Mode Fracture Parameters . Fracture parameters like Fracture energy G f , Cohesive fracture process Zone C F , Stress intensity factor K I would be determined by using size effect method.
International Nuclear Information System (INIS)
Fatigue life of nuclear facilities tends to be decreased by the influence of reactor coolant, which is called environmental effect. The effect accelerates crack growth rate but the influence for crack initiation is not clarified. This study intends to discuss the environmental effect in crack initiation. The crack length and the number of cracks are measured from the investigation of fatigue test specimens in reactor coolant and air. The behavior of crack initiation is revealed from the measurement of number of cracks, crack sizes and fatigue life. From this study, environmental effect of reactor coolant is considered to influence crack initiation and increase the number of micro crack. It is also estimated that the coalescence of cracks influences the acceleration of crack growth. (author)
Modelling probabilistic fatigue crack propagation rates for a mild structural steel
J.A.F.O. Correia; A.M.P. de Jesus; A. Fernández-Canteli; Calçada, R.A.B.
2015-01-01
A class of fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and local strain-life damage models have been proposed in literature. The fatigue crack growth is regarded as a process of continuous crack initializations over successive elementary material blocks, which may be governed by smooth strain-life damage data. Some approaches account for the residual stresses developing at the crack tip in the actual crack driving force asse...
Saito, Youichi; Tanaka, Shun-Ichiro
2016-04-01
Initiation, propagation, and termination of internal cracks in a continuously cast austenitic stainless steel has been investigated with emphasis on stress loading of the solidified shell during casting. Cracks were formed at the center of the slab, parallel to the width of the cast, and were observed near the narrow faces. Optimized two-dimensional X-ray diffraction method was employed to measure residual stress tensor distributions around the cracks in the as-cast slab with coarse and strongly preferentially oriented grains. The tensor distributions had a sharp peak, as high as 430 MPa, at the crack end neighboring the columnar grains. On the other hand, lower values were measured at the crack end neighboring the equiaxed grains, where the local temperatures were higher during solidification. The true residual stress distributions were determined by evaluating the longitudinal elastic constant for each measured position, resulting in more accurate stress values than before. Electron probe micro-analysis at the terminal crack position showed that Ni, Ti, and Si were concentrated at the boundaries of the equiaxed grains, where the tensile strength was estimated to be lower than at the primary grains. A model of the crack formation and engineering recommendations to reduce crack formation are proposed.
Corrosion Fatigue Crack Propagation Study on a High Toughness and High Strength 3Ni-Cr-Mo Steel
International Nuclear Information System (INIS)
Corrosion fatigue crack propagation study was performed on a high toughness and high strength 3Ni-Cr-Mo steel(or DS-100 steel) which has been developed for the deep sea structural materials. Since multipass welding in generally applied for DS-100 steel, corrosion properties on weldment are the key factors to determine design limit. In this study, the crack propagation behavior was investigated in terms of welding process, heat input rate, and electrode potential. Corrosion fatigue crack propagation rates of DS-100 steel base and weld metals were faster when tested at-1100mV than at any other potentials. These results clearly demonstrate that DS-100 steel is sensitive to the hydrogen embrittlement. Compressive residual stress (CRS) developed during multipass welding played an improtant role to determine the fatigue crack propagation rate(FCPR) in the stress intensity range of low to medium. The greater the CRS, the slower the FCPR. In the range of high stress intensity factor, however, the effect of metallographic parameters on FCPR became dominant over the effect of both CRS and hydrogen
Directory of Open Access Journals (Sweden)
Guian Qian
2013-07-01
Full Text Available The influence of environmental medias on crack propagation of a structural steel at high and very-high-cycle fatigue (VHCF regimes is investigated based on the fatigue tests performed in air, water and 3.5% NaCl aqueous solution. Crack propagation mechanisms due to different crack driving forces are investigated in terms of fracture mechanics. A model is proposed to study the relationship between fatigue life, applied stress and material property in different environmental medias, which reflects the variation of fatigue life with the applied stress, grain size, inclusion size and material yield stress in high cycle and VHCF regimes. The model prediction is in good agreement with experimental observations.
Energy Technology Data Exchange (ETDEWEB)
Junru, Yang; Chuanjuan, Song; Minglan, Wang; Yeukan, Zhang; Jing, Sun [College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao (China)
2016-01-15
The interface crack propagation problem in the cermet cladding material structure was studied. A comparative propagation property parameter (CP) suitable to judge the propagation direction of the interface crack in the cermet cladding material structure was proposed. The interface crack propagation criterion was established. Theoretical models of the CPs for the crack normal to and dwelling on the interface deflecting separately into the clad, the interface and the substrate were built, and the relations between the CPs and the load action angle, the clad thickness ratio and the load were investigated with an example. The research results show that, under the research conditions, the interface crack will more easily propagate into the clad layer than into the substrate.
International Nuclear Information System (INIS)
The interface crack propagation problem in the cermet cladding material structure was studied. A comparative propagation property parameter (CP) suitable to judge the propagation direction of the interface crack in the cermet cladding material structure was proposed. The interface crack propagation criterion was established. Theoretical models of the CPs for the crack normal to and dwelling on the interface deflecting separately into the clad, the interface and the substrate were built, and the relations between the CPs and the load action angle, the clad thickness ratio and the load were investigated with an example. The research results show that, under the research conditions, the interface crack will more easily propagate into the clad layer than into the substrate
Overcoming the brittleness of glass through bio-inspiration and micro-architecture
Mirkhalaf, M.; Dastjerdi, A. Khayer; Barthelat, F.
2014-01-01
Highly mineralized natural materials such as teeth or mollusk shells boast unusual combinations of stiffness, strength and toughness currently unmatched by engineering materials. While high mineral contents provide stiffness and hardness, these materials also contain weaker interfaces with intricate architectures, which can channel propagating cracks into toughening configurations. Here we report the implementation of these features into glass, using a laser engraving technique. Three-dimensional arrays of laser-generated microcracks can deflect and guide larger incoming cracks, following the concept of ‘stamp holes’. Jigsaw-like interfaces, infiltrated with polyurethane, furthermore channel cracks into interlocking configurations and pullout mechanisms, significantly enhancing energy dissipation and toughness. Compared with standard glass, which has no microstructure and is brittle, our bio-inspired glass displays built-in mechanisms that make it more deformable and 200 times tougher. This bio-inspired approach, based on carefully architectured interfaces, provides a new pathway to toughening glasses, ceramics or other hard and brittle materials.
International Nuclear Information System (INIS)
In present work, after finding the anisotropy resulting in dissimilar properties in different orientations of a thick-walled cylinder, experimental and numerical study was performed to reveal the fatigue crack growth behavior of the cylinder under cyclic hoop stress. Fatigue crack growth experiments were conducted on middle tension M(T) samples prepared in an orientation to simulate the hoop stress on the cylinder. The tests were conducted under constant amplitude loading at R ratio 0.1. The fatigue crack growth data was compiled and applied to simulate and predict the crack growth process using two dimensional parametric finite element technique. The fatigue crack propagation was simulated, based on linear elastic fracture mechanics and stress intensity factor determination. Both the experimental and numerical results of crack growth data, at stress levels of 10 to 40 percent of the yield stress of the material, were found in close agreement. The disparity observed was concluded in the range of statistical scatter in the experimental data. The crack growth rate and the fatigue life of the samples obtained from the experiments and the simulation were also in good agreement at all the stress levels analyzed. (author)
International Nuclear Information System (INIS)
In present work, after finding the anisotropy resulting in dissimilar properties in different orientations of a thick-walled cylinder, experimental and numerical study was performed to reveal the fatigue crack growth behavior of the cylinder under cyclic hoop stress. Fatigue crack growth experiments were conducted on middle tension M(T) samples prepared in an orientation to simulate the hoop stress on the cylinder. The tests were conducted under constant amplitude loading at R ratio 0.1. The fatigue crack growth data was compiled and applied to simulate and predict the crack growth process using two dimensional parametric finite element technique. The fatigue crack propagation was simulated, based on linear elastic fracture mechanics and stress intensity factor determination. Both the experimental and numerical results of crack growth data, at stress levels of 10 to 40 per cent of the yield stress of the material, were found in close agreement. The disparity observed was concluded in the range of statistical scatter in the experimental data. The crack growth rate and the fatigue life of the samples obtained from the experiments and the simulation were also in good agreement at all the stress levels analyzed
Schiøtz, J; Carlsson, A E
1997-01-01
We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60$^\\circ$ corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughne...
International Nuclear Information System (INIS)
The integrity of the reactor pressure vessel (RPV) must be guaranteed for all failure and loading conditions which may occur during operation and accidents. The impact of a pressurized thermal shock on postulated cracks is to be considered as one of the worst cases. In a loss-of-coolant accident with injection of cooling water such loads are to be expected. It was the essential aim of these investigations to qualify the knowledge of the temperature gradients, of the flow paths in the medium, of the stresses in the structure, of the crack loading and of the stable crack growth of the RPV-material. A result of that is high thermal stresses in wall areas adjacent to the inner surface and great thermal stress gradients in the wall. Critical areas for pressurized thermal shock transients are particularly cracked zones. Then the secondary thermal stresses can considerably increase the loading at the crack tip relative to the primary mechanical stresses resulting from the internal pressure, on the basis of the cooling process the toughness and the fracture toughness of the ferritic material will be reduced. In the Federal Republic of Germany extensive thermal shock examinations are being conducted at a decommissioned superheated steam reactor (HDR). Since at this real component the parameter variation in a wide spectrum is not possible, especially with regard to the material toughness, such parameter variations are being conducted at the MPA Stuttgart in connection with the research project emergency cooling simulation (NKS)
Energy Technology Data Exchange (ETDEWEB)
Shin, Jeong Woo [Korea Aerospace Research Institute, Daejeon (Korea, Republic of); Lee, Young Shin [Chungnam National University, Daejeon (Korea, Republic of)
2011-10-15
The dynamic propagation of an interface crack between two functionally graded material (FGM) layers under anti-plane shear is analyzed using the integral transform method. The properties of the FGM layers vary continuously along their thicknesses. The properties of the two FGM layers vary and the two layers are connected weak-discontinuously. A constant velocity Yoffe-type moving crack is considered. The Fourier transform is used to reduce the problem to a dual integral equation, which is then expressed to a Fredholm integral equation of the second kind. Numerical values on the dynamic energy release rate (DERR) are presented for the FGM to show the effect of the gradient of material properties, crack moving velocity, and thickness of FGM layers. The following are helpful to increase resistance to interface crack propagation in FGMs: a) increasing the gradient of material properties, b) an increase of shear modulus and density from the interface to the upper and lower free surface, and c) increasing the thickness of the FGM layer. The DERR increases or decreases with increase of the crack moving velocity.
International Nuclear Information System (INIS)
The dynamic propagation of an interface crack between two functionally graded material (FGM) layers under anti-plane shear is analyzed using the integral transform method. The properties of the FGM layers vary continuously along their thicknesses. The properties of the two FGM layers vary and the two layers are connected weak-discontinuously. A constant velocity Yoffe-type moving crack is considered. The Fourier transform is used to reduce the problem to a dual integral equation, which is then expressed to a Fredholm integral equation of the second kind. Numerical values on the dynamic energy release rate (DERR) are presented for the FGM to show the effect of the gradient of material properties, crack moving velocity, and thickness of FGM layers. The following are helpful to increase resistance to interface crack propagation in FGMs: a) increasing the gradient of material properties, b) an increase of shear modulus and density from the interface to the upper and lower free surface, and c) increasing the thickness of the FGM layer. The DERR increases or decreases with increase of the crack moving velocity
International Nuclear Information System (INIS)
This PhD thesis is a study of dynamic cleavage crack propagation and arrest in a PWR steel. It is first recalled that a good understanding of phenomena involved in dynamic fracture mechanics implies good experimental data as well as an efficient numerical tool. The extended Finite Element Method is implanted in the French software Cast3M. It enables to simulate crack growth without any re-meshing. Two techniques are proposed: the level set functions update on an auxiliary grid, and the non-conforming partitioning integration to avoid fields projection in plasticity case. Experimental fracture tests are performed on three configurations: CT specimen, ring under compression in both mode I and mixed mode. Crack speed is measured. Fractography assign cleavage as responsible of fracture. A propagation model based on the principal stress evaluated at the crack tip is identified. Critical cleavage stress is found to depend on rate of phenomena. This model permits to predict accurately by numerical simulation, the crack behavior which is observed experimentally. (author)
In-situ observation of crack propagation at the interface in SiC and W joining by HVEM
International Nuclear Information System (INIS)
SiC/SiC composites are candidate materials for fusion applications due to their potential to retain strength and exhibit tough behavior at elevated temperatures. The irradiation stability of monolithic beta-SiC has been well studied and the mechanical property evaluation of SiC/SiC composites has been started under the standard test method of Continuous Fiber reinforced Ceramic Composites (CFCC's). It is important to know the initiation, coalescence and growth of crack in SiC/SiC composites. However there are no good tests for measuring the crack propagation at fracture of SiC/SiC composites than before. After mechanical testing, microstructure analysis of fracture region in SiC/SiC composites by Focused Ion Beam (FIB) was done, though the artifact during TEM specimen preparation would be introduced sometime. Recently we successfully developed a piezo driven nano indenting equipment for observation of crack propagation in SiC/SiC under the irradiation by High Voltage Electron Microscope (HVEM). Preliminary result of in-situ observation of shear crack propagation at the interface between SiC fiber and SiC matrix by HVEM shows a good agreement with the result of out situ experiment by using the cross section TEM specimen at the shear fracture interface between SiC fiber and SiC matrix prepared by FIB after the fiber pushing out testing by the nano indenter. The shear crack initiated and propagated at the interface between SiC matrix and carbon coated layer on the SiC fiber. (authors)
Hydrogen-induced cracking of commercial purity titanium
International Nuclear Information System (INIS)
Slow straining of compact tension specimens of commercial-purity titanium has been employed for assessing the likelihood of failure due to hydrogen pick-up in titanium containers for nuclear fuel waste disposal. Results indicate that slow crack growth occurs by a ductile tearing process at low hydrogen concentrations. No fast crack growth occurs at such hydrogen levels, apparently because ductile collapse relaxes the stresses and prevents the attainment of a sufficiently high stress intensity for fast crack initiation. Fast propagation of a brittle crack was observed only at hydrogen concentrations above a critical value that depended upon the material involved and the orientation of the crack relative to the manufactured microstructure. An empirical relationship suggests that the higher the strength of the titanium involved the lower is the critical hydrogen level for brittle failure. Both the distribution of residual β-phase and the texture of the fabricated material influence the susceptibility of a particular specimen orientation to fast fracture. (author)
Telesman, J.; Antolovich, S. D.
1986-01-01
An investigation of the fatigue crack propagation FCP behavior of two aluminum alloys is performed to simulate spectrum loading conditions found at critical locations in high performance fighter aircraft. Negative loads are shown to be eliminated for the tension-compression spectrum for low to intermediate maximum stress intensities, and load interactions are found to be more significant at higher stress intensities and with more plasticity at the crack tip. In the second part, the influence of microstructural features including grain size, inclusions, and dispersoids on constant amplitude and spectrum crack growth behavior in aluminum alloys is studied. At low stress intensities the I/M alloy demonstrated better FCP resistance than the P/M 7091 alloy for both constant amplitude and spectrum testing, and the inhomogeneous planar slip and large grain size of 7050 limit dislocation interactions, thereby improving FCP performance.
International Nuclear Information System (INIS)
Stress and displacement fields for a propagating crack in a Functionally Gradient Material (FGM) which has shear modulus as μ=μ0(1+ζX) are derived. The equations of motion in FGM which is nonhomogeneous material are different form those of homogeneous material. The stress intensity factors in stress fields have influence on odd terms of γn/2-1(n=1,3,5,...,) but stress at crack tip only retains term of γ-1/2, where the γ is a radius of cylindrical coordinates centered at crack tip. When the FGM constant ζ is zero or γ→0, the fields for FGM are almost same as the those for isotropic material
DEFF Research Database (Denmark)
Sonne, Mads Rostgaard; Carlone, P.; Citarella, R.;
2015-01-01
This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. An integrated FEM-DBEM procedure for the simulation of crack propagation is proposed and discussed. A numerical FEM model of...... the welding process of precipitation hardenable AA2024-T3 aluminum alloy is employed to infer the process induced residual stress field. The reliability of the FEM simulations with respect to the induced residual stresses is assessed comparing numerical outcomes with experimental data obtained by...... means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen. Numerical results are compared with experimental data showing good agreement and...
