A numerical study of non-linear crack tip parameters

Directory of Open Access Journals (Sweden)

F.V. Antunes

2015-07-01

Full Text Available Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i LEFM concepts are applicable to the problem under study; ii the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii the Keff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.

Sub-10-micrometer toughening and crack tip toughness of dental enamel

OpenAIRE

Ang, Siang Fung; Schulz, Anja; Pacher Fernandes, Rodrigo; Schneider, Gerold A.

2011-01-01

In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited R-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip toughness (KI0, KIII0), the crack closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine en...

Crack Tip Parameters for Growing Cracks in Linear Viscoelastic Materials

DEFF Research Database (Denmark)

Brincker, Rune

In this paper the problem of describing the asymptotic fields around a slowly growing crack in a linearly viscoelastic material is considered. It is shown that for plane mixed mode problems the asymptotic fields must be described by 6 parameters: 2 stress intensity factors and 4 deformation...... intensity factors. In the special case of a constant Poisson ratio only 2 deformation intensity factors are needed. Closed form solutions are given both for a slowly growing crack and for a crack that is suddenly arrested at a point at the crack extension path. Two examples are studied; a stress boundary...... value problem, and a displacement boundary value problem. The results show that the stress intensity factors and the displacement intensity factors do not depend explicitly upon the velocity of the crack tip....

Semi-empirical crack tip analysis

Science.gov (United States)

Chudnovsky, A.; Ben Ouezdon, M.

1988-01-01

Experimentally observed crack opening displacements are employed as the solution of the multiple crack interaction problem. Then the near and far fields are reconstructed analytically by means of the double layer potential technqiue. Evaluation of the effective stress intensity factor resulting from the interaction of the main crack and its surrounding crazes in addition to the remotely applied load is presented as an illustrative example. It is shown that crazing (as well as microcracking) may constitute an alternative mechanism to Dugdale-Berenblatt models responsible for the cancellation of the singularity at the crack tip.

Sub-10-micrometer toughening and crack tip toughness of dental enamel.

Science.gov (United States)

Ang, Siang Fung; Schulz, Anja; Pacher Fernandes, Rodrigo; Schneider, Gerold A

2011-04-01

In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited R-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip's toughness (K(I0),K(III0)), the crack's closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine enamel was studied using atomic force microscopy (AFM). The mode I crack tip toughness K(I0) of cracks along enamel rod boundaries and across enamel rods exhibit a similar range of values: K(I0,Ir)=0.5-1.6MPa m(0.5) (based on Irwin's 'near-field' solution) and K(I0,cz)=0.8-1.5MPa m(0.5) (based on the cohesive zone solution of the Dugdale-Muskhelishvili (DM) crack model). The mode III crack tip toughness K(III0,Ir) was computed as 0.02-0.15MPa m(0.5). The crack-closure stress at the crack tip was computed as 163-770 MPa with a cohesive zone length and width 1.6-10.1μm and 24-44 nm utilizing the cohesive zone solution. Toughening elements were observed under AFM and SEM: crack bridging due to protein ligament and hydroxyapatite fibres (micro- and nanometer scale) as well as microcracks were identified. Crown Copyright © 2010. Published by Elsevier Ltd. All rights reserved.

Role of hydrogen in stress corrosion cracking

International Nuclear Information System (INIS)

Mehta, M.L.

1981-01-01

Electrochemical basis for differentiation between hydrogen embrittlement and active path corrosion or anodic dissolution crack growth mechanisms is examined. The consequences of recently demonstrated acidification in crack tip region irrespective of electrochemical conditions at the bulk surface of the sample are that the hydrogen can evolve within the crack and may be involved in the cracking process. There are basically three aspects of hydrogen involvement in stress corrosion cracking. In dissolution models crack propagation is assumed to be caused by anodic dissolution on the crack tip sustained by cathodic reduction of hydrogen from electrolyte within the crack. In hydrogen induced structural transformation models it is postulated that hydrogen is absorbed locally at the crack tip producing structural changes which facilitate crack propagation. In hydrogen embrittlement models hydrogen is absorbed by stressed metal from proton reduction from the electrolyte within the crack and there is interaction between lattice and hydrogen resulting in embrittlement of material at crack tip facilitating crack propagation. In the present paper, the role of hydrogen in stress corrosion crack growth in high strength steels, austenitic stainless steels, titanium alloys and high strength aluminium alloys is discussed. (author)

Characterization of SCC crack tip and hydrogen distribution in alloy 600

Energy Technology Data Exchange (ETDEWEB)

Fujii, Katsuhiko; Nakajima, Nobuo; Fukuya, Koji [Institute of Nuclear Safety System Inc., Mihama, Fukui (Japan); Hatano, Yuji [Toyama Univ. (Japan)

2001-09-01

In order to identify the mechanism of primary water stress corrosion cracking (SCC), direct observations of SCC crack tip microstructure and hydrogen distribution in alloy 600 were carried out. A new technique has been developed to prepare electron transparent foils including the crack tip using focused-ion beam (FIB) micro-processing technique. Cr-rich oxide and metal-Ni phase were identified in the crack tip and grain boundary ahead of the crack. >From the fact that similar microstructure was observed in the surface oxide layer, it is suggested that the oxidation mechanism is identical at the crack tip region and the surface. It became clear that the crack tip region and the oxidized grain boundary don't work as strong trapping sites of solute hydrogen under unloaded condition, because a homogeneous hydrogen distribution around the crack tip region was detected by tritium microautoradiography. (author)

Analysis of crack opening stresses for center- and edge-crack tension specimens

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Tong Di-Hua

2014-04-01

Full Text Available Accurate determination of crack opening stress is of central importance to fatigue crack growth analysis and life prediction based on the crack-closure model. This paper studies the crack opening behavior for center- and edge-crack tension specimens. It is found that the crack opening stress is affected by the crack tip element. By taking the crack tip element into account, a modified crack opening stress equation is given for the center-crack tension specimen. Crack surface displacement equations for an edge crack in a semi-infinite plate under remote uniform tension and partially distributed pressure are derived by using the weight function method. Based on these displacements, a crack opening stress equation for an edge crack in a semi-infinite plate under uniform tension has been developed. The study shows that the crack opening stress is geometry-dependent, and the weight function method provides an effective and reliable tool to deal with such geometry dependence.

Observations on Hydride Structures at the Tip of Arrested Cracks Grown under Conditions of Delayed Hydride Cracking

International Nuclear Information System (INIS)

Pettersson, Kjell; Oskarsson, Magnus; Bergqvist, Hans

2003-04-01

One sample of Zr2.5%Nb and one sample of cold worked and stress relieved Zircaloy-4 which have been tested for hydrogen induced crack growth have been examined in the crack tip region with the aim of determining the mechanism behind the growth of cracks. The proposed mechanisms are brittle failure of a crack tip hydride and hydrogen enhanced localized shear. The examinations were done by TEM and SEM. However attempts to produce a TEM specimen with a thinned region at the tip of the crack were unsuccessful in both samples. One feature observed in the Zr2.5%Nb material may however be an indication of intense shear deformation at the tip of the crack. On the other hand all observations on the Zircaloy-4 sample indicate precipitation of hydrides ahead of the crack tip and the presence of hydrides on the crack flanks

Mixed-mode crack tip loading and crack deflection in 1D quasicrystals

Science.gov (United States)

Wang, Zhibin; Scheel, Johannes; Ricoeur, Andreas

2016-12-01

Quasicrystals (QC) are a new class of materials besides crystals and amorphous solids and have aroused much attention of researchers since they were discovered. This paper presents a generalized fracture theory including the J-integral and crack closure integrals, relations between J1, J2 and the stress intensity factors as well as the implementation of the near-tip stress and displacement solutions of 1D QC. Different crack deflection criteria, i.e. the J-integral and maximum circumferential stress criteria, are investigated for mixed-mode loading conditions accounting for phonon-phason coupling. One focus is on the influence of phason stress intensity factors on crack deflection angles.

Crack tip fracture toughness of base glasses for dental restoration glass-ceramics using crack opening displacements.

Science.gov (United States)

Deubener, J; Höland, M; Höland, W; Janakiraman, N; Rheinberger, V M

2011-10-01

The critical stress intensity factor, also known as the crack tip toughness K(tip), was determined for three base glasses, which are used in the manufacture of glass-ceramics. The glasses included the base glass for a lithium disilicate glass-ceramic, the base glass for a fluoroapatite glass-ceramic and the base glass for a leucite glass-ceramic. These glass-ceramic are extensively used in the form of biomaterials in restorative dental medicine. The crack tip toughness was established by using crack opening displacement profiles under experimental conditions. The crack was produced by Vickers indentation. The crack tip toughness parameters determined for the three glass-ceramics differed quite significantly. The crack tip parameters of the lithium disilicate base glass and the leucite base glass were higher than that of the fluoroapatite base glass. This last material showed glass-in-glass phase separation. The discussion of the results clearly shows that the droplet glass phase is softer than the glass matrix. Therefore, the authors conclude that a direct relationship exists between the chemical nature of the glasses and the crack tip parameter. Copyright © 2011 Elsevier Ltd. All rights reserved.

Characterization of SCC crack tips and surface oxide layers in alloy 600

Energy Technology Data Exchange (ETDEWEB)

Fujii, Katsuhiko; Fukuya, Koji [Inst. of Nuclear Safety System Inc., Mihama, Fukui (Japan)

2002-09-01

In order to investigate the mechanism of primary water stress corrosion cracking (SCC), direct observation of microstructures of SCC crack tips and surface oxide layers in alloy 600 were carried out. A focused-ion beam (FIB) micro-processing technique was applied to prepare electron transparent foils including the crack tip and the surface oxide layer without any damage to those microstructures. Transmission electron microscopy and analysis were used to characterize the crack tips and surface oxide layers. Cr-rich oxides and a metal-Ni phase were identified in the crack tips and grain boundaries ahead of the crack tips independent of dissolved hydrogen concentrations. >From the fact that the Cr-rich oxides and metal-Ni phase were observed in the inner surface oxide layer, the same oxidation mechanism as the surface is proposed for the crack tip region and internal oxidation accompanying selective Cr oxidation is suggested as the mechanism. (author)

Direct observation of microtwin formation at crack tips in InP

International Nuclear Information System (INIS)

Vanderschaeve, G.; Caillard, D.; Peyrade, J.P.

1992-01-01

This paper reports that in brittle materials which contain cracks, stress concentrations arise at crack tips. At low temperatures, when the load is increased, brittle fracture happens for a critical stress intensity factor, which is an intrinsic material property, depending on the loading mode and on the cleavage plane. At higher temperatures dislocations may be emitted at the crack tip: a plastic zone is formed, which releases the stresses and increases the critical load for crack propagation. It is generally accepted that the brittle-to ductile transition is controlled directly or indirectly by dislocation mobility. During the course of an in situ transmission electron microscopy, study of dislocation mobility in the III-V compound InP, we have observed the nucleation at a crack tip and the propagation of partial dislocations of same Burgers vectors, resulting in a microtwinning of the crystal. Such an observation provides information on both the way stress relaxation occurs and the relative mobilities of the partial dislocations in this material. In spite of the importance of twin formation on the quality of the material used as substrate in semiconducting devices, this last point is rather poorly documented

A linear least squares approach for evaluation of crack tip stress field parameters using DIC

Science.gov (United States)

Harilal, R.; Vyasarayani, C. P.; Ramji, M.

2015-12-01

In the present work, an experimental study is carried out to estimate the mixed-mode stress intensity factors (SIF) for different cracked specimen configurations using digital image correlation (DIC) technique. For the estimation of mixed-mode SIF's using DIC, a new algorithm is proposed for the extraction of crack tip location and coefficients in the multi-parameter displacement field equations. From those estimated coefficients, SIF could be extracted. The required displacement data surrounding the crack tip has been obtained using 2D-DIC technique. An open source 2D DIC software Ncorr is used for the displacement field extraction. The presented methodology has been used to extract mixed-mode SIF's for specimen configurations like single edge notch (SEN) specimen and centre slant crack (CSC) specimens made out of Al 2014-T6 alloy. The experimental results have been compared with the analytical values and they are found to be in good agreement, thereby confirming the accuracy of the algorithm being proposed.

Behavior of Fatigue Crack Tip Opening in Air and Corrosive Atmosphere

Science.gov (United States)

Hayashi, Morihito; Toeda, Kazunori

In the study, a formula for predicting fatigue crack tip opening displacement is deduced firstly. And then, due to comparing actual crack growth rate with the deduced formula, the crack tip configuration factor is defined to figure out the crack tip opening configuration that is useful to clarify the behavior of fatigue crack tip formation apparently. Applying the concept, the crack growth of 7/3 brass and 6/4 brass is predicted from the formula, by replacing material properties such as plastic flow resistance, Young modulus, the Poisson ratio, and fatigue toughness, and fatigue test conditions such as the stress intensity factor range, the load ratio, and cycle frequency. Furthermore, the theoretically expected results are verified with the fatigue tests which were carried out on CT specimens under different load conditions of load ratio, cycle frequency, and cyclic peak load, in different environments of air or corrosive ammonia atmosphere, for various brasses. And by comparing and discussing the calculated crack growth rate with attained experimental results, the apparent configuration factor at the crack tip is determined. And through the attained factor which changes along with crack growth, the behaviors of fatigue crack tip formation under different test conditions have been found out.

On the measurement of the crack tip stress field as a means of determining Delta K(sub eff) under conditions of fatigue crack closure

Science.gov (United States)

Wallhead, Ian R.; Edwards, Lyndon; Poole, Peter

1994-01-01

The optical method of caustics has been successfully extended to enable stress intensity factors as low as 1MPa square root of m to be determined accurately for central fatigue cracks in 2024-T3 aluminium alloy test panels. The feasibility of using this technique to study crack closure, and to determine the effective stress intensity factor range, Delta K(sub eff), has been investigated. Comparisons have been made between the measured values of stress intensity factor, K(sub caus), and corresponding theoretical values, K(sub theo), for a range of fatigue cracks grown under different loading conditions. The values of K(sub caus) and K(sub theo) were in good agreement at maximum stress, where the cracks are fully open, while K(sub caus) exceeded K(sub theo) at minimum stress, due to crack closure. However, the levels of crack closure and values of Delta K(sub eff) obtained could not account for the variations of crack growth rate with loading conditions. It is concluded that the values of Delta K(sub eff), based on caustic measurements in a 1/square root of r stress field well outside the plastic zone, do not fully reflect local conditions which control crack tip behavior.

T-stresses for internally cracked components

International Nuclear Information System (INIS)

Fett, T.

1997-12-01

The failure of cracked components is governed by the stresses in the vicinity of the crack tip. The singular stress contribution is characterised by the stress intensity factor K, the first regular stress term is represented by the so-called T-stress. T-stress solutions for components containing an internal crack were computed by application of the Bundary Collocation Method (BCM). The results are compiled in form of tables or approximative relations. In addition a Green's function of T-stresses is proposed for internal cracks which enables to compute T-stress terms for any given stress distribution in the uncracked body. (orig.) [de

The effect of crack branching on the residual lifetime of machine components containing stress corrosion cracks

International Nuclear Information System (INIS)

Magdowski, R.M.; Uggowitzer, P.J.; Speidel, M.O.

1985-01-01

A comparison is presented of theoretical, numerical and experimental investigations concerning the effect of crack branching on the reduction of stress intensity at the tip of single cracks. The results indicate that the division of a single crack into n branches reduces the stress intensity at the branch tips by a factor of about 1/√n. This permits branched cracks to grow to larger depths before becoming critical. The implication is that longer residual lifetimes and longer operating times between inspections can be calculated for machine components with growing branched stress corrosion cracks. (author)

Mode I and mixed mode crack-tip fields in strain gradient plasticity

DEFF Research Database (Denmark)

Goutianos, Stergios

2011-01-01

Strain gradients develop near the crack-tip of Mode I or mixed mode cracks. A finite strain version of the phenomenological strain gradient plasticity theory of Fleck–Hutchinson (2001) is used here to quantify the effect of the material length scales on the crack-tip stress field for a sharp...... stationary crack under Mode I and mixed mode loading. It is found that for material length scales much smaller than the scale of the deformation gradients, the predictions converge to conventional elastic–plastic solutions. For length scales sufficiently large, the predictions converge to elastic solutions....... Thus, the range of length scales over which a strain gradient plasticity model is necessary is identified. The role of each of the three material length scales, incorporated in the multiple length scale theory, in altering the near-tip stress field is systematically studied in order to quantify...

Analytical description of brittle-to-ductile transition in bcc metals. Nucleation of dislocation loop at the crack tip

International Nuclear Information System (INIS)

Voskoboinikov, R.E.

2002-03-01

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.)

Role of hydrogen on the incipient crack tip deformation behavior in α-Fe: An atomistic perspective

Science.gov (United States)

Adlakha, I.; Solanki, K. N.

2018-01-01

A crack tip in α-Fe presents a preferential trap site for hydrogen, and sufficient concentration of hydrogen can change the incipient crack tip deformation response, causing a transition from a ductile to a brittle failure mechanism for inherently ductile alloys. In this work, the effect of hydrogen segregation around the crack tip on deformation in α-Fe was examined using atomistic simulations and the continuum based Rice-Thompson criterion for various modes of fracture (I, II, and III). The presence of a hydrogen rich region ahead of the crack tip was found to cause a decrease in the critical stress intensity factor required for incipient deformation for various crack orientations and modes of fracture examined here. Furthermore, the triaxial stress state ahead of the crack tip was found to play a crucial role in determining the effect of hydrogen on the deformation behavior. Overall, the segregation of hydrogen atoms around the crack tip enhanced both dislocation emission and cleavage behavior suggesting that hydrogen has a dual role during the deformation in α-Fe.

Crack Tip Creep Deformation Behavior in Transversely Isotropic Materials

International Nuclear Information System (INIS)

Ma, Young Wha; Yoon, Kee Bong

2009-01-01

Theoretical mechanics analysis and finite element simulation were performed to investigate creep deformation behavior at the crack tip of transversely isotropic materials under small scale creep (SCC) conditions. Mechanical behavior of material was assumed as an elastic-2 nd creep, which elastic modulus ( E ), Poisson's ratio (v ) and creep stress exponent ( n ) were isotropic and creep coefficient was only transversely isotropic. Based on the mechanics analysis for material behavior, a constitutive equation for transversely isotropic creep behavior was formulated and an equivalent creep coefficient was proposed under plain strain conditions. Creep deformation behavior at the crack tip was investigated through the finite element analysis. The results of the finite element analysis showed that creep deformation in transversely isotropic materials is dominant at the rear of the crack-tip. This result was more obvious when a load was applied to principal axis of anisotropy. Based on the results of the mechanics analysis and the finite element simulation, a corrected estimation scheme of the creep zone size was proposed in order to evaluate the creep deformation behavior at the crack tip of transversely isotropic creeping materials

New theory for crack-tip twinning in fcc metals

Science.gov (United States)

Andric, Predrag; Curtin, W. A.

2018-04-01

Dislocation emission from a crack tip is a necessary mechanism for crack tip blunting and toughening. In fcc metals under Mode I loading, a first partial dislocation is emitted, followed either by a trailing partial dislocation ("ductile" behaviour) or a twinning partial dislocation ("quasi-brittle"). The twinning tendency is usually estimated using the Tadmor and Hai extension of the Rice theory. Extensive molecular statics simulations reveal that the predictions of the critical stress intensity factor for crack tip twinning are always systematically lower (20-35%) than observed. Analyses of the energy change during nucleation reveal that twin partial emission is not accompanied by creation of a surface step while emission of the trailing partial creates a step. The absence of the step during twinning motivates a modified model for twinning nucleation that accounts for the fact that nucleation does not occur directly at the crack tip. Predictions of the modified theory are in excellent agreement with all simulations that show twinning. Emission of the trailing partial dislocation, including the step creation, is predicted using a model recently introduced to accurately predict the first partial emission and shows why twinning is preferred. A second mode of twinning is found wherein the crack first advances by cleavage and then emits the twinning partial at the new crack tip; this mode dominates for emission beyond the first twinning partial. These new theories resolve all the discrepancies between the Tadmor twinning analysis and simulations, and have various implications for fracture behaviour and transitions.

Crack-tip chemistry modeling of stage I stress corrosion cracking

International Nuclear Information System (INIS)

Jones, R.H.; Simonen, E.P.

1991-10-01

Stage I stress corrosion cracking usually exhibits a very strong K dependence with Paris law exponents of up to 30. 2 Model calculations indicate that the crack velocity in this regime is controlled by transport through a salt film and that the K dependence results from crack opening controlled salt film dissolution. An ionic transport model that accounts for both electromigration through the resistive salt film and Fickian diffusion through the aqueous solution was used for these predictions. Predicted crack growth rates are in excellent agreement with measured values for Ni with P segregated to the grain boundaries and tested in IN H 2 SO 4 at +900 mV. This salt film dissolution may be applicable to stage I cracking of other materials

Some aspects of forces and fields in atomic models of crack tips

International Nuclear Information System (INIS)

Hoagland, R.G.; Daw, M.S.; Hirth, J.P.

1991-01-01

This paper examines the stresses and displacement gradients in atomistic models of cracks based on an EAM potential devised for aluminum. Methods for computing these quantities are described. Results are presented for two models differing in terms of the orientations of the crack relative to the crystal, a [100](010) orientation that behaves in a brittle fashion and a [111](110) orientation which emits partial dislocations prior to extending. Both models display lattice trapping. The stresses in the brittle crack model are compared with the linear elastic prediction and found to be in remarkably good agreement to within distances of about one lattice parameter of the crack tip and at the free surface where contributions from sources other than strain energy (e.g., surface tension) influence the results. Similar results are observed for the ductile model until dislocation emission occurs. The largest stresses that develop just prior to crack extension or dislocation emission are used to estimate the ratio of theoretical tensile strength to shear strength in this material. Eshelby's conservation integrals, F and M, are also computed. F is found to be essentially contour independent and in agreement with the linear elastic prediction in both models until dislocation emission occurs, at which point a large screening contribution arises from the emitted partials. The contour size dependence of M reveals some interesting features of the crack tip including a slight wobble of the crack tip inside its potential well with changing applied K and the existence of forces acting to move the crack faces apart as blunting occurs

A study on fatigue crack growth behavior subjected to a single tensile overload: Part II. Transfer of stress concentration and its role in overload-induced transient crack growth

International Nuclear Information System (INIS)

Lee, S.Y.; Choo, H.; Liaw, P.K.; An, K.; Hubbard, C.R.

2011-01-01

The combined effects of overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are suggested to be responsible for the observed changes in the crack opening load and resultant post-overload transient crack growth behavior [Lee SY, Liaw PK, Choo H, Rogge RB, Acta Mater 2010;59:485-94]. In this article, in situ neutron diffraction experiments were performed to quantify the influence of the combined effects by investigating the internal-stress evolution at various locations away from the crack tip. In the overload-retardation period, stress concentration occurs in the crack blunting region (an overload point) until a maximum crack arrest load is reached. The stress concentration is then transferred from the blunting region to the propagating crack tip (following the overload), requiring a higher applied load, as the closed crack is gradually opened. The transfer phenomena of the stress concentration associated with a crack opening process account for the nonlinearity of strain response in the vicinity of the crack tip. The delaying action of stress concentration at the crack tip is understood in conjunction with the concept of a critical stress (i.e. the stress required to open the closed crack behind the crack tip). A linear relationship between Δε eff and ΔK eff provides experimental support for the hypothesis that ΔK eff can be considered as the fatigue crack tip driving force.

High-voltage electron-microscopical observation of crack-tip dislocations in silicon crystals

International Nuclear Information System (INIS)

Tanaka, Masaki; Higashida, Kenji

2005-01-01

Crack-tip dislocations in silicon single crystals were observed by high-voltage electron microscopy. Cracks were introduced into silicon wafers at room temperature by a Vickers indenter. The indented specimens were annealed at 823 K in order to activate dislocation emission from the crack tip under the residual stress due to the indentation. In the specimen without annealing, no dislocations were observed around the crack. On the other hand, in the specimen after the annealing, the aspect of the early stage of dislocation emission was observed, where dislocations were emitted not as a perfect dislocation but as a partial dislocation in the hinge-type plastic zone. Prominent dislocation arrays that were emitted from a crack tip were also observed, and they were found to be of shielding type, which increases the fracture toughness of those crystals

Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture toughness

International Nuclear Information System (INIS)

Bass, B.R.; Bryson, J.W.; Theiss, T.J.; Rao, M.C.

1994-01-01

A program to develop and evaluate fracture methodologies for the assessment of crack-tip constraint effects on fracture toughness of reactor pressure vessel (RPV) steels has been initiated in the Heavy-Section Steel Technology (HSST) Program. Crack-tip constraint is an issue that significantly impacts fracture mechanics technologies employed in safety assessment procedures for commercially licensed nuclear RPVs. The focus of studies described herein is on the evaluation of two stressed-based methodologies for quantifying crack-tip constraint (i.e., J-Q theory and a micromechanical scaling model based on critical stressed volumes) through applications to experimental and fractographic data. Data were utilized from single-edge notch bend (SENB) specimens and HSST-developed cruciform beam specimens that were tested in HSST shallow-crack and biaxial testing programs. Results from applications indicate that both the J-Q methodology and the micromechanical scaling model can be used successfully to interpret experimental data from the shallow- and deep-crack SENB specimen tests. When applied to the uniaxially and biaxially loaded cruciform specimens, the two methodologies showed some promising features, but also raised several questions concerning the interpretation of constraint conditions in the specimen based on near-tip stress fields. Fractographic data taken from the fracture surfaces of the SENB and cruciform specimens are used to assess the relevance of stress-based fracture characterizations to conditions at cleavage initiation sites. Unresolved issues identified from these analyses require resolution as part of a validation process for biaxial loading applications. This report is designated as HSST Report No. 142

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth

Science.gov (United States)

Telesman, Jack; Gabb, Tim; Ghosn, Louis J.

2016-01-01

Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress intensity parameter, Kmax, was developed by incorporating into the formulation the remaining stress level concept as measured by simple stress relaxation tests. The newly proposed parameter, Ksrf, did an excellent job in correlating the dwell crack growth rates for the four heat treatments which were shown to have similar intrinsic environmental cyclic fatigue crack growth resistance.

On fatigue crack growth in ductile materials by crack-tip blunting

DEFF Research Database (Denmark)

Tvergaard, Viggo

2004-01-01

One of the basic mechanisms for fatigue crack growth in ductile metals is that depending on crack-tip blunting under tensile loads and re-sharpening of the crack-tip during unloading. In a standard numerical analysis accounting for finite strains it is not possible to follow this process during...

Friction stress effects on mode I crack growth predictions

NARCIS (Netherlands)

Chen, Q.; Deshpande, V.S.; Giessen, E. van der; Needleman, A.

2003-01-01

The effect of a lattice friction stress on the monotonic growth of a plane strain mode I crack under small-scale yielding conditions is analyzed using discrete dislocation plasticity. When the friction stress is increased from zero to half the dislocation nucleation stress, the crack tip stress

Investigation on the Crack Initiation of V-Shaped Notch Tip in Precision Cropping

Directory of Open Access Journals (Sweden)

Lijun Zhang

2014-01-01

Full Text Available The crack initiation of V-shaped notch tip has a very important influence on the cross-section quality and the cropping time for every segment of metal bar in course of low stress precision cropping. By the finite element method, the influence of machining precision of V-shaped notch bottom corner on the crack initiation location is analyzed and it is pointed out that the crack initiation point locates in the place at the maximal equivalent stress change rate on V-shaped notch surface. The judgment criterion of the crack initiation direction is presented and the corresponding crack initiation angle can be calculated by means of the displacement extrapolation method. The factual crack initiation angle of the metal bar has been measured by using the microscopic measurement system. The formula of the crack initiation life of V-shaped notch tip is built, which mainly includes the stress concentration factor of V-shaped notch, the tensile properties of metal material, and the cyclic loading conditions. The experimental results show that the obtained theoretical analyses about the crack initiation location, the crack initiation direction, and the crack initiation time in this paper are correct. It is also shown that the crack initiation time accounts for about 80% of the cropping time for every segment of the metal bar.

Analysis of accuracy of Williams series approximation of stress field in cracked body – influence of area of interest around crack-tip on multi-parameter regression performance

Directory of Open Access Journals (Sweden)

J. Sobek

2017-01-01

Full Text Available A study on the accuracy of an approximation of the stress field in a cracked body is presented. Crack-tip stress tensor is expressed using the linear elastic fracture mechanics (LEFM theory in this work, more precisely via its multi-parameter formulation, i.e. by Williams power series (WPS. Determination of coefficients of terms of this series is performed using a least squares-based regression technique known as over-deterministic method (ODM for which results from finite element (FE method computation are usually taken as inputs. Main attention is paid to a detailed analysis of a suitable selection of FE nodes whose results serve as the inputs to the employed method. Two different ways of FE nodal selection are compared – nodes selected from the crack tip vicinity lying at a ring of a certain radius versus nodes selected more or less uniformly from a specified part of the test specimen body. Comparison of these approaches is made with the help of procedures developed by the authors which enable both the determination of the coefficients of terms of the analytical WPS approximation of the stress field based on the FE results and the backward reconstruction of the field (again using WPS from those determined terms’ coefficients/functions. The wedge-splitting test (WST specimen with a crack is taken as example for the study.

TEM observations of crack tip: cavity interactions

International Nuclear Information System (INIS)

Horton, J.A.; Ohr, S.M.; Jesser, W.A.

1981-01-01

Crack tip-cavity interactions have been studied by performing room temperature deformation experiments in a transmission electron microscope on ion-irradiated type 316 stainless steel with small helium containing cavities. Slip dislocations emitted from a crack tip cut, sheared, and thereby elongated cavities without a volume enlargement. As the crack tip approached, a cavity volume enlargement occurred. Instead of the cavities continuing to enlarge until they touch, the walls between the cavities fractured. Fracture surface dimples do not correlate in size or density with these enlarged cavities

Effects of External Hydrogen on Hydrogen Transportation and Distribution Around the Fatigue Crack Tip in Type 304 Stainless Steel

Science.gov (United States)

Chen, Xingyang; Zhou, Chengshuang; Cai, Xiao; Zheng, Jinyang; Zhang, Lin

2017-10-01

The effects of external hydrogen on hydrogen transportation and distribution around the fatigue crack tip in type 304 stainless steel were investigated by using hydrogen microprint technique (HMT) and thermal desorption spectrometry. HMT results show that some silver particles induced by hydrogen release are located near the fatigue crack and more silver particles are concentrated around the crack tip, which indicates that hydrogen accumulates in the vicinity of the crack tip during the crack growth in hydrogen gas environment. Along with the crack propagation, strain-induced α' martensite forms around the crack tip and promotes hydrogen invasion into the matrix, which will cause the crack initiation and propagation at the austenite/ α' martensite interface. In addition, the hydrogen content in the vicinity of the crack tip is higher than that at the crack edge far away from the crack tip, which is related to the stress state and strain-induced α' martensite.

Linear least squares approach for evaluating crack tip fracture parameters using isochromatic and isoclinic data from digital photoelasticity

Science.gov (United States)

Patil, Prataprao; Vyasarayani, C. P.; Ramji, M.

2017-06-01

In this work, digital photoelasticity technique is used to estimate the crack tip fracture parameters for different crack configurations. Conventionally, only isochromatic data surrounding the crack tip is used for SIF estimation, but with the advent of digital photoelasticity, pixel-wise availability of both isoclinic and isochromatic data could be exploited for SIF estimation in a novel way. A linear least square approach is proposed to estimate the mixed-mode crack tip fracture parameters by solving the multi-parameter stress field equation. The stress intensity factor (SIF) is extracted from those estimated fracture parameters. The isochromatic and isoclinic data around the crack tip is estimated using the ten-step phase shifting technique. To get the unwrapped data, the adaptive quality guided phase unwrapping algorithm (AQGPU) has been used. The mixed mode fracture parameters, especially SIF are estimated for specimen configurations like single edge notch (SEN), center crack and straight crack ahead of inclusion using the proposed algorithm. The experimental SIF values estimated using the proposed method are compared with analytical/finite element analysis (FEA) results, and are found to be in good agreement.

Numerical analysis of interacting cracks in biaxial stress field

International Nuclear Information System (INIS)

Kovac, M.; Cizelj, L.

1999-01-01

The stress corrosion cracks as seen for example in PWR steam generator tubing made of Inconel 600 usually produce highly irregular kinked and branched crack patterns. Crack initialization and propagation depends on stress state underlying the crack pattern. Numerical analysis (such as finite element method) of interacting kinked and branched cracks can provide accurate solutions. This paper discusses the use of general-purpose finite element code ABAQUS for evaluating stress fields at crack tips of interacting complex cracks. The results obtained showed reasonable agreement with the reference solutions and confirmed use of finite elements in such class of problems.(author)

Influence of the non-singular stress on the crack extension and fatigue life

International Nuclear Information System (INIS)

Cheng, C.Z.; Recho, N.; Niu, Z.R.

2012-01-01

Highlights: ► BEM is combined by characteristic analysis to calculate the singular stress field. ► A new method is proposed to evaluate the full stress field at crack tip region. ► Effect of non-singular stress on the propagation direction of the fatigue crack is analyzed. ► The influence of non-singular stress on the fatigue crack life is evaluated. - Abstract: The complete elasticity stress field at a crack tip region can be presented by the sum of the singular stress and several non-singular stress terms according to the Williams asymptotic expansion theory. The non-singular stress has a non-negligible influence on the prediction of the crack extension direction and crack growth rate under the fatigue loading. A novel method combining the boundary element method and the singularity characteristic analysis is proposed here to evaluate the complete stress field at a crack tip region. In this new method, any non-singular stress term in the Williams series expansion can be evaluated according to the computational accuracy requirement. Then, a modified Paris law is introduced to predict the crack propagation under the mixed-mode loading for exploring the influence of the non-singular stress on the fatigue life duration. By comparing with the existed experimental results, the predicted crack fatigue life when the non-singular stress is taken into consideration is more accurate than the predicted ones only considering the singular stress.