Institute of Scientific and Technical Information of China (English)
WAN Zhen-ping; LIU Ya-jun; TANG Yong; YE Bang-yan
2006-01-01
With more and more applications of glass in advanced fields of science,the demand for glass machining precision has increased greatly.More and more attention is being paid to glass cutting because precise glass parts with various shapes can be obtained at high efficiency and low cost.To improve the machining precision of part surfaces and to facilitate tool design and cutting parameter selection,the initiation and propagation laws of glass cracks in specimens subjected to normal loading by symmetric wedges were investigated.Research results show that initiation and propagation laws are the same with interior symmetric wedge angles of 30°-120°,while the laws are different with interior symmetric wedge angles equal to or more than ≥150°.The relationship between medial crack length and normal loading was also investigated when specimens were indented by symmetrical wedges with interior angles of 30° -120°.
International Nuclear Information System (INIS)
Because duplex stainless steel shows the good strength and corrosion resistance properties, the necessity of duplex stainless steel, which has long life in severe environments, has been increased with industrial development. The fatigue crack propagation behavior of Heat Affected Zone(HAZ) has been investigated in super duplex stainless steel. The fatigue crack propagation rate of HAZ of super duplex stainless steel was faster than that of base metal of super duplex stainless steel. We also analysed acoustic emission signals during the fatigue test with time-frequency analysis method. According to the results of time-frequency analysis, the frequency ranges of 200-400 kHz were obtained by striation and the frequency range of 500 kHz was obtained due to dimple and separate of inclusion
Wang, Julia; Kaplan, Jonah A; Colson, Yolonda L; Grinstaff, Mark W
2016-02-18
The concept of using crack propagation in polymeric materials to control drug release and its first demonstration are reported. The composite drug delivery system consists of highly-textured superhydrophobic electrosprayed microparticle coatings, composed of biodegradable and biocompatible polymers poly(caprolactone) and poly(glycerol monostearate carbonate-co-caprolactone), and a cellulose/polyester core. The release of entrapped agents is controlled by the magnitude of applied strain, resulting in a graded response from water infiltration through the propagating patterned cracks in the coating. Strain-dependent delivery of the anticancer agents cisplatin and 7-ethyl-10-hydroxycamptothecin to esophageal cancer cells (OE33) in vitro is observed. Finally the device is integrated with an esophageal stent to demonstrate delivery of fluorescein diacetate, using applied tension, to an ex vivo esophagus. PMID:26804182
International Nuclear Information System (INIS)
stress corrosion cracking of austenitic stainless steels (SS) and to quantify the effect on the crack propagation rate, an experimental research program was performed using cold and warm worked 304, 316L and 347 SS. Stress corrosion crack growth rate tests, under BWR and PWR environments have been carried out. The results obtained have permitted to determine the yield strength effect in the crack propagation of austenitic stainless steels in PWR and BWR conditions. In addition, similarities on cold work and radiation hardening in enhancing the yield strength and the stress corrosion cracking propagation at high temperature water have been evaluated. (authors)
Behavior of Fatigue Crack Propagation for Grade 2 Titanium on the Hydride Formation
International Nuclear Information System (INIS)
Fatigue crack growth tests were carried out to investigate the crack growth behavior of hydrided ASTM Grade 2 Titanium plate at 25 .deg. C and 300 .deg. C. The hydride in the α type titanium was formed by ingressing H2 gas(100-300mbar) at 300 .deg. C-500 .deg. C. Fatigue crack growth rates of the Ti specimens at 300 .deg. C were higher than the growth rates at room temperature. The fatigue crack growth rates along the rolling direction were always higher than those of along the transverse rolling direction. The fatigue crack growth rate of hydrogen charged Ti was strongly dependent on the crack plane orientation in the rolled sheet and the arrangement of hydrides relative to the crack growth direction
International Nuclear Information System (INIS)
The fatigue crack propagation and fracture toughness of a Ni-free Zr60.14Cu22.31Fe4.85Al9.7Ag3 bulk metallic glass (BMG) have been studied by using single edge notched beam (SE(B)) samples prepared from the as-cast BMG plates. It is shown that the metallic glass follows similar fatigue crack propagation behavior to traditional ductile polycrystalline metals, i.e., exhibiting “S” shape of da/dN ∼ ΔK curve with a fatigue threshold (ΔKth) of 3.71 MPa√m and a Paris law exponent of 3.52 in Paris regime. The fatigue-fractured surfaces show universal fatigue striations at different stress intensity levels. The striation spacing (d) fits well with the fatigue crack propagation rate (da/dN) in high stress intensity regime, but deviates significantly from crack propagation rate in low stress intensity regime, i.e., d is larger than da/dN, reflecting that the crack propagation in low stress intensity range was driven by the accumulation of damage from a number of cyclic loadings. A shear slip pair model based on STZ theory and linear-elastic-fracture mechanics (LEFM) analysis are established to interpret the mechanism of fatigue striation formation and crack propagation of the BMG. In addition, the fracture toughness of the BMG was also studied by three-point bending test of pre-cracked samples, which yields a high KJ value of 116.7 MPa√m. The high fracture toughness is believed to be related to the high Poisson’s ratio (ν = 0.368) of the BMG, which helps to the formation of abundant shear bands in front of crack tip and to the retardation of crack propagation by crack path deflection
G. Klimaytys, A.P. Jivkov, D.L. Engelberg
2013-01-01
Exposure of Alloy 82 welds to hydrogen containing, de-oxygenated aqueous environments at temperatures below 150°C can result in a reduction of fracture toughness. The work reported in this paper aims to provide a better understanding of the effect of grain boundary micro- and meso-structure on Low Temperature Crack Propagation (LTCP) susceptibility. Grain boundary morphology of an Alloy 82 weld microstructure was characterised using an image analysis routine, and microstructurally-faithful gr...
O'Hara, P.; Hollkamp, J.; Duarte, C. A.; Eason, T.
2016-01-01
This paper presents a two-scale extension of the generalized finite element method (GFEM) which allows for static fracture analyses as well as fatigue crack propagation simulations on fixed, coarse hexahedral meshes. The approach is based on the use of specifically-tailored enrichment functions computed on-the-fly through the use of a fine-scale boundary value problem (BVP) defined in the neighborhood of existing mechanically-short cracks. The fine-scale BVP utilizes tetrahedral elements, and thus offers the potential for the use of a highly adapted fine-scale mesh in the regions of crack fronts capable of generating accurate enrichment functions for use in the coarse-scale hexahedral model. In this manner, automated hp-adaptivity which can be used for accurate fracture analyses, is now available for use on coarse, uniform hexahedral meshes without the requirements of irregular meshes and constrained approximations. The two-scale GFEM approach is verified and compared against alternative approaches for static fracture analyses, as well as mixed-mode fatigue crack propagation simulations. The numerical examples demonstrate the ability of the proposed approach to deliver accurate results even in scenarios involving multiple discontinuities or sharp kinks within a single computational element. The proposed approach is also applied to a representative panel model similar in design and complexity to that which may be used in the aerospace community.
Chongchong, Li; Lihong, Dong; Haidou, Wang; Guolu, Li; Binshi, Xu
2016-05-01
Monitoring fatigue crack propagation behavior of ferromagnetic components is very important. In this paper, the tension-tension fatigue tests of center cracked tension (CCT) specimens were carried out; the variation regularity of both tangential and normal components of magnetic signals during fatigue process were investigated. The results showed that the initial abnormal signals which appeared at the notch were reversed after cyclic loading. The abnormal magnetic signals became more significant with the increase of fatigue cycles and reversed again after failure. The characteristic parameters, i.e., the peak value of tangential component, Btp, and maximum gradient value of normal component, Km, showed similar variation trends during the fatigue process, which can be divided into three different stages. An approximate linear relationship was found between the characteristic parameters and fatigue crack length 2a. The feasibility of predicting the fatigue crack propagation using the abnormal magnetic signals was discussed. What's more, the variation and distribution of the magnetic signals were also analyzed based on the theory of magnetic charge.
Qiu, Lei; Yuan, Shenfang; Bao, Qiao; Mei, Hanfei; Ren, Yuanqiang
2016-05-01
For aerospace application of structural health monitoring (SHM) technology, the problem of reliable damage monitoring under time-varying conditions must be addressed and the SHM technology has to be fully validated on real aircraft structures under realistic load conditions on ground before it can reach the status of flight test. In this paper, the guided wave (GW) based SHM method is applied to a full-scale aircraft fatigue test which is one of the most similar test status to the flight test. To deal with the time-varying problem, a GW-Gaussian mixture model (GW-GMM) is proposed. The probability characteristic of GW features, which is introduced by time-varying conditions is modeled by GW-GMM. The weak cumulative variation trend of the crack propagation, which is mixed in time-varying influence can be tracked by the GW-GMM migration during on-line damage monitoring process. A best match based Kullback–Leibler divergence is proposed to measure the GW-GMM migration degree to reveal the crack propagation. The method is validated in the full-scale aircraft fatigue test. The validation results indicate that the reliable crack propagation monitoring of the left landing gear spar and the right wing panel under realistic load conditions are achieved.
Liu, X.; Y Luo, Y.; Wang, Z. W.
2014-03-01
As an important component of the blade-control system in Kaplan turbines, piston rods are subjected to fluctuating forces transferred by the turbines blades from hydraulic pressure oscillations. Damage due to unsteady hydraulic loads might generate unexpected down time and high repair cost. In one running hydropower plant, the fracture failure of the piston rod was found twice at the same location. With the transient dynamic analysis, the retainer ring structure of the piston rod existed a relative high stress concentration. This predicted position of the stress concentration agreed well with the actual fracture position in the plant. However, the local strain approach was not able to explain why this position broke frequently. Since traditional structural fatigue analyses use a local stress strain approach to assess structural integrity, do not consider the effect of flaws which can significantly degrade structural life. Using linear elastic fracture mechanism (LEFM) approaches that include the effect of flaws is becoming common practice in many industries. In this research, a case involving a small semi-ellipse crack was taken into account at the stress concentration area, crack growth progress was calculated by FEM. The relationship between crack length and remaining life was obtained. The crack propagation path approximately agreed with the actual fracture section. The results showed that presence of the crack had significantly changed the local stress and strain distributions of the piston rod compared with non-flaw assumption.
Directory of Open Access Journals (Sweden)
Roberto Tovo
2016-03-01
Full Text Available This paper reports results of fatigue tests of friction stir welded (FSW aluminium tubes. Relatively small 38 mm diameter tubes were used and hence an automated FSW process using a retracting tool was designed for this project, as the wall thickness of the aluminium tube was similar to the diameter of the FSW tool. This is a more complex joint geometry to weld than the more usual larger diameter tube reported in the literature. S-N fatigue testing was performed using load ratios of R = 0.1 and R = -1. Crack path analysis was performed using both low magnification stereo microscopy and scanning electron microscopy, in order to identify crack initiation sites and to determine the direction of crack propagation. Work is still in progress to follow the crack path through the various microstructural zones associated with the weld. A simple statistical analysis was used to characterize the most typical crack initiation site. This work forms part of a wider project directed at determining multiaxial fatigue design rules for small diameter 6082-T6 aluminium tubes that could be of use in the ground vehicle industry.
International Nuclear Information System (INIS)
As an important component of the blade-control system in Kaplan turbines, piston rods are subjected to fluctuating forces transferred by the turbines blades from hydraulic pressure oscillations. Damage due to unsteady hydraulic loads might generate unexpected down time and high repair cost. In one running hydropower plant, the fracture failure of the piston rod was found twice at the same location. With the transient dynamic analysis, the retainer ring structure of the piston rod existed a relative high stress concentration. This predicted position of the stress concentration agreed well with the actual fracture position in the plant. However, the local strain approach was not able to explain why this position broke frequently. Since traditional structural fatigue analyses use a local stress strain approach to assess structural integrity, do not consider the effect of flaws which can significantly degrade structural life. Using linear elastic fracture mechanism (LEFM) approaches that include the effect of flaws is becoming common practice in many industries. In this research, a case involving a small semi-ellipse crack was taken into account at the stress concentration area, crack growth progress was calculated by FEM. The relationship between crack length and remaining life was obtained. The crack propagation path approximately agreed with the actual fracture section. The results showed that presence of the crack had significantly changed the local stress and strain distributions of the piston rod compared with non-flaw assumption
International Nuclear Information System (INIS)
The primary aim of this paper is to evaluate the probabilistic fatigue crack propagation models using the residual of a random variable and to present the probabilistic model fit for the probabilistic fatigue crack growth behavior in Mg-Al-Zn alloys under maximum load conditions. The models used in this study were prepared by applying a random variable to empirical fatigue crack propagation models such as the Paris-Erdogan model, Walker model, Forman model, and modified Forman model. It was verified that the good models for describing the stochastic variation of the fatigue crack propagation behavior in Mg-Al-Zn alloys under maximum load conditions were the ‘probabilistic Paris-Erdogan model’ and ‘probabilistic Walker model’. The influence of the maximum load conditions on the stochastic variation of fatigue crack growth is also considered
Energy Technology Data Exchange (ETDEWEB)
Choi, Seon Soon [Sahmyook University, Seoul (Korea, Republic of)
2015-01-15
The primary aim of this paper is to evaluate the probabilistic fatigue crack propagation models using the residual of a random variable and to present the probabilistic model fit for the probabilistic fatigue crack growth behavior in Mg-Al-Zn alloys under maximum load conditions. The models used in this study were prepared by applying a random variable to empirical fatigue crack propagation models such as the Paris-Erdogan model, Walker model, Forman model, and modified Forman model. It was verified that the good models for describing the stochastic variation of the fatigue crack propagation behavior in Mg-Al-Zn alloys under maximum load conditions were the ‘probabilistic Paris-Erdogan model’ and ‘probabilistic Walker model’. The influence of the maximum load conditions on the stochastic variation of fatigue crack growth is also considered.
Piascik, Robert S.; Gangloff, Richard P.
1991-01-01
Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant delta K and K(sub max) loading. The da/dN are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 percent NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depends on delta K(sup 4.0) for the inert gases, water vapor and chloride induced multiple power-laws, and a transition growth rate 'plateau'. Environmental effects are strongest at low delta K. Crack tip damage is ascribed to hydrogen embrittlement because of the following: (1) accelerated da/dN due to part-per-million levels of H2O without condensation; (2) impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening; (3) the lack of an effect of film-forming O2; (4) the likelihood for crack tip hydrogen production in NaCl, and (5) the environmental and delta K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3, and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.
Piascik, Robert S.; Gangloff, Richard P.
1991-01-01
Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant delta K and K(sub max) loading. The da/dN are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness of aqueous 1 percent NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depends on delta K(sup 4.0) for the inert gases, water vapor and chloride induced multiple power-laws, and a transition growth rate 'plateau'. Environmental effects are strongest at low delta K. Crack tip damage is ascribed to hydrogen embrittlement because of the following: (1) accelerated da/dN due to part-per-million levels of H2O without condensation; (2) impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening; (3) the lack of an effect of film-forming O2; (4) the likelihood for crack tip hydrogen production in NaCl; and (5) the environmental and delta K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3, and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.