Crack Tip Mechanics in Distortion Gradient Plasticity

DEFF Research Database (Denmark)

Fuentes-Alonso, Sandra; Martínez Pañeda, Emilio

2017-01-01

Gradient Plasticity (DGP), the influence on crack tip mechanics of DGP's distinguishing features that entail superior modelling capabilities has not been investigated yet. In this work crack tip fields are thoroughly examined by implementing the higher order theory of DGP in an implicit finite element...

Stress intensity factors and constant stress terms for interface cracks

International Nuclear Information System (INIS)

Fett, T.; Rizzi, G.

2004-01-01

In bi-material joints cracks can propagate along the interface or kink into one of the two materials. Whereas the energy release rate can be applied for interface cracks in the same way as usual for homogeneous materials, the computation of stresses in the vicinity of the crack tip is significantly more complicated. In order to assess crack kinking, it is necessary to know the mixed-mode stress intensity factor contributions K I and K II as well as the constant stress terms in the two materials. Whereas the stress intensity factors are available for a large number of infinite and semi-infinite bodies, there is experimental interest in practically used test specimens. This especially holds for the constant x-stress terms. Finite element computations are performed for the special case of a disappearing second Dundurs parameter, i.e. β=0. The fracture mechanics parameters K I , K II , σ 0 for the interface crack are reported in the form of diagrams and approximate relations. (orig.)

Multi-scale analysis of deformation behavior at SCC crack tip (2). (Contract research)

International Nuclear Information System (INIS)

Kaji, Yoshiyuki; Miwa, Yukio; Tsukada, Takashi; Hayakawa, Masao; Nagashima, Nobuo

2007-03-01

This report describes a result of the research conducted by the Japan Atomic Energy Agency and the National Institute for Materials Science under contract with Japan Nuclear Energy Safety Organization (JNES) that was concerned with a multi-scale analysis of plastic deformation behavior at the crack tip of stress corrosion cracking (SCC). The research was carried out to evaluate the validity of the SCC growth data acquired in the intergranular SCC (IGSCC) project based on a mechanistic understanding of SCC. For the purpose, in this research, analyses of the plastic deformation behavior and microstructure around the crack tip were performed in a nano-order scale. The hardness measured in nano, meso and macro scales was employed as a common index of the strength, and the essential data necessary to understand the SCC propagation behavior were acquired and analyzed that are mainly a size of plastic deformation region and a microstructural information in the region, e.g. data of crystallografy, microscopic deformation and dislocations at the inside of grains and grain boundaries. In this year, we analyzed the state of plastic deformation region at the crack tip of IGSCC under various conditions and investigated relationship between crack growth behavior and stress intensity factor. Especially, we investigated in detail about two different hardened specimens used in the SCC growth tests in the IGSCC project. (J.P.N.)

Crack-tip constraint analyses and constraint-dependent LBB curves for circumferential through-wall cracked pipes

Energy Technology Data Exchange (ETDEWEB)

Chen, Y.L.; Wang, G.Z., E-mail: gzwang@ecust.edu.cn; Xuan, F.Z.; Tu, S.T.

2015-04-15

Highlights: • Solution of constraint parameter τ* for through-wall cracked pipes has been obtained. • Constraint increases with increasing crack length and radius–thickness ratio of pipes. • Constraint-dependent LBB curve for through-wall cracked pipes has been constructed. • For increasing accuracy of LBB assessments, constraint effect should be considered. - Abstract: The leak-before-break (LBB) concept has been widely applied in the structural integrity assessments of pressured pipes in nuclear power plants. However, the crack-tip constraint effects in LBB analyses and designs cannot be incorporated. In this paper, by using three-dimensional finite element calculations, the modified load-independent T-stress constraint parameter τ* for circumferential through-wall cracked pipes with different geometries and crack sizes has been analyzed under different loading conditions, and the solutions of the crack-tip constraint parameter τ* have been obtained. Based on the τ* solutions and constraint-dependent J–R curves of a steel, the constraint-dependent LBB (leak-before-break) curves have been constructed. The results show that the constraint τ* increases with increasing crack length θ, mean radius R{sub m} and radius–thickness ratio R{sub m}/t of the pipes. In LBB analyses, the critical crack length calculated by the J–R curve of the standard high constraint specimen for pipes with shorter cracks is over-conservative, and the degree of conservatism increases with decreasing crack length θ, R{sub m} and R{sub m}/t. Therefore, the constraint-dependent LBB curves should be constructed to modify the over-conservatism and increase accuracy of LBB assessments.

A phenomenological model for iodine stress corrosion cracking of zircaloy

International Nuclear Information System (INIS)

Miller, A.K.; Tasooji, A.

1981-01-01

To predict the response of Zircaloy tubing in iodine environments under conditions where either crack initiation or crack propagation predominates, a unified model of the SCC process has been developed based on the local conditions (the local stress, local strain, and local iodine concentration) within a small volume of material at the cladding inner surface or the crack tip. The methodology used permits computation of these values from simple equations. A nonuniform distribution of local stress and strain results once a crack has initiated. The local stress can be increased due to plastic constraint and triaxiality at the crack tip. Iodine penetration is assumed to be a surface diffusion-controlled process. Experimental data are used to derive criteria for intergranular failure, transgranular failure, and ductile rupture in terms of the local conditions. The same failure criteria are used for both crack initiation and crack propagation. Irradiation effects are included in the model by changing the value of constants in the equation governing iodine penetration and by changing the values used to represent the mechanical properties of the Zircaloy. (orig./HP)

Water radiolysis in a crack tip under gamma ray irradiation

International Nuclear Information System (INIS)

Satoh, Tomonori; Uchida, Shunsuke; Satoh, Yoshiyuki

2002-01-01

Under a non-irradiation condition, oxidant, e.g., O 2 and H 2 O 2 , in a crack tip is supplied from the bulk water. But under irradiation conditions, even if the diffusion of radiolytic species is not sufficient, direct radiolysis in the crack tip causes high concentrations of radiolytic species. As a result of measurements and Monte Carlo calculation of gamma ray energy deposition, it has been confirmed that the energy deposition rate in the gap water is larger than that in the bulk water. The energy absorption rate increases as the gap width decreases and reaches 1.3 times that in the bulk water. In order to evaluate crack propagation rate for irradiation assisted stress corrosion cracking (IASCC) of stainless steel, a water radiolysis model in a crevice is proposed. A larger energy deposition rate in the crevice water produces many more radiolytic species, which causes high oxidant concentrations in spite of enhanced recombination of the species at the crevice inner surface. So, for IASCC evaluation, crevice water chemistry plays an important role to determine the crack propagation rate under irradiation. (authors)

Cleavage crack growth resistance due to plastic flow around a near-tip dislocation-free region

DEFF Research Database (Denmark)

Tvergaard, Viggo

1997-01-01

; but plastic yielding far from the tip still adds to the fracture toughness. The model employed makes use of a dislocation-free strip of elastic material, inside which the crack propagates, while the material outside the strip is described by continuum plasticity. The approximation involved in assuming......Crack growth resistance curves are computed numerically for cases where fracture occurs by atomic separation, so that the length scale of the fracture process is typically much smaller than the dislocation spacing. Here, continuum plasticity would not give realistic stress levels near the crack tip...

The interaction of fatigue cracks with a residual stress field using thermoelastic stress analysis and synchrotron X-ray diffraction experiments

Science.gov (United States)

Amjad, Khurram; Asquith, David; Sebastian, Christopher M.; Wang, Wei-Chung

2017-01-01

This article presents an experimental study on the fatigue behaviour of cracks emanating from cold-expanded holes utilizing thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques with the aim of resolving the long-standing ambiguity in the literature regarding potential relaxation, or modification, of beneficial compressive residual stresses as a result of fatigue crack propagation. The crack growth rates are found to be substantially lower as the crack tip moved through the residual stress zone induced by cold expansion. The TSA results demonstrated that the crack tip plastic zones were reduced in size by the presence of the residual compressive stresses induced by cold expansion. The crack tip plastic zones were found to be insignificant in size in comparison to the residual stress zone resulting from cold expansion, which implied that they were unlikely to have had a notable impact on the surrounding residual stresses induced by cold expansion. The residual stress distributions measured along the direction of crack growth, using SXRD, showed no signs of any significant stress relaxation or redistribution, which validates the conclusions drawn from the TSA data. Fractographic analysis qualitatively confirmed the influence on crack initiation of the residual stresses induced by the cold expansion. It was found that the application of single compressive overload caused a relaxation, or reduction in the residual stresses, which has wider implications for improving the fatigue life. PMID:29291095

Analysis of short and long crack behavior and single overload effect by crack opening stress

International Nuclear Information System (INIS)

Song, Sam Hong; Lee, Kyeong Ro

1999-01-01

The study analyzed the behaviors of short and long crack as well as the effect of single tensile overload on the crack behaviors by using fatigue crack opening behavior. Crack opening stress is measured by an elastic compliance method which may precisely and continuously provide many data using strain gages during experiment. The unusual growth behaviors of short crack and crack after the single tensile overload applied, was explained by the variations of crack opening stress. In addition, fatigue crack growth rate was expressed as a linear form for short crack as for long crack by using effective stress intensity factor range as fracture mechanical parameter, which is based on crack closure concept. And investigation is performed with respect to the relation between plastic zone size formed at the crack tip and crack retardation, crack length and the number of cycles promoted or retarded, and the overload effect on the fatigue life

The elastic T-stress for slightly curved or kinked cracks

DEFF Research Database (Denmark)

Li, Dong-Feng; Li, Chen-Feng; Qing, Hai

2010-01-01

This work presents a solution for the elastic T-stress at the tip of a slightly curved or kinked crack based on a perturbation approach. Compared to other exact or numerical solutions the present solution is accurate for considerable deviations from straightness. The T-stress variation as crack...... extends along a curved trajectory is subsequently examined. It is predicted that T-stress always keeps negative during crack extension when the crack has an initial negative T-stress. In the case of a positive T-stress and non-zero first and second stress intensity factors initially accompanying the crack......, the T-stress is not positive with increasing the extension length until a threshold is exceeded. Based on directional stability criterion with respect to the sign of the T-stress, this result implies that for a straight crack with a positive T-stress, the crack extension path will not turn immediately...

Stress corrosion cracking of titanium alloys

Science.gov (United States)

May, R. C.; Beck, F. H.; Fontana, M. G.

1971-01-01

Experiments were conducted to study (1) the basic electrochemical behavior of titanium in acid chloride solutions and (2) the response of the metal to dynamic straining in the same evironment. The aim of this group of experiments was to simulate, as nearly as possible, the actual conditions which exist at the tip of a crack. One of the foremost theories proposed to explain the propagation of stress corrosion cracks is a hydrogen embrittlement theory involving the precipitation of embrittling titanium hydrides inside the metal near the crack tip. An initial survey of the basic electrochemical literature indicated that surface hydrides play a critical role in the electrochemistry of titanium in acid solutions. A comprehensive analysis of the effect of surface films, particularly hydrides, on the electrochemical behavior of titanium in these solution is presented.

Propagation of stress corrosion cracks in alpha-brasses

Energy Technology Data Exchange (ETDEWEB)

Beggs, Dennis Vinton [Univ. of Illinois, Urbana-Champaign, IL (United States)

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, Δt 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, Δx, decreased linearly with Δt. With Δt 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, Δx = Δx* which approached a limiting value of 1 μm. 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.

Multi-parameter crack tip stress state description for estimation of fracture process zone extent in silicate composite WST specimens

Czech Academy of Sciences Publication Activity Database

Veselý, V.; Sobek, J.; Šestáková, L.; Frantík, P.; Seitl, Stanislav

2013-01-01

Roč. 7, č. 25 (2013), s. 69-78 ISSN 1971-8993 R&D Projects: GA ČR(CZ) GAP104/11/0833; GA ČR(CZ) GAP105/11/1551 Institutional support: RVO:68081723 Keywords : Near-crack tip fields * Williams series * higher-order terms * stress field approximation * wedge splitting test * fracture process zone Subject RIV: JL - Materials Fatigue, Friction Mechanics

Resolved shear stress intensity coefficient and fatigue crack growth in large crystals

Science.gov (United States)

Chen, QI; Liu, Hao-Wen

1988-01-01

Fatigue crack growth in large grain Al alloy was studied. Fatigue crack growth is caused primarily by shear decohesion due to dislocation motion in the crack tip region. The crack paths in the large crystals are very irregular and zigzag. The crack planes are often inclined to the loading axis both in the inplane direction and the thickness direction. The stress intensity factors of such inclined cracks are approximated from the two dimensional finite element calculations. The plastic deformation in a large crystal is highly anisotropic, and dislocation motion in such crystals are driven by the resolved shear stress. The resolved shear stress intensity coefficient in a crack solid, RSSIC, is defined, and the coefficients for the slip systems at a crack tip are evaluated from the calculated stress intensity factors. The orientations of the crack planes are closely related to the slip planes with the high RSSIC values. If a single slip system has a much higher RSSIC than all the others, the crack will follow the slip plane, and the slip plane becomes the crack plane. If two or more slip systems have a high RSSIC, the crack plane is the result of the decohesion processes on these active slip planes.

Stress intensity factor at the tip of cladding incipient crack in RIA-simulating experiments for high-burnup PWR fuels

International Nuclear Information System (INIS)

Udagawa, Yutaka; Suzuki, Motoe; Sugiyama, Tomoyuki; Fuketa, Toyoshi

2009-01-01

RIA-simulating experiments for high-burnup PWR fuels have been performed in the NSRR, and the stress intensity factor K 1 at the tip of cladding incipient crack has been evaluated in order to investigate its validity as a PCMI failure threshold under RIA conditions. An incipient crack depth was determined by observation of metallographs. The maximum hydride-rim thickness in the cladding of the test fuel rod was regarded as the incipient crack depth in each test case. Hoop stress in the cladding periphery during the pulse power transient was calculated by the RANNS code. K 1 was calculated based on crack depth and hoop stress. According to the RANNS calculation, PCMI failure cases can be divided into two groups: failure in the elastic phase and failure in the plastic phase. In the former case, elastic deformation was predominant around the incipient crack at failure time. K 1 is available only in this case. In the latter, plastic deformation was predominant around the incipient crack at failure time. Failure in the elastic phase never occurred when K 1 was less than 17 MPa m 1/2 . For failure in the plastic phase, the plastic hoop strain of the cladding periphery at failure time clearly showed a tendency to decrease with incipient crack depth. The combination of K 1 , for failure in the elastic phase, and plastic hoop strain at failure, for failure in the plastic phase, can be an effective index of PCMI failure under RIA conditions. (author)

A quantitative prediction model of SCC rate for nuclear structure materials in high temperature water based on crack tip creep strain rate

International Nuclear Information System (INIS)

Yang, F.Q.; Xue, H.; Zhao, L.Y.; Fang, X.R.

2014-01-01

Highlights: • Creep is considered to be the primary mechanical factor of crack tip film degradation. • The prediction model of SCC rate is based on crack tip creep strain rate. • The SCC rate calculated at the secondary stage of creep is recommended. • The effect of stress intensity factor on SCC growth rate is discussed. - Abstract: The quantitative prediction of stress corrosion cracking (SCC) of structure materials is essential in safety assessment of nuclear power plants. A new quantitative prediction model is proposed by combining the Ford–Andresen model, a crack tip creep model and an elastic–plastic finite element method. The creep at the crack tip is considered to be the primary mechanical factor of protective film degradation, and the creep strain rate at the crack tip is suggested as primary mechanical factor in predicting the SCC rate. The SCC rates at secondary stage of creep are recommended when using the approach introduced in this study to predict the SCC rates of materials in high temperature water. The proposed approach can be used to understand the SCC crack growth in structural materials of light water reactors

Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2

KAUST Repository

Ly, Thuc Hue

2017-01-18

Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS2. Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation.

Separating the Influence of Environment from Stress Relaxation Effects on Dwell Fatigue Crack Growth in a Nickel-Base Disk Alloy

Science.gov (United States)

Telesman, J.; Gabb, T. P.; Ghosn, L. J.

2016-01-01

Both environmental embrittlement and crack tip visco-plastic stress relaxation play a significant role in determining the dwell fatigue crack growth (DFCG) resistance of nickel-based disk superalloys. In the current study performed on the Low Solvus High Refractory (LSHR) disk alloy, the influence of these two mechanisms were separated so that the effects of each could be quantified and modeled. Seven different microstructural variations of LSHR were produced by controlling the cooling rate and the subsequent aging and thermal exposure heat treatments. Through cyclic fatigue crack growth testing performed both in air and vacuum, it was established that four out of the seven LSHR heat treatments evaluated, possessed similar intrinsic environmental resistance to cyclic crack growth. For these four heat treatments, it was further shown that the large differences in dwell crack growth behavior which still persisted, were related to their measured stress relaxation behavior. The apparent differences in their dwell crack growth resistance were attributed to the inability of the standard linear elastic fracture mechanics (LEFM) stress intensity parameter to account for visco-plastic behavior. Crack tip stress relaxation controls the magnitude of the remaining local tensile stresses which are directly related to the measured dwell crack growth rates. It was hypothesized that the environmentally weakened grain boundary crack tip regions fail during the dwells when their strength is exceeded by the remaining local crack tip tensile stresses. It was shown that the classical creep crack growth mechanisms such as grain boundary sliding did not contribute to crack growth, but the local visco-plastic behavior still plays a very significant role by determining the crack tip tensile stress field which controls the dwell crack growth behavior. To account for the influence of the visco-plastic behavior on the crack tip stress field, an empirical modification to the LEFM stress

Strain gradient plasticity modeling of hydrogen diffusion to the crack tip

DEFF Research Database (Denmark)

Martínez Pañeda, Emilio; del Busto, S.; Niordson, Christian Frithiof

2016-01-01

to characterize the gradient-enhanced stress elevation and subsequent diffusion of hydrogen towards the crack tip. Results reveal that GNDs, absent in conventional plasticity predictions, play a fundamental role on hydrogen transport ahead of a crack. SGP estimations provide a good agreement with experimental......In this work hydrogen diffusion towards the fracture process zone is examined accounting for local hardening due to geometrically necessary dislocations (GNDs) by means of strain gradient plasticity (SGP). Finite element computations are performed within the finite deformation theory...

Effect of residual stresses on interface crack growth by void expansion mechanism

DEFF Research Database (Denmark)

Tvergaard, Viggo

2006-01-01

Crack growth along an interface between two adjacent elastic-plastic materials in a layered solid is analysed, using special interface elements to represent the fracture process ahead of the crack-tip. These interface elements account for ductile failure by the nucleation and growth of voids to c....... The results show that the value of the T-stress component in the softer material adjacent to the interface crack plays the dominant role, such that a negative value of this stress component gives a significant increase of the interface fracture toughness.......Crack growth along an interface between two adjacent elastic-plastic materials in a layered solid is analysed, using special interface elements to represent the fracture process ahead of the crack-tip. These interface elements account for ductile failure by the nucleation and growth of voids...... to coalescence. In these elements the stress components normal to the interface and the shear stresses are given by equilibrium with the surrounding material, and the stress component tangential to the interface is determined by the requirement of compatibility with the surrounding material in the tangential...

Fatigue crack closure behavior at high stress ratios

Science.gov (United States)

Turner, C. Christopher; Carman, C. Davis; Hillberry, Ben M.

1988-01-01

Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.

On the variation in crack-opening stresses at different locations in a three-dimensional body

Science.gov (United States)

Chermahini, R. G.; Blom, Anders F.

1990-01-01

Crack propagation and closure behavior of thin, and thick middle crack tension specimens under constant amplitude loading were investigated using a three dimensional elastic plastic finite element analysis of fatigue crack propagation and closure. In the thin specimens the crack front closed first on the exterior (free) surface and closed last in the interior during the unloading portion of cyclic loading; a load reduced displacement technique was used to determine crack opening stresses at specified locations in the plate from the displacements calculated after the seven cycle. All the locations were on the plate external surface and were located near the crack tip, behind the crack tip, at the centerline of the crack. With this technique, the opening stresses at the specified points were found to be 0.52, 0.42, and 0.39 times the maximum applied stress.

Application of computer assisted moire to the study of a crack tip

Science.gov (United States)

Sciammarella, C. A.; Albertazzi, A., Jr.; Mourikes, J.

The basic principles of computer assisted moire are discussed. The influence of the image sensor and its finite dimensions on the sampling theorem requirements is discussed. Criteria for the selection of grating pitch on the basis of the spatial bandwidth of the pattern to be observed and the requirements arising from sensitivity considerations are given. The method is used to analyze the strain field in the neighborhood of the crack tip of a standard ASTM compact tension specimen. From the displacements the strains are computed, and from the strains the stresses are obtained using the generalized Ramberg-Osgood stress strain relationship. The stresses are used to compute the values for the J-integral in several circuits surrounding the crack. Good agreement is obtained between the values of the stress intensity factors obtained by different methods. The plastic region surrounding the crack does not show a HRR field and thus the usual rationale to justify the J-integral methods must be re-evaluated.

Critique of the Ford-Andresen film rupture model for aqueous stress corrosion cracking

International Nuclear Information System (INIS)

Hall, M.M.

2009-01-01

The Ford-Andresen film rupture model for aqueous stress corrosion cracking has obtained a prominent position in the nuclear reactor industry. The model is said to have superior predictive capabilities because it is derived from a fundamental understanding of the film rupture-repassivation mechanism of crack advance. However, a critical review shows that there are conceptual and mathematical problems with the Ford-Andresen model development; there are inconsistencies among the stated and implied assumptions, the crack tip current density expression lacks the necessary dependence on crack tip strain rate and the fundamental proportionality that exists between crack tip strain rate and crack growth rate is overlooked and omitted from the model development. Consequently, the Ford-Andresen model must be considered neither phenomenologically nor fundamentally supported.

Stress Intensity Factor for Interface Cracks in Bimaterials Using Complex Variable Meshless Manifold Method

Directory of Open Access Journals (Sweden)

Hongfen Gao

2014-01-01

Full Text Available This paper describes the application of the complex variable meshless manifold method (CVMMM to stress intensity factor analyses of structures containing interface cracks between dissimilar materials. A discontinuous function and the near-tip asymptotic displacement functions are added to the CVMMM approximation using the framework of complex variable moving least-squares (CVMLS approximation. This enables the domain to be modeled by CVMMM without explicitly meshing the crack surfaces. The enriched crack-tip functions are chosen as those that span the asymptotic displacement fields for an interfacial crack. The complex stress intensity factors for bimaterial interfacial cracks were numerically evaluated using the method. Good agreement between the numerical results and the reference solutions for benchmark interfacial crack problems is realized.

Prediction of pure water stress corrosion cracking (PWSCC) in nickel base alloys using crack growth rate models

International Nuclear Information System (INIS)

Thompson, C.D.; Krasodomski, H.T.; Lewis, N.; Makar, G.L.

1995-01-01

The Ford/Andresen slip dissolution SCC model, originally developed for stainless steel components in BWR environments, has been applied to Alloy 600 and Alloy X-750 tested in deaerated pure water chemistry. A method is described whereby the crack growth rates measured in compact tension specimens can be used to estimate crack growth in a component. Good agreement was found between model prediction and measured SCC in X-750 threaded fasteners over a wide range of temperatures, stresses, and material condition. Most data support the basic assumption of this model that cracks initiate early in life. The evidence supporting a particular SCC mechanism is mixed. Electrochemical repassivation data and estimates of oxide fracture strain indicate that the slip dissolution model can account for the observed crack growth rates, provided primary rather than secondary creep rates are used. However, approximately 100 cross-sectional TEM foils of SCC cracks including crack tips reveal no evidence of enhanced plasticity or unique dislocation patterns at the crack tip or along the crack to support a classic slip dissolution mechanism. No voids, hydrides, or microcracks are found in the vicinity of the crack tips creating doubt about classic hydrogen related mechanisms. The bulk oxide films exhibit a surface oxide which is often different than the oxides found within a crack. Although bulk chromium concentration affects the rate of SCC, analytical data indicates the mechanism does not result from chromium depletion at the grain boundaries. The overall findings support a corrosion/dissolution mechanism but not one necessarily related to slip at the crack tip

Numerical investigation of J-characterization of growing crack tips

International Nuclear Information System (INIS)

Nilsson, F.

1992-01-01

Two different geometries, a centrally cracked panel and a three-point bend bar, are modelled with aid of the finite element program ABAQUS. Elastic-plastic behaviour with a realistic linear hardening modulus is assumed. By simulation of the growth with the aid of nodal relaxation, the J-value for a remote path around the growing tip is obtained for some different local-crack growth histories. The J F -value is compared to the J D -value that results if the crack tip is assumed to be stationary at the current length. It is found that the J C - and J F -values agree well for crack growth histories satisfying the criteria for J-characterization. However, after examination of the crack surface displacements it was found that the results for the bend geometry and the tension geometry, respectively, did not coincide for corresponding J-values, except at low load levels. This raises doubt about the abilities of J to characterize the state at a growing tip. (orig.)

The influence of surface stress on dislocation emission from sharp and blunt cracks in f.c.c. metals

DEFF Research Database (Denmark)

Schiøtz, Jakob

2000-01-01

We use computer simulations to study the behaviour of atomically sharp and blunted cracks in various fee metals. The simulations use effective medium potentials which contain many-body interactions. We find that when using potentials representing platinum and gold a sharp crack is stable with res......We use computer simulations to study the behaviour of atomically sharp and blunted cracks in various fee metals. The simulations use effective medium potentials which contain many-body interactions. We find that when using potentials representing platinum and gold a sharp crack is stable...... with respect to the emission of a dislocation from the crack tip, whereas for all other metals studied the sharp crack is unstable. This result cannot be explained by existing criteria for the intrinsic ductile/brittle behaviour of crack tips, but is probably caused by surface stresses. When the crack...... is no longer atomically sharp dislocation emission becomes easier in all the studied metals. The effect is relatively strong; the critical stress intensity factor for emission to occur is reduced by up to 20%. This behaviour appears to be caused by the surface stress near the crack tip. The surface stress...

Stress-intensity factors for a thick-walled cylinder containing an annular imbedded or external or internal surface crack

Science.gov (United States)

Erdol, R.; Erdogan, F.

1976-01-01

The elastostatic axisymmetric problem for a long thick-walled cylinder containing a ring-shaped internal or edge crack is considered. Using the standard transform technique the problem is formulated in terms of an integral equation which has a simple Cauchy kernel for the internal crack and a generalized Cauchy kernel for the edge crack as the dominant part. As examples the uniform axial load and the steady-state thermal stress problems have been solved and the related stress intensity factors have been calculated. Among other findings the results show that in the cylinder under uniform axial stress containing an internal crack the stress intensity factor at the inner tip is always greater than that at the outer tip for equal net ligament thicknesses and in the cylinder with an edge crack which is under a state of thermal stress the stress intensity factor is a decreasing function of the crack depth, tending to zero as the crack depth approaches the wall thickness.

Assisted crack tip flipping under Mode I thin sheet tearing

DEFF Research Database (Denmark)

Felter, Christian Lotz; Nielsen, Kim Lau

2017-01-01

Crack tip flipping, where the fracture surface alternates from side to side in roughly 45° shear bands, seems to be an overlooked propagation mode in Mode I thin sheet tearing. In fact, observations of crack tip flipping is rarely found in the literature. Unlike the already established modes...

Actual light deflections in regions of crack tips and their influence on measurements in photomechanics

Science.gov (United States)

Hecker, Friedrich W.; Pindera, Jerzy T.; Wen, Baicheng

Crack-tip photomechanics procedures are based on certain simplifying assumptions that are seldom discussed. In a recent paper the theoretical bases of the shadow optical methods of caustics have been analysed and tested using the results obtained by three analytical-experimental procedures, namely classical strain gage techniques, isodynes, and strain-gradient index method. It has been concluded that the straing-radient index method appears to be a suitable tool for analysis of stress states near crack tips and notches and, in particular, for testing the predictive power of the pertinent singular solutions of the linear elastic fracture mechanics and the ranges of their applicability. In the present paper, a more detailed analysis of all results obtained in light deflection experiments allows to quantify the contribution of both involved effects and to determine the distortion of the faces of the investigated plates along their crack planes. The ability of the strain-gradient light bending method to analyse some features of the three-dimensional stress-state is reported. Finally, the presented experimental evidence allows to draw conclusions related to limits of applicability of certain photomechanical measurements near crack tips. An extensive summary of this paper is published in the Proceedings of the Second International Conference on Photomechanics and Speckle Metrology, Vol. 1554A, part of SPIE's 1991 International Symposium on Optical Applied Science and Engineering, 22-26 July 1991, San Diego, CA, USA. 1

The effect of residual thermal stresses on the fatigue crack growth of laser-surface-annealed AISI 304 stainless steel Part I: computer simulation

International Nuclear Information System (INIS)

Shiue, R.K.; Chang, C.T.; Young, M.C.; Tsay, L.W.

2004-01-01

The effect of residual thermal stresses on the fatigue crack growth of the laser-surface-annealed AISI 304 stainless steel, especially the effect of stress redistribution ahead of the crack tip was extensively evaluated in the study. Based on the finite element simulation, the longitudinal residual tensile stress field has a width of roughly 20 mm on the laser-irradiated surface and was symmetric with respect to the centerline of the laser-annealed zone (LAZ). Meanwhile, residual compressive stresses distributed over a wide region away from the LAZ. After introducing a notch perpendicular to the LAZ, the distribution of longitudinal residual stresses became unsymmetrical about the centerline of LAZ. High residual compressive stresses exist within a narrow range ahead of notch tip. The improved crack growth resistance of the laser-annealed specimen might be attributed to those induced compressive stresses. As the notch tip passed through the centerline of the LAZ, the residual stress ahead of the notch tip was completely reverted into residual tensile stresses. The existence of unanimous residual tensile stresses ahead of the notch tip was maintained, even if the notch tip extended deeply into the LAZ. Additionally, the presence of the residual tensile stress ahead of the notch tip did not accelerate the fatigue crack growth rate in the compact tension specimen

Stress intensity factors for complete internal and external cracks in spherical shells

International Nuclear Information System (INIS)

Chao, Y.J.; Chen, H.

1989-01-01

Cracks or flows found in the nuclear structure must be fully evaluated to assure the safety of the plant. The weight function method has been widely used in the determination of stress intensity factors for cracks under stress gradient e.g. for thermal shock loading. The unique features of the weight function method is that once the weight function for a particular cracked geometry is determined the stress intensity factors at the crack tip for any loading applied to the flawed structure can be calculated by a simple integration. In this paper the stress intensity factors of the complete, part-through internal and external cracks in a spherical shell are determined. The finite element method was used to develop the weight functions for the flawed geometry. The approximate crack surface profile was used to derive the weight functions. The stress intensity factors associated with the cracks in spherical shells under internal pressure are determined by both the weight functions and the direct finite element method

Study of stress corrosion cracking initiation of high alloy materials

Energy Technology Data Exchange (ETDEWEB)

Blahetova, Marie; Cihal, Vladimir; Lasek, Stanislav [Department of Materials Engineering, VSB - Technical University of Ostrava, tr. 17. listopadu 15, 708 33 Ostrava - Poruba (Czech Republic)

2004-07-01

The stainless steels and related alloys with sufficient resistance to a general corrosion can be susceptible to a localized corrosion (pitting, cracking, intergranular corrosion) in certain environment under specific conditions. The Drop Evaporation Test (DET) was developed for study of stainless materials resistance to stress corrosion cracking (SCC) at elevated temperatures 100 - 300 deg. C under constant external load using a chloride containing water solution. In the contribution the initiation and propagation of short cracks as well as pits were observed during the test. The crack initiation and/or propagation can be influenced by the cyclic thermal stresses, when the diluted water solution drops cool down the hot sample. The coordinates measurement of microscopic pits and sharp corrosion crack tips by the travelling microscope method allowed to derive the crack growth lengths and rates of short cracks. (authors)

Study of stress corrosion cracking initiation of high alloy materials

International Nuclear Information System (INIS)

Blahetova, Marie; Cihal, Vladimir; Lasek, Stanislav

2004-01-01

The stainless steels and related alloys with sufficient resistance to a general corrosion can be susceptible to a localized corrosion (pitting, cracking, intergranular corrosion) in certain environment under specific conditions. The Drop Evaporation Test (DET) was developed for study of stainless materials resistance to stress corrosion cracking (SCC) at elevated temperatures 100 - 300 deg. C under constant external load using a chloride containing water solution. In the contribution the initiation and propagation of short cracks as well as pits were observed during the test. The crack initiation and/or propagation can be influenced by the cyclic thermal stresses, when the diluted water solution drops cool down the hot sample. The coordinates measurement of microscopic pits and sharp corrosion crack tips by the travelling microscope method allowed to derive the crack growth lengths and rates of short cracks. (authors)

Strain energy density-distance criterion for the initiation of stress corrosion cracking of alloy X-750

Energy Technology Data Exchange (ETDEWEB)

Hall, M.M. Jr.; Symons, D.M.

1996-05-01

A strain energy density-distance criterion was previously developed and used to correlate rising-load K{sub c} initiation data for notched and fatigue precracked specimens of hydrogen precharged Alloy X-750. This criterion, which was developed for hydrogen embrittlement (HE) cracking, is used here to correlate static-load stress corrosion cracking (SCC) initiation times obtained for smooth geometry, notched and fatigue precracked specimens. The onset of SCC crack growth is hypothesized to occur when a critical strain, which is due to environment-enhanced creep, is attained within the specimen interior. For notched and precracked specimens, initiation is shown by analysis to occur at a variable distance from notch and crack tips. The initiation site varies from very near the crack tip, for highly loaded sharp cracks, to a site that is one grain diameter from the notch, for lower loaded, blunt notches. The existence of hydrogen gradients, which are due to strain-induced hydrogen trapping in the strain fields of notch and crack tips, is argued to be controlling the site for initiation of cracking. By considering the sources of the hydrogen, these observations are shown to be consistent with those from the previous HE study, in which the characteristic distance for crack initiation was found to be one grain diameter from the notch tip, independent of notch radius, applied stress intensity factor and hydrogen level.

Multi-parameter crack tip stress state description for evaluation of nonlinear zone width in silicate composite specimens in component splitting/bending test geometry

Czech Academy of Sciences Publication Activity Database

Veselý, V.; Frantík, P.; Sopek, J.; Malíková, L.; Seitl, Stanislav

2015-01-01

Roč. 38, č. 2 (2015), s. 200-214 ISSN 8756-758X R&D Projects: GA ČR(CZ) GAP104/11/0833 Institutional support: RVO:68081723 Keywords : near-crack tip fields * Williams series * higher-order terms * stress field * failure criterion * nonlinear zone * quasi-brittle fracture * splitting-bending geometry Subject RIV: JL - Materials Fatigue, Friction Mechanics Impact factor: 1.838, year: 2015

Tensile cracks in creeping solids

International Nuclear Information System (INIS)

Riedel, H.; Rice, J.R.