Theory of tough propagation of a crack in fibrous composite material
International Nuclear Information System (INIS)
The velocity v of thermally activated crack growth is calculated for a composite with a small volume fraction of high-module fibres in a plastic matrix. Account is taken of the repeated breaking of the fibres, the influence of the line tension of the crack front on this breaking, stress concentration at the tip of the crack pushing into the fibre due to the delay of plastic shears on the other side of the fibre in the matrix, and the thermally activated growth of the crack through the matrix by the nucleation of microcracks in front of it. At a certain critical size of crack R=Rsub(c), the tearing and pulling out of fibres from the matrix is replaced by tearing in the plane of the crack with a corresponding increase of dv/dR
Analytical Model for Fictitious Crack Propagation in Reinforced Concrete Beams without Debonding
Ulfkjær, J. P.; Brincker, Rune
1996-01-01
The non-linear fracture mechanical problem of combined crack growth and reinforcement action is modelled by adopting a simplified fictitious crack model for concrete and a linear elastic-plastic action for the reinforcement. The softening relation of the concrete is assumed to be linear, however, the crack growth is further simplified by introducing a continuous layer of springs at the midsection mainly representing a simplified material response around the fracture zone. In the reinforcement...
Multiple crack propagation by DBEM in a riveted butt-joint: a simplified bidimensional approach
Directory of Open Access Journals (Sweden)
R. Citarella
2016-03-01
Full Text Available A Multi-Site Damage (MSD crack growth simulation is presented, carried out by means of Dual Boundary Element Method (DBEM, in a two-dimensional analysis of a cracked butt-joint made of aluminium 2024 T3. An equivalent crack length is proposed for an approximated 2D analysis of a 3D problem where the crack front assumes a part elliptical shape due to secondary bending effects. The assumptions made to perform such simplified bidimensional analyses are validated by comparing numerical results with experimental data, the latter obtained from a fatigue tested riveted butt-joint.
Energy Technology Data Exchange (ETDEWEB)
Ancelet, O
2005-07-01
The incident which has occurred on the Civaux power plant has shown the noxiousness of thermal loading and the difficulty to take it into account at design level. The objective of this report is to study the initiation and the propagation of crack under thermal loading. In this aim the CEA has developed a new experiment named FAT3D. The various experiments carried out showed the harmfulness of a thermal loading, which makes it possible to rapidly initiate a network of cracks and to propagate one (or some) cracks through the totally thickness of the component under certain conditions. These experimental results associated with a mechanical analysis put at fault the usual criteria of damage based on the variations of the equivalent strain. In addition, the study of the propagation stage shows the importance of the plasticity which, in the case of a thermal loading, slows down the propagation of the crack. (author)
Energy Technology Data Exchange (ETDEWEB)
Assire, A
2000-10-13
results. Concerning local approach, intergranular creep damage model of the Ecole des Mines de Paris is used for crack initiation and growth under creep and creep fatigue loading. Crack propagations are simulated with an adaptive re-meshing technique and are compared with experimental results for crack located in weld and base metal. A sensibility analysis shows that crack propagation strongly depend on the characteristic distance, which is needed to simulate crack growth with local approach, for the weld metal parameters. (author)
International Nuclear Information System (INIS)
The crack growth rates during room temperature low-cycle fatigue of two austenitic stainless steels are evaluated through striation space measurements on the fracture surfaces of axisymmetric smooth specimens. The effect of nitrogen interstitials and of previous aging on the initiation and crack propagation phase durations is discussed
Institute of Scientific and Technical Information of China (English)
LU Xia-mei; MA Yun-fei; ZHANG Jia-zhen
2008-01-01
In-situ SEM (Scanning Electron Microscope) observation of fatigue crack propagation in aluminium alloys reveals that crack growth occurs in a continuous way over the time period during the load cycle. Based on this observation, a new parameter da/dS is introduced to describe the fatigue crack propagation rate, which de-fines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship is given between this new parameter and the conventional used parameter da/dN which de-scribes the fatigue crack propagation rate per stress cycle. Using this new parameter, an analysis has been per-formed and a model has been set up to consider the effect of the applied stress ratio on the fatigue crack propagation rate. The obtained results have been used to correlate the published test data and a good correlation has been achieved. This method is very easy to use and no fatigue crack closure measurement is needed, therefore this model is significant in engineering application.
Crack Propagation on ESE(T) Specimens Strengthened with CFRP Sheets
DEFF Research Database (Denmark)
Hansen, Christian Skodborg; Jensen, Peter Holmstrøm; Dyrelund, Jens;
2009-01-01
In this paper fatigue tests on side notched steel test specimens strengthened with adhesive bonded fibre reinforced polymer (FRP) sheets are presented. The specimens are subject to crack growth both in the steel and bond line. Influence of the load ratio and initial crack length on the overall...
Substructuring FE-XFE approaches applied to three-dimensional crack propagation
Wyart, E.; Duflot, M.; Coulon, D.; Martiny, P.; Pardoen, T.; Remacle, J. F.; Lani, F.
2008-06-01
Two substructuring methods are investigated in order to allow for the use of the eXtended Finite Element Method (X-FEM) within commercial finite element (FE) codes without need for modifying their kernel. The global FE problem is decomposed into two subdomains, the safe domain and the cracked domain based on the value of the level sets representing the crack. The safe domain is treated by the host FE software while the cracked domain is treated by an independent XFE code. The first substructuring method consists of calculating the Schur matrix of a cracked super-element with the XFE code. The second technique introduces the finite element tearing and interconnecting method (FETI) which ensures the compatibility of the displacements at the interface between the cracked and safe subdomains. The stiffness matrices and nodal forces are provided by the XFE and FE codes for the cracked and safe subdomains, respectively. The solutions obtained with these two techniques are rigorously equivalent to those computed with the stand-alone XFE code. First, the computational efficiency of the two approaches is demonstrated. Second, a validation is proposed towards comparison with reference values of the stress intensity factors in simple 3D cracked geometries. Finally, this contribution presents an application of the FE-XFE-FETI method to the computation of the stress intensity factor induced by a crack inside a hydraulic cylinder under internal pressure.
Shear crack formation and propagation in fiber reinforced cementitious composites (FRCC)
DEFF Research Database (Denmark)
Paegle, Ieva; Fischer, Gregor
2011-01-01
strain hardening of both FRCC materials was observed under shear loading. The influence of fibers, fiber type, including polyvinyl alcohol (PVA) and polypropylene (PP) fibers, and shear crack angle were investigated. Based upon photogrammetric results, fundamental descriptions of shear crack opening...
Shear crack formation and propagation in fiber reinforced cementitious composites (FRCC)
DEFF Research Database (Denmark)
Paegle, Ieva; Fischer, Gregor
2012-01-01
strain hardening of both FRCC materials was observed under shear loading. The influence of fibers, fiber type, including polyvinyl alcohol (PVA) and polypropylene (PP) fibers, and shear crack angle were investigated. Based upon photogrammetric results, fundamental descriptions of shear crack opening...
Eddy current testing. Evaluation of cracks propagation in austenitic steel cladding
International Nuclear Information System (INIS)
A low frequency eddy current method has been developed to evaluate the ligament between crack front and cladding surface and measure crack length. It uses a large surface probe to obtain a low sensitivity on surface variations and a good penetration of eddy current
Wave propagation in cracked frame structures by the spectral element method
Izadifard, Ramezan Ali; Khaleseh Ranjbar, Reza; Mohebi, Benyamin
2014-09-01
This paper deals with the detection of crack in frame structures based on Euler-Bernoulli beams theorem by the Spectral Element Method. The effect of cracking is modeled using Castigliano's theorem and laws of fracture mechanics as mass-less rotational and translational springs which are embedded in different locations of the steel frame structure in both beam and column. The crack location is revealed precisely without prior knowledge of their positions in frame structure. This means that there is no necessity to know the location and the node number which is assigned to crack. Finally the effect of crack when it is embedded simultaneously in both beam and column members, is also studied.
Steadiness and stop of brittle fracture driven by the forces in different distances
Institute of Scientific and Technical Information of China (English)
和雪松; 李世愚; 滕春凯
2005-01-01
Based on the principle of fracture mechanics, the stop criterion of brittle fracture is proposed and the equation of minimal crack stop is given. By using the zero frequency Green function, the steadiness and stop of brittle fracture driven by the concentrated force and simple distributed forces in different locales are analyzed. The critical loading, unsteady boundary line and location of stop points under some typical conditions are calculated. The steady growth caused by the near forces is significant in interpreting the creep and the forming of some tectonics. Whereas the unsteady propagation caused by the forces in different distances from the crack is significant in interpreting the occurring and stop of earthquakes. It is suggested that the strong earthquakes may be the result of compound of the near-field and far-field forces. The results of this paper are also valuable for investigation of the mechanism of induced earthquake.
Chong, Alexander C M; Miller, Forrest; Buxton, McKee; Friis, Elizabeth A
2007-08-01
Third-generation mechanical analogue bone models and synthetic analogue cortical bone materials manufactured by Pacific Research Laboratories, Inc. (PRL) are popular tools for use in mechanical testing of various orthopedic implants and biomaterials. A major issue with these models is that the current third-generation epoxy-short fiberglass based composite used as the cortical bone substitute is prone to crack formation and failure in fatigue or repeated quasistatic loading of the model. The purpose of the present study was to compare the tensile and fracture mechanics properties of the current baseline (established PRL "third-generation" E-glass-fiber-epoxy) composite analogue for cortical bone to a new composite material formulation proposed for use as an enhanced fourth-generation cortical bone analogue material. Standard tensile, plane strain fracture toughness, and fatigue crack propagation rate tests were performed on both the third- and fourth-generation composite material formulations using standard ASTM test techniques. Injection molding techniques were used to create random fiber orientation in all test specimens. Standard dog-bone style tensile specimens were tested to obtain ultimate tensile strength and stiffness. Compact tension fracture toughness specimens were utilized to determine plane strain fracture toughness values. Reduced thickness compact tension specimens were also used to determine fatigue crack propagation rate behavior for the two material groups. Literature values for the same parameters for human cortical bone were compared to results from the third- and fourth-generation cortical analogue bone materials. Tensile properties of the fourth-generation material were closer to that of average human cortical bone than the third-generation material. Fracture toughness was significantly increased by 48% in the fourth-generation composite as compared to the third-generation analogue bone. The threshold stress intensity to propagate the crack
Hueckel, T.; Hu, M.
2015-12-01
Crack propagation in a subcritically stressed rock subject to chemically aggressive environment is analyzed and numerically simulated. Chemically induced weakening is often encountered in hydraulic fracturing of low-permeability oil/gas reservoirs and heat reservoirs, during storage of CO2 and nuclear waste corroding canisters, and other circumstances when rock matrix acidizing is involved. Upon acidizing, mineral mass dissolution is substantially enhanced weakening the rock and causing crack propagation and eventually permeability changes in the medium. The crack process zone is modeled mathematically via a chemo-plastic coupling and chemo-elastic coupling model. In plasticity a two-way coupling is postulated between mineral dissolution and a yield limit of rock matrix. The rate of dissolution is described by a rate law, but the mineral mass removal per unit volume is also a function of a variable internal specific surface area, which is in turn affected by the micro-cracking (treated as a plastic strain). The behavior of the rock matrix is modeled as rigid-plastic adding a chemical softening capacity to Cam-Clay model. Adopting the Extended Johnson's approximation of processes around the crack tip, the evolution of the stress field and deformation as a function of the chemically enhanced rock damage is modeled in a simplified way. In addition, chemical reactive transport is made dependent on plastic strain representing micro-cracking. Depending on mechanical and chemical boundary conditions, the area of enhanced chemical softening is near or somewhat away from the crack tip.In elasticity, chemo-mechanical effect is postulated via a chemical volumetric shrinkage strain proportional to mass removal variable, conceived analogously to thermal expansion. Two versions are considered: of constant coefficient of shrinkage and a variable one, coupled to deviatoric strain. Airy Potential approach used for linear elasticity is extended considering an extra term, which is
International Nuclear Information System (INIS)
Five of the world's best laboratories at performing stress corrosion crack growth studies - ABB Atom AB, AEA Technology, GE Corporate Research and Development Center, Studsvik Material AB, and VTT Manufacturing Technology, were selected to participate in a round robin to evaluate the quality and reproducibility of testing conditions and resulting stress corrosion crack growth rates in sensitized type 304 stainless steel in 288 deg C water. Heat treated, machined and fatigue pre-cracked specimens were provided to all laboratories, and detailed test procedures prescribed the use of active loading, reversed dc potential drop crack monitoring, a common reference electrode supplied to all laboratories by GE CRD (to be used along side each laboratory's own reference electrode), and highly specified water chemistry conditions. The ability of each laboratory to achieve optimal testing conditions varied, although all laboratories achieved an impressive standard of testing control. The most significant laboratory-to-laboratory differences were associated with their ability to achieve high purity autoclave outlet water, reproduce accurate measurements of corrosion potential on the test specimen, and provide high resolution crack following using a reversed dc potential drop. However, the most notable outcome of the program is the consistent observation by all laboratories that initiating and sustaining stress corrosion crack growth at constant load in sensitized type 304 stainless steel is difficult, despite the use of a moderately high stress intensity, and high dissolved oxygen and corrosion potential conditions. Concerns for specimen machining and pre-cracking were identified, although these factors were not the sole cause of difficulty in initiating and sustaining stress corrosion cracking. It was shown that many phases of specimen preparation and testing can have a large influence on the measured SCC response. Even under the best test conditions it is critical to ensure
International Nuclear Information System (INIS)
Microstructural refinement of structural materials generally improves their tensile properties but deteriorates their fatigue properties. However, pipeline steels with ultra-fine acicular ferrite (UFAF) possess not only high strength and toughness, but also a low fatigue-crack-growth rate (FCGR) and long fatigue-propagation life. In this paper, the micro-fracture mechanisms of an UFAF pipeline steel are investigated by in situ tensile testing in a transmission electron microscope. The results indicate that a grain-boundary-film structure composed of martensite/austenite could significantly influence the crack propagating behavior in the UFAF steel, consequently lowering the FCGR by enhancing roughness-induced crack closure during cyclic loading
Gubeljak, N.; Predan, J.; Senčič, B.; Chapetti, M. D.
2016-03-01
An integrated fracture mechanics approach is proposed to account for the estimation of the fatigue resistance of component. Applications, estimations and results showed very good agreements with experimental results. The model is simple to apply, accounts for the main geometrical, mechanical and material parameters that define the fatigue resistance, and allows accurate predictions. It offers a change in design philosophy: It could be used for design, while simultaneously dealing with crack propagation thresholds. Furthermore, it allows quantification of the material defect sensitivity. In the case of the set of fatigue tests carried out by rotational bending of specimens without residual stresses, the estimated results showed good agreement and that an initial crack length of 0.5 mm can conservatively explain experimental data. In the case of fatigue tests carried out on the springs at their final condition with bending at R = 0.1 our data shows the influence of compressive residual stresses on fatigue strength. Results also showed that the procedures allow us to analyze the different combinations of initial crack length and residual stress levels, and how much the fatigue resistance can change by changing that configuration. For this set of tests, the fatigue resistance estimated for an initial crack length equal to 0.35 mm, can explain all testing data observed for the springs.