1979-02-01

The loading parameter determining the stress and strain fields near a crack tip, and thereby the growth of the crack, under creep conditions is discussed. Relevant loading parameters considered are the stress intensity factor K/sub I/, the path-independent integral C*, and the net section stress sigma/sub net/. The material behavior is modelled as elastic-nonlinear viscous where the nonlinear term describes power law creep. At the time t = 0 load is applied to the cracked specimen, and in the first instant the stress distribution is elastic. Subsequently, creep deformation relaxes the initial stress concentration at the crack tip, and creep strains develop rapidly near the crack tip. These processes may be analytically described by self-similar solutions for short times t. Small scale yielding may be defined. In creep problems, this means that elastic strains dominate almost everywhere except in a small creep zone which grows around the crack tip. If crack growth ensues while the creep zone is still small compared with the crack length and the specimen size, the stress intensity factor governs crack growth behavior. If the calculated creep zone becomes larger than the specimen size, the stresses become finally time-independent and the elastic strain rates can be neglected. In this case, the stress field is the same as in the fully-plastic limit of power law hardening plasticity. The loading parameter which determines the near tip fields uniquely is then the path-independent integral C*.K/sub I/ and C* characterize opposite limiting cases. The case applied in a given situation is decided by comparing the creep zone size with the specimen size and the crack length. Besides several methods of estimating the creep zone size, a convenient expression for a characteristic time is derived, which characterizes the transition from small scale yielding to extensive creep of the whole specimen

The determination of the local conditions for void initiation in front of a crack tip for materials with second-phase particles

Energy Technology Data Exchange (ETDEWEB)

Sabirov, I. [Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, A-8700 Leoben (Austria)]. E-mail: sabirov@unileoben.ac.at; Duschlbauer, D. [Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gusshausstrasse 27-29, A-1040 Vienna (Austria); Pettermann, H.E. [Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Gusshausstrasse 27-29, A-1040 Vienna (Austria); Kolednik, O. [Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, A-8700 Leoben (Austria)

2005-02-25

A procedure is proposed to determine, for second-phase particles near a crack tip, the maximum particle stresses at the moment of void initiation by either particle fracture or particle/matrix interface separation. A digital image analysis system is applied to perform a quantitative analysis of corresponding fracture surface regions from stereo image pairs taken in the scanning electron microscope. The fracture surface analysis is used to measure, for individual particles, the crack tip opening displacement at the moment of void initiation and the particle location with respect to the crack tip. From these data, the stress tensor at the moment of void initiation is calculated from the Hutchinson-Rice-Rosengren (HRR) field theory. The corresponding average local stresses within the particle are evaluated by a non-linear Mori-Tanaka-type approach. These stresses are compared to estimates according to the models by Argon et al. [A.S. Argon, J. Im, R. Safoglu, Metall. Trans. 6 (1975) 825] and Beremin [F.M. Beremin, Metall. Trans. 12 (1981) 723]. The procedure is demonstrated on an Al6061-10% Al{sub 2}O{sub 3} metal matrix composite.

Stress corrosion cracking of stainless steel under deaerated high-temperature water. Influence of cold work and processing orientation

International Nuclear Information System (INIS)

Terachi, Takumi; Yamada, Takuyo; Chiba, Goro; Arioka, Koji

2006-01-01

The influence of cold work and processing orientation on the propagation of stress corrosion cracking (SCC) of stainless steel under hydrogenated high-temperature water was examined. It was shown that (1) the crack growth rates increased with heaviness of cold work, and (2) processing orientation affected crack growth rate with cracking direction. Crack growth rates showed anisotropy of T-L>>T-S>L-S, with T-S and L-S branches representing high shear stress direction. Geometric deformation of crystal grains due to cold work caused the anisotropy and shear stress also assisted the SCC propagation. (3) The step intervals of slip like patterns observed on intergranular facets increased cold work. (4) Nano-indentation hardness of the crack tip together with EBSD measurement indicated that the change of hardness due to crack propagation was less than 5% cold-work, even though the distance from the crack tip was 10μm. (author)

Transient hydrogen diffusion analyses coupled with crack-tip plasticity under cyclic loading

International Nuclear Information System (INIS)

Kotake, Hirokazu; Matsumoto, Ryosuke; Taketomi, Shinya; Miyazaki, Noriyuki

2008-01-01

The effect of hydrogen on the material strengths of metals is known as the hydrogen embrittlement, which affects the structural integrity of a hydrogen energy system. In the present paper, we developed a computer program for a transient hydrogen diffusion-elastoplastic coupling analysis by combining an in-house finite element program with a general purpose finite element computer program to analyze hydrogen diffusion. In this program, we use a hypothesis that the hydrogen absorbed in the metal affects the yield stress of the metal. In the present paper, we discuss the effects of the cyclic loading on the hydrogen concentration near the crack tip. An important finding we obtained here is the fact that the hydrogen concentration near the crack tip greatly depends on the loading frequency. This result indicates that the fatigue lives of the components in a hydrogen system depend not only on the number of loading cycles but also on the loading frequency

Stress fields around a crack lying parallel to a free surface

International Nuclear Information System (INIS)

Higashida, Yutaka; Kamada, K.

1980-12-01

A method of stress analysis for a two dimentional crack, which is subjected to internal gas pressure, and situated parallel to a free surface of a material, is presented. It is based on the concept of continuously distributed edge dislocations of two kinds, i.e. one with Burgers vector normal to the free surface and the other with parallel to it. Stress fields of individual dislocations are chosen so as to satisfy stress free boundary conditions at the free surface, by taking account of image dislocations. Distributions of the both kinds of dislocations in the crack are derived so as to give the internal gas pressure and, at the same time, to satisfy shear stress free boundary condition on the crack surface. Stress fields σsub(xx), σsub(yy) and σsub(xy) in the sub-surface layer are then determined from them. They have square root singularities at the crack-tip. (author)

The concept of the average stress in the fracture process zone for the search of the crack path

Directory of Open Access Journals (Sweden)

Yu.G. Matvienko

2015-10-01

Full Text Available The concept of the average stress has been employed to propose the maximum average tangential stress (MATS criterion for predicting the direction of fracture angle. This criterion states that a crack grows when the maximum average tangential stress in the fracture process zone ahead of the crack tip reaches its critical value and the crack growth direction coincides with the direction of the maximum average tangential stress along a constant radius around the crack tip. The tangential stress is described by the singular and nonsingular (T-stress terms in the Williams series solution. To demonstrate the validity of the proposed MATS criterion, this criterion is directly applied to experiments reported in the literature for the mixed mode I/II crack growth behavior of Guiting limestone. The predicted directions of fracture angle are consistent with the experimental data. The concept of the average stress has been also employed to predict the surface crack path under rolling-sliding contact loading. The proposed model considers the size and orientation of the initial crack, normal and tangential loading due to rolling–sliding contact as well as the influence of fluid trapped inside the crack by a hydraulic pressure mechanism. The MATS criterion is directly applied to equivalent contact model for surface crack growth on a gear tooth flank.

Strain gradient plasticity-based modeling of hydrogen environment assisted cracking

DEFF Research Database (Denmark)

Martínez Pañeda, Emilio; Niordson, Christian Frithiof; P. Gangloff, Richard

2016-01-01

Finite element analysis of stress about a blunt crack tip, emphasizing finite strain and phenomenologicaland mechanism-based strain gradient plasticity (SGP) formulations, is integrated with electrochemical assessment of occluded-crack tip hydrogen (H) solubility and two H-decohesion models...... to predict hydrogen environment assisted crack growth properties. SGP elevates crack tip geometrically necessary dislocation density and flow stress, with enhancement declining with increasing alloy strength. Elevated hydrostatic stress promotes high-trapped H concentration for crack tip damage......; it is imperative to account for SGP in H cracking models. Predictions of the threshold stress intensity factor and H-diffusion limited Stage II crack growth rate agree with experimental data for a high strength austenitic Ni-Cusuperalloy (Monel®K-500) and two modern ultra-high strength martensitic steels (Aer...

Role of plasticity-induced crack closure in fatigue crack growth

Directory of Open Access Journals (Sweden)

Jesús Toribio

2013-07-01

Full Text Available The premature contact of crack surfaces attributable to the near-tip plastic deformations under cyclic loading, which is commonly referred to as plasticity induced crack closure (PICC, has long been focused as supposedly controlling factor of fatigue crack growth (FCG. Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insights into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been performed. Modeling visualizes the Laird-Smith conceptual mechanism of FCG by plastic blunting and re-sharpening. Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC has never been detected. Near-tip deformation patterns discard the filling-in a crack with material stretched out of the crack plane in the wake behind the tip as supposed PICC origin. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This demonstrates ambiguities of PICC as a supposedly intrinsic factor of FCG and, by implication, favors the stresses and strains in front of the crack tip as genuine fatigue drivers.

Multi-scale analysis of deformation behavior at SCC crack tip (3) (Contract research)

International Nuclear Information System (INIS)

Kaji, Yoshiyuki; Miwa, Yukio; Tsukada, Takashi; Hayakawa, Masao; Nagashima, Nobuo

2008-08-01

In recent years, incidents of the stress corrosion cracking (SCC) were frequently reported that occurred to the various components of domestic boiling water reactors (BWR), and the cause investigation and measure become the present important issue. By the Japan nuclear energy safety organization (JNES), a research project on the intergranular SCC (IGSCC) in nuclear grade stainless steels (henceforth, IGSCC project) is under enforcement from a point of view to secure safety and reliability of BWR, and SCC growth data of low carbon stainless steels are being accumulated for the weld part or the work-hardened region adjacent to the weld metal. In the project, it has been an important subject to guarantee the validity of accumulated SCC data. At a crack tip of SCC in compact tension (CT) type specimen used for the SCC propagation test, a macroscopic plastic region is formed where heterogeneity of microstructure developed by microscopic sliding and dislocations is observed. However, there is little quantitative information on the plastic region, and therefore, to assess the data of macroscopic SCC growth rate and the validity of propagation test method, it is essentially required to investigate the plastic region at the crack tip in detail from a microscopic viewpoint. This report describes a result of the research conducted by the Japan Atomic Energy Agency and the National Institute for Materials Science under contract with JNES that was concerned with a multi-scale analysis of plastic deformation behavior at the crack tip of SCC. The research was carried out to evaluate the validity of the SCC growth data acquired in the IGSCC project based on a mechanistic understanding of SCC. For the purpose, in this research, analyses of the plastic deformation behavior and microstructure around the crack tip were performed in a nano-order scale. The hardness measured in nano, meso and macro scales was employed as a common index of the strength, and the essential data necessary

Fatigue crack growth in 2024-T3 aluminum under tensile and transverse shear stresses

Science.gov (United States)

Viz, Mark J.; Zehnder, Alan T.

1994-01-01

The influence of transverse shear stresses on the fatigue crack growth rate in thin 2024-T3 aluminum alloy sheets is investigated experimentally. The tests are performed on double-edge cracked sheets in cyclic tensile and torsional loading. This loading generates crack tip stress intensity factors in the same ratio as the values computed for a crack lying along a lap joint in a pressurized aircraft fuselage. The relevant fracture mechanics of cracks in thin plates along with the details of the geometrically nonlinear finite element analyses used for the test specimen calibration are developed and discussed. Preliminary fatigue crack growth data correlated using the fully coupled stress intensity factor calibration are presented and compared with fatigue crack growth data from pure delta K(sub I)fatigue tests.

Modified Dugdale crack models - some easy crack relations

DEFF Research Database (Denmark)

Nielsen, Lauge Fuglsang

1997-01-01

the same strength as a plain Dugdale model. The critical energy release rates Gamma_CR, however, become different. Expressions (with easy computer algorithms) are presented in the paper which relate critical energy release rates and crack geometry to arbitrary cohesive stress distributions.For future...... lifetime analysis of viscoelastic materials strain energy release rates, crack geometries, and cohesive stress distributions are considered as related to sub-critical loads sigma stress-deformation tests......The Dugdale crack model is widely used in materials science to predict strength of defective (cracked) materials. A stable Dugdale crack in an elasto-plastic material is prevented from spreading by uniformly distributed cohesive stresses acting in narrow areas at the crack tips. These stresses...

Stress Concentration Factor and Stress Intensity Factor with U-notch and Crack in the Beam

Energy Technology Data Exchange (ETDEWEB)

Seo, Bo Seong; Lee, Kwang Ho [Kyungpook National Univ., Daegu (Korea, Republic of)

2016-05-15

The stress concentration factors and stress intensity factors for a simple beam and a cantilever are analyzed by using finite element method and photoelasticity. Using the analyzed results, the estimated graphs on stress concentration factors and stress intensity factors are obtained. To analyze stress concentration factors of notch, the dimensionless notch length H(height of specimen)/h=1.1-2 and dimensionless gap space r(radius at the notch tip)/h=0.1~0.5 are used, where h=H-c and c is the notch length. As the notch gap length increases and the gap decreases, the stress concentration factors increase. Stress concentration factors of a simple beam are greater than those of a cantilever beam. However, actually, the maximum stress values under a load, a notch length and a gap occur more greatly in the cantilever beam than in the simple beam. To analyze stress intensity factors, the normalized crack length α(crack length)/H=0.2~0.5 is used. As the length of the crack increases, the normalized stress intensity factors increase. The stress intensity factors under a constant load and a crack length occur more greatly in the cantilever beam than in the simple beam.

Estimation of Fatigue Crack Growth Behavior of Cracked Specimen Under Mixed-mode Loads

International Nuclear Information System (INIS)

Han, Jeong Woo; Woo, Eun Taek; Han, Seung Ho

2015-01-01

To estimate the fatigue crack propagation behavior of compact tension shear (CTS) specimen under mixed-mode loads, crack path prediction theories and Tanaka’s equation were applied. The stress intensity factor at a newly created crack tip was calculated using a finite element method via ANSYS, and the crack path and crack increment were then obtained from the crack path prediction theories, Tanaka’s equation, and the Paris’ equation, which were preprogrammed in Microsoft Excel. A new method called the finite element crack tip updating method (FECTUM) was developed. In this method, the finite element method and Microsoft Excel are used to calculate the stress intensity factors and the crack path, respectively, at the crack tip per each crack increment. The developed FECTUM was applied to simulate the fatigue crack propagation of a single-edge notched bending (SENB) specimen under eccentric three-point bending loads. The results showed that the number of cycles to failure of the specimen obtained experimentally and numerically were in good agreement within an error range of less than 3%

Estimation of Fatigue Crack Growth Behavior of Cracked Specimen Under Mixed-mode Loads

Energy Technology Data Exchange (ETDEWEB)

Han, Jeong Woo [KIMM, Daejeon (Korea, Republic of); Woo, Eun Taek; Han, Seung Ho [Dong-A University, Busan (Korea, Republic of)

2015-07-15

To estimate the fatigue crack propagation behavior of compact tension shear (CTS) specimen under mixed-mode loads, crack path prediction theories and Tanaka’s equation were applied. The stress intensity factor at a newly created crack tip was calculated using a finite element method via ANSYS, and the crack path and crack increment were then obtained from the crack path prediction theories, Tanaka’s equation, and the Paris’ equation, which were preprogrammed in Microsoft Excel. A new method called the finite element crack tip updating method (FECTUM) was developed. In this method, the finite element method and Microsoft Excel are used to calculate the stress intensity factors and the crack path, respectively, at the crack tip per each crack increment. The developed FECTUM was applied to simulate the fatigue crack propagation of a single-edge notched bending (SENB) specimen under eccentric three-point bending loads. The results showed that the number of cycles to failure of the specimen obtained experimentally and numerically were in good agreement within an error range of less than 3%.

A consistent partly cracked XFEM element for cohesive crack growth

DEFF Research Database (Denmark)

Asferg, Jesper L.; Poulsen, Peter Noe; Nielsen, Leif Otto

2007-01-01

Present extended finite element method (XFEM) elements for cohesive crack growth may often not be able to model equal stresses on both sides of the discontinuity when acting as a crack-tip element. The authors have developed a new partly cracked XFEM element for cohesive crack growth with extra...... enrichments to the cracked elements. The extra enrichments are element side local and were developed by superposition of the standard nodal shape functions for the element and standard nodal shape functions for a sub-triangle of the cracked element. With the extra enrichments, the crack-tip element becomes...... capable of modelling variations in the discontinuous displacement field on both sides of the crack and hence also capable of modelling the case where equal stresses are present on each side of the crack. The enrichment was implemented for the 3-node constant strain triangle (CST) and a standard algorithm...

A study on fatigue crack growth behavior subjected to a single tensile overload

International Nuclear Information System (INIS)

Lee, S.Y.; Liaw, P.K.; Choo, H.; Rogge, R.B.

2011-01-01

Neutron diffraction and electric potential experiments were carried out to investigate the growth behavior of a fatigue crack subjected to a single tensile overload. The specific objectives were to (i) probe the crack tip deformation and fracture behaviors under applied loads; (ii) examine the overload-induced transient crack growth micromechanism; (iii) validate the effective stress intensity factor range based on the crack closure approach as the fatigue crack tip driving force; and (iv) establish a quantitative relationship between the crack tip driving force and crack growth behavior. Immediately after a single tensile overload was introduced and then unloaded, the crack tip became blunt and enlarged compressive residual stresses in both magnitude and zone size were observed around the crack tip. The results show that the combined contributions of the overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are responsible for the observed changes in the crack opening load and the resultant post-overload transient crack growth behavior.

Threshold intensity factors as lower boundaries for crack propagation in ceramics

Directory of Open Access Journals (Sweden)

Walter Per-Ole

2004-11-01

Full Text Available Abstract Background Slow crack growth can be described in a v (crack velocity versus KI (stress intensity factor diagram. Slow crack growth in ceramics is attributed to corrosion assisted stress at the crack tip or at any pre-existing defect in the ceramic. The combined effect of high stresses at the crack tip and the presence of water or body fluid molecules (reducing surface energy at the crack tip induces crack propagation, which eventually may result in fatigue. The presence of a threshold in the stress intensity factor, below which no crack propagation occurs, has been the subject of important research in the last years. The higher this threshold, the higher the reliability of the ceramic, and consequently the longer its lifetime. Methods We utilize the Irwin K-field displacement relation to deduce crack tip stress intensity factors from the near crack tip profile. Cracks are initiated by indentation impressions. The threshold stress intensity factor is determined as the time limit of the tip stress intensity when the residual stresses have (nearly disappeared. Results We determined the threshold stress intensity factors for most of the all ceramic materials presently important for dental restorations in Europe. Of special significance is the finding that alumina ceramic has a threshold limit nearly identical with that of zirconia. Conclusion The intention of the present paper is to stress the point that the threshold stress intensity factor represents a more intrinsic property for a given ceramic material than the widely used toughness (bend strength or fracture toughness, which refers only to fast crack growth. Considering two ceramics with identical threshold limits, although with different critical stress intensity limits, means that both ceramics have identical starting points for slow crack growth. Fast catastrophic crack growth leading to spontaneous fatigue, however, is different. This growth starts later in those ceramic materials

Multi-parameter approximation of stress field in a cracked specimen using purpose-built Java applications

Czech Academy of Sciences Publication Activity Database

Veselý, V.; Sopek, J.; Tesař, D.; Frantík, P.; Pail, T.; Seitl, Stanislav

2015-01-01

Roč. 9, č. 33 (2015), s. 120-133 ISSN 1971-8993 Institutional support: RVO:68081723 Keywords : Cracked specimen * Near-crack-tip fields * Williams expansion * Higher order terms * Stress field reconstruction * Finite element analysis * Java application Subject RIV: JL - Materials Fatigue, Friction Mechanics

Method of measurement of near tip field of fast propagating cracks by means of interferometry; Hikari kansho ni yoru kosoku shinten kiretsu sentanbu no oryokuba keisokuho

Energy Technology Data Exchange (ETDEWEB)

Suzuki, s.; Miyazaki, F. [Toyohashi University of Technology, Aichi (Japan); Nakane, K. [Nitto Denko Corp., Osaka (Japan)

1994-12-15

The measurement by an interference method at the tip of fast propagating cracks was investigated. To clarify the direction-dependent problem of dynamic cracks in a higher-order term, a high-precision stress field must be measured in every direction. In this method, the propagated interference fringes near the crack tip are shot at a speed of some hundreds of m/sec, and the coefficient of dynamic stress extension is obtained from the expression given when the number of fringes (`m`) in the interferences fringes is differentiated partially. The information below was obtained. The stress field in every direction can be analyzed by the interference method. However, the interference fringes in an area of absolute {theta} < 120{degree} are radially spread from the crack tip, so the precision of the interference fringes in the {gamma} direction deteriorates. In this area, the above partial differential is higher in precision when {alpha}m/{alpha}{theta} is used instead of {alpha}m/{alpha}{gamma}. The stress extension coefficient obtained by a caustic method can be verified if a stress field of {theta} = {plus_minus}72{degree} is analyzed by the interference method. By shooting in this optical system, the COD measurement, the K-value measurement of caustic light, and the stress field measurement by an interference method can be done simultaneously. 17 refs., 11 figs.

The dislocation distribution function near a crack tip generated by external sources

International Nuclear Information System (INIS)

Lung, C.W.; Deng, K.M.

1988-06-01

The dislocation distribution function near a crack tip generated by external sources is calculated. It is similar to the shape of curves calculated for the crack tip emission case but the quantative difference is quite large. The image forces enlarges the negative dislocation zone but does not change the form of the curve. (author). 10 refs, 3 figs

Critical applied stresses for a crack initiation from a sharp V-notch

Directory of Open Access Journals (Sweden)

L. Náhlík

2014-10-01

Full Text Available The aim of the paper is to estimate a value of the critical applied stress for a crack initiation from a sharp V-notch tip. The classical approach of the linear elastic fracture mechanics (LELM was generalized, because the stress singularity exponent differs from 0.5 in the studied case. The value of the stress singularity exponent depends on the V-notch opening angle. The finite element method was used for a determination of stress distribution in the vicinity of the sharp V-notch tip and for the estimation of the generalized stress intensity factor depending on the V-notch opening angle. Critical value of the generalized stress intensity factor was obtained using stability criteria based on the opening stress component averaged over a critical distance d from the V-notch tip and generalized strain energy density factor. Calculated values of the critical applied stresses were compared with experimental data from the literature and applicability of the LEFM concept is discussed.

Determination of Stress Coefficient Terms in Cracked Solids for Monoclinic Materials with Plane Symmetry at x3 = 0

Science.gov (United States)

Yuan, F. G.

1998-01-01

Determination of all the coefficients in the crack tip field expansion for monoclinic materials under two-dimensional deformation is presented in this report. For monoclinic materials with a plane of material symmetry at x(sub 3) = 0, the in-plane deformation is decoupled from the anti-plane deformation. In the case of in-plane deformation, utilizing conservation laws of elasticity and Betti's reciprocal theorem, together with selected auxiliary fields, T-stress and third-order stress coefficients near the crack tip are evaluated first from path-independent line integrals. To determine the T-stress terms using the J-integral and Betti's reciprocal work theorem, auxiliary fields under a concentrated force and moment acting at the crack tip are used respectively. Through the use of Stroh formalism in anisotropic elasticity, analytical expressions for all the coefficients including the stress intensity factors are derived in a compact form that has surprisingly simple structure in terms of the Barnett-Lothe tensors, L. The solution forms for degenerated materials, orthotropic, and isotropic materials are presented.

Stress corrosion cracking of Zircaloy-4 in non-aqueous iodine solutions

International Nuclear Information System (INIS)

Gomez Sanchez, Andrea V.

2006-01-01

In the present work the susceptibility to intergranular attack and stress corrosion cracking of Zircaloy-4 in different iodine alcoholic solutions was studied. The influence of different variables such as the molecular weight of the alcohols, the water content of the solutions, the alcohol type (primary, secondary or tertiary) and the temperature was evaluated. To determine the susceptibility to stress corrosion cracking the slow strain rate technique was used. Specimens of Zircaloy-4 were also exposed between 0.5 and 300 hours to the solutions without applied stress to evaluate the susceptibility to intergranular attack. The electrochemical behavior of the material in the corrosive media was studied by potentiodynamic polarization tests. It was determined that the active species responsible for the stress corrosion cracking of Zircaloy-4 in iodine alcoholic solutions is a molecular complex between the alcohol and iodine. The intergranular attack precedes the 'true' stress corrosion cracking phenomenon (which is associated to the transgranular propagation of the crack) and it is controlled by the diffusion of the active specie to the tip of the crack. Water acts as inhibitor to intergranular attack. Except for methanolic solutions, the minimum water content necessary to inhibit stress corrosion cracking was determined. This critical water content decreases when increasing the molecular weight of the alcohol. An explanation for this behavior is proposed. The susceptibility to stress corrosion cracking also depends on the type of the alcohol used as solvent. The temperature dependence of the crack propagation rate is in agreement with a thermal activated process, and the activation energy is consistent with a process controlled by the volume diffusion of the active species. (author) [es

The effect of a single tensile overload on stress corrosion cracking growth of stainless steel in a light water reactor environment

International Nuclear Information System (INIS)

Xue He; Li Zhijun; Lu Zhanpeng; Shoji, Tetsuo

2011-01-01

Research highlights: → The affect of a single tensile overload on SCC growth rate is investigated. → A single tensile overload would produce a residual plastic strain in the SCC tip. → The residual plastic strain would decrease the plastic strain rate in the SCC tip. → A single tensile overload would cause crack growth rate retardation in the SCC tip. → SCC growth rate in the quasi-stationary crack tip is relatively lower. - Abstract: It has been found that a single tensile overload applied during constant load amplitude might cause crack growth rate retardation in various crack propagating experiments which include fatigue test and stress corrosion cracking (SCC) test. To understand the affecting mechanism of a single tensile overload on SCC growth rate of stainless steel or nickel base alloy in light water reactor environment, based on elastic-plastic finite element method (EPFEM), the residual plastic strain in both tips of stationary and growing crack of contoured double cantilever beam (CDCB) specimen was simulated and analyzed in this study. The results of this investigation demonstrate that a residual plastic strain in the region immediately ahead of the crack tips will be produced when a single tensile overload is applied, and the residual plastic strain will decrease the plastic strain rate level in the growing crack tip, which will causes crack growth rate retardation in the tip of SCC.

Elastic crack-tip stress field in a semi-strip

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Victor Reut

2018-04-01

Full Text Available In this article the plain elasticity problem for a semi-strip with a transverse crack is investigated in the different cases of the boundary conditions at the semi-strips end. Unlike many works dedicated to this subject, the fixed singularities in the singular integral equation�s kernel are considered. The integral transformations� method is applied by the generalized scheme to reduce the initial problem to a one-dimensional problem. The one-dimensional problem is formulated as the vector boundary value problem which is solved with the help of matrix differential calculations and Green�s matrix apparatus. The solution of the problem is reduced to the solving of the system of three singular integral equations. Depending on the conditions given on the short edge of the semi-strip, the constructed singular integral equation can have one, or two fixed singularities. A special method is applied to solve this equation in regard of the singularities existence. Hence the system of the singular integral equations (SSIE is solved with the help of the generalized method. The stress intensity factors (SIF are investigated for different lengths of crack. The novelty of this work is in the application of new approach allowing the consideration of the fixed singularities in the problem about a transverse crack in the elastic semi-strip. The comparison of the numerical results� accuracy during the usage of the different approaches to the solving of SSIE is worked out

Size-effects at a crack-tip interacting with a number of voids

DEFF Research Database (Denmark)

Tvergaard, Viggo; Niordson, Christian Frithiof

2008-01-01

A strain gradient plasticity theory is used to analyse the growth of discretely represented voids in front of a blunting crack tip, in order to study the influence of size effects on two competing mechanisms of crack growth. For a very small void volume fraction the crack tip tends to interact...... of the characteristic material length relative to the initial void radius. For a case showing the multiple void mechanism, it is found that the effect of the material length can change the behaviour towards the void by void mechanism. A material model with three characteristic length scales is compared with a one...

Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

Science.gov (United States)

Telesman, J.; Gabb, T. P.; Ghosn, L. J.; Smith, T.

2017-01-01

Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

Mechanisms of hydrogen induced delayed cracking in hydride forming materials

International Nuclear Information System (INIS)

Dutton, R.; Nuttall, K.; Puls, M.P.; Simpson, L.A.

1977-01-01

Mechanisms which have been formulated to describe delayed hydrogen cracking in hydride-forming metals are reviewed and discussed. Particular emphasis is placed on the commercial alloy Zr--2.5% Nb (Cb) which is extensively used in nuclear reactor core components. A quantitative model for hydrogen cracking in this material is presented and compared with available experimental data. The kinetics of crack propagation are controlled by the growth of hydrides at the stressed crack tip by the diffusive ingress of hydrogen into this region. The driving force for the diffusion flux is provided by the local stress gradient which interacts with both hydrogen atoms in solution and hydrogen atoms being dissolved and reprecipitated at the crack tip. The model is developed using concepts of elastoplastic fracture mechanics. Stage I crack growth is controlled by hydrides growing in the elastic stress gradient, while Stage II is controlled by hydride growth in the plastic zone at the crack tip. Recent experimental observations are presented which indicate that the process occurs in an intermittent fashion; hydride clusters accumulate at the crack tip followed by unstable crack advance and subsequent crack arrest in repeated cycles

Mechanisms of hydrogen induced delayed cracking in hydride forming materials

International Nuclear Information System (INIS)

Dutton, R.; Nuttall, K.; Puls, M.P.; Simpson, L.A.

1977-01-01

Mechanisms which have been formulated to describe delayed hydrogen cracking in hydride-forming metals are reviewed and discussed. Particular emphasis is placed on the commercial alloy Zr-2.5 pct Nb which is extensively used in nuclear reactor core components. A quantitative model for hydrogen cracking in this material is presented and compared with available experimental data. The kinetics of crack propagation are controlled by the growth of hydrides at the stressed crack tip by the diffusive ingress of hydrogen into this region. The driving force for the diffusion flux is provided by the local stress gradient which interacts with both hydrogen atoms in solution and hydrogen atoms being dissolved and reprecipitated at the crack tip. The model is developed using concepts of elastoplastic fracture mechanics. Stage I crack growth is controlled by hydrides growing in the elastic stress gradient, while Stage II is controlled by hydride growth in the plastic zone at the crack tip. Recent experimental observations are presented which indicate that the process occurs in an intermittent fashion; hydride clusters accumulate at the crack tip followed by unstable crack advance and subsequent crack arrest in repeated cycles. 55 refs., 6 figs

Transport of lead to crack tips in steam generator tubes

International Nuclear Information System (INIS)

Adler, G.D.; Marks, C.R.; Fruzzetti, K.

2009-01-01

The mechanisms by which lead is transported from its ultimate source to steam generator tubes and into cracks are not well understood and, to date, a comprehensive evaluation of possible mechanisms has not previously been performed. Specifically, local lead concentrations up to 20 wt. percent have been measured at crack tips, and it is not fully understood how lead concentrations of this magnitude occur, since lead concentrations in SG feedwater are typically quite low (on the order of a few parts per trillion). Additionally, there is evidence that at secondary side conditions, lead is essentially entirely adsorbed onto solid surfaces. Furthermore, if lead were present in the liquid phase, it would not be expected to be in a form that would facilitate concentration in a crevice (crack) by electrochemical means. There has previously been some speculation that lead transport to crack tips may occur through surface diffusion of adsorbed species. It has also been postulated that lead transport may occur via diffusion through the oxide layer along crack walls or via diffusion of lead out of the bulk Alloy 600 to grain boundaries exposed to secondary water by advancing cracks. However, there have been no critical evaluations of these hypotheses. With the current state of knowledge, it is difficult for utilities to determine whether additional efforts to further reduce the inventory of lead in the secondary system are justified. Furthermore, specific sources of lead that are especially likely to accelerate SCC cannot be identified (e.g., significant masses of lead are present in SG deposits, but it is not known if this lead can be transported to crack tips). The work presented in this paper quantitatively evaluates (based on the published literature, not new experimental work) a number of hypothesized lead transport mechanisms, including: Liquid phase diffusion; Electrochemically influenced diffusion of cations and anions; Bulk alloy diffusion; Surface diffusion; Solid

Short intergranular cracks in the piecewise anisotropic continuum model of the microstructure

International Nuclear Information System (INIS)

Cizelj, L.; Kovse, I.

2001-01-01

Computational algorithms aiming at modeling and visualization of the initiation and growth of intergranular stress corrosion cracks (e.g., in the steam generator tubes) on the grain-size scale have already been proposed [6]. The main focus of the paper is given to the influence of randomly oriented neighboring grains on the microscopic stress fields at crack tips. The incompatibility strains, which develop along the boundaries of randomly oriented anisotropic grains, are shown to influence the local stress fields at crack tips significantly. Special attention has been paid to the implementation and comparison of different numerical methods estimating the local stress fields at crack tips, aiming at optimizing the computational time and the numerical accuracy of the results. The limited number of calculations indicate that the anisotropic arrangement of grains with local incompatibility strains causes on average about 10% (plane strain) and 26% (plane stress) higher J-integral values at the crack tips than expected from the calculations in the isotropic case.(author)

A study on the fractures of iodine induced stress corrosion cracking of new zirconium alloys

International Nuclear Information System (INIS)

Peng Qian; Zhao Wenjin; Li Weijun; Tang Zhenghua; Heng Xuemei

2005-10-01

The morphology and chemical compositions of I-SCC fractures of new zirconium alloys were investigated by SEM and EDXA. The feature on fracture surface for I-SCC samples, such as corrosion products, the secondary cracking, intergranular cracking and pseudo-cleavage transgranular cracking, have been observed. And the fluting, the typical characteristic of I-SCC also has been found. Intergranular cracking is visible at crack initiation stage and transgranular cracking is distinguished at crack propagation stage. The corrosion products are mainly composed of Zr and O; and I can be detected on the local pseudocleavage zone. The most of grooves on the fractures of relieved-stress annealing samples are parallel with the roll plane. The intergranular cracking in relieved-stress annealing samples is not obvious. When the test temperature increases, the activity of iodine increases and the stress on crack tip is easier to be released, thus the corrosion products on fracture also increase and intergranular cracking is visible. The partial pressure of iodine influents the thickness of corrosion products, and intergranular cracking is easier to be found when iodine partial pressure is high enough. (authors)

Facts and views on the role of anionic impurities, crack tip chemistry and oxide films in environmentally assisted cracking

International Nuclear Information System (INIS)

Aaltonen, P.; Bojinov, M.; Helin, M.

2002-01-01

The aim of this literature study has been to evaluate the level of understanding of the role of anionic impurities in environmentally assisted cracking (EAC) of iron- and nickel-based alloys in the coolant conditions of a boiling water reactor (BWR) - type nuclear power plant, mainly under normal water chemistry (NWC). The study has been motivated by a need to find the most relevant experimental approaches that can be applied when looking for correlations between crack growth rate and measurable electrochemical and chemical parameters. Special crack tip chemistry conditions are established, when trace amounts are present in the BWR coolant and become enriched within a crack. Anions may influence both the conductivity and the pH of the coolant within the crack. In addition, they may influence the composition, structure and properties of the oxide films formed on crack walls either directly via adsorption or incorporation or indirectly via the effect of changes in pH within the crack. Based on the proposed mechanisms for EAC, oxide films formed on crack wall surfaces are likely to play a key role in determing the crack growth rate of structural materials. The prediction of the influence of anionic impurities is thus likely to be facilitated by means of understanding their effect on the films on crack walls. One of the most promising approaches to experimentally clarify this influence is based on investigating the electrochemical behaviour of oxide films Fe- and Ni-based materials in high-temperature conditions simulating the special chemistry within a stress corrosion crack. Results from such studies should be compared and combined with ex situ analytical results obtained using modern electron microscopic techniques. In addition to crack growth, currently available electro-chemical techniques should also be applied to find out whether crack initiation can be explained and modelled on the basis of the electrochemical behaviour of oxide films. (orig.)

Dislocation structures around crack tips of fatigued polycrystalline copper

International Nuclear Information System (INIS)

Kaneko, Yoshihisa; Ishikawa, Masao; Hashimoto, Satoshi

2005-01-01

Dislocation structures near fatigue cracks of polycrystalline copper specimens were analyzed using the electron channelling contrast imaging (ECCI) technique. Prior to the ECCI observations, optical microscopy was conducted to classify the fatigue crack morphologies into several kinds. It was found that the dislocation structures were correlated with the slip morphologies observed using the optical microscope. The cell structure almost corresponded to the severely deformed plastic zone where the individual slip bands could not be identified. The labyrinth dislocation structure was detected at the double-slip region. Ladder-like dislocation structure was detected ahead of the Stage I type fatigue crack. Hence, it can be said that the persistent slip band (PSB) was a favorable crack path. However, the microscopic route of the crack growth was not along the PSB but along the cell structure, which was developed locally in the vicinity of the crack tip

Effect of plastic strain on elastic-plastic fracture toughness of SM490 carbon steel. Assessment by stress-based criterion for ductile crack initiation

International Nuclear Information System (INIS)

Kamaya, Masayuki

2012-01-01

Although the plastic strain induced in materials increases the mechanical strength, it may reduce the fracture toughness. In this study, the change in fracture toughness of SM490 carbon steel due to pre-straining was investigated using a stress-based criterion for ductile crack initiation. The specimens with blunt notch of various radiuses were used in addition to those with conventional fatigue pre-cracking. The degree of applied plastic strain was 5%, 10% or 20%. The fracture toughness was largest when the induced plastic strain was 5%, although it decreased for the plastic strains of 10% and 20%. The stress and strain distributions near the crack tip of fracture toughness test specimens was investigated by elastic-plastic finite element analyses using a well-correlated stress-strain curve for large strain. It was shown that the critical condition at the onset of the ductile crack was better correlated with the equivalent stress than the plastic strain at the crack tip. By using the stress-based criterion, which was represented by the equivalent stress and stress triaxiality, the change in the fracture toughness due to pre-straining could be reasonably explained. Based on these results, it was concluded that the stress-based criterion should be used for predicting the ductile crack initiation. (author)

Dynamic ductile fracture of a central crack

Science.gov (United States)

Tsai, Y. M.

1976-01-01

A central crack, symmetrically growing at a constant speed in a two dimensional ductile material subject to uniform tension at infinity, is investigated using the integral transform methods. The crack is assumed to be the Dugdale crack, and the finite stress condition at the crack tip is satisfied during the propagation of the crack. Exact expressions of solution are obtained for the finite stress condition at the crack tip, the crack shape, the crack opening displacement, and the energy release rate. All those expressions are written as the product of explicit dimensional quantities and a nondimensional dynamic correction function. The expressions reduce to the associated static results when the crack speed tends to zero, and the nondimensional dynamic correction functions were calculated for various values of the parameter involved.

Stress Intensity Factors of Slanted Cracks in Bi-Material Plates

Science.gov (United States)

Ismail, Al Emran; Azhar Kamarudin, Kamarul; Nor, Nik Hisyamudin Muhd

2017-10-01

In this study, the stress intensity factors (SIF) of slanted cracks in bi-material plates subjected to mode I loading is numerically solved. Based on the literature survey, tremendous amount of research works are available studying the normal cracks in both similar and dissimilar plates. However, lack of SIF behavior for slanted cracks especially when it is embedded in bi-material plates. The slanted cracks are then modelled numerically using ANSYS finite element program. Two plates of different in mechanical properties are firmly bonded obliquely and then slanted edge cracks are introduced at the lower inclined edge. Isoparametric singular element is used to model the crack tip and the SIF is determined which is based on the domain integral method. Three mechanical mismatched and four slanted angles are used to model the cracks. According to the present results, the effects of mechanical mismatch on the SIF for normal cracks are not significant. However, it is played an important role when slanted angles are introduced. It is suggested that higher SIF can be obtained when the cracks are inclined comparing with the normal cracks. Consequently, accelerating the crack growth at the interface between two distinct materials.

Subsurface metals fatigue cracking without and with crack tip

Directory of Open Access Journals (Sweden)

Andrey Shanyavskiy

2013-07-01

Full Text Available Very-High-Cycle-Fatigue regime for metals was considered and mechanisms of the subsurface crack origination were introduced. In many metals first step of crack origination takes place with specific area formation because of material pressing and rotation that directed to transition in any volume to material ultra-high-plasticity with nano-structure appearing. Then by the border of the nano-structure takes place volume rotation and fracture surface creates with spherical particles which usually named Fine-Granular-Area. In another case there takes place First-Smooth-Facet occurring in area of origin due to whirls appearing by the one of the slip systems under discussed the same stress-state conditions. Around Fine-Granular-Area or First-Smooth-Facet there plastic zone appeared and, then, subsurface cracking develops by the same manner as for through cracks. In was discussed quantum-mechanical nature of fatigue crack growth in accordance with Yang’s modulus quantization for low level of deformations. New simply equation was considered for describing subsurface cracking in metals out of Fine-Granular-Area or Fist-Smooth-Facet.

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 Tip Flipping Under Mode I/III Tearing

DEFF Research Database (Denmark)

Felter, Christian Lotz; Specht Jensen, Lasse; Nielsen, Kim Lau

Crack tip flipping, where the fracture surface alternates from side to side in 45° shear bands, seems to be an overlooked propagation mode in Mode I sheet tearing often disregarded as  “transitional” or tied to randomness in the material. In fact, such observations rarely make it to the literature...

Stress corrosion cracking resistance of aluminum alloy 7000 series after two-step aging

Directory of Open Access Journals (Sweden)

Jegdić Bore V.

2015-01-01

Full Text Available The effect of one step-and a new (short two-step aging on the resistance to stress corrosion cracking of an aluminum alloy 7000 series was investigated, using slow strain rate test and fracture mechanics method. Aging level in the tested alloy was evaluated by means of scanning electron microscopy and measurements of electrical resistivity. It was shown that the alloy after the new two-step aging is significantly more resistant to stress corrosion cracking. Values of tensile properties and fracture toughness are similar for both thermal states. Processes that take place at the crack tip have been considered. The effect of the testing solution temperature on the crack growth rate on the plateau was determined. Two values of the apparent activation energy were obtained. These values correspond to different processes that control crack growth rate on the plateau at higher and lower temperatures. [Projekat Ministarstva nauke Republike Srbije, br. TR 34028 i br. TR 34016

Modelling of stress corrosion cracking in zirconium alloys

International Nuclear Information System (INIS)

Fandeur, O.; Rouillon, L.; Pilvin, P.; Jacques, P.; Rebeyrolle, V.

2001-01-01

During normal and incidental operating conditions, PWR power plants must comply with the first safety requirement, which is to ensure that the cladding wall is sound. Indeed some severe power transients potentially induce Stress Corrosion Cracking (SCC) of the zirconium alloy clad, due to strong Pellet Cladding Interaction (PCI). Since, at present, the prevention of this risk has some consequences on the French reactors manoeuvrability, a better understanding and forecast of the clad damage related to SCC/PCI is needed. With this aim, power ramp tests are performed in experimental reactors to assess the fuel rod behaviour and evaluate PCI failure risks. To study in detail SCC mechanisms, additional laboratory experiments are carried out on non-irradiated and irradiated cladding tubes. Numerical simulations of these tests have been developed aiming, on the one hand, to evaluate mechanical state variables and, on the other hand, to study consistent mechanical parameters for describing stress corrosion clad failure. The main result of this simulation is the determination of the validity ranges of the stress intensity factor, which is frequently used to model SCC. This parameter appears to be valid only at the onset of crack growth, when crack length remains short. In addition, the role of plastic strain rate and plastic strain as controlling parameters of the SCC process has been analysed in detail using the above mechanical description of the crack tip mechanical fields. Finally, the numerical determination of the first-order parameter(s) in the crack propagation rate law is completed by the development of laboratory tests focused on these parameters. These tests aim to support experimentally the results of the FE simulation. (author)

The relative stress-corrosion-cracking susceptibility of candidate aluminum-lithium alloys for aerospace applications

Science.gov (United States)

Pizzo, P. P.

1982-01-01

Stress corrosion tests of Al-Li-Cu powder metallurgy alloys are described. Alloys investigated were Al-2.6% Li-1.4% and Al-2.6% Li-1.4% Cu-1.6% Mg. The base properties of the alloys were characterized. Process, heat treatment, and size/orientational effects on the tensile and fracture behavior were investigated. Metallurgical and electrochemical conditions are identified which provide reproducible and controlled parameters for stress corrosion evaluation. Preliminary stress corrosion test results are reported. Both Al-Li-Cu alloys appear more susceptible to stress corrosion crack initiation than 7075-T6 aluminum, with the magnesium bearing alloy being the most susceptible. Tests to determine the threshold stress intensity for the base and magnesium bearing alloys are underway. Twelve each, bolt loaded DCB type specimens are under test (120 days) and limited crack growth in these precracked specimens has been observed. General corrosion in the aqueous sodium chloride environment is thought to be obscuring results through crack tip blunting.

Influence of microstructure on stress corrosion cracking of mild steel in synthetic caustic-nitrate nuclear waste solution

International Nuclear Information System (INIS)

Sarafian, P.G.

1975-12-01

The influence of alloy microstructure on stress corrosion cracking of mild steel in caustic-nitrate synthetic nuclear waste solutions was studied. An evaluation was made of the effect of heat treatment on a representative material (ASTM A 516 Grade 70) used in the construction of high activity radioactive waste storage tanks at Savannah River Plant. Several different microstructures were tested for susceptibility to stress corrosion cracking. Precracked fracture specimens loaded in either constant load or constant crack opening displacement were exposed to a variety of caustic-nitrate and nitrate solutions. Results were correlated with the mechanical and corrosion properties of the microstructures. Crack velocity and crack arrest stress intensity were found to be related to the yield strength of the steel microstructures. Fractographic evidence indicated pH depletion and corrosive crack tip chemistry conditions even in highly caustic solutions. Experimental results were compatible with crack growth by a strain-assisted anodic dissolution mechanism; however, hydrogen embrittlement also was considered possible

Numerical simulations of material mismatch and ductile crack growth

Energy Technology Data Exchange (ETDEWEB)

Oestby, Erling

2002-07-01

Both the global geometry and inhomogeneities in material properties will influence the fracture behaviour of structures in presence of cracks. In this thesis numerical simulations have been used to investigate how some aspects of both these issues affect the conditions at the crack-tip. The thesis is organised in an introduction chapter, summarising the major findings and conclusions, a review chapter, presenting the main aspects of the developments in the field of fracture mechanics, and three research papers. Paper I considers the effect of mismatch in hardening exponent on the local near-tip stress field for stationary interface cracks in bi-materials under small scale yielding conditions. It is demonstrated that the stress level in the weaker material increases compared to what is found in the homogeneous material for the same globally applied load level, with the effect being of increasing importance as the crack-tip is approached. Although a coupling between the radial and angular dependence of the stress fields exists, the evolving stress field can still be normalised with the applied J. The effect on the increase in stress level can closely be characterised by the difference in hardening exponent, {delta}n, termed the hardening mismatch, and is more or less independent of the absolute level of hardening in the two materials. Paper II and Ill deal with the effects of geometry, specimen size, hardening level and yield stress mismatch in relation to ductile crack growth. The ductile crack growth is simulated through use of the Gurson model. In Paper H the effect of specimen size on the crack growth resistance is investigated for deep cracked bend and shallow cracked tensile specimens. At small amounts of crack growth the effect of specimen size on the crack growth resistance is small, but a more significant effect is found for larger amounts of crack growth. The crack growth resistance decreases in smaller specimens loaded in tension, whereas the opposite is

Pipe stress intensity factors and coupled depressurization and dynamic crack propagation. 1976 Annual report

International Nuclear Information System (INIS)

Emery, A.F.; Kobayashi, A.S.; Love, W.J.

1978-04-01

This report contains the description of predictive models for the initiation and propagation of cracks in pipes and the numerical results obtained. The initiation of the crack was studied by evaluating stress intensity factors under static conditions for a series of representative flaws. Three-dimensional static stress intensity factors were determined for quarter-elliptical cracks at the corner of a hole in an infinite plate and at the corner of a bore in a rotating disk. Semi-elliptical cracks for plates in bending and in pressurized and thermally stressed hollow cylinders were also evaluated. The stress fields, in the absence of a crack, were used in the ''alternating technique'' to compute the stress intensity factors along the crack front. Parametric studies were made to assess the effects of crack thickness, the ratio of the major and minor axes of the ellipse and the thickness of the cylinders or plates. These parametric results may be used to predict critical flaw sizes for the initiation of the running crack. The initiation and propagation of axial through cracks in pressurized pipes was studied by using an elastic-plastic finite different shell code coupled with a one-dimensional thermal-hydraulic code which computed the leakage through the crack opening and the depressurization of the fluid in the pipe. The effects of large deflections and different fluid pressure profiles were investigated. The results showed that the crack opening shape is dependent upon the fracture criterion used and upon the average pressure on the crack flaps, but not upon the specific pressure profile. The consideration of large deflections changed the opening size of the crack and through the coupling with the pipe pressures, strongly affected the crack tip speed. However, for equal crack lengths, there was little difference between calculations made for large and small deflection

Neutron diffraction studies on lattice strain evolution around a crack-tip during tensile loading and unloading cycles

Energy Technology Data Exchange (ETDEWEB)

Sun Yinan [Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 (United States)]. E-mail: ysun1@utk.edu; Choo, Hahn [Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 (United States); Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 (United States); Liaw, Peter K. [Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 (United States); Lu Yulin [Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 (United States); Yang Bing [Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 (United States); Brown, Donald W. [Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Bourke, Mark A.M. [Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

2005-10-15

Elastic lattice-strain profiles ahead of a fatigue-crack-tip were measured during tensile loading and unloading cycles using neutron diffraction. The crack-closure phenomenon after an overload was observed. Furthermore, the plastic-zone size in front of the crack-tip was estimated from the diffraction-peak broadening, which showed good agreement with the calculated result.

Characterization of Residual Stress Effects on Fatigue Crack Growth of a Friction Stir Welded Aluminum Alloy

Science.gov (United States)

Newman, John A.; Smith, Stephen W.; Seshadri, Banavara R.; James, Mark A.; Brazill, Richard L.; Schultz, Robert W.; Donald, J. Keith; Blair, Amy

2015-01-01

An on-line compliance-based method to account for residual stress effects in stress-intensity factor and fatigue crack growth property determinations has been evaluated. Residual stress intensity factor results determined from specimens containing friction stir weld induced residual stresses are presented, and the on-line method results were found to be in excellent agreement with residual stress-intensity factor data obtained using the cut compliance method. Variable stress-intensity factor tests were designed to demonstrate that a simple superposition model, summing the applied stress-intensity factor with the residual stress-intensity factor, can be used to determine the total crack-tip stress-intensity factor. Finite element, VCCT (virtual crack closure technique), and J-integral analysis methods have been used to characterize weld-induced residual stress using thermal expansion/contraction in the form of an equivalent delta T (change in local temperature during welding) to simulate the welding process. This equivalent delta T was established and applied to analyze different specimen configurations to predict residual stress distributions and associated residual stress-intensity factor values. The predictions were found to agree well with experimental results obtained using the crack- and cut-compliance methods.

[Mechanism of the dentino-enamel junction on the resist-crack propagation of human teeth by the finite element method].

Science.gov (United States)

Jingjing, Zheng; Tiezhou, Hou; Hong, Tao; Xueyan, Guo; Cui, Wu

2014-10-01

This study aims to identify the crack tip stress intensity factor of the propagation process, crack propagation path, and the changes in the shape of the crack tip by the finite element method. The finite element model of dentino-enamel junction was established with ANSYS software, and the length of the initial crack in the single edge was set to 0.1 mm. The lower end of the sample was fixed. The tensile load of 1 MPa with frequency of 5 Hz was applied to the upper end. The stress intensity factor, deflection angle, and changes in the shape of the crack tip in the crack propagation were calculated by ANSYS. The stress intensity factor suddenly and continuously decreased in dentino-enamel junction as the crack extended. A large skewed angle appeared, and the stress on crack tip was reduced. The dentino-enamel junction on human teeth may resist crack propagation through stress reduction.

Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture-toughness

International Nuclear Information System (INIS)

Pennell, W.E.; Bass, B.R.; Bryson, J.W.; McAfee, W.J.; Theiss, T.J.; Rao, M.C.

1993-01-01

Uniaxial tests of single-edged notched bend (SENB) specimens with both deep- and shallow-flaws have shown elevated fracture-toughness for the shallow flaws. The elevation in fracture-toughness for shallow flaws has been shown to be the result of reduced constraint at the crack-tip. Biaxial loading has the potential to increase constraint at the crack-tip and thereby reduce some of the shallow-flaw, fracture-toughness elevation. Biaxial fracture-toughness tests have shown that the shallow-flaw, fracture-toughness elevation is reduced but not eliminated by biaxial loading. Dual-parameter, fracture-toughness correlations have been proposed to reflect the effect of crack-tip constraint on fracture-toughness. Test results from the uniaxial and biaxial tests were analyzed using the dual-parameter technology. Discrepancies between analysis results and cleavage initiation site data from fractographic examinations indicate that the analysis models are in need of further refinement. Addition of a precleavage, ductile-tearing element to the analysis model has the potential to resolve the noted discrepancies

Stress intensities for nozzle cracks in reactor vessels. Reporting period, January 1, 1976--October 31, 1976

International Nuclear Information System (INIS)

Smith, C.W.; Jolles, M.; Peters, W.H.

1976-11-01

A series of six frozen stress photoelastic tests was conducted to investigate the distribution of stress intensity factor (SIF) along a crack which occurred at the juncture of a pipe (nozzle) with a cylindrical pressure vessel. Typical photoelastic fringe patterns are shown for slices which were taken mutually orthogonal to the flaw border and the flaw surface. A typical plot of normalized apparent SIF versus square root of normalized distance from the crack tip is presented. The variation in SIF along the flaw border is given for all six different crack geometries and also the variation of SIF with varying a/T is presented. 40 references

The pipeline fracture behavior and pressure assessment under HIC (Hydrogen induced cracking) environment

Energy Technology Data Exchange (ETDEWEB)

Shaohua, Dong [China National Petroleum Corporation (CNPC), Beijing (China); Lianwei, Wang [University of Science and Technology Beijing (USTB), Beijing (China)

2009-07-01

As Hydrogen's transmit and diffuse, after gestating for a while, the density of hydrogen around crack tip of pipeline will get to the critical density, and the pipeline material will descend, make critical stress factor, the reason of pipeline Hydrogen Induced Cracking is Hydrogen's transmit and diffuse. The stress factor of Hydrogen Induced Cracking under surroundings-condition of stress is the key that estimate material's rupture behavior. The paper study the relationship among hydrogen concentrate, crack tip stress, stain field, hydrogen diffusion and inner pressure for crack tip process zone, then determined the length of HIC (hydrogen induced cracking) process zone. Based on the theory of propagation which reason micro-crack making core, dislocation model is produced for fracture criteria of HIC, the influence between material and environments under the HIC is analyzed, step by step pipeline maximum load pressure and threshold of J-integrity ( J{sub ISCC} ) is calculated, which is very significant for pipeline safety operation. (author)

Analysis of stress corrosion cracking in alloy 718 following commercial reactor exposure

Energy Technology Data Exchange (ETDEWEB)

Leonard, Keith J., E-mail: leonardk@ornl.gov [Oak Ridge National Laboratory, Oak Ridge, TN (United States); Gussev, Maxim N. [Oak Ridge National Laboratory, Oak Ridge, TN (United States); Stevens, Jacqueline N. [AREVA Inc., Lynchburg, VA (United States); Busby, Jeremy T. [Oak Ridge National Laboratory, Oak Ridge, TN (United States)

2015-11-15

Alloy 718 is generally considered a highly corrosion-resistant material but can still be susceptible to stress corrosion cracking (SCC). The combination of factors leading to SCC susceptibility in the alloy is not always clear enough. In the present work, alloy 718 leaf spring (LS) materials that suffered stress corrosion damage during two 24-month cycles in pressurized water reactor service, operated to >45 MWd/mtU burn-up, was investigated. Compared to archival samples fabricated through the same processing conditions, little microstructural and property changes occurred in the material with in-service irradiation, contrary to high dose rate laboratory-based experiments reported in literature. Though the lack of delta phase formation along grain boundaries would suggest a more SCC resistant microstructure, grain boundary cracking in the material was extensive. Crack propagation routes were explored through focused ion beam milling of specimens near the crack tip for transmission electron microscopy as well as in polished plan view and cross-sectional samples for electron backscatter diffraction analysis. It has been shown in this study that cracks propagated mainly along random high-angle grain boundaries, with the material around cracks displaying a high local density of dislocations. The slip lines were produced through the local deformation of the leaf spring material above their yield strength. The cause for local SCC appears to be related to oxidation of both slip lines and grain boundaries, which under the high in-service stresses resulted in crack development in the material.

A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum

Science.gov (United States)

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.

The importance of the strain rate and creep on the stress corrosion cracking mechanisms and models

International Nuclear Information System (INIS)

Aly, Omar F.; Mattar Neto, Miguel; Schvartzman, Monica M.A.M.

2011-01-01

Stress corrosion cracking is a nuclear, power, petrochemical, and other industries equipment and components (like pressure vessels, nozzles, tubes, accessories) life degradation mode, involving fragile fracture. The stress corrosion cracking failures can produce serious accidents, and incidents which can put on risk the safety, reliability, and efficiency of many plants. These failures are of very complex prediction. The stress corrosion cracking mechanisms are based on three kinds of factors: microstructural, mechanical and environmental. Concerning the mechanical factors, various authors prefer to consider the crack tip strain rate rather than stress, as a decisive factor which contributes to the process: this parameter is directly influenced by the creep strain rate of the material. Based on two KAPL-Knolls Atomic Power Laboratory experimental studies in SSRT (slow strain rate test) and CL (constant load) test, for prediction of primary water stress corrosion cracking in nickel based alloys, it has done a data compilation of the film rupture mechanism parameters, for modeling PWSCC of Alloy 600 and discussed the importance of the strain rate and the creep on the stress corrosion cracking mechanisms and models. As derived from this study, a simple theoretical model is proposed, and it is showed that the crack growth rate estimated with Brazilian tests results with Alloy 600 in SSRT, are according with the KAPL ones and other published literature. (author)

The influence of residual stresses on small through-clad cracks in pressure vessels

International Nuclear Information System (INIS)

deLorenzi, H.G.; Schumacher, B.I.

1984-01-01

The influence of cladding residual stresses on the crack driving force for shallow cracks in the wall of a nuclear pressure vessel is investigated. Thermo-elastic-plastic analyses were carried out on long axial through-clad and sub-clad flaws on the inside of the vessel. The depth of the flaws were one and three times the cladding thickness, respectively. An analysis of a semielliptical axial through-clad flaw was also performed. It was assumed that the residual stresses arise due to the difference in the thermal expansion between the cladding and the base material during the cool down from stress relieving temperature to room temperature and due to the subsequent proof test before the vessel is put into service. The variation of the crack tip opening displacement during these loadings and during a subsequent thermal shock on the inside wall is described. The analyses for the long axial flaws suggest that the crack driving force is smaller for this type of flaw if the residual stresses in the cladding are taken into account than if one assumes that the cladding has no residual stresses. However, the analysis of the semielliptical flaw shows significantly different results. Here the crack driving force is higher than when the residual stresses are not taken into account and is maximum in the cladding at or near the clad/base material interface. This suggests that the crack would propagate along the clad/base material interface before it would penetrate deeper into the wall. The elastic-plastic behavior found in the analyses show that the cladding and the residual stresses in the cladding should be taken into acocunt when evaluating the severity of shallow surface cracks on the inside of a nuclear pressure vessel

Thermometric measurements in notches and crack tips in steels under cyclic stress

International Nuclear Information System (INIS)

Mueller, K.

1989-01-01

The present study reports on temperature measurements with notched samples with and without incipient cracks of unalloyed steels (St 37-2 and Ck 45). Investigations were conducted on thermometric stress determination and on cyclic deformation behaviour. A thermometric concept is presented with which an effective threshold value of cyclic stress intensity can be successfully determined at a low cost with the help of a thermometric estimation method. Thermocouple measurements were performed in all of the experiments, measurements which permitted the registration of temperature range due to thermoelastic effect, besides the registration of the dissipation of deformation work due, particularly, to plastic deformations. (orig./MM) [de

Stress corrosion cracking behaviour of low alloy steels in high temperature water: Description and results from modelling

International Nuclear Information System (INIS)

Tirbonod, B.

2001-01-01

The initiation and growth of a crack by stress and corrosion in the low alloy steels used for the pressure vessels of Boiling Water Reactors may affect the availability and safety of the plant. This paper presents a new model for stress corrosion cracking of the low alloy steels in high temperature water. The model, based on observations, assumes the crack growth mechanism to be based on an anodic dissolution and cleavage. The main results deal with the position of the dissolution cell found at the crack tip, and with the identification of the parameters sensitive to crack growth, among which are the electrolyte composition and the cleavage length. The model is conservative, in qualitative agreement with measurements conducted at PSI, and may be extended to other metal-environment systems. (author)

Influence of tip mass on dynamic behavior of cracked cantilever pipe conveying fluid with moving mass

International Nuclear Information System (INIS)

Yoon, Han Ik; Son, In Soo

2005-01-01

In this paper, we studied about the effect of the open crack and a tip mass on the dynamic behavior of a cantilever pipe conveying fluid with a moving mass. The equation of motion is derived by using Lagrange's equation and analyzed by numerical method. The cantilever pipe is modelled by the Euler-Bernoulli beam theory. The crack section is represented by a local flexibility matrix connecting two undamaged pipe segments. The influences of the crack, the moving mass, the tip mass and its moment of inertia, the velocity of fluid, and the coupling of these factors on the vibration mode, the frequency, and the tip-displacement of the cantilever pipe are analytically clarified

Creep crack growth in phosphorus alloyed oxygen free copper

Energy Technology Data Exchange (ETDEWEB)

Wu, Rui; Seitisleam, Facredin (Swerea KIMAB (Sweden)); Sandstroem, Rolf; Jin, Lai-Zhe (Materials Science and Engineering, Royal Inst. of Technology (Sweden))

2011-01-15

Using standard compact tension (CT) specimens taken from a pierce and draw cylinder, creep crack growth (CCG) has been studied in phosphorus-alloyed oxygen-free copper (Cu-OFP) parent metal at 22, 75, 175, and 215 deg C. Pre- and post-test metallography are performed. At higher temperatures the rupture time of CCG is shorter by a factor up of 65 than that of uniaxial at same stress/reference stress. At 175 and 215 deg C, crack does grow by creep about 10 mm before final instantaneous failure. In contrast, there is hardly any visible crack growth at 22 and 75 deg C. The tests were interrupted after 5000 to 13000 hours. For ruptured tests at 175 and 215 deg C, strongly elongated and deformed grains are observed adjacent to crack. Extensive and intergranular creep cavities and microcracks are found several mm around crack. For interrupted tests at 22 and 75 deg C, strongly elongated and deformed grains, creep cavities, as well as microcracks are observed close to crack tip. Surface cracks from both sides have initiated and grown about 45 deg to the load direction towards inside. For the interrupted tests, hardness adjacent to crack tip has more than doubled because of work hardening, or heavy deformation. This is consistent with large crack tip opening. The true strain at the crack tip is estimated to 10 and 4 for the tests at 22 and 75 deg C, respectively. The stress state behind the crack tip has been modelled with FEM. Stress relaxation after loading has also been taken into account. A model for the creep damage based on the creep strain rate has been formulated that can describe the uniaxial creep rupture data without fitting parameters. Based on the formulation for the creep damage, a model for the crack propagation has been set up. When the creep damage has reached the value unity in front of the crack tip, the crack is assumed to propagate. Taking multiaxial effects into account the observed life times of the CT specimens can be well described. The multiaxial

Creep crack growth in phosphorus alloyed oxygen free copper

International Nuclear Information System (INIS)

Wu, Rui; Seitisleam, Facredin; Sandstroem, Rolf; Jin, Lai-Zhe

2011-01-01

Using standard compact tension (CT) specimens taken from a pierce and draw cylinder, creep crack growth (CCG) has been studied in phosphorus-alloyed oxygen-free copper (Cu-OFP) parent metal at 22, 75, 175, and 215 deg C. Pre- and post-test metallography are performed. At higher temperatures the rupture time of CCG is shorter by a factor up of 65 than that of uniaxial at same stress/reference stress. At 175 and 215 deg C, crack does grow by creep about 10 mm before final instantaneous failure. In contrast, there is hardly any visible crack growth at 22 and 75 deg C. The tests were interrupted after 5000 to 13000 hours. For ruptured tests at 175 and 215 deg C, strongly elongated and deformed grains are observed adjacent to crack. Extensive and intergranular creep cavities and microcracks are found several mm around crack. For interrupted tests at 22 and 75 deg C, strongly elongated and deformed grains, creep cavities, as well as microcracks are observed close to crack tip. Surface cracks from both sides have initiated and grown about 45 deg to the load direction towards inside. For the interrupted tests, hardness adjacent to crack tip has more than doubled because of work hardening, or heavy deformation. This is consistent with large crack tip opening. The true strain at the crack tip is estimated to 10 and 4 for the tests at 22 and 75 deg C, respectively. The stress state behind the crack tip has been modelled with FEM. Stress relaxation after loading has also been taken into account. A model for the creep damage based on the creep strain rate has been formulated that can describe the uniaxial creep rupture data without fitting parameters. Based on the formulation for the creep damage, a model for the crack propagation has been set up. When the creep damage has reached the value unity in front of the crack tip, the crack is assumed to propagate. Taking multiaxial effects into account the observed life times of the CT specimens can be well described. The multiaxial

Effect of T-stress on the cleavage crack growth resistance resulting from plastic flow

DEFF Research Database (Denmark)

Tvergaard, Viggo

1998-01-01

Crack growth is studied numerically for cases where fracture occurs by atomic separation, sc that the length scale of the fracture process is typically much smaller than the dislocation spacing. Thus, the crack growth mechanism is brittle, but due to plastic flow at some distance from the crack tip......, the materials show crack growth resistance. It is shown here that the resistance is strongly dependent on the value of the non-singular T-stress, acting parallel to the crack plane. The numerical technique employed makes use of a thin dislocation-free strip of elastic material inside which the crack propagates......, with the material outside described by continuum plasticity. Thus the width of the strip is a material length scale comparable to the dislocation spacing or the dislocation cell size....

Relationship between stress corrosion cracking and low frequency fatigue-corrosion of alloy 600 in PWR primary water

International Nuclear Information System (INIS)

Bosch, C.

1998-01-01

Stress corrosion cracking of PWR vessel head adapters is a main problem for nuclear industry. With the aim to better understand the influence of the mechanical parameters on the cracking phenomena (by stress corrosion (SCC) or fatigue corrosion (FC)) of alloy 600 exposed to primary PWR coolant, a parametrical study has been carried out. Crack propagation tests on CT test specimens have been implemented under static loads (stress corrosion tests) or low frequency cyclic loads (fatigue corrosion tests). Results (frequency influence, type of cycles, ratio charge on velocities and propagation modes of cracks) have allowed to characterize the transition domain between the crack phenomena of SCC and FC. With the obtained results, it has been possible too to differentiate the effects due to environmental factors and the effects due to mechanical factors. At last, a quantitative fractographic study and the observations of the microstructure at the tip of crack have led to a better understanding of the transitions of the crack propagation mode between the SCC and the FC. (O.M.)

Investigation of plastic zones near SCC tips in a pipeline after hydrostatic testing

International Nuclear Information System (INIS)

Li Jian; Elboujdaini, M.; Gao, M.; Revie, R.W.