Experimental simulation of frost wedging-induced crack propagation in alpine rockwall
Jia, Hailiang; Leith, Kerry; Krautblatter, Michael
2016-04-01
Frost wedging is widely presumed to be the principal mechanism responsible for shattering jointed low-porosity rocks in high alpine rockwalls. The interaction of ice and rock physics regulates the efficacy of frost wedging. In order to better understand temporal aspects of this interaction, we present results of a series of laboratory experiments monitoring crack widening as a result of ice formation in an artificial crack (4mm wide, 80mm deep) cut 20 mm from the end of a rectangular granite block. Our results indicate that i) freezing direction plays a key role in determining the magnitude of crack widening; in short-term (1 day) experiments, maximum crack widening during top-down freezing (associated with 'autumn' conditions) was around 0.11mm, while inside-out freezing (resulting from 'spring' conditions) produced only 0.02 mm of deformation; ii) neither ice, nor water pressure (direct tension and hydraulic fracturing respectively) caused measurable irreversible crack widening during short-term tests, as the calculated maximum stress intensity at the crack tip was less than the fracture toughness of our granite sample; iii) development of ice pressure is closely related to the mechanical properties of the fracture in which it forms, and as such, the interaction of ice and rock is intrinsically dynamic; iv) irreversible crack widening (about 0.03mm) was only observed following a long-term (53 day) experiment representing a simplified transition from autumn to winter conditions. We suggest this is the result of stress corrosion aided by strong opening during freezing, and to a lesser degree by ice segregation up to one week after the initial freezing period, and downward migration of liquid water during the remainder of the test. Our results suggest the fundamental assumption of frost wedging, that rapid freezing from open ends of cracks can seal water inside the crack and thus cause damage through excessive stresses induced by volumetric expansion seems
Energy Technology Data Exchange (ETDEWEB)
Henninger, C
2007-11-15
Many material assemblies subjected to thermo-mechanical loadings develop thermal residual stresses which modify crack onset conditions. Besides if one of the components has a plastic behaviour, plastic residual deformations may also have a contribution. One of the issues in brittle fracture mechanics is to predict crack onset without any pre-existing defect. Leguillon proposed an onset criterion based on both a Griffth-like energetic condition and a maximum stress criterion. The analysis uses matched asymptotics and the theory of singularity. The good fit between the model and experimental measurements led on homogeneous isotropic materials under pure mechanical loading incited us to take into account residual stresses in the criterion. The comparison between the modified criterion and the experimental measurements carried out on an aluminum/epoxy assembly proves to be satisfying concerning the prediction of failure of the interface between the two components. Besides, it allows, through inversion, identifying the fracture properties of this interface. The modified criterion is also applied to the delamination of the tile/structure interface in the plasma facing components of the Tore Supra tokamak. Indeed thermal and plastic residual stresses appear in the metallic part of these coating tiles. (author)
Institute of Scientific and Technical Information of China (English)
S.H. Dong; Y.M. Lu; Y. Zhang; Q. Wu
2001-01-01
Based on the theory of hydrogen enhanced localized plasticity of the hydrogen induced cracking and considered the effect of residual stress produced by eliminated stress heattreatment, a fractal model of hydrogen induced cracking was presented, and a relationship among the effective surface energy T( H), fractal dimension D and stress intensity factor of hydrogen induced cracking, KIH, for welding pipeline under hydrogen environment was set up, from which the relationship of D and KIsCC is obtained. The model was verified experimentally to be correct.
International Nuclear Information System (INIS)
The effect of oxidation behavior on the corrosion fatigue crack initiation and propagation of 316LN austenitic stainless steel (SS) in 320 °C water and air environments was investigated by corrosion fatigue cracking test system, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nanoindentation. The experimental results indicated that the cracks were found to be initiated at first 20% of fatigue life of the specimens tested in 320 °C water, while only a few cracks were found until fracture when tested in 320 °C air. The crack propagation rates of the specimens tested in 320 °C water were obviously faster than those tested in 320 °C air especially at lower strain amplitude of ±0.5%. This can be attributed to the oxidation behavior of the 316LN SS specimens tested in 320 °C water was more serious than those tested in 320 °C air. The compositions were more complex of the oxide films formed in 320 °C water than those tested in 320 °C air, which lead to crack initiation easier in the former. Moreover, the 320 °C water made the mechanical properties of the oxide films worse, resulting in a faster fatigue crack propagation rate at crack tip
Energy Technology Data Exchange (ETDEWEB)
Wu, H.C. [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China); Yang, B., E-mail: byang@ustb.edu.cn [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China); Collaborative Innovation Center of Universal Iron & Steel Technology, Beijing 100083 (China); Wang, S.L.; Zhang, M.X. [State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 (China)
2015-05-01
The effect of oxidation behavior on the corrosion fatigue crack initiation and propagation of 316LN austenitic stainless steel (SS) in 320 °C water and air environments was investigated by corrosion fatigue cracking test system, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nanoindentation. The experimental results indicated that the cracks were found to be initiated at first 20% of fatigue life of the specimens tested in 320 °C water, while only a few cracks were found until fracture when tested in 320 °C air. The crack propagation rates of the specimens tested in 320 °C water were obviously faster than those tested in 320 °C air especially at lower strain amplitude of ±0.5%. This can be attributed to the oxidation behavior of the 316LN SS specimens tested in 320 °C water was more serious than those tested in 320 °C air. The compositions were more complex of the oxide films formed in 320 °C water than those tested in 320 °C air, which lead to crack initiation easier in the former. Moreover, the 320 °C water made the mechanical properties of the oxide films worse, resulting in a faster fatigue crack propagation rate at crack tip.
Finite-element blunt-crack propagation: a modified J-integral approach
International Nuclear Information System (INIS)
In assessing the safety of a liquid metal fast breeder reactor (LMFBR), a major concern is the behavior of concrete structures subjected to high temperatures. The potential of concrete cracking is an important parameter which could significantly influence the safety assessment of thermally attacked concrete. A new modified J-integral approach for the blunt crack model has been derived to provide a general procedure to accurately predict the direction of crack growth. This formulation has been incorporated into the coupled heat transfer-stress analysis finite element code TEMP-STRESS. A description of the formulation is presented in this paper. Results for the problems of a Mode I and mixed mode crack in a plate using regular and slanted meshes subjected to uniaxial and shear loading are presented
The use of COD and plastic instability in crack propagation and arrest in shells
Erdogan, F.; Ratwani, M.
1974-01-01
The initiation, growth, and possible arrest of fracture in cylindrical shells containing initial defects are dealt with. For those defects which may be approximated by a part-through semi-elliptic surface crack which is sufficiently shallow so that part of the net ligament in the plane of the crack is still elastic, the existing flat plate solution is modified to take into account the shell curvature effect as well as the effect of the thickness and the small scale plastic deformations. The problem of large defects is then considered under the assumptions that the defect may be approximated by a relatively deep meridional part-through surface crack and the net ligament through the shell wall is fully yielded. The results given are based on an 8th order bending theory of shallow shells using a conventional plastic strip model to account for the plastic deformations around the crack border.
Institute of Scientific and Technical Information of China (English)
无
2001-01-01
The stress-induced martensitic transformation and its relation with crack nucleation and propagation in CuNiAl shape memory alloy were investigated through in-situ tensile tests by SEM and TEM.The results indicated that the stress concentration ahead of the crack tip could induce formation of stacking faults and different types of martensites.TEM observations showed that the martensites could transform from one type to another and even reversely to the parent during loading.The micro-cracks nucleated along the martensite/parent interface and intersection between two martensites.When the crack propagated a certain distance,the stress concentration ahead of the crack tip was large enough to result in formation of slip bands,and in this condition the microcrack nucleated along slip bands more easily.
Wide plate crack arrest testing
International Nuclear Information System (INIS)
To predict the behavior of a nuclear pressure vessel undergoing pressurized thermal shock, certain information on dynamic crack propagation and arrest is required. The purpose of the work described is to provide such data on wide plates fracturing at temperatures up to the upper shelf region. Four tests have been completed on the 26 MN Universal Testing Machine at NBS. The specimens are to be fractured in a thermal gradient that, in the most extreme case, might extend from -1000C to 2000 across the 1 meter specimen width. This is done so that the crack will initiate in a cold, brittle region and arrest in a hot, tough region. An important part of this study is data acquisition from the numerous strain gages, thermocouples, timing wires, crack mouth opening displacement gages, and acoustic emission transducers that are mounted on the specimen. Each test has been different with respect to conditions of testing, specimen configuration, and instrumentation used. The progressive changes in test procedure represent attempts to obtain the desired crack run and arrest behavior and to improve upon the quality of the data collected. In particular, efforts were made to initiate crack propagation at lower stress intensity factors. Also, strain gage combinations and locations were optimized to better deduce the crack position as a function of time. Another result of great interest that can be deduced from these tests is the initiation of fracture toughness and the arrest toughness
Fatigue crack propagation rate in EUROFER 97 estimated using small specimens
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Náhlík, Luboš; Ševčík, Martin; Seitl, Stanislav; Kruml, Tomáš; Polák, Jaroslav
2011-01-01
Roč. 452-453, - (2011), s. 325-328. ISSN 1013-9826 R&D Projects: GA ČR GA106/09/1954; GA ČR GA101/09/0867 Institutional research plan: CEZ:AV0Z20410507 Keywords : small fatigue cracks * Eurofer 97 * J-integral * fatigue crack growth rate Subject RIV: JL - Materials Fatigue, Friction Mechanics
Effect of specimen geometry on fatigue crack propagation in threshold region
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Seitl, Stanislav; Kruml, Tomáš
Ottawa : NRCan - CANMET, 2009, s. 1-9. ISBN N. [International Conference on Fracture /12./. Ottawa (CA), 12.07.2009-17.07.2009] R&D Projects: GA ČR GA101/08/1623; GA ČR GA101/09/0867; GA ČR GA106/09/1954 Institutional research plan: CEZ:AV0Z20410507 Keywords : constraint * fatigue crack growth rate * crack closure Subject RIV: JL - Materials Fatigue, Friction Mechanics
Leise, Tanya L.
2009-08-19
We consider the problem of the dynamic, transient propagation of a semi-infinite, mode I crack in an infinite elastic body with a nonlinear, viscoelastic cohesize zone. Our problem formulation includes boundary conditions that preclude crack face interpenetration, in contrast to the usual mode I boundary conditions that assume all unloaded crack faces are stress-free. The nonlinear viscoelastic cohesive zone behavior is motivated by dynamic fracture in brittle polymers in which crack propagation is preceeded by significant crazing in a thin region surrounding the crack tip. We present a combined analytical/numerical solution method that involves reducing the problem to a Dirichlet-to-Neumann map along the crack face plane, resulting in a differo-integral equation relating the displacement and stress along the crack faces and within the cohesive zone. © 2009 Springer Science+Business Media B.V.
International Nuclear Information System (INIS)
The crack initiation and propagation characteristics of two medium grained polygranular graphites, nuclear block graphite (NBG10) and Gilsocarbon (GCMB grade) graphite, have been studied using the Double Torsion (DT) technique. The DT technique allows stable crack propagation and easy crack tip observation of such brittle materials. The linear elastic fracture mechanics (LEFM) methodology of the DT technique was adapted for elastic-plastic fracture mechanics (EPFM) in conjunction with a methodology for directly calculating the J-integral from in-plane displacement fields (JMAN) to account for the non-linearity of graphite deformation. The full field surface displacement measurement techniques of electronic speckle pattern interferometry (ESPI) and digital image correlation (DIC) were used to observe and measure crack initiation and propagation. Significant micro-cracking in the fracture process zone (FPZ) was observed as well as crack bridging in the wake of the crack tip. The R-curve behaviour was measured to determine the critical J-integral for crack propagation in both materials. Micro-cracks tended to nucleate at pores, causing deflection of the crack path. Rising R-curve behaviour was observed, which is attributed to the formation of the FPZ, while crack bridging and distributed micro-cracks are responsible for the increase in fracture resistance. Each contributes around 50% of the irreversible energy dissipation in both graphites.
Assessment of thermal fatigue crack propagation in safety injection PWR lines
International Nuclear Information System (INIS)
Cyclic thermal stratification resulting in alternating thermal stresses in pipe cross sections has been identified as the primary cause of high cycle thermal fatigue failure. A number of piping lines in operating plants around the world, susceptible to thermal stratification, have experienced circumferential cracking as a result of high levels of alternating bending stresses. This paper addresses the mechanisms of crack initiation and crack growth and provides estimates of fatigue cycles to failure for a typical safety injection line with such cyclic load history. Utilizing a 3-D finite element analysis, the temperature profile and the corresponding thermal stress field of a complete thermal cycle in a safety injection line consisting of a horizontal pipe section and an elbow, is obtained. Since the observed cracking occurred in the region of the elbow-to-horizontal pipe weld, the analysis performed assessed (1) the impact of the level of local geometric discontinuities on the initiation of an inside surface flaw is greatest and (2) the number of thermal cycles required to drive a small surface crack through the pipe wall. 12 refs., 14 figs., 2 tabs
Fracture of brittle materials under compression
International Nuclear Information System (INIS)
Kinetics of crack development in plates of brittle materials under uniaxial compression is studied by the calculated-experimental method. Failure diagrams for single sloping cracks as well as for the periodical system of parallel cracks with due regrd for curvikinearity of their trajectory are plotted using the method of singular integral equations. Effect of the crack interaction on the plate failure kinetics is estimated. The calculated data are compared with experimental results obtained on graphite ARW, zirconium carbide, soda-lime and acrylic glass sepcimens with atificial nothes
Universal behaviour in compressive failure of brittle materials.
Renshaw, C E; Schulson, E M
2001-08-30
Brittle failure limits the compressive strength of rock and ice when rapidly loaded under low to moderate confinement. Higher confinement or slower loading results in ductile failure once the brittle-ductile transition is crossed. Brittle failure begins when primary cracks initiate and slide, creating wing cracks at their tips. Under little to no confinement, wing cracks extend and link together, splitting the material into slender columns which then fail. Under low to moderate confinement, wing crack growth is restricted and terminal failure is controlled by the localization of damage along a narrow band. Early investigations proposed that localization results from either the linkage of wing cracks or the buckling of microcolumns created between adjacent wing cracks. Observations of compressive failure in ice suggest a mechanism whereby localization initiates owing to the bending-induced failure of slender microcolumns created between sets of secondary cracks emanating from one side of a primary crack. Here we analyse this mechanism, and show that it leads to a closed-form, quantitative model that depends only on independently measurable mechanical parameters. Our model predictions for both the brittle compressive strength and the brittle-ductile transition are consistent with data from a variety of crystalline materials, offering quantitative evidence for universal processes in brittle failure and for the broad applicability of the model. PMID:11528475
Modeling of Fatigue Crack Propagation in Aluminum Alloys Using an Energy Based Approach
Directory of Open Access Journals (Sweden)
F. Khelil
2013-08-01
Full Text Available Materials fatigue is a particularly serious and unsafe kind of material destruction. Investigations of the fatigue crack growth rate and fatigue life constitute very important and complex problems in mechanics. The understanding of the cracking mechanisms, taking into account various factors such as the load pattern, the strain rate, the stress ratio, etc., is of a first need. In this work an energy approach of the Fatigue Crack Growth (FCG was proposed. This approach is based on the numerical determination of the plastic zone by introducing a novel form of plastic radius. The experimental results conducted on two aluminum alloys of types 2024-T351 and 7075-T7351 were exploited to validate the developed numerical model. A good agreement has been found between the two types of results.
The Cracking of Irradiated Uranium
International Nuclear Information System (INIS)
Structural features other than fission-product gas bubbles seen in unrestrained, unalloyed uranium irradiated in the alpha range to burn-ups of up to 0.7% of all atoms are described. The main features are: (1) Brittle grain-boundary cracks in which the grains appear to have parted without plastic deformation. These cracks are associated with thermal cycling and high maximum temperatures (~600°C). (2) Transgranular cracks, which are comparatively rare. It is suggested that they may be related to the pseudo-cleavage or ''twin parting'' observed in unirradiated uranium by Cahn. (3) Grain boundary ''creep voids'' believed to arise from the coalescence of vacancies under tensile stress. The voids may form a path for crack propagation or may themselves coalesce to form cracks. Apart from the direct effects of these features in increasing the volume and changing the thermal conductivity and mechanical properties of the uranium, a mechanism is discussed by which cracks may accumulate fission-product gas and cause further volume increases. The possible effects of structural variations in the uranium and of restraint during irradiation on the incidence of cracking and void formation are considered. (author)
Energy Technology Data Exchange (ETDEWEB)
Poitou, B
2007-11-15
In this study, criterions are proposed to describe crack initiation in the vicinity of an interface in brittle bi-materials. The purpose is to provide a guide for the elaboration of ceramic multi-layer structures being able to develop damage tolerance by promoting crack deflection along interfaces. Several cracking mechanisms are analyzed, like the competition between the deflection of a primary crack along the interface or its penetration in the second layer. This work is first completed in a general case and is then used to describe the crack deviation at the interface in ceramic matrix composites and nuclear fuels. In this last part, experimental tests are carried out to determine the material fracture properties needed to the deflection criteria. An optimization of the fuel coating can be proposed in order to increase its toughness. (author)
Interface fatigue crack propagation in sandwich X-joints – Part I: Experiments
DEFF Research Database (Denmark)
Moslemian, Ramin; Berggreen, Christian
2013-01-01
/core interface of the joints. Sandwich tear test specimens with a face/core debond representing a debonded sandwich X-joint were tested under cyclic loading. Fatigue tests were conducted on the sandwich tear test specimens with H45, H100 and H250 PVC cores and glass/polyester face sheets. The Digital Image...... interface. The interface crack eventually kinked into the face sheet, resulting in large-scale fiber bridging. Finally, mixed mode bending tests were conducted to measure crack growth rates of the face/core interface at mode-mixity phase angles similar to those calculated for the sandwich tear test...