2008-01-01

Stress corrosion cracking (SCC) is an important failure mechanism for oil and gas pipelines. In the past, hydrostatic testing has been frequently used to assess and mitigate stress corrosion cracking. It is commonly agreed that an effective hydrostatic test not only eliminates critical crack-like flaws, but also blunts the sub-critical crack tip thereby suppressing further SCC propagation. However, little study has been done on the plastic deformation that results from the high stress intensity at the crack tip due to hydrostatic testing pressure and its possible role in subsequent SCC propagation. In this study, microstructural details were examined of an API 5L X52 SCC-containing pipe removed from field service. Plastic deformation generated by the hydrostatic testing pressure was revealed by using high-resolution imaging of a focused ion beam (FIB) microscope. The existence of the microscopic plastic zones around some crack tips suggests that caution should be taken when setting up pipeline hydrostatic tests

Investigation of plastic zones near SCC tips in a pipeline after hydrostatic testing

Energy Technology Data Exchange (ETDEWEB)

Li Jian [Materials Technology Laboratory, Natural Resources Canada, 568 Booth Street, Ottawa, Ont., K1A 0G1 (Canada)], E-mail: jili@nrcan.gc.ca; Elboujdaini, M [Materials Technology Laboratory, Natural Resources Canada, 568 Booth Street, Ottawa, Ont., K1A 0G1 (Canada); Gao, M [Blade Energy Partners, 16225 Park Ten Place, Suite 450, Houston, TX 77084 (United States); Revie, R W [Materials Technology Laboratory, Natural Resources Canada, 568 Booth Street, Ottawa, Ont., K1A 0G1 (Canada)

2008-07-15

Stress corrosion cracking (SCC) is an important failure mechanism for oil and gas pipelines. In the past, hydrostatic testing has been frequently used to assess and mitigate stress corrosion cracking. It is commonly agreed that an effective hydrostatic test not only eliminates critical crack-like flaws, but also blunts the sub-critical crack tip thereby suppressing further SCC propagation. However, little study has been done on the plastic deformation that results from the high stress intensity at the crack tip due to hydrostatic testing pressure and its possible role in subsequent SCC propagation. In this study, microstructural details were examined of an API 5L X52 SCC-containing pipe removed from field service. Plastic deformation generated by the hydrostatic testing pressure was revealed by using high-resolution imaging of a focused ion beam (FIB) microscope. The existence of the microscopic plastic zones around some crack tips suggests that caution should be taken when setting up pipeline hydrostatic tests.

Analysis of steady-state ductile crack growth

DEFF Research Database (Denmark)

Niordson, Christian

1999-01-01

The fracture strength under quasi-static steady-state crack growth in an elastic-plastic material joined by a laser weld is analyzed. Laser welding gives high mismatch between the yield stress within the weld and the yield stress in the base material. This is due to the fast termic cycle, which...... the finite element mesh remains fixed relative to the tip of the growing crack. Fracture is modelled using two different local crack growth criteria. One is a crack opening displacement criterion, while the other is a model in which a cohesive zone is imposed in front of the crack tip along the fracture zone....... Both models predict that in general a thinner laser weld gives higher interface strength. Furthermore, both fracture criteria show, that the preferred path of the crack is close outside the weld material; a phenomenon also observed in experiments....

Stress corrosion cracking of Ni-Fe-Cr alloys in acid sulfate environments relevant to CANDU steam generators

Energy Technology Data Exchange (ETDEWEB)

Persaud, S.Y.; Carcea, A.G., E-mail: suraj.persaud@mail.utoronto.ca [Univ. of Toronto, Toronto, ON (Canada); Huang, J.; Korinek, A.; Botton, G.A. [McMaster Univ., Hamilton, ON (Canada); Newman, R.C. [Univ. of Toronto, Toronto, ON (Canada)

2014-07-01

Ni-Fe-Cr alloys used in nuclear plants have been found susceptible to stress corrosion cracking (SCC) in acid sulfate environments. Electrochemical measurements and SCC tests were done using Ni, Alloy 600, and Alloy 800 in acid sulfate solutions at 315 {sup o}C. Electrochemical measurements suggested that sulfate is a particularly aggressive anion in mixed chloride systems. Cracks with lengths in excess of 300 μm were present on stressed Alloy 800 samples after 60 hours. High resolution analytical electron microscopy was used to extract a crack tip from an Alloy 800 sample and draw final conclusions with respect to the mechanism of SCC. (author)

Effect of thermomechanical treatment of the stress corrosion cracking of metastable beta III titanium

International Nuclear Information System (INIS)

Seats, J.H.; Condit, D.O.

1974-01-01

Results of studies on the relations of microstructural changes with stress corrosion of Ti--11.5 Mo--6 Zr--4.5 Sn (Beta III) alloys are presented. It was found that this alloy is virtually immune to stress corrosion cracking if no imperfections in the surface are present. Specimens that had not been cold worked showed surface deterioration, but it was not serious enough to cause any marked reduction in yield strengths. The alloy is, however, susceptible to SCC if the surface contains an imperfection such as a fatigue crack where high stresses can concentrate during testing. These high stress levels at the crack tip may cause mechanical destruction of the passivating oxide and allow a higher concentration of chloride ions near the fresh metal surfaces. However, even with precracked specimens, crack propagation is slow as evidenced by no failures within the 720 hour test period. The extreme notch sensitivity of Beta III prevented initiation of fatigue cracks in the sections of the alloy with 20 and 50 percent cold work. More research must be done to test Beta III in this condition. However, on the basis of the research conducted thus far, SCC susceptibility of Beta III titanium alloy appears to be independent of thermomechanical pretreatment. (U.S.)

Factors controlling nitrate cracking of mild steel

International Nuclear Information System (INIS)

Donovan, J.A.

1977-01-01

Nitrite and hydroxide ions inhibit the growth of nitrate stress corrosion cracks in mild steel. Crack growth measurements showed that sufficient concentrations of nitrite and hydroxide ions can prevent crack growth; however, insufficient concentrations of these ions did not influence the Stage II growth rate or the threshold stress intensity, but extended the initiation time. Stage III growth was discontinuous. Oxide formed in the grain boundaries ahead of the crack tip and oxide dissolution (Stage II) and fracture (Stage III) are the proposed mechanisms of nitrate stress corrosion crack growth

Fatigue crack growth behavior of RAFM steel in Paris and threshold regimes at different temperatures

Energy Technology Data Exchange (ETDEWEB)

Babu, M. Nani; Sasikala, G., E-mail: gsasi@igcar.gov.in; Dutt, B. Shashank; Venugopal, S.; Bhaduri, A.K.; Jayakumar, T.

2014-04-01

Fatigue crack growth (FCG) behavior of a reduced activation ferritic martensitic (indigenous RAFM) steel has been evaluated at 300, 653 and 823 K in Paris and threshold regimes. The effect of temperature on threshold stress intensity factor range and associated crack closure mechanisms is highlighted. The FCG results were compared with those for EUROFER 97. Further, crack tip effective stress intensity factor ranges (ΔK{sub tip,eff}) have been evaluated by taking crack tip shielding into account in order to examine the effect of temperature on true intrinsic FCG behavior.

Hydrogen embrittlement and stress corrosion cracking in metals

Energy Technology Data Exchange (ETDEWEB)

Kim, Young Suk; Cheong, Yong Mu; Im, Kyung Soo

2004-10-15

The objective of this report is to elucidate the mechanism for hydrogen embrittlement (HE) and stress corrosion cracking (SCC) in metals. To this end, we investigate the common features between delayed hydride cracking (DHC) in zirconium alloys and HE in metals with no precipitation of hydrides including Fe base alloys, Nickel base alloys, Cu alloys and Al alloys. Surprisingly, as with the crack growth pattern for the DHC in zirconium alloy, the metals mentioned above show a discontinuous crack growth, striation lines and a strong dependence of yield strength when exposed to hydrogen internally and externally. This study, for the first time, analyzes the driving force for the HE in metals in viewpoints of Kim's DHC model that a driving force for the DHC in zirconium alloys is a supersaturated hydrogen concentration coming from a hysteresis of the terminal solid solubility of hydrogen, not by the stress gradient, As with the crack growing only along the hydride habit plane during the DHC in zirconium alloys, the metals exposed to hydrogen seem to have the crack growing by invoking the dislocation slip along the preferential planes as a result of some interactions of the dislocations with hydrogen. Therefore, it seems that the hydrogen plays a role in inducing the slip only on the preferential planes so as to cause a strain localization at the crack tip. Sulfur in metals is detrimental in causing a intergranular cracking due to a segregation of the hydrogens at the grain boundaries. In contrast, boron in excess of 500 ppm added to the Ni3Al intermetallic compound is found to be beneficial in suppressing the HE even though further details of the mechanism for the roles of boron and sulfur are required. Carbon, carbides precipitating semi-continuously along the grain boundaries and the CSL (coherent site lattice) boundaries is found to suppress the intergranular stress corrosion cracking (IGSCC) in Alloy 600. The higher the volume fraction of twin boundaries, the

Hydrogen embrittlement and stress corrosion cracking in metals

International Nuclear Information System (INIS)

Kim, Young Suk; Cheong, Yong Mu; Im, Kyung Soo

2004-10-01

The objective of this report is to elucidate the mechanism for hydrogen embrittlement (HE) and stress corrosion cracking (SCC) in metals. To this end, we investigate the common features between delayed hydride cracking (DHC) in zirconium alloys and HE in metals with no precipitation of hydrides including Fe base alloys, Nickel base alloys, Cu alloys and Al alloys. Surprisingly, as with the crack growth pattern for the DHC in zirconium alloy, the metals mentioned above show a discontinuous crack growth, striation lines and a strong dependence of yield strength when exposed to hydrogen internally and externally. This study, for the first time, analyzes the driving force for the HE in metals in viewpoints of Kim's DHC model that a driving force for the DHC in zirconium alloys is a supersaturated hydrogen concentration coming from a hysteresis of the terminal solid solubility of hydrogen, not by the stress gradient, As with the crack growing only along the hydride habit plane during the DHC in zirconium alloys, the metals exposed to hydrogen seem to have the crack growing by invoking the dislocation slip along the preferential planes as a result of some interactions of the dislocations with hydrogen. Therefore, it seems that the hydrogen plays a role in inducing the slip only on the preferential planes so as to cause a strain localization at the crack tip. Sulfur in metals is detrimental in causing a intergranular cracking due to a segregation of the hydrogens at the grain boundaries. In contrast, boron in excess of 500 ppm added to the Ni3Al intermetallic compound is found to be beneficial in suppressing the HE even though further details of the mechanism for the roles of boron and sulfur are required. Carbon, carbides precipitating semi-continuously along the grain boundaries and the CSL (coherent site lattice) boundaries is found to suppress the intergranular stress corrosion cracking (IGSCC) in Alloy 600. The higher the volume fraction of twin boundaries, the more

The effect of crack blunting on the competition between dislocation nucleation and cleavage

Science.gov (United States)

Fischer, Lisa L.; Beltz, Glenn E.

2001-03-01

To better understand the ductile versus brittle fracture behavior of crystalline materials, attention should be directed towards physically realistic crack geometries. Currently, continuum models of ductile versus brittle behavior are typically based on the analysis of a pre-existing sharp crack in order to use analytical solutions for the stress fields around the crack tip. This paper examines the effects of crack blunting on the competition between dislocation nucleation and atomic decohesion using continuum methods. We accomplish this by assuming that the crack geometry is elliptical, which has the primary advantage that the stress fields are available in closed form. These stress field solutions are then used to calculate the thresholds for dislocation nucleation and atomic decohesion. A Peierls-type framework is used to obtain the thresholds for dislocation nucleation, in which the region of the slip plane ahead of the crack develops a distribution of slip discontinuity prior to nucleation. This slip distribution increases as the applied load is increased until an instability is reached and the governing integral equation can no longer be solved. These calculations are carried out for various crack tip geometries to ascertain the effects of crack tip blunting. The thresholds for atomic decohesion are calculated using a cohesive zone model, in which the region of the crack front develops a distribution of opening displacement prior to atomic decohesion. Again, loading of the elliptical crack tip eventually results in an instability, which marks the onset of crack advance. These calculations are carried out for various crack tip geometries. The results of these separate calculations are presented as the critical energy release rates versus the crack tip radius of curvature for a given crack length. The two threshold curves are compared simultaneously to determine which failure mode is energetically more likely at various crack tip curvatures. From these

Cracks propagation by stress corrosion cracking in conditions of Boiling Water Reactor (BWR)

International Nuclear Information System (INIS)

Fuentes C, P.

2003-01-01

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 O 2 ; 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

Modelling probabilistic fatigue crack propagation rates for a mild structural steel

OpenAIRE

Correia, J.A.F.O.; de Jesus, A.M.P.; Fernández-Canteli, A.

2014-01-01

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...

Deformation localization at the tips of shear fractures: An analytical approach

Science.gov (United States)

Misra, Santanu

2011-04-01

Mechanical heterogeneities are important features in rocks which trigger deformation localization in brittle, ductile or brittle-ductile modes during deformation. In a recent study Misra et al. (2009) have investigated these different processes of deformation localization at the tips of pre-existing planar shear fractures. The authors identified four mechanisms of deformation, ranging from brittle to ductile, operating at the crack tips. Mechanism A: brittle deformation is the dominant process that forms a pair of long tensile fractures at the two crack tips. Mechanism B: nature of deformation is mixed where the tensile fractures grow to a finite length with incipient plastic deformation at the tips. Mechanism C: mixed mode deformation characterized by dominating macro-scale shear bands, and short, opened-out tensile fissures. Mechanism D: localization of plastic bands in the form of a pair of shear bands at each tip without any discernible brittle fracturing. The transition of the mechanisms is a function of orientation ( α) of the crack with respect to the bulk compression direction and the finite length ( l) of the crack. The aim of this study is to present a theoretical analysis to account for the variability of deformation localization in the vicinity of pre-existing shear cracks considering an elastic-plastic rheological model. The analysis calculates the principal tensile stress ( σ1) and the second stress invariant ( I2) of the stress field at the fracture tip to explain the transition from Mechanism A (tensile fracturing) to Mechanism D (ductile strain). The results show that σ1 at the fracture tip increases non-linearly with increasing α and Ar (aspect ratio of the shear crack), and assumes a large value when α > 50 o, promoting tensile fractures. On the other hand, I2 is a maximum at α < 45°, resulting in nucleation of ductile shear bands.

Crack propagation rate modelling for 316SS exposed to PWR-relevant conditions

International Nuclear Information System (INIS)

Vankeerberghen, M.; Weyns, G.; Gavrilov, S.; Martens, B.; Deconinck, J.

2009-01-01

The crack propagation rate of Type 316 stainless steel in boric acid-lithium hydroxide solutions under PWR-relevant conditions was modelled. A film rupture/dissolution/repassivation mechanism is assumed and extended to cold worked materials by including a stress-dependent bare metal dissolution current density. The chemical and electrochemical conditions within the crack are calculated by finite element calculations, an analytical expression is used for the crack-tip strain rate and the crack-tip stress is assumed equal to 2.5 times the yield stress (plane-strain). First the model was calibrated against a literature published data set. Afterwards, the influence of various variables - dissolved hydrogen, boric acid and lithium hydroxide content, stress intensity, crack length, temperature, flow rate - was studied. Finally, other published crack growth rate tests were modelled and the calculated crack growth rates were found to be in reasonable agreement with the reported ones

Hydride redistribution and crack growth in Zr-2.5 wt.% Nb stressed in torsion

International Nuclear Information System (INIS)

Puls, M.P.; Rogowski, A.J.

1980-11-01

The effect of applied shear stresses on zirconium hydride solubility in a zirconium alloy was investigated. Recent studies have shown that zirconium hydride precipiates probably nucleate and grow by means of a shear transformation mechanism. It is postulated that these transformation shear strains can interact with applied shear stress gradients in the same way that the dilatational strains can interact with a dilatational stress gradient, providing a driving force for hydride accumulation, hydride embrittlement and crack propagation. To test this proposition, crack growth experiments were carried out under torsional loading conditions on hydrided, round notched bar specimens of cold-worked Zr-2.5 wt.% Nb cut from Pickering-type pressure tube material. Postmortem metallographic examination of the hydride distribution in these samples showed that, in many cases, the hydrides appeared to have reoriented in response to the applied shear stress and that hydride accumulation at the notch tip had occurred. However, except in a few cases, the rate of accumulation of reoriented hydrides at the notch tip due to applied shear stresses was much less than the rate due to corresponding applied uniaxial stresss. Moreover, the process in shear appears to be more sensitive to the inital hydride size. Attempts to elucidate the fracture mechanism by fractographic examination using scanning and replica transmission electron microscopy proved to be inconclusive because of smearing of the fracture face. (auth)

Stress corrosion cracking of iron-nickel-chromium alloys in primary circuit environment of PWR-type reactors

International Nuclear Information System (INIS)

Boursier, Jean-Marie

1993-01-01

Stress corrosion cracking of Alloy 600 steam generator tubing is a great concern for pressurized water reactors. The mechanism that controls intergranular stress corrosion cracking of Alloy 600 in primary water (lithiated-borated water) has yet to be clearly identified. A study of stress corrosion cracking behaviour, which can identify the main parameters that control the cracking phenomenon, was so necessary to understand the stress corrosion cracking process. Constant extension rate tests, and constant load tests have evidenced that Alloy 600 stress corrosion cracking involves firstly an initiation period, then a slow propagation stage with crack less than 50 to 80 micrometers, and finally a rapid propagation stage leading to failure. The influence of mechanical parameters have shown the next points: - superficial strain hardening and cold work have a strong effect of stress corrosion cracking resistance (decrease of initiation time and increase of crack growth rate), - strain rate was the most suitable parameter for describing the different stage of propagation. The creep behaviour of alloy 600 has shown an increase of creep rate in primary water compared to air, which implies a local interaction plasticity/corrosion. An assessment of the durations of the initiation and the propagation stages was attempted for the whole uniaxial tensile tests, using the macroscopic strain rate: - the initiation time is less than 100 hours and seems to be an electrochemical process, - the durations of the propagation stage are strongly dependent on the strain rate. The behaviour in high primary water temperature of Alloys 690 and 800, which replace Alloy 600, was studied to appraise their margin, and validate their choice. Then the last chapter has to objective to evaluate the crack tip strain rate, in order to better describe the evolution of the different stages of cracking. (author) [fr

The interaction of two collinear cracks in a rectangular superconductor slab under an electromagnetic force

International Nuclear Information System (INIS)

Gao Zhiwen; Zhou Youhe; Lee, Kang Yong

2010-01-01

The interaction of two collinear cracks is obtained for a type-II superconducting under electromagnetic force. Fracture analysis is performed by means of finite element method and the magnetic behavior of superconductor is described by the critical-state Bean model. The stress intensity factors at the crack tips can be obtained and discussed for decreasing field after zero-field cooling. It is revealed that the stress intensity factor decreases as applied field increases. The crack-tip stress intensity factors decrease when the distance between the two collinear cracks increases and the superconductors with smaller crack has more remarkable shielding effect than those with larger cracks.

Quarter elliptical crack growth using three dimensional finite element method and crack closure technique

Energy Technology Data Exchange (ETDEWEB)

Gozin, Mohammad-Hosein; Aghaie-Khafri, Mehrdad [K. N. Toosi University of Technology, Tehran (Korea, Republic of)

2014-06-15

Shape evolution of a quarter-elliptical crack emanating from a hole is studied. Three dimensional elastic-plastic finite element analysis of the fatigue crack closure was considered and the stress intensity factor was calculated based on the duplicated elastic model at each crack tip node. The crack front node was advanced proportional to the imposed effective stress intensity factor. Remeshing was applied at each step of the crack growth and solution mapping algorithm was considered. Crack growth retardation at free surfaces was successfully observed. A MATLAB-ABAQUS interference code was developed for the first time to perform crack growth on the basis of crack closure. Simulation results indicated that crack shape is sensitive to the remeshing strategy. Predictions based on the proposed models were in good agreement with Carlson's experiments results.

Numerical investigation on the prefabricated crack propagation of FV520B stainless steel

Directory of Open Access Journals (Sweden)

Juyi Pan

Full Text Available FV520B is a common stainless steel for manufacturing centrifugal compressor impeller and shaft. The internal metal flaw destroys the continuity of the material matrix, resulting in the crack propagation fracture of the component, which seriously reduces the service life of the equipment. In this paper, Abaqus software was used to simulate the prefabricated crack propagation of FV520B specimen with unilateral gap. The results of static crack propagation simulation results show that the maximum value of stress–strain located at the tip of the crack and symmetrical distributed like a butterfly along the prefabricated crack direction, the maximum stress is 1990 MPa and the maximum strain is 9.489 × 10−3. The Mises stress and stress intensity factor KI increases with the increase of the expansion step, the critical value of crack initiation is reached at the 6th extension step. The dynamic crack propagation simulation shows that the crack propagation path is perpendicular to the load loading direction. Similarly, the maximum Mises stress located at the crack tip and is symmetrically distributed along the crack propagation direction. The critical stress range of the crack propagation is 23.3–43.4 MPa. The maximum value of stress–strain curve located at the 8th extension step, that is, the crack initiation point, the maximum stress is 55.22 MPa, and the maximum strain is 2.26 × 10−4. On the crack tip, the stress changed as 32.24–40.16 MPa, the strain is at 1.292 × 10−4–1.897 × 10−4. Keywords: FV520B, Crack propagation, Mises stress, Stress–strain, Numerical investigation

Analysis on the stress corrosion crack inception based on pit shape and size of the FV520B tensile specimen

Science.gov (United States)

Xiang, Longhao; Pan, Juyi; Chen, Songying

2018-06-01

The influence of pit shape and size on local stress concentration in the tensile specimen and the stress corrosion cracks inception was studied by employing the element remove technique. The maximum stress located in the bottom of pit on FV520B tensile specimen. The location of maximum strain was near the mouth of the pit or the shoulder and plastic strain existed in this region. Stress concentration factor and plastic deformation on four different geometrical shape pits of hemisphere, semi-ellipsoid, bullet and butterfly were numerically investigated, respectively. The simulation results showed that butterfly pit got the biggest stress concentration factor. The plastic strain rate during pit growth was in the sensitivity range of stress corrosion cracks inception, indicating that stress corrosion cracks were more likely to nucleate near the pit tip or the shoulder.

Attractive and repulsive cracks in a heterogeneous material

International Nuclear Information System (INIS)

Cortet, Pierre-Philippe; Huillard, Guillaume; Vanel, Loïc; Ciliberto, Sergio

2008-01-01

We study experimentally the paths of an assembly of cracks growing in interaction in a heterogeneous two-dimensional elastic brittle material submitted to uniaxial stress. For a given initial crack assembly geometry, we observe two types of crack path. The first one corresponds to a repulsion followed by an attraction on one end of the crack and a tip-to-tip attraction on the other end. The second one corresponds to a pure attraction. Only one of the crack path types is observed in a given sample. Thus, selection between the two types appears as a statistical collective process

A short summary on finite element modelling of fatigue crack closure

Energy Technology Data Exchange (ETDEWEB)

Singh, Konjengbam Darunkumar [Indian Institute of Technology, Guwahati (India); Parry, Matthew Roger [Airbus Operations Ltd, Bristol(United Kingdom); Sinclair, Ian [University of Southampton, Southampton (United Kingdom)

2011-12-15

This paper presents a short summary pertaining to the finite element modelling of fatigue crack closure. Several key issues related to finite element modelling of fatigue crack closure are highlighted: element type, mesh refinement, stabilization of crack closure, crack-tip node release scheme, constitutive model, specimen geometry, stress-states (i.e., plane stress, plane strain), crack closure monitoring. Reviews are presented for both straight and deflected cracks.

Assessment of foam fracture in sandwich beams using thermoelastic stress analysis

DEFF Research Database (Denmark)

Dulieu-Barton, J.M.; Berggreen, Christian; Mettemberg, C.

2009-01-01

Thermoelastic Stress Analysis (TSA) has been well established for determining crack-tip parameters in metallic materials. This paper examines its ability to determine accurately the crack-tip parameters for PVC foam used in sandwich structures.......Thermoelastic Stress Analysis (TSA) has been well established for determining crack-tip parameters in metallic materials. This paper examines its ability to determine accurately the crack-tip parameters for PVC foam used in sandwich structures....

Subcritical crack growth in a phosphate laser glass

Energy Technology Data Exchange (ETDEWEB)

Crichton, S.N.; Tomozawa, M.; Hayden, J.S.; Suratwala, T.I.; Campbell, J.H.

1999-11-01

The rate of subcritical crack growth in a metaphosphate Nd-doped laser glass was measured using the double-cleavage-drilled compression (DCDC) method. The crack velocity is reported as a function of stress intensity at temperatures ranging from 296 to 573 K and in nitrogen with water vapor pressures ranging from 40 Pa (0.3 mmHg) to 4.7 x 10{sup 4} Pa (355 mmHg). The measured crack velocities follow region I, II, and III behavior similar to that reported for silicate glasses. A chemical and mass-transport-limited reaction rate model explains the behavior of the data except at high temperatures and high water vapor pressures where crack tip blunting is observed. Blunting is characterized to reinitiate slow crack growth at higher stresses. A dynamic crack tip blunting mechanism is proposed to explain the deviation from the reaction rate model.

Evaluation of the presence of constraint in crack run/arrest events

International Nuclear Information System (INIS)

Schwartz, C.W.; Bass, B.R.

1988-01-01

Crack arrest studies currently being conducted by the Heavy-Section Steel Technology Program are designed to improve our understanding of the conditions contributing to the arrest of a propagating fracture in a pressure vessel. These studies are generating data spanning a wide temperature range for a variety of experimental configurations. Dynamic crack arrest parameters are back-figured from these experiments through 'generation mode' dynamic viscoplastic finite element calculations driven by the measured crack tip history input. A major approximation in these analyses, which is dictated by the practical limitations of current supercomputer hardware, is the assumption of two-dimensional plane stress conditions. Although this approximation is reasonable over most of the problem domain for many test specimen geometries, it deteriorates at locations near the crack tip due to triaxial constraint effects. This paper describes plans for a fine-grained three-dimensional computational study to investigate the importance of these near-tip triaxial constraint effects on crack tip yielding and to develop appropriate algorithms for incorporating these effects into conventional two-dimensional plane stress approximations. (author)

Towards a quantification of stress corrosion mechanisms: numerical simulations of hydrogen-dislocations at the very crack tip

International Nuclear Information System (INIS)

Chateau, J.P.

1999-01-01

We discuss the respective roles played by anodic dissolution and hydrogen in SCC mechanisms of f.c.c. materials, by studying the fracture of copper in nitrite for which we compare the results with that previously obtained in 316L steel in hot chloride. It is surprising to note that even the crystallographies at the scale of the micron are different, the macroscopic inclination of the fracture surfaces are the same. In the case of 316L steel, the formation of strong pile-ups in the presence of hydrogen leads to a zigzag fracture along alternated slip planes in the most general case. In the absence of hydrogen, as in copper, this mechanism effectively disappears. Furthermore, numerical simulations of crack shielding by dislocations emitted on one plane predict the macroscopic inclination. It shows that it is due to the mere dissolution which confines slip activity at the very crack tip in f.c.c. materials. In order to quantify the mechanism involved in 316L steel, we developed simulations which numerically solve the coupled diffusion and elasticity equations for hydrogen in the presence of a crack and shielding dislocations. They reproduce the mechanisms of hydrogen segregation on edge dislocations and of a localised softening effect by decreasing pair interactions. These mechanisms lead to i) a localisation of hydrogen embrittlement along the activated slip planes, ii) an increase of the dislocation density in pile-ups, and iii) a decrease of the cross slip probability. These three factors enhance micro-fracture at the head of a pile-up, which is responsible of the zigzag fracture. Introducing the free surface effects for hydrogen, we point out a new mechanism: the inhibition of dislocation sources at the crack tip, which is relevant with the brittle fracture surfaces observed in some cases in 316L steel. The quantification of these different mechanisms allows to give a relation between the local fracture possibility and the macroscopic parameters. A general law for

Development of nondestructive method for prediction of crack instability

International Nuclear Information System (INIS)

Schroeder, J.L.; Eylon, D.; Shell, E.B.; Matikas, T.E.

2000-01-01

A method to characterize the deformation zone at a crack tip and predict upcoming fracture under load using white light interference microscopy was developed and studied. Cracks were initiated in notched Ti-6Al-4V specimens through fatigue loading. Following crack initiation, specimens were subjected to static loading during in-situ observation of the deformation area ahead of the crack. Nondestructive in-situ observations were performed using white light interference microscopy. Profilometer measurements quantified the area, volume, and shape of the deformation ahead of the crack front. Results showed an exponential relationship between the area and volume of deformation and the stress intensity factor of the cracked alloy. These findings also indicate that it is possible to determine a critical rate of change in deformation versus the stress intensity factor that can predict oncoming catastrophic failure. In addition, crack front deformation zones were measured as a function of time under sustained load, and crack tip deformation zone enlargement over time was observed

Analytical Tem Comparisons of Stress-Corrosion-Crack Microstructures in Alloy 600 under Steam-Generator Service and Laboratory Test Conditions

International Nuclear Information System (INIS)

Thomas, L.E.; Bruemmer, S.M.; Scott, P.M.

2002-01-01

High-resolution analytical transmission electron microscopy (ATEM) has been used to characterize stress-corrosion cracks (SCC) in Alloy 600 steam-generator (SG) tubing from tests with caustic and acid-sulfate solutions. The aim of this work was to identify the microstructural and microchemical signatures of intergranular attack and cracking produced under well-controlled test conditions in order to determine the local environments promoting degradation in service. Cross-sectioned cracks and crack tips were examined in samples of mill-annealed alloy 600 tested in concentrated caustic and acid-sulfate solutions at 320 C. Characteristic microstructures observed in the caustic (10% NaOH) test sample included deeply penetrative attack along crack-intersected grain boundaries, with Cr-rich spinel and NiO structure oxides ranging from random nanocrystalline to oriented epitaxial films filling cracks up to the tips. Sodium was readily detectable in the oxides (up to 5 wt.% in the spinel corrosion product) along with S and Cu enrichment at crack-wall metal/oxide interfaces and local attack of the metal matrix around IG carbide particles. In the sulfate (Na 2 SO 4 + FeSO 4 ) test sample, the grain boundaries were also deeply attacked/cracked. Epitaxial NiO-structure oxide formed on the crack walls and S, sometimes with Cu, was concentrated between the oriented oxide layers rather than along the metal/oxide interfaces. Carbides were attacked and partially converted to fine-grained oxide containing up to several percent S. Observations of crack tips in the acid sulfate sample also revealed nm-wide cracks preceding the oxide along grain boundaries. The SCC structures produced in the laboratory tests differed in most details from the secondary-side SCC structures observed in pulled SG tubes. Important differences included the oxide morphologies, the presence of easily detectable Na and absence of sulfides in the test samples, different types of attack on IG carbide particles

On the controlling parameters for fatigue-crack threshold at low homologous temperatures

International Nuclear Information System (INIS)

Yu, W.; Gerberich, W.W.

1983-01-01

Fatigue crack propagation phenomena near the threshold stress intensity level ΔK /SUB TH/ , has been a vigorously studied topic in recent years. Near threshold the crack propagates rather slowly, thus giving enough time for various physical and chemical reactions to take place. Room air, which is the most commonly encountered environment, can still supply various ingredients such as oxygen, water vapor (and thus hydrogen) to support these reactions. Much effort had been directed toward the environmental aspects of near threshold fatigue crack growth. By conducting tests under vacuum, Suresh and coworkers found that the crack propagation rate in a 2-1/4 Cr-1Mo steel was higher in vacuum than in air. An oxide induced closure, which served to reduce the effective stress intensity at the crack tip, seems to furnish a good explanation. Neumann and coworkers proposed that during the fatigue process, extrusion-intrusion pairs can develop as a consequence of reversed slip around the crack tip when the crack was propagated near threshold stress intensity. Beevers demonstrated that fatigue fracture surfaces contact each other during unloading even under tension-tension cycling. Kanninen and Atkinson also reached the conclusion that the compressive stress acting at the crack tip due to residual plasticity can induce closure. Microstructural effects have also been cited as important factors in near threshold crack growth. It is generally accepted that coarser grains have a beneficial effect on the resistance to the near threshold crack propagation

Dislocations, the elastic energy momentum tensor and crack propagation

International Nuclear Information System (INIS)

Lung, Chi-wei

1979-07-01

Based upon dislocation theory, some stress intensity factors can be calculated for practical cases. The results obtained by this method have been found to agree fairly well with the results obtained by the conventional fracture mechanics. The elastic energy momentum tensor has been used to calculate the force acting on the crack tip. A discussion on the kinetics of migration of impurities to the crack tip was given. It seems that the crack tip sometimes may be considered as a singularity in an elastic field and the fundamental law of classical field theory is applicable on the problem in fracture of materials. (author)

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.

Crack-tips enriched platinum-copper superlattice nanoflakes as highly efficient anode electrocatalysts for direct methanol fuel cells.

Science.gov (United States)

Zheng, Lijun; Yang, Dachi; Chang, Rong; Wang, Chengwen; Zhang, Gaixia; Sun, Shuhui

2017-07-06

We have developed "crack-tips" and "superlattice" enriched Pt-Cu nanoflakes (NFs), benefiting from the synergetic effects of "crack-tips" and "superlattice crystals"; the Pt-Cu NFs exhibit 4 times higher mass activity, 6 times higher specific activity and 6 times higher stability than those of the commercial Pt/C catalyst, respectively. Meanwhile, the Pt-Cu NFs show more enhanced CO tolerance than the commercial Pt/C catalyst.

Uncertainty quantification methodologies development for stress corrosion cracking of canister welds

Energy Technology Data Exchange (ETDEWEB)

Dingreville, Remi Philippe Michel [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Bryan, Charles R. [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

2016-09-30

This letter report presents a probabilistic performance assessment model to evaluate the probability of canister failure (through-wall penetration) by SCC. The model first assesses whether environmental conditions for SCC – the presence of an aqueous film – are present at canister weld locations (where tensile stresses are likely to occur) on the canister surface. Geometry-specific storage system thermal models and weather data sets representative of U.S. spent nuclear fuel (SNF) storage sites are implemented to evaluate location-specific canister surface temperature and relative humidity (RH). As the canister cools and aqueous conditions become possible, the occurrence of corrosion is evaluated. Corrosion is modeled as a two-step process: first, pitting is initiated, and the extent and depth of pitting is a function of the chloride surface load and the environmental conditions (temperature and RH). Second, as corrosion penetration increases, the pit eventually transitions to a SCC crack, with crack initiation becoming more likely with increasing pit depth. Once pits convert to cracks, a crack growth model is implemented. The SCC growth model includes rate dependencies on both temperature and crack tip stress intensity factor, and crack growth only occurs in time steps when aqueous conditions are predicted. The model suggests that SCC is likely to occur over potential SNF interim storage intervals; however, this result is based on many modeling assumptions. Sensitivity analyses provide information on the model assumptions and parameter values that have the greatest impact on predicted storage canister performance, and provide guidance for further research to reduce uncertainties.

Stress corrosion cracking of X80 pipeline steel exposed to high pH solutions with different concentrations of bicarbonate

Science.gov (United States)

Fan, Lin; Du, Cui-wei; Liu, Zhi-yong; Li, Xiao-gang

2013-07-01

Susceptibilities to stress corrosion cracking (SCC) of X80 pipeline steel in high pH solutions with various concentrations of HCO{3/-} at a passive potential of -0.2 V vs. SCE were investigated by slow strain rate tensile (SSRT) test. The SCC mechanism and the effect of HCO{3/-} were discussed with the aid of electrochemical techniques. It is indicated that X80 steel shows enhanced susceptibility to SCC with the concentration of HCO{3/-} increasing from 0.15 to 1.00 mol/L, and the susceptibility can be evaluated in terms of current density at -0.2 V vs. SCE. The SCC behavior is controlled by the dissolution-based mechanism in these circumstances. Increasing the concentration of HCO{3/-} not only increases the risk of rupture of passive films but also promotes the anodic dissolution of crack tips. Besides, little susceptibility to SCC is found in dilute solution containing 0.05 mol/L HCO{3/-} for X80 steel. This can be attributed to the inhibited repassivation of passive films, manifesting as a more intensive dissolution in the non-crack tip areas than at the crack tips.

Influence of surrounding environment on subcritical crack growth in marble

Science.gov (United States)

Nara, Yoshitaka; Kashiwaya, Koki; Nishida, Yuki; , Toshinori, Ii

2017-06-01

Understanding subcritical crack growth in rock is essential for determining appropriate measures to ensure the long-term integrity of rock masses surrounding structures and for construction from rock material. In this study, subcritical crack growth in marble was investigated experimentally, focusing on the influence of the surrounding environment on the relationship between the crack velocity and stress intensity factor. The crack velocity increased with increasing temperature and/or relative humidity. In all cases, the crack velocity increased with increasing stress intensity factor. However, for Carrara marble (CM) in air, we observed a region in which the crack velocity still increased with temperature, but the increase in the crack velocity with increasing stress intensity factor was not significant. This is similar to Region II of subcritical crack growth observed in glass in air. Region II in glass is controlled by mass transport to the crack tip. In the case of rock, the transport of water to the crack tip is important. In general, Region II is not observed for subcritical crack growth in rock materials, because rocks contain water. Because the porosity of CM is very low, the amount of water contained in the marble is also very small. Therefore, our results imply that we observed Region II in CM. Because the crack velocity increased in both water and air with increasing temperature and humidity, we concluded that dry conditions at low temperature are desirable for the long-term integrity of a carbonate rock mass. Additionally, mass transport to the crack tip is an important process for subcritical crack growth in rock with low porosity.

Non-self-similar cracking in unidirectional metal-matrix composites

International Nuclear Information System (INIS)

Rajesh, G.; Dharani, L.R.

1993-01-01

Experimental investigations on the fracture behavior of unidirectional Metal Matrix Composites (MMC) show the presence of extensive matrix damage and non-self-similar cracking of fibers near the notch tip. These failures are primarily observed in the interior layers of an MMC, presenting experimental difficulties in studying them. Hence an investigation of the matrix damage and fiber fracture near the notch tip is necessary to determine the stress concentration at the notch tip. The classical shear lag (CLSL) assumption has been used in the present study to investigate longitudinal matrix damage and nonself-similar cracking of fibers at the notch tip of an MMC. It is seen that non-self-similar cracking of fibers reduces the stress concentration at the notch tip considerably and the effect of matrix damage is negligible after a large number of fibers have broken beyond the notch tip in a non-self-similar manner. Finally, an effort has been made to include non-self-similar fiber fracture and matrix damage to model the fracture behavior of a unidirectional boron/aluminum composite for two different matrices viz. a 6061-0 fully annealed aluminum matrix and a heat treated 6061-T6 aluminum matrix. Results have been drawn for several characteristics pertaining to the shear stiffnesses and the shear yield stresses of the two matrices and compared with the available experimental results

Evaluation of crack interaction effect for in-plane surface cracks using elastic finite element analyses

International Nuclear Information System (INIS)

Huh, Nam Su; Choi, Suhn; Park, Keun Bae; Kim, Jong Min; Choi, Jae Boong; Kim, Young Jin

2008-01-01

The crack-tip stress fields and fracture mechanics assessment parameters, such as the elastic stress intensity factor and the elastic-plastic J-integral, for a surface crack can be significantly affected by adjacent cracks. Such a crack interaction effect due to multiple cracks can magnify the fracture mechanics assessment parameters. There are many factors to be considered, for instance the relative distance between adjacent cracks, crack shape and loading condition, to quantify a crack interaction effect on the fracture mechanics assessment parameters. Thus, the current guidance on a crack interaction effect (crack combination rule), including ASME Sec. XI, BS7910, British Energy R6 and API RP579, provide different rules for combining multiple surface cracks into a single surface crack. The present paper investigates a crack interaction effect by evaluating the elastic stress intensity factor of adjacent surface cracks in a plate along the crack front through detailed 3-dimensional elastic finite element analyses. The effects of the geometric parameters, the relative distance between cracks and the crack shape, on the stress intensity factor are systematically investigated. As for the loading condition, only axial tension is considered. Based on the elastic finite element results, the acceptability of the crack combination rules provided in the existing guidance was investigated, and the relevant recommendations on a crack interaction for in-plane surface cracks in a plate were discussed

Stress corrosion cracking of duplex stainless steels in caustic solutions

Science.gov (United States)

Bhattacharya, Ananya

susceptibility. Annealed and water quenched specimens were found to be immune to SCC in caustic environment. Aging treatment at 800°C gave rise to sigma and chi precipitates in the DSS. However, these sigma and chi precipitates, known to initiate cracking in DSS in chloride environment did not cause any cracking of DSS in caustic solutions. Aging of DSS at 475°C had resulted in '475°C embrittlement' and caused cracks to initiate in the ferrite phase. This was in contrast to the cracks initiating in the austenite phase in the as-received DSS. Alloy composition and microstructure of DSS as well as solution composition (dissolved ionic species) was also found to affect the electrochemical behavior and passivation of DSS which in turn plays a major role in stress corrosion crack initiation and propagation. Corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS, characterized using XRD and X-ray photoelectron spectroscopy, indicated that the metal sulfide compounds were formed along with oxides at the metal surface in the presence of sulfide containing caustic environments. These metal sulfide containing passive films are unstable and hence breaks down under mechanical straining, leading to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.

The role of local strains from prior cold work on stress corrosion cracking of α-brass in Mattsson's solution

International Nuclear Information System (INIS)

Ulaganathan, Jaganathan; Newman, Roger C.

2014-01-01

The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously within the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work. - Highlights: • Plastic strain localization has a complex relationship with SCC susceptibility. • Surface relief created by cold work creates its own granular strain localization. • Cold work promotes crack growth but several other factors are involved

Sizing of cracks by ultrasonic testing - Diffraction methods

International Nuclear Information System (INIS)

Hoegberg, K.; Sattari-Far, I.; Pers-Anderson, E.B.

1989-01-01

The work has been concentrated on manual ultrasonic testing of plates in carbon and austenitic steel with thicknesses of 10-40 mm. Evaluation of data was performed by studying the amplitude, accuracy (crack depth) and visibility. The experience from the project showed that identification of the weak signals from the crack tips requires well-trained personnel. Besides that, the following can be recommended: Estimate if the crack has compressive stresses. Especially shallow cracks are exposed for compressive stresses. Chose a refraction angle ≥ 60 degrees if the crack is deep. Try both low (approx equivalent to 2MHz) and high (approx equivalent to 4-5MHz) frequency. Lower frequencies often increase amplitude response. Avoid the combination of refraction angle greater than 60 degrees and low frequency. Inspect with half as well as full skip. Sometimes a stronger signal is received for full skip, because the amplitude of the diffracted signal is higher from the cracked side. Use complementary measurements with mode conversion techniques. Focused probes can improve the results, especially for complicated geometries. Do not use reference reflectors of EDM-notch type for verification of signal amplitude. No correlation between amplitude from an EDM-notch tip and a crack tip exists. Reference reflectors of EDM-notch type can be used to verify the resolution of the system. A shallow EDM-notch can show if the probe can separate the tip and corner. It is our experience that general solutions does not exist, and each case needs an individual solution

Mechanism of crack healing at room temperature revealed by atomistic simulations

International Nuclear Information System (INIS)

Li, J.; Fang, Q.H.; Liu, B.; Liu, Y.; Liu, Y.W.; Wen, P.H.

2015-01-01

Three dimensional molecular dynamics (MD) simulations are systematically carried out to reveal the mechanism of the crack healing at room temperature, in terms of the dislocation shielding and the atomic diffusion to control the crack closure, in a copper (Cu) plate suffering from a shear loading. The results show that the process of the crack healing is actualized through the dislocation emission at a crack tip accompanied with intrinsic stacking faults ribbon forming in the crack tip wake, the dislocation slipping in the matrix and the dislocation annihilation in the free surface. Dislocation included stress compressing the crack tip is examined from the MD simulations and the analytical models, and then the crack closes rapidly due to the assistance of the atomic diffusion induced by the thermal activation when the crack opening displacement is less than a threshold value. This phenomenon is very different from the previous results for the crack propagation under the external load applied because of the crack healing (advancing) largely dependent on the crystallographic orientations of crack and the directions of external loading. Furthermore, based on the energy characteristic and considering the crack size effect, a theoretical model is established to predict the relationships between the crack size and the shear stress which qualitatively agree well with that obtained in the MD simulations

Extended timescale atomistic modeling of crack tip behavior in aluminum

International Nuclear Information System (INIS)

Baker, K L; Warner, D H

2012-01-01

Traditional molecular dynamics (MD) simulations are limited not only by their spatial domain, but also by the time domain that they can examine. Considering that many of the events associated with plasticity are thermally activated, and thus rare at atomic timescales, the limited time domain of traditional MD simulations can present a significant challenge when trying to realistically model the mechanical behavior of materials. A wide variety of approaches have been developed to address the timescale challenge, each having their own strengths and weaknesses dependent upon the specific application. Here, we have simultaneously applied three distinct approaches to model crack tip behavior in aluminum at timescales well beyond those accessible to traditional MD simulation. Specifically, we combine concurrent multiscale modeling (to reduce the degrees of freedom in the system), parallel replica dynamics (to parallelize the simulations in time) and hyperdynamics (to accelerate the exploration of phase space). Overall, the simulations (1) provide new insight into atomic-scale crack tip behavior at more typical timescales and (2) illuminate the potential of common extended timescale techniques to enable atomic-scale modeling of fracture processes at typical experimental timescales. (paper)

Quasi-static crack tip fields in rate-sensitive FCC single crystals

Indian Academy of Sciences (India)

In this work, the effects of loading rate, material rate sensitivity and constraint level on quasi-static crack tip fields in a FCC single crystal are studied. ... Global General Motors R&D, India Science Lab, GM Technical Centre (India), Bangalore 560 066, India; Department of Mechanical Engineering, Indian Institute of Science, ...

Compressive failure with interacting cracks

International Nuclear Information System (INIS)

Yang Guoping; Liu Xila

1993-01-01

The failure processes in concrete and other brittle materials are just the results of the propagation, coalescence and interaction of many preexisting microcracks or voids. To understand the real behaviour of the brittle materials, it is necessary to bridge the gap from the relatively matured one crack behaviour to the stochastically distributed imperfections, that is, to concern the crack propagation and interaction of microscopic mechanism with macroscopic parameters of brittle materials. Brittle failure in compression has been studied theoretically by Horii and Nemat-Nasser (1986), in which a closed solution was obtained for a preexisting flaw or some special regular flaws. Zaitsev and Wittmann (1981) published a paper on crack propagation in compression, which is so-called numerical concrete, but they did not take account of the interaction among the microcracks. As for the modelling of the influence of crack interaction on fracture parameters, many studies have also been reported. Up till now, some researcher are working on crack interaction considering the ratios of SIFs with and without consideration of the interaction influences, there exist amplifying or shielding effects of crack interaction which are depending on the relative positions of these microcracks. The present paper attempts to simulate the whole failure process of brittle specimen in compression, which includes the complicated coupling effects between the interaction and propagation of randomly distributed or other typical microcrack configurations step by step. The lengths, orientations and positions of microcracks are all taken as random variables. The crack interaction among many preexisting random microcracks is evaluated with the help of a simple interaction matrix (Yang and Liu, 1991). For the subcritically stable propagation of microcracks in mixed mode fracture, fairly known maximum hoop stress criterion is adopted to compute branching lengths and directions at each tip of the crack

Electrochemical study of stress corrosion cracking of copper alloys

International Nuclear Information System (INIS)

Malki, Brahim

1999-01-01

This work deals with the electrochemical study of stress corrosion of copper alloys in aqueous environment. Selective dissolution and electrochemical oxidation are two key-points of the stress corrosion of these alloys. The first part of this thesis treats of these aspects applied to Cu-Au alloys. Measurements have been performed using classical electrochemical techniques (in potentio-dynamic, potentio-static and galvano-static modes). The conditions of occurrence of an electrochemical noise is analysed using signal processing techniques. The impact on the behavior of Cu 3 Au are discussed. In the second part, the stress corrosion problem is addressed in the case of surface oxide film formation, in particular for Cu-Zn alloys. We have found useful to extend this study to mechanical stress oxidation mechanisms in the presence of an oscillating potential electrochemical system. The aim is to examine the influence of these new electrochemical conditions (galvano-static mode) on the behavior of stressed brass. Finally, the potential distribution at crack tip is calculated in order to compare the different observations [fr

Exploring How Weathering Related Stresses and Subcritical Crack Growth May Influence the Size of Sediment Produced From Different Rock Types.

Science.gov (United States)

Eppes, M. C.; Hallet, B.; Hancock, G. S.; Mackenzie-Helnwein, P.; Keanini, R.

2016-12-01

The formation and diminution of rock debris, sediment and soil at and near Earth's surface is driven in large part by in situ, non-transport related, rock cracking. Given the relatively low magnitude stresses that arise in surface and near-surface settings, this production and diminution of granular material is likely strongly influenced and/or driven by subcritical crack growth (Eppes et al., 2016), cracking that occurs under stress loading conditions much lower than a rock's strength as typically measured in the laboratory under rapid loading. Despite a relatively sound understanding of subcritical crack growth through engineering and geophysical studies, its geomorphic and sedimentologic implications have only been minimally explored. Here, based on existing studies, we formulate several hypotheses to predict how weathering-induced stresses combined with the subcritical crack growth properties of rock may influence sediment size distribution. For example, subcritical crack growth velocity (v) can be described by v = CKIn where KI is the mode I (simple opening mode) stress intensity factor, a function of tensile stress at the crack tip and crack length; C is a rock- and environment-dependent constant; and n is material constant, the subcritical crack growth index. Fracture length and spacing in rock is strongly dependent on n, where higher n values result in fewer, more distally spaced cracks (e.g. Olsen, 1993). Thus, coarser sediment might be expected from rocks with higher n values. Weathering-related stresses such as thermal stresses and mineral hydration, however, can disproportionally stress boundaries between minerals with contrasting thermal or chemical properties and orientation, resulting in granular disintegration. Thus, rocks with properties favorable to inducing these stresses might produce sediment whose size is reflective of its constituent grains. We begin to test these hypotheses through a detailed examination of crack and rock characteristics in

Experimental Study on Wing Crack Behaviours in Dynamic-Static Superimposed Stress Field Using Caustics and High-Speed Photography

Directory of Open Access Journals (Sweden)

L.Y. Yang

2014-07-01

Full Text Available During the drill-and-blast progress in rock tunnel excavation of great deep mine, rock fracture is evaluated by both blasting load and pre-exiting earth stress (pre-compression. Many pre-existing flaws in the rock mass, like micro-crack, also seriously affect the rock fracture pattern. Under blasting load with pre-compression, micro-cracks initiate, propagate and grow to be wing cracks. With an autonomous design of static-dynamic loading system, dynamic and static loads were applied on some PMMA plate specimen with pre-existing crack, and the behaviour of the wing crack was tested by caustics corroding with a high-speed photography. Four programs with different static loading modes that generate different pre-compression fields were executed, and the length, velocity of the blasting wing crack and dynamic stress intensity factor (SIF at the wing crack tip were analyzed and discussed. It is found that the behaviour of blasting-induced wing crack is affected obviously by blasting and pre-compression. And pre-compression, which is vertical to the direction of the wing crack propagation, hinders the crack propagation. Furthermore, the boundary constraint condition plays an important role on the behaviour of blasting induced crack during the experiment.

Photoelastic Analysis of Cracked Thick Walled Cylinders

Science.gov (United States)

Pastramă, Ştefan Dan

2017-12-01

In this paper, the experimental determination of the stress intensity factor in thick walled cylinders subject to uniform internal pressure and having longitudinal non-penetrating cracks is presented. Photoelastic measurements were used together with the expressions of the stress field near the crack tip for Mode I crack extension and a specific methodology for stress intensity factor determination. Two types of longitudinal cracks - internal and external - were considered. Four plane models were manufactured and analyzed in a plane polariscope at different values of the applied internal pressure. The values of the normalized stress intensity factor were calculated and the results were compared to those reported by other authors. A good accuracy was noticed, showing the reliability of the experimental procedure.

Theoretical and numerical studies of crack initiation and propagation in rock masses under freezing pressure and far-field stress

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.

Stress Corrosion Cracking of Ni-Fe-Cr Alloys Relevant to Nuclear Power Plants

Science.gov (United States)

Persaud, Suraj

Stress corrosion cracking (SCC) of Ni-Fe-Cr alloys and weld metals was investigated in simulated environments representative of high temperature water used in the primary and secondary circuits of nuclear power plants. The mechanism of primary water SCC (PWSCC) was studied in Alloys 600, 690, 800 and Alloy 82 dissimilar metal welds using the internal oxidation model as a guide. Initial experiments were carried out in a 480°C hydrogenated steam environment considered to simulate high temperature reducing primary water. Ni alloys underwent classical internal oxidation intragranularly resulting in the expulsion of the solvent metal, Ni, to the surface. Selective intergranular oxidation of Cr in Alloy 600 resulted in embrittlement, while other alloys were resistant owing to their increased Cr contents. Atom probe tomography was used to determine the short-circuit diffusion path used for Ni expulsion at a sub-nanometer scale, which was concluded to be oxide-metal interfaces. Further exposures of Alloys 600 and 800 were done in 315°C simulated primary water and intergranular oxidation tendency was comparable to 480°C hydrogenated steam. Secondary side work involved SCC experiments and electrochemical measurements, which were done at 315°C in acid sulfate solutions. Alloy 800 C-rings were found to undergo acid sulfate SCC (AcSCC) to a depth of up to 300 microm in 0.55 M sulfate solution at pH 4.3. A focused-ion beam was used to extract a crack tip from a C-ring and high resolution analytical electron microscopy revealed a duplex oxide structure and the presence of sulfur. Electrochemical measurements were taken on Ni alloys to complement crack tip analysis; sulfate was concluded to be the aggressive anion in mixed sulfate and chloride systems. Results from electrochemical measurements and crack tip analysis suggested a slip dissolution-type mechanism to explain AcSCC in Ni alloys.

Crack Tip Flipping under Mode I Tearing: Investigated by X-Ray Tomography

DEFF Research Database (Denmark)

Nielsen, Kim Lau; Gundlach, Carsten

2017-01-01

The fracture surface morphology that results from mode I tearing of ductile plate metals depends heavily on both the elastic-plastic material properties and the microstructure. Severe tunneling of the advancing crack tip (resulting in cup-cup, or bath-tub like fracture surfaces) can take place...... in a range of materials, often of low strength, while tearing of high strength metals typically progress by the shear band failure mechanism (slanting). In reality, however, most fracture surfaces display a mixture of morphologies. For example, slant crack propagation can be accompanied by large shear lips...... near the outer free plate surface or a complete shear band switch - seemingly distributed randomly on the fracture surface. The occasionally observed shear band switch of mode I slant cracks, related to ductile plate tearing, is far from random as the crack can flip systematically from one side...

The Reflective Cracking in Flexible Pavements

Directory of Open Access Journals (Sweden)

Pais Jorge

2013-07-01

Full Text Available Reflective cracking is a major concern for engineers facing the problem of road maintenance and rehabilitation. The problem appears due to the presence of cracks in the old pavement layers that propagate into the pavement overlay layer when traffic load passes over the cracks and due to the temperature variation. The stress concentration in the overlay just above the existing cracks is responsible for the appearance and crack propagation throughout the overlay. The analysis of the reflective cracking phenomenon is usually made by numerical modeling simulating the presence of cracks in the existing pavement and the stress concentration in the crack tip is assessed to predict either the cracking propagation rate or the expected fatigue life of the overlay. Numerical modeling to study reflective cracking is made by simulating one crack in the existing pavement and the loading is usually applied considering the shear mode of crack opening. Sometimes the simulation considers the mode I of crack opening, mainly when temperature effects are predominant.

Alloy SCR-3 resistant to stress corrosion cracking

International Nuclear Information System (INIS)

Kowaka, Masamichi; Fujikawa, Hisao; Kobayashi, Taiki

1977-01-01

Austenitic stainless steel is used widely because the corrosion resistance, workability and weldability are excellent, but the main fault is the occurrence of stress corrosion cracking in the environment containing chlorides. Inconel 600, most resistant to stress corrosion cracking, is not necessarily safe under some severe condition. In the heat-affected zone of SUS 304 tubes for BWRs, the cases of stress corrosion cracking have occurred. The conventional testing method of stress corrosion cracking using boiling magnesium chloride solution has been problematical because it is widely different from actual environment. The effects of alloying elements on stress corrosion cracking are remarkably different according to the environment. These effects were investigated systematically in high temperature, high pressure water, and as the result, Alloy SCR-3 with excellent stress corrosion cracking resistance was found. The physical constants and the mechanical properties of the SCR-3 are shown. The states of stress corrosion cracking in high temperature, high pressure water containing chlorides and pure water, polythionic acid, sodium phosphate solution and caustic soda of the SCR-3, SUS 304, Inconel 600 and Incoloy 800 are compared and reported. (Kako, I.)

On the application of cohesive crack modeling in cementitious materials

DEFF Research Database (Denmark)

Stang, Henrik; Olesen, John Forbes; Poulsen, Peter Noe

2007-01-01

typically for multi scale problems such as crack propagation in fiber reinforced composites. Mortar and concrete, however, are multi-scale materials and the question naturally arises, if bridged crack models in fact are more suitable for concrete and mortar as well? In trying to answer this question a model......Cohesive crack models-in particular the Fictitious Crack Model - are applied routinely in the analysis of crack propagation in concrete and mortar. Bridged crack models-where cohesive stresses are assumed to exist together with a stress singularity at the crack tip-on the other hand, are used...

Dynamic steady-state analysis of crack propagation in rubber-like solids using an extended finite element method

Science.gov (United States)

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.

Modeling of a Curvilinear Planar Crack with a Curvature-Dependent Surface Tension

KAUST Repository

Zemlyanova, A. Y.; Walton, J. R.

2012-01-01

An approach to modeling fracture incorporating interfacial mechanics is applied to the example of a curvilinear plane strain crack. The classical Neumann boundary condition is augmented with curvature-dependent surface tension. It is shown that the considered model eliminates the integrable crack-tip stress and strain singularities of order 1/2 present in the classical linear fracture mechanics solutions, and also leads to the sharp crack opening that is consistent with empirical observations. Unlike for the case of a straight crack, for a general curvilinear crack some components of the stresses and the derivatives of the displacements may still possess weaker singularities of a logarithmic type. Generalizations of the present study that lead to complete removal of all crack-tip singularities, including logarithmic, are the subject of a future paper. © 2012 Society for Industrial and Applied Mathematics.

Curvilinear crack layer propagation

Science.gov (United States)

Chudnovsky, Alexander; Chaoui, Kamel; Moet, Abdelsamie

1987-01-01

An account is given of an experiment designed to allow observation of the effect of damage orientation on the direction of crack growth in the case of crack layer propagation, using polystyrene as the model material. The direction of crack advance under a given loading condition is noted to be determined by a competition between the tendency of the crack to maintain its current direction and the tendency to follow the orientation of the crazes at its tip. The orientation of the crazes is, on the other hand, determined by the stress field due to the interaction of the crack, the crazes, and the hole. The changes in craze rotation relative to the crack define the active zone rotation.

Fringe Analysis around an Inclined Crack Tip of Finite-Width Plate under Tensile Load by Photoelastic Phase-Shifting Method

International Nuclear Information System (INIS)

Li, Weizheng; Baek, Tae Hyun; Lee, Byung Hee; Seo, Jin; Hong, Dong Pyo

2012-01-01

Photoelasticity is a technique of experimental methods and has been widely used in various domains of engineering to determine the stress distribution of structures. Without complicated mathematical formulation, this technique can conveniently provide a fairly accurate whole-field stress analysis for a mechanical structure. Here, stress distribution around an inclined crack tip of finite-width plate is studied by 8-step phase-shifting method. This method is a kind of photoelastic phase-shifting techniques and can be used for the determination of the phase values of isochromatics and isoclinics. According to stress-optic law, the stress distribution could be obtained from fringe patterns. The results obtained by polariscope arrangement combined with 8-step method and ABAQUS FEM simulations are compared with each other. Good agreement between them shows that 8-step phase-shifting method is reliable and can be used for determination of stress by experiment

Molecular dynamics simulation of mode-I-crack propagation and dislocation generation processes in α-Fe

International Nuclear Information System (INIS)

Wang Jianwei; Lu Guocai; Shang Xinchun

2011-01-01

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 K I increased into 0. 566 MPam 1/2 , one layer of atoms near crack tip would be separated into two layers which produced a dislocation. The first dislocation was emitted when stress K I reached 0.669 MPam 1/2 . With the temperature increasing, the critical stress intensity factor decreased gradually and the dislocation emission correspondingly became faster as well. (authors)

Fatigue crack growth retardation in spot heated mild steel sheet

Indian Academy of Sciences (India)

A fatigue crack can be effectively retarded by heating a spot near the crack tip under nil remote stress condition. The subcritical spot heating at a proper position modifies the crack growth behaviour in a way, more or less, similar to specimen subjected to overload spike. It is observed that the extent of crack growth retardation ...

Strain rate effects in stress corrosion cracking

Energy Technology Data Exchange (ETDEWEB)

Parkins, R.N. (Newcastle upon Tyne Univ. (UK). Dept. of Metallurgy and Engineering Materials)

1990-03-01

Slow strain rate testing (SSRT) was initially developed as a rapid, ad hoc laboratory method for assessing the propensity for metals an environments to promote stress corrosion cracking. It is now clear, however, that there are good theoretical reasons why strain rate, as opposed to stress per se, will often be the controlling parameter in determining whether or not cracks are nucleated and, if so, are propagated. The synergistic effects of the time dependence of corrosion-related reactions and microplastic strain provide the basis for mechanistic understanding of stress corrosion cracking in high-pressure pipelines and other structures. However, while this may be readily comprehended in the context of laboratory slow strain tests, its extension to service situations may be less apparent. Laboratory work involving realistic stressing conditions, including low-frequency cyclic loading, shows that strain or creep rates give good correlation with thresholds for cracking and with crack growth kinetics.

Retarding effect of prior-overloading on stress corrosion cracking of cold rolled 316L SS in simulated PWR water environment

Science.gov (United States)

Chen, Junjie; Lu, Zhanpeng; Xiao, Qian; Ru, Xiangkun; Ma, Jiarong; Shoji, Tetsuo

2017-12-01

The effect of prior single tensile overloading on the stress corrosion cracking behavior of cold rolled 316L in a simulated PWR water environment at 310 °C was investigated. SCC growth retardation by overloading was observed in cold rolled 316L specimens in both the T-L and L-T orientations. The stretch zone observed on the fracture surfaces of the overloaded specimens affected SCC propagation. The compressive residual stress induced by overloading process reduced the effective driving force of SCC propagation. The negative dK/da effect ahead of the crack tip likely contributes to the retardation of SCC growth. The duration of overloading is dependent on water chemistry and the local stress conditions.

A new procedure for investigating three-dimensional stress fields in a thin plate with a through-the-thickness crack

Science.gov (United States)

Yi, Dake; Wang, TzuChiang

2018-06-01

In the paper, a new procedure is proposed to investigate three-dimensional fracture problems of a thin elastic plate with a long through-the-thickness crack under remote uniform tensile loading. The new procedure includes a new analytical method and high accurate finite element simulations. In the part of theoretical analysis, three-dimensional Maxwell stress functions are employed in order to derive three-dimensional crack tip fields. Based on the theoretical analysis, an equation which can describe the relationship among the three-dimensional J-integral J( z), the stress intensity factor K( z) and the tri-axial stress constraint level T z ( z) is derived first. In the part of finite element simulations, a fine mesh including 153360 elements is constructed to compute the stress field near the crack front, J( z) and T z ( z). Numerical results show that in the plane very close to the free surface, the K field solution is still valid for in-plane stresses. Comparison with the numerical results shows that the analytical results are valid.

A crack opening stress equation for fatigue crack growth

Science.gov (United States)

Newman, J. C., Jr.

1984-01-01

A general crack opening stress equation is presented which may be used to correlate crack growth rate data for various materials and thicknesses, under constant amplitude loading, once the proper constraint factor has been determined. The constraint factor, alpha, is a constraint on tensile yielding; the material yields when the stress is equal to the product of alpha and sigma. Delta-K (LEFM) is plotted against rate for 2024-T3 aluminum alloy specimens 2.3 mm thick at various stress ratios. Delta-K sub eff was plotted against rate for the same data with alpha = 1.8; the rates correlate well within a factor of two.

Crack propagation at stresses below the fatigue limit.

Science.gov (United States)

Holden, F. C.; Hyler, W. S.; Marschall, C. W.

1967-01-01

Crack propagation for stainless steel and Ti alloy at stresses below fatigue limit, noting of alternating stress cycles crack propagation for stainless steel and Ti alloy at stresses below fatigue limit, noting role of alternating stress cycles

Effects of the Crack Tip Constraint on the Fracture Assessment of an Al 5083-O Weldment for Low Temperature Applications

Directory of Open Access Journals (Sweden)

Dong Hyun Moon

2017-07-01

Full Text Available The constraint effect is the key issue in structural integrity assessments based on two parameter fracture mechanics (TPFM to make a precise prediction of the load-bearing capacity of cracked structural components. In this study, a constraint-based failure assessment diagram (FAD was used to assess the fracture behavior of an Al 5083-O weldment with various flaws at cryogenic temperature. The results were compared with those of BS 7910 Option 1 FAD, in terms of the maximum allowable stress. A series of fracture toughness tests were conducted with compact tension (CT specimens at room and cryogenic temperatures. The Q parameter for the Al 5083-O weldment was evaluated to quantify the constraint level, which is the difference between the actual stress, and the Hutchinson-Rice-Rosengren (HRR stress field near the crack tip. Nonlinear 3D finite element analysis was carried out to calculate the Q parameter at cryogenic temperature. Based on the experimental and numerical results, the influence of the constraint level correction on the allowable applied stress was investigated using a FAD methodology. The results showed that the constraint-based FAD procedure is essential to avoid an overly conservative allowable stress prediction in an Al 5083-O weldment with flaws.

Experimental estimation of the heat energy dissipated in a volume surrounding the tip of a fatigue crack

Directory of Open Access Journals (Sweden)

G. Meneghetti

2016-01-01

Full Text Available Fatigue crack initiation and propagation involve plastic strains that require some work to be done on the material. Most of this irreversible energy is dissipated as heat and consequently the material temperature increases. The heat being an indicator of the intense plastic strains occurring at the tip of a propagating fatigue crack, when combined with the Neuber’s structural volume concept, it might be used as an experimentally measurable parameter to assess the fatigue damage accumulation rate of cracked components. On the basis of a theoretical model published previously, in this work the heat energy dissipated in a volume surrounding the crack tip is estimated experimentally on the basis of the radial temperature profiles measured by means of an infrared camera. The definition of the structural volume in a fatigue sense is beyond the scope of the present paper. The experimental crack propagation tests were carried out on hot-rolled, 6-mm-thick AISI 304L stainless steel specimens subject to completely reversed axial fatigue loading.

Thermal analysis of cracked bodies using finite element techniques

International Nuclear Information System (INIS)

Hellen, T.K.; Price, R.H.; Harrison, R.P.

1975-01-01

The paper develops the potential energy equation in terms of finite element theory including thermal loads. Following this, the energy release rate and consequently the stress intensity factors are derived. Considerations of the classical near crack tip equations are made and deficiencies with the popular substitution methods are highlighted. A method of removing these deficiencies is described. Various energy methods are reconsidered in terms of the role of the thermal energy contribution to the potential energy. These methods include work of crack closure, energy compliance and virtual crack extensions with no other change in nodal geometry, and therefore only requires the recalculation of the stiffness matrices of the crack tip elements. An example of a quadratic temperature gradient parallel to the crack plane in an edge cracked plate is described. Comparisons of the various finite element methods are made and generally show good agreement. A second application compares the virtual crack extension method with an approximate analytical solution in determining stress intensity factors for a thick hollow cylinder with an axial crack for various depths through the wall thickness and for different times. Initially the cylinder is at a uniform high temperature and is then subjected to a sustained cooling shock. Analytical solutions are available for temperature and stress distributions in the uncracked pipe. The stress intensity for a shallow crack in the early stages of the transient has been determined using a superposition procedure. Comparison of the analytical and computed results shows good agreement between the methods

Mechanism of crack initiation and crack growth under thermal and mechanical fatigue loading

Energy Technology Data Exchange (ETDEWEB)

Utz, S.; Soppa, E.; Silcher, H.; Kohler, C. [Stuttgart Univ. (Germany). Materials Testing Inst.