International Nuclear Information System (INIS)
Fracture surfaces are considered as a useful source of informations: an introduction to fractography is presented; the fracture surface may be observed through X ray microanalysis, and other physical methods such as Auger electron spectroscopy or secundary ion emission. The mechanisms of macroscopic and microscopic crackgrowth and fracture are described, in the case of unstable fracture (cleavage, ductile with shear, intergranular brittleness) and of progressive crack propagation (creep, fatigue). Exemples of cracks are presented in the last chapter
Han, Xianhong; Yang, Kun; Chen, Sisi; Chen, Jun
2015-10-01
Mechanical trimming process for ultra-high strength boron steel after hot stamping was carried out in this study. Shear and tensile tests were designed to analyze the influences of stress state on the fracture mode; trimmed fracture surface and profile were observed and compared to other commonly used steels such as DP980 and Q235 etc.; the crack propagation during trimming process was studied through step-by-step tests. The observation and analysis reveal that the fracture mode of hot-stamped boron steel is highly related to the stress state, it belongs to cleavage fracture on low stress triaxiality but dimple fracture on high stress triaxiality. Such phenomenon is reflected in the trimming process, during which the stress state changes from shear-dominated state to tensile-dominated state. In addition, the burnish zone of trimmed boron steel is much smaller than other high strength steels, and the profile of cutting surface shows an `S'-like shape which is destructive to the trimming tool. Moreover, during the trimming process, most martensite laths near the cutting edge are stretched and rotated markedly to the direction of the shear band, and the main crack expands along those grain boundaries, which may penetrate through a few martensite laths and form small crack branches.
G-control fatigue testing for cyclic crack propagation in composite structures
DEFF Research Database (Denmark)
Manca, Marcello; Berggreen, Christian; Carlsson, Leif A.
2015-01-01
This paper presents a computer controlled testing methodology called “The G-control Method” which allows cyclic crack growth testing using real-time control of the cyclic energy release rate. The advantages of using this approach are described and compared with traditional fatigue testing methods...
Fatigue crack propagation in drawn filled nitinol wires for medical applications
Czech Academy of Sciences Publication Activity Database
Major, Štěpán; Vlach, P.; Trudič, P.
Kazan : Foliant Kazan, 2012 - (Shlyannikov, V.; Goldstein, R.; Makhutov, N.), s. 232-236 ISBN 978-5-905576-18-8. [European conference on fracture /19./. Kazan (RU), 26.08.2012-31.08.2012] Institutional support: RVO:68378297 Keywords : nitinol * drawn filled wires, * fatigue crack growth Subject RIV: JL - Materials Fatigue, Friction Mechanics
Wave propagation analysis of edge cracked circular beams under impact force.
Directory of Open Access Journals (Sweden)
Şeref Doğuşcan Akbaş
Full Text Available This paper presents responses of an edge circular cantilever beam under the effect of an impact force. The beam is excited by a transverse triangular force impulse modulated by a harmonic motion. The Kelvin-Voigt model for the material of the beam is used. The cracked beam is modelled as an assembly of two sub-beams connected through a massless elastic rotational spring. The considered problem is investigated within the Bernoulli-Euler beam theory by using energy based finite element method. The system of equations of motion is derived by using Lagrange's equations. The obtained system of linear differential equations is reduced to a linear algebraic equation system and solved in the time domain by using Newmark average acceleration method. In the study, the effects of the location of crack, the depth of the crack, on the characteristics of the reflected waves are investigated in detail. Also, the positions of the cracks are calculated by using reflected waves.
Brittle fracture of T91 steel in liquid lead–bismuth eutectic alloy
Energy Technology Data Exchange (ETDEWEB)
Ye, Changqing, E-mail: Changqing.ye@ed.univ-lille1.fr; Vogt, Jean-Bernard, E-mail: jean-bernard.vogt@univ-lille1.fr; Proriol-Serre, Ingrid, E-mail: ingrid.proriol-serre@univ-lille1.fr
2014-12-15
Highlights: • Tempering temperature is important for LBE embrittlement occurrence. • Brittle behaviour in LBE evidenced by small punch test and fatigue test. • Brittle behaviour in low oxygen LBE observed for low loading rate. - Abstract: The mechanical behaviour of the T91 martensitic steel has been studied in liquid lead–bismuth eutectic (LBE) and in inert atmosphere. Several conditions were considered to point out the most sensitive embrittling factors. Smooth and notched specimens were employed for respectively monotonic and cyclic loadings. The present investigation showed that T91 appeared in general as a ductile material, and became brittle in the considered conditions only if at least tests were performed in LBE. It turns out that the loading rate appeared as a critical parameter for the occurrence of liquid metal embrittlement of T91 in LBE. For the standard heat treatment condition, loading monotonically the T91 very slowly instead of rapidly in LBE resulted in brittle fracture. Also, under cyclic loading, the crack propagated in a brittle manner in LBE.
Crack detection in a wheel end spindle using wave propagation via modal impacts and piezo actuation
Ackers, Spencer; Evans, Ronald; Johnson, Timothy; Kess, Harold; White, Jonathan; Adams, Douglas E.; Brown, Pam
2006-03-01
This research demonstrates two methodologies for detecting cracks in a metal spindle housed deep within a vehicle wheel end assembly. First, modal impacts are imposed on the hub of the wheel in the longitudinal direction to produce broadband elastic wave excitation spectra out to 7000 Hz. The response data on the flange is collected using 3000 Hz bandwidth accelerometers. It is shown using frequency response analysis that the crack produces a filter, which amplifies the elastic response of the surrounding components of the wheel assembly. Experiments on wheel assemblies mounted on the vehicle with the vehicle lifted off the ground are performed to demonstrate that the modal impact method can be used to nondestructively evaluate cracks of varying depths despite sources of variability such as the half shaft angular position relative to the non-rotating spindle. Second, an automatic piezo-stack actuator is utilized to excite the wheel hub with a swept sine signal extending from 20 kHz. Accelerometers are then utilized to measure the response on the flange. It is demonstrated using frequency response analysis that the crack filters waves traveling from the hub to the flange. A simple finite element model is used to interpret the experimental results. Challenges discussed include variability from assembly to assembly, the variability in each assembly, and the high amount of damping present in each assembly due to the transmission gearing, lubricant, and other components in the wheel end. A two-channel measurement system with a graphical user interface for detecting cracks was also developed and a procedure was created to ensure that operators properly perform the test.
Gołebiowski, B; Swiatnicki, W A; Gaspérini, M
2010-03-01
Microstructural changes occurring during fatigue tests of austenitic-ferritic duplex stainless steel (DSS) in air and in hydrogen-generating environment have been investigated. Hydrogen charging of steel samples during fatigue crack growth (FCG) tests was performed by cathodic polarization of specimens in 0.1M H(2)SO(4) aqueous solution. Microstructural investigations of specimens after FCG tests were carried out using transmission electron microscopy to reveal the density and arrangement of dislocations formed near crack tip. To determine the way of crack propagation in the microstructure, electron backscatter diffraction investigations were performed on fatigue-tested samples in both kinds of environment. To reveal hydrogen-induced phase transformations the atomic force microscopy was used. The above investigations allowed us to define the character of fatigue crack propagation and microstructural changes near the crack tip. It was found that crack propagation after fatigue tests in air is accompanied with plastic deformation; a high density of dislocations is observed at large distance from the crack. After fatigue tests performed during hydrogen charging the deformed zone containing high density of dislocations is narrow compared to that after fatigue tests in air. It means that hydrogenation leads to brittle character of fatigue crack propagation. In air, fatigue cracks propagate mostly transgranularly, whereas in hydrogen-generating environment the cracks have mixed transgranular/interfacial character. PMID:20500395
Molecular dynamics simulations of edge cracks in copper and aluminum single crystals
International Nuclear Information System (INIS)
Edge cracks in Cu and Al single crystals under mode I loading conditions are investigated using molecular dynamics simulations. Calculations are carried out at 0 K and the embedded atom method potentials are adopted for (100)[011] edge crack systems. Five different crack lengths are employed to examine the effects of crack length on the fracture behavior of each material. The results show that Cu and Al exhibit different fracture mechanisms. The overall failure feature of Cu is brittle except for the shortest crack, for which the emission of dislocations preceded crack propagation. All the edge cracks in Al are extended through void nucleation and coalescence, and a zigzag fracture pattern is observed for each crack. Detailed analysis shows that the strikingly different fracture behavior of the two materials is the result of their different vacancy-formation energies and surface energies
International Nuclear Information System (INIS)
Initiation stage of cracks is considered as a key issue, but more and more component integrity analyses investigate the crack propagation and arrest possibility. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture. Dynamic effects, involved in unstable cleavage crack propagation, have to be taken into account to properly depict brittle crack propagation, arrest and possible propagation re-initiation events. Experiments were carried out on thin CT specimens made of 16MND5 PWR vessel steel at five temperatures (-150 degrees C, -125 degrees C, -100 degrees C, -75 degrees C, -50 degrees C). In addition to standard crack gages, an innovative experimental technique has been used to determine crack propagation. By the means of developments on the experimental protocol (improvements of isolation and airtightness of the thermal chamber, optimization of the experimental protocol to eliminate ice in the thermal chamber and in order to have a good acquisition quality), use of a high speed framing camera was made possible to measure crack propagation on a CT mirror polished surface. This optical device, combined with this optimized experimental process, has allowed the study of straight and branching crack paths with high accuracy. The framing camera (520 000 fps up to 1 100 000 fps) has allowed to have a very accurate estimation of crack speed even up to 1000 m.s-1 and also to detect some phases of crack branching during propagation and phases of arrest-re-initiation. Numerical computations, based on X-FEM and combining a local non linear dynamic approach with a RKR type fracture stress criterion, have been performed to depict experimental crack behavior. This paper describes this innovative experimentation and the interpretation by FE calculations and SEM observations associated with quantitative 3D optical microscopy. (authors)
Energy Technology Data Exchange (ETDEWEB)
Horstmann, M.; Ventzke, V.; Petrovski, B.; Kocak, M. [GKSS Research Centre Geesthacht, Institute of Materials Research, Materials Mechanics, Geesthacht (Germany); Kocik, R.; Tempus, G. [AIRBUS Deutschland GmbH, Metal Technology, Bremen (Germany); Vaidya, W.V.
2009-10-15
Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti-sheet into the profiled Al-sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non-uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: crack parallel to the weld and crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation ''crack parallel to the weld'' was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al-side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl{sub 3} had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition. (Abstract Copyright [2009], Wiley Periodicals, Inc.) [German] Teil II: Widerstand gegen Ermuedungsrissausbreitung und Bruch Die Untersuchungen an der laserstrahlgeschweissten Mischverbindung aus AA6056 und Ti6Al4V wurden fortgesetzt. Fuer die Ermuedungsrissausbreitungs
Institute of Scientific and Technical Information of China (English)
无
2001-01-01
A two-phase model of rock was proposed in order to investigate the mechanism of brittle fracture due to uniaxial compression, in which rock was considered to be a composite material consisting of hard grains and colloids. The stress state in colloid region near grains was calculated using Finite Element Metnod ( FEM). The influence of the tensile stresses on the crack initiation and failure process of brittle rock subjected to uniaxial compression was investigated by numerical experiments. The FE results show that tensile stresses are induced easily in the neighboring area of hard grains with the maximum value near grain boundaries. The distribution of tensile stresses depends on the relative position of hard grains. The cracks initiated just near the boundary area of hare grains, which was governed by tensile stress. These results dearly reveal the micro-fracture mechanism of brittle rock loaded by uniaxial compression. It can be concluded that the failure mode of brittle rock under uniaxial compression is still tensile fracture from the point view of microstructure. However,since the wide colloid region is still under compressive stress state, further propagation of boundary, cracks through this region obviously needs more external load, thus causing the uniaxial compressive strength of rock much higher than its tensile strength obtained via Brazilian (splitting) experiment.
Effect of crystallinity on crack propagation and mineralization of bioactive glass 45S5
Kashyap, Satadru
Bioactive glasses are a type of ceramic material designed to be used as bioresorbable therapeutic bone implants. Thermal treatment of bioactive glass ceramics dictates many important features such as microstructure, degree of crystallinity, mechanical properties, and mineralization. This study investigates the effects of temperature, time, and heating rates on the crystallization kinetics of melt cast bioactive glass 45S5. Bulk crystallization (three dimensional crystallite formation) was found to always occur in bulk bioactive glass 45S5 irrespective of the processing conditions. A comparative study of crack paths in amorphous and crystalline phases of bioactive glass 45S5 revealed crack deflections and higher fracture resistance in partially crystallized bioactive glass. Such toughening is likely attributed to different crystallographic orientations of crystals or residual thermal mismatch strains. Furthermore, in vitro immersion testing of partially crystalline glass ceramic revealed higher adhesion capabilities of the mineralized layer formed on amorphous regions as compared to its crystalline counterpart.
Application of digital radiography in evaluation of crack propagation rate in cast steel specimens
Sikora, R.; B. Piekarski; K. Świadek; R. Chylińska
2009-01-01
A technology that utilizes penetrating X-rays is one of the oldest non-destructive testing methods. Digital radiography combines modern digital image processing algorithms with traditional X-ray testing method. The following paper describes the present use of digital radiograms in flaw detection, and the use of identification and classification algorithms in detection of cracks that occur under the effect of thermal fatigue process in creep-resistant steel castings operating as structural ele...