2013-07-01

measurements the deformation induced transformation of an fcc-austenite into a bcc α'-martensite was observed in different stages of the specimen lifetime. Plastic zones develop at the crack tips, in which stress and strain amplitudes are much higher than the nominal loading, and enable martensitic transformation in the surrounding of the crack tip. The consequence of this is that cracks grow in the ''martensitic tunnels''. The short and long crack growth behaviours of the steel X6CrNiNb18-10 under mechanical loading at room temperature and T = 288 C were studied for different loading parameters. Moreover, the R-ratio was modified in order to study the effect of crack closure at the crack tip for long cracks. Several FE models of specimens with different geometries and microstructures were created and cyclically loaded according to the experimental boundary conditions. A plastic constitutive law based on a Chaboche type model was implemented as a user subroutine in the FE software ABAQUS. The corresponding material parameters were identified using uniaxial LCF tests of X6CrNiNb18-10 with different strain amplitudes and at different temperatures. These calculations delivered the estimation of stress and strain distributions in the critical areas in which the crack initiation was expected.

Mechanism of crack initiation and crack growth under thermal and mechanical fatigue loading

International Nuclear Information System (INIS)

Utz, S.; Soppa, E.; Silcher, H.; Kohler, C.

2013-01-01

measurements the deformation induced transformation of an fcc-austenite into a bcc α'-martensite was observed in different stages of the specimen lifetime. Plastic zones develop at the crack tips, in which stress and strain amplitudes are much higher than the nominal loading, and enable martensitic transformation in the surrounding of the crack tip. The consequence of this is that cracks grow in the ''martensitic tunnels''. The short and long crack growth behaviours of the steel X6CrNiNb18-10 under mechanical loading at room temperature and T = 288 C were studied for different loading parameters. Moreover, the R-ratio was modified in order to study the effect of crack closure at the crack tip for long cracks. Several FE models of specimens with different geometries and microstructures were created and cyclically loaded according to the experimental boundary conditions. A plastic constitutive law based on a Chaboche type model was implemented as a user subroutine in the FE software ABAQUS. The corresponding material parameters were identified using uniaxial LCF tests of X6CrNiNb18-10 with different strain amplitudes and at different temperatures. These calculations delivered the estimation of stress and strain distributions in the critical areas in which the crack initiation was expected.

Fatigue crack growth behavior under cyclic thermal transient stress

International Nuclear Information System (INIS)

Ueda, Masahiro; Kano, Takashi; Yoshitoshi, Atsushi.

1986-01-01

Thermal fatigue tests were performed using straight pipe specimens subjected to cyclic thermal shocks of liquid sodium, and crack growth behaviors were estimated using striation patterns observed clearly on any crack surface. Crack growth rate under cyclic thermal strain reaches the maximum at one depth, and after that it decreases gradually with crack depth. The peak location of crack growth rate becomes deeper by superposition of constant primary stress. Parallel cracks co-existing in the neighborhood move the peak to shallower location and decrease the maximum crack growth rate. The equivalent stress intensity factor range calculated by Walker's formula is successfully applied to the case of negative stress ratio. Fatigue crack growth rate under cyclic thermal strain agreed well with that under the constant temperature equal to the maximum value in the thermal cycle. Simplified methods for calculating the stress intensity factor and the crack interference factor have been developed. Crack growth behavior under thermal fatigue could be well predicted using numerical analysis results. (author)

Fatigue crack growth behavior under cyclic transient thermal stress

International Nuclear Information System (INIS)

Ueda, Masahiro; Kano, Takashi; Yoshitoshi, Atsushi.

1987-01-01

Thermal fatigue tests were performed using straight pipe specimens subjected to cyclic thermal shocks of liquid sodium, and crack growth behaviors were estimated using striation patterns observed clearly on any crack surface. Crack growth rate under cyclic thermal strain reaches the maximum at one depth, and after that it decreases gradually with crack depth. The peak location of crack growth rate becomes deeper by superposition of constant primary stress. Parallel cracks co-existing in the neighborhood move the peak to shallower location and decrease the maximum crack growth rate. The equivalent stress intensity factor range calculated by Walker's formula is successfully applied to the case of negative stress ratio. Fatigue crack growth rate under cyclic thermal strain agreed well with that under the constant temperature equal to the maximum value in the thermal cycle. Simplified methods for calculating the stress intensity factor and the crack interference factor have been developed. Crack growth behavior under thermal fatigue could be well predicted using numerical analysis results. (author)

Standard test method for crack-tip opening displacement (CTOD) fracture toughness measurement

CERN Document Server

American Society for Testing and Materials. Philadelphia

2008-01-01

1.1 This test method covers the determination of critical crack-tip opening displacement (CTOD) values at one or more of several crack extension events, and may be used to measure cleavage crack initiation toughness for materials that exhibit a change from ductile to brittle behavior with decreasing temperature, such as ferritic steels. This test method applies specifically to notched specimens sharpened by fatigue cracking. The recommended specimens are three-point bend [SE(B)], compact [C(T)], or arc-shaped bend [A(B)] specimens. The loading rate is slow and influences of environment (other than temperature) are not covered. The specimens are tested under crosshead or clip gage displacement controlled loading. 1.1.1 The recommended specimen thickness, B, for the SE(B) and C(T) specimens is that of the material in thicknesses intended for an application. For the A(B) specimen, the recommended depth, W, is the wall thickness of the tube or pipe from which the specimen is obtained. Superficial surface machini...

Determination of Fracture Parameters for Multiple Cracks of Laminated Composite Finite Plate

Science.gov (United States)

Srivastava, Amit Kumar; Arora, P. K.; Srivastava, Sharad Chandra; Kumar, Harish; Lohumi, M. K.

2018-04-01

A predictive method for estimation of stress state at zone of crack tip and assessment of remaining component lifetime depend on the stress intensity factor (SIF). This paper discusses the numerical approach for prediction of first ply failure load (FL), progressive failure load, SIF and critical SIF for multiple cracks configurations of laminated composite finite plate using finite element method (FEM). The Hashin and Chang failure criterion are incorporated in ABAQUS using subroutine approach user defined field variables (USDFLD) for prediction of progressive fracture response of laminated composite finite plate, which is not directly available in the software. A tensile experiment on laminated composite finite plate with stress concentration is performed to validate the numerically predicted subroutine results, shows excellent agreement. The typical results are presented to examine effect of changing the crack tip distance (S), crack offset distance (H), and stacking fiber angle (θ) on FL, and SIF .

Three-dimensional characterization of stress corrosion cracks

DEFF Research Database (Denmark)

Lozano-Perez, S.; Rodrigo, P.; Gontard, Lionel Cervera

2011-01-01

the best spatial resolution. To illustrate the power of these techniques, different parts of dominant stress corrosion cracks in Ni-alloys and stainless steels have been reconstructed in 3D. All relevant microstructural features can now be studied in detail and its relative orientation respect......Understanding crack propagation and initiation is fundamental if stress corrosion cracking (SCC) mechanisms are to be understood. However, cracking is a three-dimensional (3D) phenomenon and most characterization techniques are restricted to two-dimensional (2D) observations. In order to overcome...

Effect of tensile overloads on fatigue crack growth of high strength steel wires

International Nuclear Information System (INIS)

Haag, J.; Reguly, A.; Strohaecker, T.R.

2013-01-01

Highlights: • A proof load process may be an option to increase the fatigue life of flexible pipelines. • There is possibility to produce plastic deformation at crack tip of tensile armor wires. • Controlled overloads provide effective crack growth retardation. • Crack growth retardation is also evident at higher stress ratios. - Abstract: Fatigue of the tensile armor wires is the main failure mode of flexible risers. Techniques to increase the life of these components are required to improve the processes safety on oil exploration. This work evaluates the crack growth retardation of high strength steel wires used in flexible pipelines. Fracture toughness tests were performed to establish the level of stress intensity factor wherein the wires present significant plastic deformation at the crack tip. The effect of tensile overload on fatigue behavior was assessed by fatigue crack growth testing under constant ΔK control and different overload ratios with two different load ratios. The outcomes show that the application of controlled overloads provides crack retardation and increases the fatigue life of the wires more than 31%. This behavior is also evident at stress ratio of 0.5, in spite of the crack closure effect being minimized by increasing the applied mean stress

On the anomalous temperature dependency of fatigue crack growth of SS 316(N) weld

Energy Technology Data Exchange (ETDEWEB)

Babu, M. Nani; Dutt, B. Shashank; Venugopal, S. [Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102 (India); Sasikala, G., E-mail: gsasi@igcar.gov.in [Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102 (India); Bhaduri, A.K.; Jayakumar, T.; Raj, Baldev [Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102 (India)

2010-07-25

Fatigue crack growth behaviour of a nuclear grade SS 316(N) weld metal was examined in the Paris and threshold regimes in the as-welded condition, at 300, 573 and 823 K. The {Delta}K{sub th} values were 11.2, 7.5, and 9.5 MPa {radical}m, respectively. These values were corrected for closure effects and the corresponding {Delta}K{sub th,eff} were found to be 7.7, 5.8 and 3.5 MPa {radical}m, respectively. The anomalous behaviour, i.e., the high value of {Delta}K{sub th} at 823 K has been explained based on crack closure effect which is roughness induced at 300 K and oxide induced at 823 K, with both these insignificant at 573 K. The effect of temperature on crack growth mechanism and the associated closure mechanisms are discussed. The stress shielding at the crack tip due to closure is accounted for and the effective stress intensity factor experienced by the crack tip, {Delta}K{sub eff,tip} is determined. It is demonstrated that {Delta}K{sub eff,tip} qualifies as a more appropriate parameter as the driving force for the temperature-dependent crack growth in the near-threshold and Paris regimes.

Determination of a cohesive law for delamination modelling - Accounting for variation in crack opening and stress state across the test specimen width

DEFF Research Database (Denmark)

Joki, R. K.; Grytten, F.; Hayman, Brian

2016-01-01

by differentiating the fracture resistance with respect to opening displacement at the initial location of the crack tip, measured at the specimen edge. 2) Extend the bridging law to a cohesive law by accounting for crack tip fracture energy. 3) Fine-tune the cohesive law through an iterative modelling approach so......The cohesive law for Mode I delamination in glass fibre Non-Crimped Fabric reinforced vinylester is determined for use in finite element models. The cohesive law is derived from a delamination test based on DCB specimens loaded with pure bending moments taking into account the presence of large...... that the changing state of stress and deformation across the width of the test specimen is taken into account. The changing state of stress and deformation across the specimen width is shown to be significant for small openings (small fracture process zone size). This will also be important for the initial part...

An application of the J-integral to an incremental analysis of blunting crack behavior

International Nuclear Information System (INIS)

Merkle, J.G.

1989-01-01

This paper describes an analytical approach to estimating the elastic-plastic stresses and strains near the tip of a blunting crack with a finite root radius. Rice's original derivation of the path independent J-integral considered the possibility of a finite crack tip root radius. For this problem Creager's elastic analysis gives the relation between the stress intensity factor K I and the near tip stresses. It can be shown that the relation K I 2 = E'J holds when the root radius is finite. Recognizing that elastic-plastic behavior is incrementally linear then allows a derivation to be performed for a bielastic specimen having a crack tip region of reduced modulus, and the result differentiated to estimate elastic-plastic behavior. The result is the incremental form of Neuber's equation. This result does not require the assumption of any particular stress-strain relation. However by assuming a pure power law stress-strain relation and using Ilyushin's principle, the ordinary deformation theory form of Neuber's equation, K σ K var epsilon = K t 2 , is obtained. Applications of the incremental form of Neuber's equation have already been made to fatigue and fracture analysis. This paper helps to provide a theoretical basis for these methods previously considered semiempirical. 26 refs., 4 figs

Experimental study on stress corrosion crack propagation rate of FV520B in carbon dioxide and hydrogen sulfide solution

Directory of Open Access Journals (Sweden)

Ming Qin

Full Text Available FV520B steel is a kind of precipitation hardening Martensitic stainless steel, it has high-strength, good plasticity and good corrosion resistance. Stress corrosion cracking (SCC is one of the main corrosion failure mode for FV520B in industrial transportation of natural gas operation. For a better understanding the effect on SCC of FV520B, the improved wedge opening loading (WOL specimens and constant displacement loading methods were employed in experimental research in carbon dioxide and hydrogen sulfide solution. The test results showed that the crack propagation rate is 1.941 × 10−7–5.748 × 10−7 mm/s, the stress intensity factor KISCC is not more than 36.83 MPa m. The rate increases with the increasing of the crack opening displacement. Under the condition of different initial loading, KISCC generally shows a decreasing tendency with the increase in H2S concentration, and the crack propagation rate showed an increasing trend substantially. For the enrichment of sulfur ion in the crack tip induced the generation of pitting corrosion, promoting the surrounding metal formed the corrosion micro batteries, the pit defects gradually extended and connected with the adjacent pit to form a small crack, leading to further propagation till cracking happened. Fracture microscopic morphology displayed typical brittle fracture phenomena, accompanying with trans-granular cracking, river shape and sector, many second cracks on the fracture surface. Keywords: FV520B, Wedge opening loading specimen, Stress corrosion cracking, Hydrogen sulfide

Analysis of Unsteady Propagation of Mode Ⅲ Crack in Arbitrary Direction in Functionally Graded Materials

Energy Technology Data Exchange (ETDEWEB)

Lee, Kwang Ho [Kyungpook National University, Daegu (Korea, Republic of); Cho, Sang Bong [Kyungnam University, Changwon (Korea, Republic of); Hawong, Jai Sug [Yeungnam University, Gyungsan (Korea, Republic of)

2015-02-15

The stress and displacement fields at the crack tip were studied during the unsteady propagation of a mode Ⅲ crack in a direction that was different from the property graduation direction in functionally graded materials (FGMs). The property graduation in FGMs was assumed based on the linearly varying shear modulus under a constant density and the exponentially varying shear modulus and density. To obtain the solution of the harmonic function, the general partial differential equation of the dynamic equilibrium equation was transformed into a Laplace equation. Based on the Laplace equation, the stress and displacement fields, which depended on the time rates of change in the crack tip speed and stress intensity factor, were obtained through an asymptotic analysis. Using the stress and displacement fields, the effects of the angled property variation on the stresses, displacements, and stress intensity factors are discussed.

Motion of Defect Clusters and Dislocations at a Crack Tip of Irradiated Material

International Nuclear Information System (INIS)

Moon, Won Jin; Kwon, Sang Chul; Kim, Whung Whoe

2007-01-01

Effects of defect clusters on mechanical properties of irradiated materials have not been clarified until now. Two radiation hardening models have been proposed. One is a dispersed barrier hardening mechanism based on the Orowan hardening model. This explains defect clusters as barriers to a dislocation motion. Generally the dislocation would rather shear or remove the defect clusters than make so-called Orowan loops. And the other is a cascade induced source hardening mechanism, which explains defect clusters as a Cottrell atmosphere for dislocation motions. However, the above mechanisms can not explain the microstructure of deformed material after irradiation and the phenomenon of yield softening. These mechanisms are based on an immobility of clusters. But we observed defect clusters could move into a specific crystallographic direction easily. Through 3 times of High Voltage Electron Microscope analysis, defect clusters have been observed to make one dimensional motion without applied external stress. If very small defect clusters could move under a stress gradient due to interactions between clusters, we can suggest that the clusters will move more actively when a stress gradient is applied externally. In-situ tensile test at TEM, we confirmed that kind of motion. We suggest defect clusters can move into crack tip, a stress-concentrated area due to tensile stress gradient and dislocations move out from the area by shear stress. Therefore radiation hardening can be explained agglomeration of defect clusters at stress concentrated area prohibits a generation of dislocation and make an increase of yield point

Stress corrosion cracking of nickel base alloys in PWR primary water

International Nuclear Information System (INIS)

Guerre, C.; Chaumun, E.; Crepin, J.; De Curieres, I.; Duhamel, C.; Heripre, E.; Herms, E.; Laghoutaris, P.; Molins, R.; Sennour, M.; Vaillant, F.

2013-01-01

Stress corrosion cracking (SCC) of nickel base alloys and associated weld metals in primary water is one of the major concerns for pressurized water reactors (PWR). Since the 90's, highly cold-worked stainless steels (non-sensitized) were also found to be susceptible to SCC in PWR primary water ([1], [2], [3]). In the context of the life extension of pressurized water reactors, laboratory studies are performed in order to evaluate the SCC behaviour of components made of nickel base alloys and of stainless steels. Some examples of these laboratory studies performed at CEA will be given in the talk. This presentation deals with both initiation and propagation of stress corrosion cracks. The aims of these studies is, on one hand, to obtain more data regarding initiation time or crack growth rate and, one the other hand, to improve our knowledge of the SCC mechanisms. The aim of these approaches is to model SCC and to predict components life duration. Crack growth rate (CGR) tests on Alloy 82 with and without post weld heat treatment are performed in PWR primary water (Figure 1). The heat treatment seems to be highly beneficial by decreasing the CGR. This result could be explained by the effect of thermal treatment on the grain boundary nano-scopic precipitation in Alloy 82 [4]. The susceptibility to SCC of cold worked austenitic stainless steels is also studied. It is shown that for a given cold-working procedure, SCC susceptibility increases with increasing cold-work ([2], [5]). Despite the fact that the SCC behaviour of Alloy 600 has been widely studied for many years, recent laboratory experiments and analysis ([6], [7], [8]) showed that oxygen diffusion is not a rate-limiting step in the SCC mechanism and that chromium diffusion in the bulk close the crack tip could be a key parameter. (authors)

Innovative Approach to Establish Root Causes for Cracking in Aggressive Reactor Environments

International Nuclear Information System (INIS)

Bruemmer, Stephen M.; Thomas, Larry E.; Vetrano, John S.; Simonen, Edward P.

2003-01-01

The research focuses on the high-resolution characterization of degradation microstructures and microchemistries in specimens tested under controlled conditions for the environment and for the material where in-service complexities can be minimized. Thermodynamic and kinetic modeling of crack-tip processes is employed to analyze corrosion-induced structures and gain insights into degradation mechanisms. Novel mechanistic ''fingerprinting'' of crack-tip structures is used to isolate causes of environmental cracking in tandem with quantitative measurements of crack growth. Sample preparation methods and advanced analytical techniques are used to characterize corrosion/oxidation reactions and crack-tip structures at near atomic dimensions in order to gain insight into fundamental environmental cracking mechanisms. Reactions at buried interfaces, not accessible by conventional approaches, are being systematically interrogated. Crack-growth experiments in high-temperature water environments are evaluating and isolating the effects of material condition (matrix strength, grain boundary composition and precipitation) on stress corrosion cracking (SCC). The fundamental understanding of crack advance mechanisms will establish the basis to design new corrosion-resistant alloys for current light-water reactors and advanced reactor systems

Development of a computer code 'CRACK' for elastic and elastoplastic fracture mechanics analysis of 2-D structures by finite element technique

International Nuclear Information System (INIS)

Dutta, B.K.; Kakodkar, A.; Maiti, S.K.

1986-01-01

The fracture mechanics analysis of nuclear components is required to ensure prevention of sudden failure due to dynamic loadings. The linear elastic analysis near to a crack tip shows presence of stress singularity at the crack tip. The simulation of this singularity in numerical methods enhance covergence capability. In finite element technique this can be achieved by placing mid nodes of 8 noded or 6 noded isoparametric elements, at one fourth ditance from crack tip. Present report details this characteristic of finite element, implementation of this element in a code 'CRACK', implementation of J-integral to compute stress intensity factor and solution of number of cases for elastic and elastoplastic fracture mechanics analysis. 6 refs., 6 figures. (author)

Crack turning in integrally stiffened aircraft structures

Science.gov (United States)

Pettit, Richard Glen

Current emphasis in the aircraft industry toward reducing manufacturing cost has created a renewed interest in integrally stiffened structures. Crack turning has been identified as an approach to improve the damage tolerance and fail-safety of this class of structures. A desired behavior is for skin cracks to turn before reaching a stiffener, instead of growing straight through. A crack in a pressurized fuselage encounters high T-stress as it nears the stiffener---a condition favorable to crack turning. Also, the tear resistance of aluminum alloys typically varies with crack orientation, a form of anisotropy that can influence the crack path. The present work addresses these issues with a study of crack turning in two-dimensions, including the effects of both T-stress and fracture anisotropy. Both effects are shown to have relation to the process zone size, an interaction that is central to this study. Following an introduction to the problem, the T-stress effect is studied for a slightly curved semi-infinite crack with a cohesive process zone, yielding a closed form expression for the future crack path in an infinite medium. For a given initial crack tip curvature and tensile T-stress, the crack path instability is found to increase with process zone size. Fracture orthotropy is treated using a simple function to interpolate between the two principal fracture resistance values in two-dimensions. An extension to three-dimensions interpolates between the six principal values of fracture resistance. Also discussed is the transition between mode I and mode II fracture in metals. For isotropic materials, there is evidence that the crack seeks out a direction of either local symmetry (pure mode I) or local asymmetry (pure mode II) growth. For orthotropic materials the favored states are not pure modal, and have mode mixity that is a function of crack orientation. Drawing upon these principles, two crack turning prediction approaches are extended to include fracture

Nonlinear crack mechanics

International Nuclear Information System (INIS)

Khoroshun, L.P.

1995-01-01

The characteristic features of the deformation and failure of actual materials in the vicinity of a crack tip are due to their physical nonlinearity in the stress-concentration zone, which is a result of plasticity, microfailure, or a nonlinear dependence of the interatomic forces on the distance. Therefore, adequate models of the failure mechanics must be nonlinear, in principle, although linear failure mechanics is applicable if the zone of nonlinear deformation is small in comparison with the crack length. Models of crack mechanics are based on analytical solutions of the problem of the stress-strain state in the vicinity of the crack. On account of the complexity of the problem, nonlinear models are bason on approximate schematic solutions. In the Leonov-Panasyuk-Dugdale nonlinear model, one of the best known, the actual two-dimensional plastic zone (the nonlinearity zone) is replaced by a narrow one-dimensional zone, which is then modeled by extending the crack with a specified normal load equal to the yield point. The condition of finite stress is applied here, and hence the length of the plastic zone is determined. As a result of this approximation, the displacement in the plastic zone at the abscissa is nonzero

A Fourth Order Formulation of DDM for Crack Analysis in Brittle Solids

Directory of Open Access Journals (Sweden)

Abolfazl Abdollahipour

2017-01-01

Full Text Available A fourth order formulation of the displacement discontinuity method (DDM is proposed for the crack analysis of brittle solids such as rocks, glasses, concretes and ceramics. A fourth order boundary collocation scheme is used for the discretization of each boundary element (the source element. In this approach, the source boundary element is divided into five sub-elements each recognized by a central node where the displacement discontinuity components are to be numerically evaluated. Three different formulating procedures are presented and their corresponding discretization schemes are discussed. A new discretization scheme is also proposed to use the fourth order formulation for the special crack tip elements which may be used to increase the accuracy of the stress and displacement fields near the crack ends. Therefore, these new crack tips discretizing schemes are also improved by using the proposed fourth order displacement discontinuity formulation and the corresponding shape functions for a bunch of five special crack tip elements. Some example problems in brittle fracture mechanics are solved for estimating the Mode I and Mode II stress intensity factors near the crack ends. These semi-analytical results are compared to those cited in the fracture mechanics literature whereby the high accuracy of the fourth order DDM formulation is demonstrated.

Numerical analysis for prediction of fatigue crack opening level

International Nuclear Information System (INIS)

Choi, Hyeon Chang

2004-01-01

Finite Element Analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials

Variable amplitude fatigue crack growth behavior - a short overview

International Nuclear Information System (INIS)

Singh, Konjengbam Darunkumar; Parry, Matthew Roger; Sinclair, Ian

2011-01-01

A short overview concerning variable amplitude (VA) fatigue crack growth behavior is presented in this paper. The topics covered in this review encompass important issues pertaining to both single and repeated overload transients. Reviews on transient post overload effects such as plasticity induced crack closure, crack tip blunting, residual stresses, crack deflection and branching, activation of near threshold mechanisms, strain hardening are highlighted. A brief summary on experimental trends and finite element modelling of overload induced crack closure is also presented

Variable amplitude fatigue crack growth behavior - a short overview

Energy Technology Data Exchange (ETDEWEB)

Singh, Konjengbam Darunkumar [Indian Institute of Technology, Guwahati (India); Parry, Matthew Roger [Airbus Operations Ltd, Bristol (United Kingdom); Sinclair, Ian [University of Southampton, Southampton (United Kingdom)

2011-03-15

A short overview concerning variable amplitude (VA) fatigue crack growth behavior is presented in this paper. The topics covered in this review encompass important issues pertaining to both single and repeated overload transients. Reviews on transient post overload effects such as plasticity induced crack closure, crack tip blunting, residual stresses, crack deflection and branching, activation of near threshold mechanisms, strain hardening are highlighted. A brief summary on experimental trends and finite element modelling of overload induced crack closure is also presented.

Crack Tip Flipping: A New Phenomenon yet to be Resolved in Ductile Plate Tearing

DEFF Research Database (Denmark)

Nielsen, Kim Lau

2017-01-01

Conclusive insight to the mechanics that govern so-called “crack tip flipping”remains to be revealed, but details continue to fall into place as researcher dig deeper. The work presents an overview of the latest findings and the next steps to be made....

Countermeasures to stress corrosion cracking by stress improvement

International Nuclear Information System (INIS)

Umemoto, Tadahiro

1983-01-01

One of the main factors of the grain boundary stress corrosion cracking occurred in the austenitic stainless steel pipes for reactor cooling system was the tensile residual stress due to welding, and a number of methods have been proposed to reduce the residual stress or to change it to compressive stress. In this paper, on the method of improving residual stress by high frequency heating, which has been applied most frequently, the principle, important parameters and the range of application are explained. Also the other methods of stress improvement are outlined, and the merit and demerit of respective methods are discussed. Austenitic stainless steel and high nickel alloys have good corrosion resistance, high toughness and good weldability, accordingly they have been used for reactor cooling system, but stress corrosion cracking was discovered in both BWRs and PWRs. It occurs when the sensitization of materials, tensile stress and the dissolved oxygen in high temperature water exceed certain levels simultaneously. The importance of the residual stress due to welding, induction heating stress improvement, and other methods such as heat sink welding, last pass heat sink welding, back lay welding and TIG torch heating stress improvement are described. (Kako, I.)

Observation and simulation of crack growth in Zry-4

International Nuclear Information System (INIS)

Bertolino, Graciela; Meyer, Gabriel; Perez Ipina, J

2003-01-01

Security and life extension of components of nuclear reactors are the most motivating aspects that encourage to study embrittlement processes of zirconium alloys by reaction with hydrogen.Here, the use of fracture mechanics tests are suitable to monitor the material resistance of components under service.Because many times is difficult to obtain normalized probes from real size components, researchers look for alternative experimental techniques or crack growth simulation from the knowledge of particular material properties.In this work we present the results obtained after experimental observation and computer simulation of crack growth in Zry-4 probes.Experimental observation were obtained by performing flexion tests in three point probes SSEN(B) of 3 x 7 x 32 mm 3 located in the chamber of a scanning electron microscope, measuring in situ the crack length and opening when an external load is applied.Using the information obtained from stress-displacement measurements after tensile tests and the empiric relationship between crack opening and crack length, the crack growth process was simulated.Displacement field in the zone close to the crack tip was obtained by finite elements technique (Castem, DMT, CEA) assuming plain stress, a plastic bilinear homogeneous material and neglecting texture or directional anisotropy.To compare experimental observation and simulation, a grid (10 x 10 μm 2 each square) was drawn in the zone close to the crack tip by selective sputtering.Following the movement of two (three) points of the surface allows to compare uni (bi) dimensional deformation.A good agreement between observation and simulation was observed: after the crack opening grew 28 times (from 1.5 to 42 μm) the base-height relationship of a triangle involving the crack tip change 40% (35%) in the experimental observation (simulation)

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

Directory of Open Access Journals (Sweden)

Masahiro Goto

2015-10-01

Full Text Available In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress (  a = 90 MPa, the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a of b/a = 0.82. At a high stress (  a = 240 MPa, although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Stress corrosion crack growth in unirradiated zircaloy

International Nuclear Information System (INIS)

Pettersson, K.

1978-10-01

Experimental techniques suitable for the determination of stress corrosion crack growth rates in irradiated Zircaloy tube have been developed. The techniques have been tested on unirradiated. Zircaloy and it was found that the results were in good agreement with the results of other investigations. Some of the results were obtained at very low stress intensities and the crack growth rates observed, gave no indication of the existance of a K sub(ISCC) for iodine induced stress corrosion cracking in Zircaloy. This is of importance both for fuel rod behavior after a power ramp and for long term storage of spent Zircaloy-clad fuel. (author)

A study on stress corrosion cracking of explosive plugged part

International Nuclear Information System (INIS)

Kaga, Seiichi; Fujii, Katsuhiro; Yamamoto, Yoshiaki; Sakuma, Koosuke; Hibi, Seiji; Morimoto, Hiroyoshi.

1986-01-01

Studies on the stress corrosion cracking of explosive plugged part are conducted. SUS 304 stainless steel is used as testing material. The distribution of residual stress in plug and tube plate after plugging is obtained. The effect of residual stress on the stress corrosion cracking is studied. Residual stress in tube plate near the plug is compressive and stress corrosion cracking dose not occur in the tube plate there, and it occurs on the inner surface of plug because of residual tensile stress in axial direction of the plug. Stress corrosion test in MgCl 2 solution under constant load is conducted. The susceptibility to stress corrosion cracking of the explosive bonded boundary is lower than that of base metal because of greater resistance to plastic deformation. Stress corrosion test in high temperature and high pressure pure water is also conducted by means of static type of autoclave but stress corrosion cracking does not occur under the testing condition used. (author)

The role of crack tip opening in corrosion fatigue for the ductile ferritic steel-water system

International Nuclear Information System (INIS)

Tomkins, B.

1977-01-01

Water vapour or a water environment can dramatically reduce the fatigue strength of structural alloys, including aluminium and steel, and this reduction can be often related to the effect of the environment on crack initiation. More recently, however, under certain circumstances, it has become clear that fatigue crack growth rates can also be increased. A limited examination of crack tip openings in ductile steels under corrosion fatigue conditions, indicates that it may be possible to develop more physically based design rules for components which operate in some aqueous environments (author)

Crack initiation life analysis in notched pipe under cyclic bending loads

International Nuclear Information System (INIS)

Lee, Joon Seong; Kwak, Sang Log; Kim, Young Jin; Park, Youn Won

2001-01-01

In order to improve leak-before-break methodology, more precisely the crack growth evaluation, a round robin analysis was proposed by the CEA Saclay. The aim of this analysis was to evaluate the crack initiation life, penetration life and shape of through wall crack under cyclic bending loads. The proposed round robin analysis is composed of three main topic; fatigue crack initiation, crack propagation and crack penetration. This paper deals with the first topic, crack initiation in a notched pipe under four point bending. Both elastic-plastic finite element analysis and Neuber's rule were used to estimate the crack initiation life and the finite element models were verified by mesh-refinement, stress distribution and global deflection. In elastic-plastic finite element analysis, crack initiation life was determined by strain amplitude at the notch tip and strain-life curve of the material. In the analytical method, Neuber's rule with the consideration of load history and mean stress effect, was used for the life estimation. The effect of notch tip radius, strain range, cyclic hardening rule were examined in this study. When these results were compared with the experimental ones, the global deformation was a good agreement but crack initiation cycle was higher than the experimental result

Role of hydrogen in stress corrosion cracking

International Nuclear Information System (INIS)

Louthan, M.R. Jr.

1975-01-01

Hydrogen embrittlement has been postulated as a cause of stress corrosion cracking in numerous alloy systems. Such an interrelationship is useful in design considerations because it permits the designer and working engineer to relate the literature from both fields to a potential environmental compatibility problem. The role of hydrogen in stress corrosion of high strength steels is described along with techniques for minimizing the susceptibility to hydrogen stress cracking. (U.S.)

Fatigue Crack Growth Rate and Stress-Intensity Factor Corrections for Out-of-Plane Crack Growth

Science.gov (United States)

Forth, Scott C.; Herman, Dave J.; James, Mark A.

2003-01-01

Fatigue crack growth rate testing is performed by automated data collection systems that assume straight crack growth in the plane of symmetry and use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which for cracks that propagate out-of-plane, under-estimates the crack growth rates and over-estimates the stress-intensity factors. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on two-dimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys. Results indicate the correction process works well for high DeltaK levels but fails to capture the mixed-mode effects at DeltaK levels approaching threshold (da/dN approximately 10(exp -10) meter/cycle).

Effect of residual stress induced by cold expansion on fatigue crack ...

African Journals Online (AJOL)

Fatigue life and fatigue crack growth rate are controlled by stress ratio, stress level, orientation of crack, temper-ature, residual stress, corrosion, etc. The effects of residual stress on fatigue crack growth in aluminium (Al) alloy 2024-T351 by Mode I crack were investigated by applying constant amplitude cycles based on ...

Stress corrosion cracking of A515 grade 60 carbon steel

International Nuclear Information System (INIS)

Moore, E.L.

1971-01-01

An investigation was conducted to evaluate the effect of welding method plate thickness, and subsequent stress relief treatment on the stress corrosion cracking propensity of ASTM A515 Grade 60 carbon steel plate exposed to a 5 M NaNO 3 solution at 190 0 F for eight weeks. It was found that all weld coupons receiving no thermal stress relief treatment cracked within eight weeks; all weld coupons given a vibratory stress relief cracked within eight weeks; two of the eight weld coupons stress relieved at 600 0 F for one hour cracked within eight weeks; none of the weld coupons stress relieved at 1100 0 F for one hour cracked within eight weeks; and that cracking was generally more severe in coupons fabricated from 7/8 inch plate by shielded metal arc welding than it was in coupons fabricated by other welding methods. (U.S.)

Effects of absorbed hydrogen on crack-tip ductility in the welded A516 steel

International Nuclear Information System (INIS)

Khattak, M.A.; Haslan, M.H.; Tamin, M.N.