On crack propagation in the welded polyolefin pipes with and without the presence of weld beads
Czech Academy of Sciences Publication Activity Database
Mikula, Jakub; Hutař, Pavel; Nezbedová, E.; Lach, R.; Arbeiter, F.; Ševčík, Martin; Pinter, G.; Grellmann, W.; Náhlík, Luboš
2015-01-01
Roč. 87, DEC (2015), s. 95-104. ISSN 0264-1275 R&D Projects: GA ČR(CZ) GAP108/12/1560; GA MŠk(CZ) EE2.3.30.0063; GA MŠk(CZ) ED1.1.00/02.0068 Institutional support: RVO:68081723 Keywords : Slow crack growth * Butt weld * Lifetime estimation * Polyolefin pipes * Weld bead Subject RIV: JL - Materials Fatigue, Friction Mechanics
Basic modes of crack propagation through the interface in polymer layered structure
Czech Academy of Sciences Publication Activity Database
Zouhar, Michal; Hutař, Pavel; Náhlík, Luboš; Knésl, Zdeněk
488-489, č. 1 (2012), s. 162-165. ISSN 1013-9826. [International Conference on Fracture and Damage Mechanics - FDM 2011 /10./. Dubrovník, 19.09.2011-21.09.2011] R&D Projects: GA ČR GD106/09/H035; GA ČR GA106/09/0279 Institutional support: RVO:68081723 Keywords : fracture mechanics * slow crack growth * multilayer polymer pipe Subject RIV: JL - Materials Fatigue, Friction Mechanics
Probabilistic evaluation of unstable crack propagation in pipeline using SBRA Method
Czech Academy of Sciences Publication Activity Database
Václavek, L.; Marek, Pavel; Gajdoš, Lubomír
Ostrava: VŠB TU, 2006 - (Bůžek, Z.; Strnadel, B.). s. 259-268 ISBN 80-248-0896-X. [New Methods of Damage and Failure Analysis of Structural Parts . 04.09.2006-08.09.2006, Ostrava] R&D Projects: GA ČR(CZ) GA103/04/1451 Institutional research plan: CEZ:AV0Z20710524 Keywords : Crack * Load effect * Failure Subject RIV: JM - Building Engineering
Crack propagation and arrest in CFRP materials with strain softening regions
Dilligan, Matthew Anthony
Understanding the growth and arrest of cracks in composite materials is critical for their effective utilization in fatigue-sensitive and damage susceptible applications such as primary aircraft structures. Local tailoring of the laminate stack to provide crack arrest capacity intermediate to major structural components has been investigated and demonstrated since some of the earliest efforts in composite aerostructural design, but to date no rigorous model of the crack arrest mechanism has been developed to allow effective sizing of these features. To address this shortcoming, the previous work in the field is reviewed, with particular attention to the analysis methodologies proposed for similar arrest features. The damage and arrest processes active in such features are investigated, and various models of these processes are discussed and evaluated. Governing equations are derived based on a proposed mechanistic model of the crack arrest process. The derived governing equations are implemented in a numerical model, and a series of simulations are performed to ascertain the general characteristics of the proposed model and allow qualitative comparison to existing experimental results. The sensitivity of the model and the arrest process to various parameters is investigated, and preliminary conclusions regarding the optimal feature configuration are developed. To address deficiencies in the available material and experimental data, a series of coupon tests are developed and conducted covering a range of arrest zone configurations. Test results are discussed and analyzed, with a particular focus on identification of the proposed failure and arrest mechanisms. Utilizing the experimentally derived material properties, the tests are reproduced with both the developed numerical tool as well as a FEA-based implementation of the arrest model. Correlation between the simulated and experimental results is analyzed, and future avenues of investigation are identified
Effect of pressurized water reactor environment on fatigue crack propagation, including hole times
International Nuclear Information System (INIS)
Results are presented from an experimental program being conducted to investigate the effects of pressurized water reactor environment on the fatigue crack growth rate of pressure vessel steels. Tests were conducted on precracked WOL type specimens under load controlled conditions. The effects of R ratio, loading rates, and loading wave form are evaluated, and the results are compared for both forging and plate material, as well as weldments
International Nuclear Information System (INIS)
Highlights: • Cracking consistent with corrosion enhanced plasticity model of Magnin. • Cracking stress threshold is 10 MPa, substantially lower than current guidance. • Humidity threshold for cracking is 30%. • Measured length of cracks very dependent on polishing practice. • Cracking could occur at 290–300 K, based on measured activation energy. - Abstract: Bending tests were used to investigate the stress-corrosion cracking of 304L stainless steel in a corrosive atmosphere containing magnesium chloride. Initially smooth specimens showed multiple closely spaced cracks after exposures of up to 500 h. These showed threshold stresses of 10 MPa and a threshold humidity of 30%. Cracking rates increased with stress but were a maximum at plastic strains of 2%. Examination of cracks using focussed ion beam milling and electron diffraction indicated a multi-stage mechanism of propagation via preferential oxidation of slip planes. The apparent activation energy was 34 kJ mol−1 in the temperature range 333–363 K
Fatigue-crack propagation behavior in monolithic and composite ceramics and intermetallics
Energy Technology Data Exchange (ETDEWEB)
Ritchie, R.O.; Dauskardt, R.H.; Venkateswara Rao, K.T. [Lawrence Berkeley Lab., CA (United States)
1994-11-01
We study microstructural mechanisms of fatigue crack growth in advanced monolithic and composite ceramics and intermetallics. Much attention is devoted to the contribution of cycling loading to the hindrance of mechanisms that lead to a considerable increase in toughness (crack-tip shielding) of these materials. For example, in intermetallics with a ductile phase, such as {Beta}-TiNb-reinforced {gamma}-TiAl or Nb-reinforced Nb{sub 3}Al, a significant increase in toughness caused by the presence of uncracked ductile phase inside a crack is retarded under cyclic loading because ductile particles immediately fail by fatigue. Similarly, in monolithic ceramics, e.g., in alumina (aluminum oxide) or silicon nitride, the significant plasticization appearing under monotonic loading is greatly diminished under cyclic loading due to gradual wear at the grain-matrix interface. In fact, the nature of fatigue in such low-plasticity materials differs essentially from the well-known mechanisms of fatigue in metals and is governed, first of all, by a decrease in shielding, which depends on the loading cycle and time. The susceptibility of intermetallics and ceramics to fatigue degradation under cyclic loading affects seriously the possibility of structural use of these materials in practice. In particular, in this case, it is difficult to apply strength calculation methods that take into account the presence of defects and to implement life-prediction procedures.
Institute of Scientific and Technical Information of China (English)
孙雅珍; 翟晓星; 李宁
2012-01-01
This paper aims to research the propagation mechanism of reflective crack and the anti-cracking effect of geosynthetics in semi-rigid base asphalt pavement. Using surface base stress and stress intensity factor analyses the propagation mechanism of semi-rigid base asphalt pavement reflective crack under different load function. Applying numerical analysis evaluates the anti-cracking effect of geosynthetics. Under symmetrical load, reflective cracks can not propagate. Under asymmetrical load, it will cause shearing type crack. Under temperature load,it will cause tensile type crack. After paving geosynthetics at asphalt surface base, the shear stress and tensile stress of surface base are decreased and the stress intensity factor of crack tip is also decreased. Under different load function, reflective cracks have different propagation mode. Geosynthetics can relieve the stress concentration of crack tip. Its anti-cracking effect is obvious.%目的 揭示半刚性基层沥青路面反射裂缝的扩展机理,确定土工合成材料的防裂效果.方法 利用面层底部应力和应力强度因子法分析合反射裂缝沥青路面结构在不同荷载作用下的裂缝扩展机理,并应用数值分析的方法来评价土工合成材料的防裂效果.结果 在正荷载作用下,裂缝不会扩展；在偏荷载作用下,会产生剪切型裂缝；在温度荷载作用下,会产生张开型裂缝.沥青面层底部加铺土工合成材料后,面层底部裂缝处产生的切应力与拉应力以及裂缝尖端的应力强度因子均得到了明显的减小.结论 在不同的荷载作用下,反射裂缝具有不同的扩展方式.土工合成材料可以有效地缓解裂缝尖端的应力集中,防裂效果明显.
Energy Technology Data Exchange (ETDEWEB)
Bohner, E.; Mueller, H.S. [University of Karlsruhe (Germany). Institute of Concrete Structures and Building Materials
2006-12-15
Since the initiation stage of the damage process due to reinforcement corrosion had been successfully investigated in the recent past, the damage progress in the propagation stage is currently in the focus of research. This work deals with the mechanisms of cracking and spalling due to corrosion of reinforcement and aims for the development of an analytic prediction model of the damage process. On this occasion the influence of shrinkage and creep on the stress condition within the concrete cover is of major importance to the subsequent analysis of the crack formation due to reinforcement corrosion in the early propagation stage. (Abstract Copyright [2006], Wiley Periodicals, Inc.)
Elshabasy, Mohamed Mostafa Yousef Bassyouny
In this research, life extending control logic is proposed to reduce the cost of treating the aging problem of military aircraft structures and to avoid catastrophic failures and fatal accidents due to undetected cracks in the airframe components. The life extending control logic is based on load tailoring to facilitate a desired stress sequence that prolongs the structural life of the cracked airframe components by exploiting certain nonlinear crack retardation phenomena. The load is tailored to include infrequent injections of a single-cycle overload or a single-cycle overload and underload. These irregular loadings have an anti-intuitive but beneficial effect, which has been experimentally validated, on the extension of the operational structural life of the aircraft. A rigid six-degree-of freedom dynamic model of a highly maneuverable air vehicle coupled with an elastic dynamic wing model is used to generate the stress history at the lower skin of the wing. A three-dimensional equivalent plate finite element model is used to calculate the stress in the cracked skin. The plate is chosen to be of uniform chord-wise and span-wise thickness where the mechanical properties are assigned using an ad-hoc approach to mimic the full scale wing model. An in-extensional 3-node triangular element is used as the gridding finite element while the aerodynamic load is calculated using the vortex-lattice method where each lattice is laid upon two triangular finite elements with common hypotenuse. The aerodynamic loads, along with the base-excitation which is due to the motion of the rigid aircraft model, are the driving forces acting on the wing finite element model. An aerodynamic control surface is modulated based on the proposed life extending control logic within an existing flight control system without requiring major modification. One of the main goals of life extending control logic is to enhance the aircraft's service life, without incurring significant loss of vehicle
Energy Technology Data Exchange (ETDEWEB)
Raquet, O.
1994-11-25
A purely phenomenological study of stress corrosion cracking was performed using the couple Z2CN 18.10 (304L) austenitic stainless steel/boiling MgCl{sub 2} aqueous solution. The exploitation of the morphological information (shape of the cracks and size distribution) available after constant elongation rate tests led to the proposal of an analytical expression of the crack initiation and growth rates. This representation allowed to quantitatively characterize the influence of the applied strain rate as well as the effect of corrosion inhibitors on the crack initiation and propagation phases. It can be used in the search for the stress corrosion cracking mechanisms as a `riddle` for the determination of the rate controlling steps. As a matter of fact, no mechanistic hypothesis has been used for its development. (author).
Crack tip blunting and cleavage under dynamic conditions
Rajan, V. P.; Curtin, W. A.
2016-05-01
In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures (Lin et al., 1987). Here, crack tip blunting and cleavage under dynamic conditions are explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: (1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and (2) what happens in the dynamic "overloaded" situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed as a "dynamic cleavage test", in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, ductile crack tip behavior ultimately prevails since a single emission event can blunt and arrest the crack, preventing further cleavage. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears unrelated to dynamic effects at the crack tip.
Molecular dynamics investigation of dynamic crack stability
International Nuclear Information System (INIS)
A series of molecular-dynamics simulations has been performed in order to evaluate the effects of several physical factors on dynamic crack stability. These factors are the crystalline structure and the interatomic interaction modeled by various empirical potentials. For brittle crack propagation at low temperature we find that steady-state crack velocities are limited to a band of accessible values. Increasing the overload beyond KIc, the crack can propagate with a steady-state velocity, which quickly reaches the terminal velocity of about 0.4 of the Rayleigh wave speed. The magnitude of the terminal velocity can be related to the nonlinearity of the interatomic interaction. Further increasing the overload does not change the steady-state velocity dramatically, but significantly increases the amplitude of acoustic emission from the crack tip. Loading the crack even further leads to instabilities which take the form of cleavage steps, dislocation emission, or branching. The instability is closely related to the buildup of a localized coherent, phononlike field generated by the bond-breaking events. The form of the instability depends critically on crystal structure and on the crystallographic orientation of the crack system but can also be correlated with the relative ease of dislocation generation (and motion). copyright 1997 The American Physical Society
Molecular dynamics simulation of crack propagation behavior of aluminum%铝裂纹扩展行为的分子动力学模拟
Institute of Scientific and Technical Information of China (English)
刘晓波; 徐庆军; 刘剑
2014-01-01
The crack propagation behavior of aluminum was studied by molecular dynamics method. The molecular dynamics simulation model of aluminum was set up, and the energy evolution map and atomic trajectory figure of aluminum crack propagation process were obtained according to the modified embedded atom method. By plotting the crack propagation graph, the change of crack propagation is observed clearly, such as the crack tip blunted, the generation of subcrack, the formation and growth process of the void, and the collection of crack and void. The whole evolution process of the system was described in detail from energy evolution map. The effects of loading rate and initial crack length on the crack propagation behavior of the system were discussed. The results show that the degree of atomic motion is intensified and the time of system completely open is shortened with increasing the loading rate. The longer the initial crack length is, the more unobvious the details of the atomic motion are.%采用分子动力学方法研究铝的裂纹扩展行为。建立铝的分子动力学模拟模型，通过嵌入原子法进行模拟，得到铝裂纹扩展过程中的原子轨迹图与能量演变图。从裂纹扩展图清楚地观察到裂纹扩展的变化情况：裂纹尖端钝化、子裂纹的产生、孔洞的生成及长大过程以及裂纹和孔洞的汇集。能量演变图中详细地说明了整个体系演变的过程。探讨加载速率和初始裂纹长度对体系裂纹扩展行为的影响。结果表明：随着加载速率的增大，原子运动程度加剧，体系完全被拉开的时间缩短；初始裂纹长度越长，原子运动的细节表现越不明显。
International Nuclear Information System (INIS)
The combining effect of artificial age plus electric field (16 kV/cm) at 190 deg. C on microstructure and fatigue crack propagation (FCP) resistance of an Al-Cu-Mg alloy was investigated by transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and fatigue testing. The results showed that, during aging process with electric field grain boundaries became coarse with S' phase precipitates, and the precipitates were much finer and dispersed more uniform in matrix. As a result, tensile properties of aged samples did not change significantly. FCP resistance of samples with electric field on was higher due to higher barriers for dislocation movement, caused by more dispersed precipitates of S' phase. A vacancy-atom diffusion model was introduced to explain microstructure evolution during electric field aging process.
Interface fatigue crack propagation in sandwich X-joints – Part II: Finite element modeling
DEFF Research Database (Denmark)
Moslemian, Ramin; Berggreen, Christian
2013-01-01
The aim of the second and final part of this study is to simulate fatigue crack growth in the tested Sandwich Tear Test specimens, described in Part I, using the finite element method. To accelerate the simulation, a cycle jump method is utilized and implemented in the finite element routine. The...... proposed method is based on conducting finite element analysis for a set of cycles to establish a trend line, extrapolating the trend line spanning many cycles, and use the extrapolated state as initial state for additional finite element simulations. The measured da/dN relations of the face/core interface...
Small fatigue crack propagation in Y2O3 strengthened steels
Czech Academy of Sciences Publication Activity Database
Hutař, Pavel; Kuběna, Ivo; Ševčík, Martin; Šmíd, Miroslav; Kruml, Tomáš; Náhlík, Luboš
2014-01-01
Roč. 452, 1-3 (2014), s. 370-377. ISSN 0022-3115 R&D Projects: GA MŠk(CZ) EE2.3.30.0063; GA MŠk(CZ) ED1.1.00/02.0068; GA ČR(CZ) GP13-28685P Institutional support: RVO:68081723 Keywords : oxide dispersion strengthened steel * small fatigue crack * J-integral * fatigue life prediction Subject RIV: JL - Materials Fatigue, Friction Mechanics Impact factor: 1.865, year: 2014
Energy Technology Data Exchange (ETDEWEB)
Kabiri, M.R
2003-12-01
This study was performed at Centre des Materiaux de l'Ecole des Mines de Paris. It deals with identification and transferability of high temperature creep cracking laws of steels. A global approach, based on C{sup *} and J non-linear fracture mechanics parameters has been used to characterize creep crack initiation and propagation. The studied materials are: the ferritic steels 1Cr-1Mo-1/4V (hot and cold parts working at 540 and 250 C) used in the thermal power stations and the austenitic stainless steel 316 L(N) used in the nuclear power stations. During this thesis a data base was setting up, it regroups several tests of fatigue, creep, creep-fatigue, and relaxation. Its particularity is to contain several creep tests (27 tests), achieved at various temperatures (550 to 650 C) and using three different geometries. The relevance of the C{sup *} parameter to describe the creep crack propagation was analysed by a means of systematic study of elasto-viscoplastic stress singularities under several conditions (different stress triaxiality). It has been shown that, besides the C{sup *} parameter, a second non singular term, denoted here as Q{sup *}, is necessary to describe the local variables in the vicinity of the crack tip. Values of this constraint parameter are always negative. Consequently, application of typical creep crack growth laws linking the creep crack growth rate to the C{sup *} parameter (da/dt - C{sup *}), will be conservative for industrial applications. Furthermore, we showed that for ferritic steels, crack incubation period is important, therefore a correlation of Ti - C{sup *} type has been kept to predict crack initiation time Ti. For the austenitic stainless steel, the relevant stage is the one of the crack propagation, so that a master curve (da/dt - C{sup *}), using a new data analysis method, was established. Finally, the propagation of cracks has been simulated numerically using the node release technique, allowing to validate analytical
Husseini, Naji Sami
Single-crystal nickel-base superalloys are ubiquitous in demanding turbine-blade applications, and they owe their remarkable resilience to their dendritic, hierarchical microstructure and complex composition. During normal operations, they endure rapid low-stress vibrations that may initiate fatigue cracks. This failure mode in the very high-cycle regime is poorly understood, in part due to inadequate testing and diagnostic equipment. Phase-contrast imaging with coherent synchrotron x rays, however, is an emergent technique ideally suited for dynamic processes such as crack initiation and propagation. A specially designed portable ultrasonic-fatigue apparatus, coupled with x-ray radiography, allows real-time, in situ imaging while simulating service conditions. Three contrast mechanisms - absorption, diffraction, and phase contrast - span the immense breadth of microstructural features in superalloys. Absorption contrast is sensitive to composition and crack displacements, and diffraction contrast illuminates dislocation aggregates and crystallographic misorientations. Phase contrast enhances electron-density gradients and is particularly useful for fatigue-crack studies, sensitive to internal crack tips and openings less than one micrometer. Superalloy samples were imaged without external stresses to study microstructure and mosaicity. Maps of rhenium and tungsten concentrations revealed strong segregation to the center of dendrites, as manifested by absorption contrast. Though nominally single crystals, dendrites were misoriented from the bulk by a few degrees, as revealed by diffraction contrast. For dynamic studies of cyclic fatigue, superalloys were mounted in the portable ultrasonic-fatigue apparatus, subjected to a mean tensile stress of ˜50-150 MPa, and cycled in tension to initiate and propagate fatigue cracks. Radiographs were recorded every thousand cycles over the multimillion-cycle lifetime to measure micron-scale crack growth. Crack openings were
Directory of Open Access Journals (Sweden)
J. Bär
2015-10-01
Full Text Available During fatigue crack propagation experiments with constant force as well as constant stress intensity lock in thermography and heat flow measurements with a new developed peltier sensor have been performed. With lock in thermography space resolved measurements are possible and the evaluation allows to distinguish between elastic and dissipated energies. The specimens have to be coated with black paint to enhance the emissivity. The thickness of the coating influences the results and therefore quantitative measurements are problematic. The heat flow measurements are easy to perform and provide quantitative results but only integral in an area given by the used peltier element. To get comparable results the values measured with thermography were summarized in an area equivalent to that of the peltier element. The experiments with constant force show a good agreement between the thermography and the heat flow measurements. In case of the experiments with a constant stress intensity some differences become visible. Whereas the thermography measurements show a linear decrease of the signal with rising crack length, the heat flow measurements show a clearly nonlinear dependency. Obviously the measured energies in thermography and peltier based heat flow measurement are not comparable
International Nuclear Information System (INIS)
Low cycle fatigue (LCF) tests have been performed at 300 and 565 degrees C on welded joints and on microstructures to be found in or near welded joints in a low alloy ferritic steel 0.5 Cr, 0.5 Mo, 0.25 V. The difference in lifetimes between the 300 degrees C and 565 degrees C tests was small comparing the same microstructures and strain ranges, although the stress amplitude was greater at 300 degrees C. Under constant stress conditions the fatigue life depended on the fatigue life of the parent metal but under constant strain conditions the lifetime was governed by that of the bainitic structures. Strain controlled LCF tests have been performed at 750 degrees C on welded joints in the austenitic steel AISI 316 and on different parent and weld metals used in these joints. In continuously cycled samples all cracks were transgranular and initiated at the surface; hold-time samples displayed internally initiated intergranular cracking in the weld metal. Under constant strain conditions the 316 parent and weld metals exhibited similar lifetimes. When considering a constant stress situation the strength of the microsturctures decreased in the following order: Sanicro weld metal, cold deformed parent metal, undeformed parent metal and weld metal (K.A.E.)
New Developments in the Embedded Statistical Coupling Method: Atomistic/Continuum Crack Propagation
Saether, E.; Yamakov, V.; Glaessgen, E.
2008-01-01
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM region, which, in turn, generates interface reaction forces that are applied as piecewise constant traction boundary conditions to the MD domain. The enhancement is based on the addition of molecular dynamics-based cohesive zone model (CZM) elements near the MD-FEM interface. The CZM elements are a continuum interpretation of the traction-displacement relationships taken from MD simulations using Cohesive Zone Volume Elements (CZVE). The addition of CZM elements to the concurrent MD-FEM analysis provides a consistent set of atomistically-based cohesive properties within the finite element region near the growing crack. Another set of CZVEs are then used to extract revised CZM relationships from the enhanced embedded statistical coupling method (ESCM) simulation of an edge crack under uniaxial loading.
Brittle and semibrittle behaviours of a carbonate rock: Influence of water and temperature
Nicolas, A.; Fortin, J.; Regnet, J. B.; Dimanov, A.; Guéguen, Y.
2016-04-01
Inelastic deformation can either occur with dilatancy or compaction, implying differences in porosity changes, failure and petrophysical properties. In this study, the roles of water as a pore fluid, and of temperature, on the deformation and failure of a micritic limestone (white Tavel limestone, porosity 14.7%) were investigated under triaxial stresses. For each sample, a hydrostatic load was applied up to the desired confining pressure (from 0 MPa up to 85 MPa) at either room temperature or at 70°C. Two pore fluid conditions were investigated at room temperature: dry and water saturated. The samples were deformed up to failure at a constant strain rate of ˜10-5s-1. The experiments were coupled with ultrasonic wave velocity surveys to monitor crack densities. The linear trend between the axial crack density and the relative volumetric strain beyond the onset of dilatancy suggests that cracks propagate at constant aspect ratio. The decrease of ultrasonic wave velocities beyond the onset of inelastic compaction in the semibrittle regime indicate the ongoing interplay of shear-enhanced compaction and crack development. Water has a weakening effect on the onset of dilatancy in the brittle regime, but no measurable influence on the peak strength. Temperature lowers the confining pressure at which the brittle-semibrittle transition is observed but does not change the stress states at the onset of inelastic compaction and at the post-yield onset of dilatancy.
International Nuclear Information System (INIS)
Highlights: ► Measured crack growth resistance of welds with 140 wppm H from gas charging. ► H reduced fracture initiation toughness by over 67% and altered fracture mode. ► With H, microcracks initiate at weld ferrite. Without H, fracture is uniformly ductile. ► With H, localized deformation in austenite creates stress concentrations at ferrite. ► In austenite/ferrite microstructures, JIC decreases with increasing vol.% ferrite. - Abstract: Elastic–plastic fracture mechanics methods were used to characterize hydrogen-assisted crack propagation in two austenitic stainless steel gas tungsten arc (GTA) welds. Thermally precharged hydrogen (140 wppm) degraded fracture initiation toughness and crack growth toughness and altered fracture mechanisms. Fracture initiation toughness in hydrogen-precharged welds represented a reduction of >67% from the estimated toughness of non-charged welds. In hydrogen-precharged welds, microcracks initiated at ferrite, and dendritic microstructure promoted crack propagation along ferrite. Deformation twinning in austenite interacts with ferrite, facilitating microcrack formation. While hydrogen altered fracture mechanisms similarly for both welds, the amount of ferrite governed the severity of hydrogen-assisted crack propagation.
Fully Noncontact Wave Propagation Imaging in an Immersed Metallic Plate with a Crack
Directory of Open Access Journals (Sweden)
Jung-Ryul Lee
2014-01-01
Full Text Available This study presents a noncontact sensing technique with ultrasonic wave propagation imaging algorithm, for damage visualization of liquid-immersed structures. An aluminum plate specimen (400 mm × 400 mm × 3 mm with a 12 mm slit was immersed in water and in glycerin. A 532 nm Q-switched continuous wave laser is used at an energy level of 1.2 mJ to scan an area of 100 mm × 100 mm. A laser Doppler vibrometer is used as a noncontact ultrasonic sensor, which measures guided wave displacement at a fixed point. The tests are performed with two different cases of specimen: without water and filled with water and with glycerin. Lamb wave dispersion curves for the respective cases are calculated, to investigate the velocity-frequency relationship of each wave mode. Experimental propagation velocities of Lamb waves for different cases are compared with the theoretical dispersion curves. This study shows that the dispersion and attenuation of the Lamb wave is affected by the surrounding liquid, and the comparative experimental results are presented to verify it. In addition, it is demonstrated that the developed fully noncontact ultrasonic propagation imaging system is capable of damage sizing in submerged structures.
Ductile-to-brittle transition in a low alloy steel
International Nuclear Information System (INIS)
The mechanical properties of pressure vessel steel (and above all its resistance to brittle fracture) are a decisive factor in the complex safety assessment of nuclear power plants. The monitoring of neutron induced embrittlement is provided using Charpy impact tests on standard V-notch specimens due to their small size. Material's ductile-to-brittle transition temperature (DBTT) can be easily characterised using this test. However, Charpy impact energy cannot be immediately used for safety assessment, since fracture toughness is required. Some empirical formulas have been developed, but no direct relationship was still found. When the specimens are tested in the ductile-to-brittle transition region, cleavage crack initiation is preceded by ductile crack growth giving a large scatter to the values of fracture toughness and/or Charpy impact energy. Even if the cleavage initiation and propagation in steels containing isolated spheroidic carbides are qualitatively well understood, no one from existing models can explain the sharp upturn in ductile-to-brittle transition region. In the present work, French tempered bainitic steel 16MND5 (considered as equivalent to the American standard A508 Cl.3) is studied: The large fractographic analysis of CT and Charpy specimens broken in the DBTT range is undertaken to account for the evolution of cleavage fracture mechanisms. In addition to classical scanning electron microscopy, transmission electron microscopy and EBSD technique are used in order to study the propagation of cleavage crack. The classical fracture mechanics using KIc or Jc concepts can hardly describe the unstable brittle fracture in the DBTT range. Hence, the local approach, which aims to predict the fracture of any structural component using local criteria, providing that the mechanical fields in the structure are known, is used. The probability of cleavage fracture in the DBTT range is predicted using the Beremin model based on weakest link theory, e.g. 2
Institute of Scientific and Technical Information of China (English)
徐芳; 陈振中
2011-01-01
The fatigue crack propagation behavior of A357 casting aluminum alloys at stress ratios of 0.05 and 0.7 under T5 and T6 heat treatments were studied and crack propagation curves were modified by a crack deflection model. The research results show that the fatigue crack grows along the boundary of dendrite cells to cause a remarkable crack deflection. The fatigue crack propagation rate at r=0.05 is smaller than that at r=0.7 for both T5 and T6 heat treatments. The effect of heat treatment on the fatigue crack propagation rates cannot be seen. The fatigue crack propagation rates modified by the crack deflection model predict no change at r=0.7, while a remarkable increase of the fatigue crack propagation rates can be seen at r=0.05. It is more reasonable and accurate to use the effective stress intensity factors for Mode Ⅰand Ⅱ to describe the fatigue crack propagation rates when a crack deflection occurs.%该文研究了A357铸造铝合金在T5／T6两种热处理条件、应力比为0．05和0．7时的疲劳裂纹扩展行为，并用裂纹偏折模型对裂纹扩展速率曲线进行修正。研究结果表明，裂纹沿着树状晶边界扩展导致显著的裂纹偏折。在T5／T6两种热处理条件下，应力比r=0．05时的裂纹扩展速率均远小于应力比为0．7时的裂纹扩展速率，热处理方式对裂纹扩展速率没有影响。应力比r=0．7时，偏折裂纹模型修正后的裂纹扩展速率没有明显变化；而应力比r=0．05时，偏折裂纹模型修正后的裂纹扩展速率较修正前有显著增加。裂纹发生偏折时，采用同时考虑Ⅰ型和Ⅱ型裂纹的裂纹尖端有效应力强度因子描述裂纹扩展速率更合理也更准确。
Institute of Scientific and Technical Information of China (English)
唐谦; 李云安
2015-01-01
Confining pressure significantly affects the crack propagation in pre-cracked rock.Particle flow simulation is a frontier approach of researching the features and mechanism of crack propagation.In this paper we present a numerical procedure for the analysis of crack propagation in rock-like materials under compressive biaxial loads. Rock specimens with intermittent single cracks were built by a bonded particle model (BPM)and biaxial compres-sion tests under five different confinements were modeled in PFC2D (2 dimensional particle flow code).Results re-veal that when the confining pressure is 0 (which means uniaxial compression),wing cracks initiate perpendicular to the pre-existing cracks and propagate towards the direction of principal stress;however,when confining pressure increases,the angle between wing cracks and pre-existing cracks gets larger,and the wing cracks get shorter and stop developing until confining pressure reaches a certain value.Also we conclude that in addition to material type, the direction of secondary crack propagation depends on the confining pressure,too.Besides,secondary cracks can be triggered by tensile stress but the further development is affected by shear stress.The increase of confining pres-sure poses constraints to the development of tensile cracks,but promotes shear cracks,and the width of shear plane gets larger with confinement increasing.%采用颗粒流数值软件 PFC 模拟预设单裂隙岩石在双向压缩状态下裂纹的扩展过程和特征。首先利用黏聚颗粒模型建立岩石试样，并在岩石试样中预置非贯通单裂隙；为研究围压对岩石裂纹扩展的影响，分别在5种围压作用下对预设裂隙岩石进行双轴压缩试验。结果表明：在围压为0（单轴压缩）时，翼裂纹扩展方向与预设裂隙基本垂直，随着加载进行沿着主压力方向扩展，而当围压增加时，翼裂纹扩展方向与预设裂隙的交角增大，且长度变短直至围压
Crack propagation in micro-chevron-test samples of direct bonded silicon-silicon wafers
Directory of Open Access Journals (Sweden)
Detlef Billep
2011-01-01
Full Text Available Wafer bonding describes all technologies for joining two or more substrates directly or using certain intermediate layers. Current investigations are focused on so-called low temperature bonding as a special direct bonding technology. It is carried out without intermediate layers and at temperatures below 400 °C. In addition to the wafer materials, the toughness of the bonded interface also depends on the bonding process itself. It can vary for different pre-treatments. Furthermore, an increase of the annealing temperature leads to a higher toughness of the bonded interface.The fracture toughness is a suitable value to describe the damage behaviour of the bonded interface. Based on a micro-chevron-specimen, the fracture toughness can be determined either numerically or by combining numerical analysis with experimental measurement of the maximum force. The maximum force is measured during a micro-chevron-test using a Mode I loading. The minimum of the stress intensity coefficient can be determined by a FE-simulation only. One possibility to estimate the stress intensity coefficient is the compliance method. The compliance of the whole specimen increases with a growing crack. The stress intensity coefficient can be directly derived from the simulated compliance and the crack length itself.The paper is focused on the micro-chevron-test for direct bonded silicon-silicon wafers. Additional to the estimation of dimensionless stress intensity coefficient as a function of geometry, the influence of different pre-treatments and annealing temperatures on the measured maximum force are analysed and discussed.
International Nuclear Information System (INIS)
The fatigue crack initiation life of AISI 1018 steel was investigated using compact tension specimens having sharp notch root radii. The data were analyzed using two methods for predicting initiation in strain cycling experiments. Also, another approach in which initiation is related to the stress intensity factor was developed. The next phase, that of propagation, was studied using AISI 1018 steel and a new high strength steel HY-180. The crack propagation data obtained for both steels tested in air can be described accurately by the power law first suggested by Paris, da/dN = C(ΔK)/sup n/, where a is the crack length, N the number of cycles, and C and n are material constants. However, the exponent n was found to be two times larger for AISI 1018 steel than HY-180 steel
Ma, Longzhou; Roy, Shawoon K.; Hasan, Muhammad H.; Pal, Joydeep; Chatterjee, Sudin
2012-02-01
The fatigue crack propagation (FCP) as well as the sustained loading crack growth (SLCG) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 (Special Metals Corporation Family of Companies) and HAYNES 230 (Haynes International, Inc., Kokomo, IN), were studied at increased temperatures in laboratory air under a constant stress-intensity-factor ( K) condition. The crack propagation tests were conducted using a baseline cyclic triangular waveform with a frequency of 1/3 Hz. Various hold times were imposed at the maximum load of a fatigue cycle to study the hold time effect. The results show that a linear elastic fracture mechanics (LEFM) parameter, stress intensity factor ( K), is sufficient to describe the FCP and SLCG behavior at the testing temperatures ranging from 873 K to 1073 K (600 °C to 800 °C). As observed in the precipitation-strengthened superalloys, both INCONEL 617 and HAYNES 230 exhibited the time-dependent FCP, steady SLCG behavior, and existence of a damage zone ahead of crack tip. A thermodynamic equation was adapted to correlate the SLCG rates to determine thermal activation energy. The fracture modes associated with crack propagation behavior were discussed, and the mechanism of time-dependent FCP as well as SLCG was identified. Compared with INCONEL 617, the lower crack propagation rates of HAYNES 230 under the time-dependent condition were ascribed to the different fracture mode and the presence of numerous W-rich M6C-type and Cr-rich M23C6-type carbides. Toward the end, a phenomenological model was employed to correlate the FCP rates at cycle/time-dependent FCP domain. All the results suggest that an environmental factor, the stress assisted grain boundary oxygen embrittlement (SAGBOE) mechanism, is mainly responsible for the accelerated time-dependent FCP rates of INCONEL 617 and HAYNES 230.
International Nuclear Information System (INIS)
The study of mechanical properties degradation of zirconium alloys due to hydrides assumes fundamental importance in the nuclear industry.During normal nuclear reactors operation, structural parts absorbed hydrogen generated from radiolysis of water, causing detrimental effects on mechanical properties.As a consequence, these materials are easily cracked in the presence of mechanical solicitation due to loss of ductility of the hydride-phase.The presence of cracks indicates fracture mechanic as the most suitable methodology in the study of mechanical properties degradation.In this work we used the crack tip opening displacement (CTOD) criteria to evaluate the detrimental effects on mechanical properties with the observation in SEM of crack propagation.The samples used were SEN (B) of Zry-4 and cathodic homogenous charged with hydrogen concentrations lower than 400 ppm
Effect of CTE on Fatigue Cracking of Stainless Steel Vessels
International Nuclear Information System (INIS)
Visual examination of lithium hydride reactor vessels revealed cracks that were adjacent to welds. Most cracks were parallel to the weld in the bottom portion of the vessel. Sections were cut out of the vessel containing these cracks and examined using the metallograph, scanning electron microscope, and microprobe to determine the cause of cracking. most of the cracks originated on the outer surface just outside the weld fusion line in the heat affected zone and propagated along grain boundaries. Crack depth of those sections examined ranged from about 300 to 500(micro)m. Other cracks were reported to have reached a maximum depth of 0.32-cm (0.125-inch). The primary cause of cracking was the creation of high tensile stresses associated with the CTE differences between the filler metal and the base metal during operation of the vessel in a thermally cyclic environment. This failure mechanism could be described as creep-type fatigue whereby crack propagation might have been aided by the presence of brittle chromium carbides along the grain boundaries, which is indicative of a slightly sensitized microstructure
International Nuclear Information System (INIS)
Duplex stainless steels are used as cast components in nuclear power plants. At the service temperature of about 320 C, the ferrite phase is thermally aged and embrittled. This induces a significant decrease in fracture properties of these materials. The aim of this work consists in studying Fatigue Crack Growth Rates (FCGR) and Fatigue Crack Growth Mechanisms (FCGM) as a function of thermal ageing and test temperature (20 C/320 C). Two cast duplex stainless steels (30% ferrite) are tested. In order to better understand the influence of the crystallographic orientation of the phases on the FCGM, the solidification structure of the material is studied by Electron Back-Scatter Diffraction (EBSD) and by Unidirectional Solidification Quenching. Fatigue crack growth tests are also performed in equiaxed and basaltic structures. Microstructure, fatigue crack growth mechanical properties and mechanisms are thus studied in relation to each other. In the studied range of delta K, the crack propagates without any preferential path by successive ruptures of phase laths. The macroscopic crack propagation plane, as determined by EBSD, depends on the crystallographic orientation of the ferrite grain. So, according to the solidification structure, secondary cracks can appear, which in turn influences the FCGR. Fatigue crack closure, which has to be determined to estimate the intrinsic FCGR, decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening. The Paris exponent as determined from intrinsic FCGR increases with ageing. Intrinsic FCGR can then be separated in two ranges: one with lower FCGR in aged materials than in un-aged and one with the reversed tendency. (author)
Energy Technology Data Exchange (ETDEWEB)
Yamaguchi, Y.; Nakakura, M.; Nonaka, H. [Osaka Gas Co. Ltd., Osaka (Japan)
1996-11-15
In this paper, the influences of fatigue frequency and hydrogen content in material were investigated for the crack propagation at 0.5 stress ratio using a low alloy steel by cyclic stress in an overprotective environment. The six frequencies between 10 Hz and 0.05 Hz, and three hydrogen contents were used as the experimental conditions. Three stages divided by crack propagation behavior existed in the relationship between the stress intensity factor and the crack propagation rate. The lower the frequency was, the higher the crack propagation was at the higher stress intensity factor. The crack propagation rate was constant at Stage II. The relationship between crack propagation rate at the Stage II and frequency was expressed as (da/dn)II = 1.4 {times} 10{sup -7} {times} f{sup -0.763}. It was supposed that the cause of the Stage II was due to the lack of hydrogen to crack tip through diffusion. The hydrogen diffusion rate was of the order of 10{sup -7} m/s. The hydrogen content enhanced the crack propagation rate at 1 Hz. The Paris Law in hydrogen embrittlement environment at 1 Hz was expressed as a function of hydrogen content, which was da/dN=C({Delta}K){sup m} where C=1.72 {times} 10{sup -10} {times} (10{sup -5.6CH}), m=3.05+4.74CH, CH: Hydrogen content in material. 12 refs., 8 figs., 2 tabs.
Cherepanov, Genady P
2015-03-28
By way of introduction, the general invariant integral (GI) based on the energy conservation law is presented, with mention of cosmic, gravitational, mass, elastic, thermal and electromagnetic energy of matter application to demonstrate the approach, including Coulomb's Law generalized for moving electric charges, Newton's Law generalized for coupled gravitational/cosmic field, the new Archimedes' Law accounting for gravitational and surface energy, and others. Then using this approach the temperature track behind a moving crack is found, and the coupling of elastic and thermal energies is set up in fracturing. For porous materials saturated with a fluid or gas, the notion of binary continuum is used to introduce the corresponding GIs. As applied to the horizontal drilling and fracturing of boreholes, the field of pressure and flow rate as well as the fluid output from both a horizontal borehole and a fracture are derived in the fluid extraction regime. The theory of fracking in shale gas reservoirs is suggested for three basic regimes of the drill mud permeation, with calculating the shape and volume of the local region of the multiply fractured rock in terms of the pressures of rock, drill mud and shale gas. PMID:25713454
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Gonzalez-Rodriguez, J.G. [UAEM, Cuernavaca (Mexico); Salinas-Bravo, V.M.; Garcia-Ochoa, E. [Inst. de Investigaciones Electricas, Temixco (Mexico). Dept. de Fisicoquimica Aplicada; Diaz-Sanchez, A. [Inst. Nacional de Investigaciones Nucleares, Toluca (Mexico). Dept. de Materiales
1997-09-01
Corrosion potential transients were associated with nucleation and propagation of stress corrosion cracks in a 17-4 precipitation-hardenable (PH) martensitic stainless steel (SS) during slow strain rate tests (SSRT) at 90 C in deaerated sodium chloride (NaCl) solutions, Test solutions included 20 wt% NaCl at pH 3 and 7, similar to normal and faulted steam turbine environments, respectively. Time series were analyzed using the fast Fourier transform method. At the beginning of straining, the consistent noise behavior was perturbed with small potential transients, probably associated with rupture of the surface oxide layer. After yielding, these transients increased in intensity. At maximum load, the transients were still higher in intensity and frequency. These potential transients were related to crack nucleation and propagation. When the steel did not fail by stress corrosion cracking (SCC), such transients were found only at the beginning of the test. The power spectra showed some differences in all cases in roll-off slope and voltage magnitude, but these were not reliable tools to monitor the initiation and propagation of stress corrosion cracks.
International Nuclear Information System (INIS)
In order to improve the estimation methods of surface crack growth propagation through the thickness of components, CEA has proposed a benchmark to members of PWG-3 Task Group on integrity of metal components and structures. The subject is a simple configuration of a plate containing a semi-elliptical crack and submitted to a cyclic bending load. An experimental data-set from CEA was used to validate two issues in the topic of Leak Before Break: crack growth estimation, crack shape evolution during the propagation through the thickness. This report presents all estimations performed by the 9 participants and collected by CEA. They are compared to the experimental results. An analysis of the used procedures is also proposed. This, associated with the study of the accuracy of different methodologies, leads to comments and recommendations on the analysis of fatigue crack growth (e.g. on elastic and elastic-plastic analysis with the cyclic tensile curve vs the monotonic tensile curve, the difficulty to use finite element analysis, etc.)
Finite element modelling of fibre-reinforced brittle materials
Kullaa, J.
1997-01-01
The tensile constitutive behaviour of fibre-reinforced brittle materials can be extended to two or three dimensions by using the finite element method with crack models. The three approaches in this study include the smeared and discrete crack concepts and a multi-surface plasticity model. The tensi
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Polvora, J.P
1998-12-31
This study deals with crack propagation at 650 deg C on an austenitic stainless steel referenced by Z2 CND 17-12 (316L(NN)). It is based on an experimental work concerning two different cracked specimens: CT specimens tested at 650 deg C in fatigue, creep and creep-fatigue with load controlled conditions (27 tests), tube specimens containing an internal circumferential crack tested in four points bending with displacement controlled conditions (10 tests). Using the fracture mechanics tools (K, J and C* parameters), the purpose here is to construct a methodology of calculation in order to predict the evolution of a crack with time for each loading condition using a fracture mechanics global approach. For both specimen types, crack growth is monitored by using a specific potential drop technique. In continuous fatigue, a material Paris law at 650 deg C is used to correlate crack growth rate with the stress intensity factor range corrected with a factor U(R) in order to take into account the effects of crack closure and loading ratio R. In pure creep on CT specimens, crack growth rate is correlated to the evolution of the C* parameter (evaluated experimentally) which can be estimated numerically with FEM calculations and analytically by using a simplified method based on a reference stress approach. A modeling of creep fatigue growth rate is obtained from a simple summation of the fatigue contribution and the creep contribution to the total crack growth. Good results are obtained when C* parameter is evaluated from the simplified expression C*{sub s}. Concerning the tube specimens tested in 4 point bending conditions, a simulation based on the actual A 16 French guide procedure proposed at CEA. (authors) 104 refs.
In the Initiation of Brittle Compressive Failure: Lessons From Ice
Renshaw, C. E.; Schulson, E. M.
2001-12-01
Brittle failure limits the compressive strength of rock and ice when rapidly loaded under low to moderate confinement. Higher confinement or slower loading results in ductile failure once the brittle-ductile transition is crossed. It is well established that the macroscopic brittle failure of rock, concrete and other brittle materials under compression is preceded by the initiation and sliding of microscopic primary cracks, creating wing cracks at their tips. In laboratory samples, microcracks begin to nucleate more or less uniformly throughout the sample at compressions equal to about 1/5 to 1/3 the terminal failure stress. Under little to no confinement, wing cracks extend and link together, splitting the material into slender columns which then fail. Under low to moderate confinement, wing crack growth is restricted and terminal failure is controlled by the localization of damage along discrete bands of intense damage inclined by approximately 30 degrees to the direction of the most compressive stress. Earlier investigators proposed that localization results from either the linkage of wing cracks or the buckling of microcolumns created between adjacent wing cracks. Observations of compressive failure in ice suggest a new mechanism whereby localization initiates due to the bending-induced failure of slender microcolumns created between sets of secondary cracks emanating from one side of a primary crack. Analysis of this mechanism leads to a closed-form, quantitative model that only depends on independently measureable mechanical parameters. We show that model predictions for both the brittle compressive strength and the brittle-ductile transition are consistent with data from a variety of crystalline materials.
Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression
Amann, Florian; Button, Edward Alan; Evans, Keith Frederick; Gischig, Valentin Samuel; Blümel, Manfred
2011-07-01
The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress-strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.
International Nuclear Information System (INIS)
Near-threshold fatigue crack propagation behavior of a high strength steel was investigated in laboratory air under closure-free testing conditions at R = 0.7 (= Reff), and at two different K-gradients. Depending on the criterion assumed, the threshold value differed; the criterion of non-propagation gave a lower threshold value than that assumed by the propagation criterion. Nevertheless, the subsequent propagation following a load increase was discontinuous in both the cases, and da/dN vs ΔK curves obtained on the same specimen during the K-decreasing and the K-increasing test were not necessarily identical in the threshold regime. This behavior, hysteresis, is analyzed mainly from the experimental viewpoint, and it is shown that hysteresis is not an artifact. (orig.) With 13 figs., 3 appendices
Kvasha, Oleg V.; Boström, Anders; Glushkova, Natalia V.; Glushkov, Evgeny V.
2011-08-01
The propagation of in-plane (P-SV) waves in a symmetrically three-layered thick plate with a periodic array of interface cracks is investigated. The exact dispersion relation is derived based on an integral equation approach and Floquet's theorem. The interface cracks can be a model for interface damage, but a much simpler model is a recently developed spring boundary condition. This boundary condition is used for the thick plate and also in the derivation of plate equations with the help of power series expansions in the thickness coordinate. For low frequencies (cracks small compared to the wavelength) the three approaches give more or less coinciding dispersion curves, and this is a confirmation that the spring boundary condition is a reasonable approximation at low frequencies.
International Nuclear Information System (INIS)
As is demonstrated by the experimental results, the method of acoustic emission testing is well suited to obtain information on the statistical-microscopic material behaviour of fracture mechanically loaded materials before and during the propagation of cracks. So this method closes the gap between the on-line determination of macroscopic parameters (stress, volume enlargement, etc) and the microscopic observations which can only be performed after the material test. Based on different models, the fundamental processes of acoustic emission are presented to determine their influence on the experimental measurements. In addition models are presented for stable and unstable crack propagation. The most essential result of this work is a demonstration of a hitherto unknown interrelation between amplitude and time statistics. (orig./RW)
International Nuclear Information System (INIS)
Ductile crack initiation and propagation in AA6061 aluminum alloy for a fatigue precrack have been studied in situ via synchrotron radiation computed laminography, a technique specifically developed for three-dimensional imaging of laterally extended sheet specimens with micrometer resolution. The influence of the microstructure, i.e. due to the presence of coarse Mg2Si precipitates and iron-rich intermetallics, on the void nucleation process is investigated. Coarse Mg2Si precipitates are found to play a preponderant role in the void nucleation and ductile fracture process. Void growth and void coalescence are then observed and quantified by three-dimensional image analysis during crack initiation and propagation. Parameters for a Gurson–Tvergaard–Needleman micromechanical damage model are identified experimentally and validated by finite-element simulations
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Menouillard, T
2007-09-15
Computerized simulation is nowadays an integrating part of design and validation processes of mechanical structures. Simulation tools are more and more performing allowing a very acute description of the phenomena. Moreover, these tools are not limited to linear mechanics but are developed to describe more difficult behaviours as for instance structures damage which interests the safety domain. A dynamic or static load can thus lead to a damage, a crack and then a rupture of the structure. The fast dynamics allows to simulate 'fast' phenomena such as explosions, shocks and impacts on structure. The application domain is various. It concerns for instance the study of the lifetime and the accidents scenario of the nuclear reactor vessel. It is then very interesting, for fast dynamics codes, to be able to anticipate in a robust and stable way such phenomena: the assessment of damage in the structure and the simulation of crack propagation form an essential stake. The extended finite element method has the advantage to break away from mesh generation and from fields projection during the crack propagation. Effectively, crack is described kinematically by an appropriate strategy of enrichment of supplementary freedom degrees. Difficulties connecting the spatial discretization of this method with the temporal discretization of an explicit calculation scheme has then been revealed; these difficulties are the diagonal writing of the mass matrix and the associated stability time step. Here are presented two methods of mass matrix diagonalization based on the kinetic energy conservation, and studies of critical time steps for various enriched finite elements. The interest revealed here is that the time step is not more penalizing than those of the standard finite elements problem. Comparisons with numerical simulations on another code allow to validate the theoretical works. A crack propagation test in mixed mode has been exploited in order to verify the simulation