2007-01-01

Effects of absorbed hydrogen on structure and properties of welded A516 Grade-70 steel are investigated. Emphasis is placed on ductility measure of the crack-tip plastic zone under Mode I loading. Specimens are cathodically charged in a cell with dilute sulphuric acid and corrosion inhibitor with uniform charging current density of 20 mA/ cm 2 and at different exposure time. Results indicate a change from coarse- to fine-grained microstructures in the weld region and heat affected zone (HAZ) of hydrogen-charged specimen. Well-defined ferrite-pearlite bands in the base metal are transformed into coarse-grain structure. Hardness variation along radial distance indicates higher values towards the center of the bar, possibly due to faster diffusion rate but limited solubility of hydrogen. Load-COD responses indicate that slow, stable crack propagation occurred in both base metal and HAZ. The measured provisional fracture toughness, K Q is higher for HAZ than that for the base metal. The toughness values decreases significantly for the initial three hours of hydrogen charging. The tensile fracture region in the immediate fatigue pre-crack tip forms a triangular (rough) zone due to limited constraint to free surface deformation in the thin specimen. Fracture surface of HAZ is dominated by intergranular fracture with localized cleavage facets. (author)

Crack initiation life analysis in notched pipe under cyclic bending loads

International Nuclear Information System (INIS)

Goak, S. R.; Kim, Y. J.; Lee, J. S.; Park, Y. W.

2000-01-01

In order to improve LBB(Leak-Before-Break) methodology, more precisely the crack growth evaluation, a benchmark problem was proposed by the CEA Saclay. The aim of this benchmark analysis was to evaluate the crack growth in a notched pipe under cyclic bending loads. The proposed benchmark analysis is composed of three main topic; fatigue crack initiation, crack propagation and crack penetration. This paper deals with the first topic, crack initiation in a notched pipe under four point bending. Both elastic-plastic finite element analysis and Neuber's rule were used to estimate the crack initiation life and the finite element models were verified by mesh-refinement, stress distribution and global deflection. In elastic-plastic finite element analysis, crack initiation life was determined by strain amplitude at the notch tip and strain-life curve of the material. In the analytical method, Neuber's rule with the consideration of load history and mean stress effect, was used for the life estimation. The effect of notch tip radius, strain range, cyclic hardening rule were examined in this study. When these results were compared with the experimental ones, the global deformation was a good agreement but the crack initiation cycle was higher than the experimental result

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

Directory of Open Access Journals (Sweden)

Li Hangyue

2014-01-01

Full Text Available At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, ΔKth−IG, analogous to a fatigue threshold (ΔKth and a critical temperature, Tth. In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower ΔKth−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher ΔKth−IG. When there is limited stress relaxation at the crack tip, similar ΔKth−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold (ΔKth despite faster crack growth rates (da/dN in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 ∘C.

Stress-intensity factor equations for cracks in three-dimensional finite bodies

Science.gov (United States)

Newman, J. C., Jr.; Raju, I. S.

1981-01-01

Empirical stress intensity factor equations are presented for embedded elliptical cracks, semi-elliptical surface cracks, quarter-elliptical corner cracks, semi-elliptical surface cracks at a hole, and quarter-elliptical corner cracks at a hole in finite plates. The plates were subjected to remote tensile loading. Equations give stress intensity factors as a function of parametric angle, crack depth, crack length, plate thickness, and where applicable, hole radius. The stress intensity factors used to develop the equations were obtained from three dimensional finite element analyses of these crack configurations.

Study on the fatigue crack initiation life under spherical contact

Energy Technology Data Exchange (ETDEWEB)

Cho, Yong Joo; Kim, Tae Wan [Busan National Univ., Busan (Korea, Republic of); Lee, Mun Ju [Samsung Electronics Co., Ltd., Suwon (Korea, Republic of)

2001-08-01

In case of contact fatigue, the accurate calculation of surface tractions and subsurface stress is essential to the prediction of crack initiation life. Surface tractions influencing shear stress amplitude have been obtained by contact analysis based on influence function. Subsurface stress has been obtained by using rectangular patch solutions. In this study, to simulate asperity contact under sliding condition, the tip of asperity was simulated by sphere and to calculate crack initiation life in the substrate, dislocation pileup theory was used.

Study on the fatigue crack initiation life under spherical contact

International Nuclear Information System (INIS)

Cho, Yong Joo; Kim, Tae Wan; Lee, Mun Ju

2001-01-01

In case of contact fatigue, the accurate calculation of surface tractions and subsurface stress is essential to the prediction of crack initiation life. Surface tractions influencing shear stress amplitude have been obtained by contact analysis based on influence function. Subsurface stress has been obtained by using rectangular patch solutions. In this study, to simulate asperity contact under sliding condition, the tip of asperity was simulated by sphere and to calculate crack initiation life in the substrate, dislocation pileup theory was used

Thermally activated low temperature creep and primary water stress corrosion cracking of NiCrFe alloys

International Nuclear Information System (INIS)

Hall, M.M. Jr.

1993-01-01

A phenomenological SCC-CGR model is developed based on an apriori assumption that the SCC-CGR is controlled by low temperature creep (LTC). This mode of low temperature time dependent deformation occurs at stress levels above the athermal flow stress by a dislocation glide mechanism that is thermally activated and may be environmentally assisted. The SCC-CGR model equations developed contain thermal activation parameters descriptive of the dislocation creep mechanism. Thermal activation parameters are obtained by fitting the CGR model to SCC-CGR data obtained on Alloy 600 and Alloy X-750. These SCC-CGR activation parameters are compared to LTC activation parameters obtained from stress relaxation tests. When the high concentration of hydrogen at the tip of an SCC crack is considered, the SCC-CGR activation energies and rate sensitivities are shown to be quantitatively consistent with hydrogen reducing the activation energy and increasing the strain rate sensitivity in LTC stress relaxation tests. Stress dependence of SCC-CGR activation energy consistent with that found for the LTC activation energy. Comparisons between temperature dependence of the SCC-CGR stress sensitivity and LTC stress sensitivity provide a basis for speculation on effects of hydrogen and solute carbon on SCC crack growth rates

A Crack Closure Model and Its Application to Vibrothermography Nondestructive Evaluation

Science.gov (United States)

Schiefelbein, Bryan Edward

Vibrothermography nondestructive evaluation (NDE) is in the early stages of research and development, and there exists uncertainty in the fundamental mechanisms and processes by which heat generation occurs. Holland et al. have developed a set of tools which simulate and predict the outcome of a vibrothermography inspection by breaking the inspection into three distinct processes: vibrational excitation, heat generation, and thermal imaging. The stage of vibrothermography which is not well understood is the process by which vibrations are converted to heat at the crack surface. It has been shown that crack closure and closure state impact the resulting heat generation. Despite this, research into the link between partial crack closure and vibrothermography is limited. This work seeks to rectify this gap in knowledge by modeling the behavior of a partially closed crack in response to static external loading and a dynamic vibration. The residual strains left by the plastic wake during fatigue crack growth manifest themselves as contact stresses acting at the crack surface interface. In response to an applied load below the crack opening stress, the crack closure state will evolve, but the crack will remain partially closed. The crack closure model developed in this work is based in linear elastic fracture mechanics (LEFM) and describes the behavior of a partially closed crack in response to a tensile external load and non-uniform closure stress distribution. The model builds on work by Fleck to describe the effective length, crack opening displacement, and crack tip stress field for a partially closed crack. These quantities are solved for by first establishing an equilibrium condition which governs the effective or apparent length of the partially closed crack. The equilibrium condition states that, under any external or crack surface loading, the effective crack tip will be located where the effective stress intensity factor is zero. In LEFM, this is equivalent to

The combined effect of gamma radiation and stress cracking in polycarbonate;Efeito combinado da radiacao gama e stress cracking no policarbonato

Energy Technology Data Exchange (ETDEWEB)

Melo, Raphaela N. de; Rabello, Marcelo S., E-mail: marcelo@dema.ufcg.edu.b [Universidade Federal de Campina Grande (DEMa/UFCG), PB (Brazil). Dept. de Engenharia de Materiais; Silva, Leonardo G.A. [Instituto de Pesquisas Energeticas e Nucleares (IPEN/CNEN-SP), Sao Paulo, SP (Brazil)

2009-07-01

In this work the combined effect of gamma irradiation and stress cracking was studied in polycarbonate (PC). Tensile test bars were produced by injection moulding and then exposed to different doses of gamma radiation. After that they were submitted to the contact with isopropanol, the stress cracking agent used in this work. The specimens were tested for mechanical properties, viscosity molecular weight and fractography. The results indicated that the previous radiation intensified the stress cracking effects, as evidenced by the reduction in tensile properties and surface damage caused to the samples. (author)

Hydrazine and hydrogen coinjection to mitigate stress corrosion cracking of structural materials in boiling water reactors (7). Effects of bulk water chemistry on ECP distribution inside a crack

International Nuclear Information System (INIS)

Wada, Yoichi; Ishida, Kazushige; Tachibana, Masahiko; Aizawa, Motohiro; Fuse, Motomasa

2007-01-01

Water chemistry in a simulated crack (crack) has been studied to understand the mechanisms of stress corrosion cracking in a boiling water reactor environment. Electrochemical corrosion potential (ECP) in a crack made in an austenite type 304 stainless steel specimen was measured. The ECP distribution along the simulated crack was strongly affected by bulk water chemistry and bulk flow. When oxygen concentration was high in the bulk water, the potential difference between the crack tip and the outside of the crack (ΔE), which must be one motive force for crack growth, was about 0.3V under a stagnant condition. When oxygen was removed from the bulk water, ECP inside and outside the crack became low and uniform and ΔE became small. The outside ECP was also lowered by depositing platinum on the steel specimen surface and adding stoichiometrically excess hydrogen to oxygen to lower ΔE. This was effective only when bulk water did not flow. Under the bulk water flow condition, water-borne oxygen caused an increase in ECP on the untreated surface inside the crack. This also caused a large ΔE. The ΔE effect was confirmed by crack growth rate measurements with a catalyst-treated specimen. Therefore, lowering the bulk oxidant concentration by such measures as hydrazine hydrogen coinjection, which is currently under development, is important for suppressing the crack growth. (author)

Stress-corrosion cracks behavior under underground disposal environment of radioactive wastes

International Nuclear Information System (INIS)

Isei, Takehiro; Seto, Masahiro; Ogata, Yuji; Wada, Yuji; Utagawa, Manabu; Kosugi, Masayuki

2000-01-01

This study is composed by two sub-theme of study on stress-corrosion cracking under an environment of disposal on radioactive wastes and control technique on microscopic crack around the disposal cavity, and aims at experimental elucidation on forming mechanism of stress-corrosion cracking phenomenon on rocks and establishment of its control technique. In 1998 fiscal year, together with an investigation on effect of temperature on fracture toughness and on stress-corrosion cracks performance of sedimentary rocks (sandy rocks), an investigation on limit of the stress-corrosion cracking by addition of chemicals and on crack growth in a rock by in-situ observation using SEM were carried out. As a result, it was formed that fracture toughness of rocks reduced at more than 100 centigrade of temperature, that a region showing an equilibrium between water supply to crack end and crack speed appeared definitely, that a limit of stress-corrosion cracking appeared by addition of chemicals, and that as a result of observing crack advancement of saturated rock by in-situ observation of crack growth using SEM, a process zone was formed at the front of main crack due to grain boundary fracture. (G.K.)

Crack propagation under conditions of low cycle fatigue

International Nuclear Information System (INIS)

Hellmann, D.

1988-01-01

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 [de

Residual stresses and stress corrosion cracking in pipe fittings

International Nuclear Information System (INIS)

Parrington, R.J.; Scott, J.J.; Torres, F.

1994-06-01

Residual stresses can play a key role in the SCC performance of susceptible materials in PWR primary water applications. Residual stresses are stresses stored within the metal that develop during deformation and persist in the absence of external forces or temperature gradients. Sources of residual stresses in pipe fittings include fabrication processes, installation and welding. There are a number of methods to characterize the magnitude and orientation of residual stresses. These include numerical analysis, chemical cracking tests, and measurement (e.g., X-ray diffraction, neutron diffraction, strain gage/hole drilling, strain gage/trepanning, strain gage/section and layer removal, and acoustics). This paper presents 400 C steam SCC test results demonstrating that residual stresses in as-fabricated Alloy 600 pipe fittings are sufficient to induce SCC. Residual stresses present in as-fabricated pipe fittings are characterized by chemical cracking tests (stainless steel fittings tested in boiling magnesium chloride solution) and by the sectioning and layer removal (SLR) technique

Stress corrosion cracking of nuclear reactor pressure vessel and piping steels

International Nuclear Information System (INIS)

Speidel, M.O.; Magdowski, R.M.

1988-01-01

This paper presents an extensive investigation of stress corrosion cracking of nuclear reactor pressure vessel and piping steels exposed to hot water. Experimental fracture mechanics results are compared with data from the literature and other laboratories. Thus a comprehensive overview of the present knowledge concerning stress corrosion crack growth rates is provided. Several sets of data confirm that 'fast' stress corrosion cracks with growth rates between 10 -8 and 10 -7 m/s and threshold stress intensities around 20 MN m -3/2 can occur under certain conditions. However, it appears possible that specific environmental, mechanical and metallurgical conditions which may prevail in reactors can result in significantly lower stress corrosion crack growth rates. The presently known stress corrosion crack growth rate versus stress intensity curves are discussed with emphasis on their usefulness in establishing safety margins against stress corrosion cracking of components in service. Further substantial research efforts would be helpful to provide a data base which permits well founded predictions as to how stress corrosion cracking in pressure vessels and piping can be reliably excluded or tolerated. It is emphasized, however, that the nucleation of stress corrosion cracks (as opposed to their growth) is difficult and may contribute substantially to the stress corrosion free service behaviour of the overwhelming majority of pressure vessels and pipes. (author)

Stress corrosion cracking and dealloying of copper-gold alloy in iodine vapor

International Nuclear Information System (INIS)

Galvez, M.F.; Bianchi, G.L.; Galvele, J.R.

1993-01-01

The susceptibility to stress corrosion cracking of copper-gold alloy in iodine vapor was studied and the results were analyzed under the scope of the surface mobility stress corrosion cracking mechanism. The copper-gold alloy undergoes stress corrosion cracking in iodine. Copper iodide was responsible of that behavior. The copper-gold alloy shows two processes in parallel: stress corrosion cracking and dealloying. As was predicted by the surface mobility stress corrosion cracking mechanism, the increase in strain rate induces an increase in the crack propagation rate. (Author)

Experimental study on stress corrosion crack propagation rate of FV520B in carbon dioxide and hydrogen sulfide solution

Science.gov (United States)

Qin, Ming; Li, Jianfeng; Chen, Songying; Qu, Yanpeng

FV520B steel is a kind of precipitation hardening Martensitic stainless steel, it has high-strength, good plasticity and good corrosion resistance. Stress corrosion cracking (SCC) is one of the main corrosion failure mode for FV520B in industrial transportation of natural gas operation. For a better understanding the effect on SCC of FV520B, the improved wedge opening loading (WOL) specimens and constant displacement loading methods were employed in experimental research in carbon dioxide and hydrogen sulfide solution. The test results showed that the crack propagation rate is 1.941 × 10-7-5.748 × 10-7 mm/s, the stress intensity factor KISCC is not more than 36.83 MPa √{ m } . The rate increases with the increasing of the crack opening displacement. Under the condition of different initial loading, KISCC generally shows a decreasing tendency with the increase in H2S concentration, and the crack propagation rate showed an increasing trend substantially. For the enrichment of sulfur ion in the crack tip induced the generation of pitting corrosion, promoting the surrounding metal formed the corrosion micro batteries, the pit defects gradually extended and connected with the adjacent pit to form a small crack, leading to further propagation till cracking happened. Fracture microscopic morphology displayed typical brittle fracture phenomena, accompanying with trans-granular cracking, river shape and sector, many second cracks on the fracture surface.

Impacts of weld residual stresses and fatigue crack growth threshold on crack arrest under high-cycle thermal fluctuations

Energy Technology Data Exchange (ETDEWEB)

Taheri, Said, E-mail: Said.taheri@edf.fr [EDF-LAB, IMSIA, 7 Boulevard Gaspard Monge, 91120 Palaiseau Cedex (France); Julan, Emricka [EDF-LAB, AMA, 7 Boulevard Gaspard Monge, 91120 Palaiseau Cedex (France); Tran, Xuan-Van [EDF Energy R& D UK Centre/School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL (United Kingdom); Robert, Nicolas [EDF-DPN, UNIE, Strategic Center, Saint Denis (France)

2017-01-15

Highlights: • For crack growth analysis, weld residual stress field must be considered through its SIF in presence of a crack. • Presence of cracks of same depth proves their arrest, where equal depth is because mean stress acts only on crack opening. • Not considering amplitudes under a fatigue crack growth threshold (FCGT) does not compensate the lack of FGCT in Paris law. • Propagation rates are close for axisymmetric and circumferential semi-elliptical cracks. - Abstract: High cycle thermal crazing has been observed in some residual heat removal (RHR) systems made of 304 stainless steel in PWR nuclear plants. This paper deals with two types of analyses including logical argumentation and simulation. Crack arrest in networks is demonstrated due to the presence of two cracks of the same depth in the network. This identical depth may be proved assuming that mean stress acts only on crack opening and that cracks are fully open during the load cycle before arrest. Weld residual stresses (WRS) are obtained by an axisymmetric simulation of welding on a tube with a chamfer. Axisymmetric and 3D parametric studies of crack growth on: representative sequences for variable amplitude thermal loading, fatigue crack growth threshold (FCGT), permanent mean stress, cyclic counting methods and WRS, are performed with Code-Aster software using XFEM methodology. The following results are obtained on crack depth versus time: the effect of WRS on crack growth cannot be determined by the initial WRS field in absence of crack, but by the associated stress intensity factor. Moreover the relation between crack arrest depth and WRS is analyzed. In the absence of FCGT Paris’s law may give a significant over-estimation of crack depth even if amplitudes of loading smaller than FCGT have not been considered. Appropriate depth versus time may be obtained using different values of FCGT, but axisymmetric simulations do not really show a possibility of arrest for shallow cracks in

Impacts of weld residual stresses and fatigue crack growth threshold on crack arrest under high-cycle thermal fluctuations

International Nuclear Information System (INIS)

Taheri, Said; Julan, Emricka; Tran, Xuan-Van; Robert, Nicolas

2017-01-01

Highlights: • For crack growth analysis, weld residual stress field must be considered through its SIF in presence of a crack. • Presence of cracks of same depth proves their arrest, where equal depth is because mean stress acts only on crack opening. • Not considering amplitudes under a fatigue crack growth threshold (FCGT) does not compensate the lack of FGCT in Paris law. • Propagation rates are close for axisymmetric and circumferential semi-elliptical cracks. - Abstract: High cycle thermal crazing has been observed in some residual heat removal (RHR) systems made of 304 stainless steel in PWR nuclear plants. This paper deals with two types of analyses including logical argumentation and simulation. Crack arrest in networks is demonstrated due to the presence of two cracks of the same depth in the network. This identical depth may be proved assuming that mean stress acts only on crack opening and that cracks are fully open during the load cycle before arrest. Weld residual stresses (WRS) are obtained by an axisymmetric simulation of welding on a tube with a chamfer. Axisymmetric and 3D parametric studies of crack growth on: representative sequences for variable amplitude thermal loading, fatigue crack growth threshold (FCGT), permanent mean stress, cyclic counting methods and WRS, are performed with Code-Aster software using XFEM methodology. The following results are obtained on crack depth versus time: the effect of WRS on crack growth cannot be determined by the initial WRS field in absence of crack, but by the associated stress intensity factor. Moreover the relation between crack arrest depth and WRS is analyzed. In the absence of FCGT Paris’s law may give a significant over-estimation of crack depth even if amplitudes of loading smaller than FCGT have not been considered. Appropriate depth versus time may be obtained using different values of FCGT, but axisymmetric simulations do not really show a possibility of arrest for shallow cracks in

Stress corrosion cracking of uranium--niobium alloys

International Nuclear Information System (INIS)

Magnani, N.J.

1978-03-01

The stress corrosion cracking behavior of U-2 1 / 4 , 4 1 / 2 , 6 and 8 wt % Nb alloys was evaluated in laboratory air and in aqueous Cl - solutions. Thresholds for crack propagation were obtained in these environments. The data showed that Cl - solutions are more deleterious than air environments. Tests were also conducted in pure gases to identify the species in the air responsible for cracking. These data showed the primary stress corrodent is water vapor for the most reactive alloy, U-2 1 / 4 % Nb, while O 2 is primarily responsible for cracking in the more corrosion resistant alloys, U-6 and 8% Nb. The 4 1 / 2 % alloy was found to be susceptible in both H 2 O and O 2 environments

Estimation of stress intensity factors for circumferential cracked pipes under welding residual stress filed

International Nuclear Information System (INIS)

Oh, Chang Young; Kim, Yun Jae; Oh, Young Jin; Song, Tae Kwang; Kim, Yong Beum; Oh, Young Jin; Song, Tae Kwang; Kim, Yong Beum

2012-01-01

Recently, stress corrosion cracking(SCC) have been found in dissimilar metal welds of nozzles in some pressurized water reactors and on low carbon stainless steel piping systems of boiling water reactors. The important factor of SCC is the residual stress field caused by weld. For the evaluation of crack growth analysis due to SCC, stress intensity factor under a residual stress field should be estimated. Several solutions for stress intensity factor under residual stress field were recommended in flaw assessment codes such as the American Society of Mechanical Engineers (ASME) Section XI, R6, American Petroleum Institute (API579). Some relevant works have been studied. Dong et al. evaluated stress intensity factors in welded structures. Miyazaki et al. estimated stress intensity factors of surface crack in simple stress fields. This paper presents a simple method to estimate stress intensity factors in welding residual stress field. For general application, results of structure integrity assessment codes KI solutions were compared Finite element analyses of welding simulation and cracked pipes are described. Comparison results of KI solutions and proposed simplified solution are presented in the works

A study of x-ray microbeam stress measurement for local area

International Nuclear Information System (INIS)

Natsume, Yoshitaka; Miyakawa, Susumu

1987-01-01

A new type X-ray microbeam equipment which is capable of three-dimensional oscillation at an X-ray radiation position was used to measure the residual stress of pure iron powder and bending plate specimens as well as that in the vicinity of fatigue crack tip of 1/2 CT specimen. The results obtained are summerized as follows. (1) Diffraction profiles of pure iron powder with diameter 0.02 ∼ 0.03 μm particle size obtained by the present microbeam technique in the beam area of 80 μm in diamether and measuring time more than 800 sec showed Kα 1 and Kα 2 peaks clearly, so that these profiles are good enough for stress measurement. (2) The measured stress of pure iron powder with diameter 10 ∼ 25 μm particle size in the beam area of 80 μm in diameter was not equal to 0 MPa. The measured stress varied greatly and was independent of measuring time. (3) The measured stress of pure iron powder with diameter 0.02 ∼ 0.03 μm particle size in the beam area of 80 μm in diameter was almost 1 MPa, which shows the good accuracy of this X-ray micro-beam equipment. (4) The relation between the X-ray measured stress and the mechanically applied stress showed good agreement. (5) The distribution of residual stress at the vicinity of fatigue crack tip showed that the residual stress at the fatigue crack tip was compression and the residual stress in front of crack tip was tension. This tensile residual stress in front of crack tip existed in the area from 0.02 mm to 2 mm. (6) The above results show that the present equipment is advantageous in the investigation of local stress, shortening the measuring time without sacrificing the accuracy. (author)

Effects of location, thermal stress and residual stress on corner cracks in nozzles with cladding

International Nuclear Information System (INIS)

McLean, J.L.; Cohen, L.M.; Besuner, P.M.

1979-01-01

The stress intensity factors (K 1 ) for corner cracks in a boiling water reactor feedwater nozzle with stainless steel cladding are obtained for loading by internal pressure and a fluid quench in the nozzle. Conditions both with and without residual stress in the component are considered. The residual stress is simulated by means of a reference temperature change. The stress distribution for the uncracked structure is obtained from a three-dimensional finite element model. A three-dimensional influence function (IF) method, in conjunction with the boundary-integral equation method for structural analysis, is employed to compute K 1 values from the uncracked stress distribution. For each type of loading K 1 values are given for cracks at 15 nozzle locations and for 6 crack depths. Reasonable agreement is noted between calculated and previously published pressure-induced K 1 values. Comparisons are made to determine the effect on K 1 of crack location, thermal stress and residual stress, as compared with pressure stress. For the thermal transient it is shown that K 1 for small crack depths is maximised early in the transient, while K 1 for large cracks is maximised later under steady state conditions. Computation should, therefore, be made for several transient time points and the maximum K 1 for a given crack depth should be used for design analysis. It is concluded that the effects on K 1 of location, thermal stresses and residual stresses are significant and generally too complex to evaluate without advanced numerical procedures. The utilised combination of finite element analysis of the uncracked structure and three-dimensional influence function analysis of the cracked structure is demonstrated and endorsed. (author)

On crack interaction effects of in-plane surface cracks using elastic and elastic-plastic finite element analyses

International Nuclear Information System (INIS)

Kim, Jong Min; Huh, Nam Su

2010-01-01

The crack-tip stress fields and fracture mechanics assessment parameters for a surface crack, such as the elastic stress intensity factor or the elastic-plastic J-integral, can be affected significantly by the adjacent cracks. Such a crack interaction effect due to multiple cracks can alter the fracture mechanics assessment parameters significantly. There are many factors to be considered, for instance the relative distance between adjacent cracks, the crack shape, and the loading condition, to quantify the crack interaction effect on the fracture mechanics assessment parameters. Thus, the current assessment codes on crack interaction effects (crack combination rules), including ASME Sec. XI, BS7910, British Energy R6 and API 579-1/ASME FFS-1, provide different rules for combining multiple surface cracks into a single surface crack. The present paper investigates crack interaction effects by evaluating the elastic stress intensity factor and the elastic-plastic J-integral of adjacent in-plane surface cracks in a plate through detailed 3-dimensional elastic and elastic-plastic finite element analyses. The effects on the fracture mechanics assessment parameters of the geometric parameters, the relative distance between two cracks, and the crack shape are investigated systematically. As for the loading condition, an axial tension is considered. Based on the finite element results, the acceptability of the crack combination rules provided in the existing guidance was investigated, and the relevant recommendations on a crack interaction for in-plane surface cracks are discussed. The present results can be used to develop more concrete guidance on crack interaction effects for crack shape characterization to evaluate the integrity of defective components

Measurements of delayed hydride cracking propagation rate in the radial direction of Zircaloy-2 cladding tubes

Energy Technology Data Exchange (ETDEWEB)

Kubo, T., E-mail: kubo@nfd.co.jp [Nippon Nuclear Fuel Development Co., Ltd., 2163 Narita-cho, Oarai-machi, Ibaraki 311-1313 (Japan); Kobayashi, Y. [M.O.X. Co., Ltd., 1828-520 Hirasu-cho, Mito, Ibaraki 311-0853 (Japan); Uchikoshi, H. [Nippon Nuclear Fuel Development Co., Ltd., 2163 Narita-cho, Oarai-machi, Ibaraki 311-1313 (Japan)

2012-08-15

Highlights: Black-Right-Pointing-Pointer The delayed hydride cracking (DHC) velocity of Zircaloy-2 was measured. Black-Right-Pointing-Pointer The velocity followed the Arrhenius law up to 270 Degree-Sign C. Activation energy was 49 kJ/mol. Black-Right-Pointing-Pointer The threshold stress intensity factor for the DHC was from 4 to 6 MPa m{sup 1/2}. Black-Right-Pointing-Pointer An increase in material strength accelerated the DHC. Black-Right-Pointing-Pointer Precipitation and fracture of hydrides at a crack tip is responsible for the DHC. - Abstract: Delayed hydride cracking (DHC) tests of Zircaloy-2 cladding tubes were performed in the chamber of a scanning electron microscope (SEM) to directly observe the crack propagation and measure the crack velocity in the radial direction of the tubes. Pre-cracks were produced at the outer surfaces of the tubes. Hydrogen contents of the tubes were from 90 ppm to 130 ppm and test temperatures were from 225 Degree-Sign C to 300 Degree-Sign C. The crack velocity followed the Arrhenius law at temperatures lower than about 270 Degree-Sign C with apparent activation energy of about 49 kJ/mol. The upper temperature limit for DHC, above which DHC did not occur, was about 280 Degree-Sign C. The threshold stress intensity factor for the initiation of the crack propagation, K{sub IH}, was from about 4 MPa m{sup 1/2} to 6 MPa m{sup 1/2}, almost independent of temperature. An increase in 0.2% offset yield stress of the material accelerated the crack velocity and slightly decreased K{sub IH}. Detailed observations of crack tip movement showed that cracks propagated in an intermittent fashion and the propagation gradually approached the steady state as the crack depth increased. The SEM observations also showed that hydrides were formed at a crack tip and a number of micro-cracks were found in the hydrides. It was presumed from these observations that the repetition of precipitation and fracture of hydrides at the crack tip would be

Stress field determination in an alloy 600 stress corrosion crack specimen

International Nuclear Information System (INIS)

Rassineux, B.; Labbe, T.

1995-05-01

In the context of EDF studies on stress corrosion cracking rates in the Alloy 600 steam generators tubes, we studied the influence of strain hardened surface layers on the different stages of cracking for a tensile smooth specimen (TLT). The stress field was notably assessed to try and explain the slow/rapid-propagation change observed beyond the strain hardened layers. The main difficulty is to simulate in a finite element model the inner and outer surfaces of these strain hardened layers, produced by the final manufacturing stages of SG tubes which have not been heat treated. In the model, the strain hardening is introduced by simulating a multi-layer material. Residual stresses are simulated by an equivalent fictitious thermomechanical calculation, realigned with respect to X-ray measurements. The strain hardening introduction method was validated by an analytical calculation giving identical results. Stress field evolution induced by specimen tensile loading were studied using an elastoplastic 2D finite element calculations performed with the Aster Code. The stress profile obtained after load at 660 MPa shows no stress discontinuity at the boundary between the strain hardened layer and the rest of the tube. So we propose that a complementary calculation be performed, taking into account the multi-cracked state of the strain hardened zones by means of a damage variable. In fact, this state could induce stress redistribution in the un-cracked area, which would perhaps provide an explanation of the crack-ground rate change beyond the strain hardened zone. The calculations also evidence the harmful effects of plastic strains on a strain hardened layer due to the initial state of the tube (not heat-treated), to grit blasting or to shot peening. The initial compressive stress condition of this surface layer becomes, after plastic strain, a tensile stress condition. These results are confirmed by laboratory test. (author). 10 refs., 18 figs., 9 tabs., 2 appends

Numerical analysis of macro-crack formation behavior within the lump coke; Cokes sonai kiretsu shinten kiko no kaiseki

Energy Technology Data Exchange (ETDEWEB)

Aoki, H; Sato, H; Miura, T [Tohoku University, Sendai (Japan). Faculty of Engineering

1995-03-15

The thermal stress analysis within lump coke was studied in order to investigate macro-crack formation and deformation behavior which strongly influence heat and mass transfer in a coke oven chamber. The dilatation of plastic layer, heating rate dependence of thermophysical and mechanical properties of coal/coke, creep in the plastic and semi-coke layers, macro-crack propagation and radiative heat transfer within the macro-crack were considered in an analytical model. The macro-crack propagation was determined from the estimated crack tip stress intensity factor, K{sub I}, at the macro-crack tip compared with the plane strain fracture toughness, K{sub IC}, through the unsteady-state calculation. Calculated results on crack formation and deformation behavior of lump coke were in good agreement with experimental observations in a laboratory-scale oven chamber. The analytical model could predict micro-crack formation within the lump coke normal to the heated wall and the coke surface close to the heated wall. 12 refs., 13 figs.

On the driving force for crack growth during thermal actuation of shape memory alloys

Science.gov (United States)

Baxevanis, T.; Parrinello, A. F.; Lagoudas, D. C.

2016-04-01

The effect of thermomechanically induced phase transformation on the driving force for crack growth in polycrystalline shape memory alloys is analyzed in an infinite center-cracked plate subjected to a thermal actuation cycle under mechanical load in plain strain. Finite element calculations are carried out to determine the mechanical fields near the static crack and the crack-tip energy release rate using the virtual crack closure technique. A substantial increase of the energy release rate - an order of magnitude for some material systems - is observed during the thermal cycle due to the stress redistribution induced by large scale phase transformation. Thus, phase transformation occurring due to thermal variations under mechanical load may result in crack growth if the crack-tip energy release rate reaches a material specific critical value.

On the strong crack-microcrack interaction problem

Science.gov (United States)

Gorelik, M.; Chudnovsky, A.

1992-07-01

The problem of the crack-microcrack interaction is examined with special attention given to the iterative procedure described by Chudnovsky and Kachanov (1983), Chudnovsky et al. (1984), and Horii and Nemat-Nasser (1983), which yields erroneous results as the crack tips become closer (i.e., for strong crack interaction). To understand the source of error, the traction distributions along the microcrack line on the n-th step of iteration representing the exact and asymptotic stress fields are compared. It is shown that the asymptotic solution gives a gross overestimation of the actual traction.

Stress corrosion and corrosion fatigue crack growth monitoring in metals

International Nuclear Information System (INIS)

Senadheera, T.; Shipilov, S.A.

2003-01-01

Environmentally assisted cracking (including stress corrosion cracking and corrosion fatigue) is one of the major causes for materials failure in a wide variety of industries. It is extremely important to understand the mechanism(s) of environmentally assisted crack propagation in structural materials so as to choose correctly from among the various possibilities-alloying elements, heat treatment of steels, parameters of cathodic protection, and inhibitors-to prevent in-service failures due to stress corrosion cracking and corrosion fatigue. An important step towards understanding the mechanism of environmentally assisted crack propagation is designing a testing machine for crack growth monitoring and that simultaneously provides measurement of electrochemical parameters. In the present paper, a direct current (DC) potential drop method for monitoring crack propagation in metals and a testing machine that uses this method and allows for measuring electrochemical parameters during stress corrosion and corrosion fatigue crack growth are described. (author)

Seacoast stress corrosion cracking of aluminum alloys

Science.gov (United States)

Humphries, T. S.; Nelson, E. E.

1981-01-01

The stress corrosion cracking resistance of high strength, wrought aluminum alloys in a seacoast atmosphere was investigated and the results were compared with those obtained in laboratory tests. Round tensile specimens taken from the short transverse grain direction of aluminum plate and stressed up to 100 percent of their yield strengths were exposed to the seacoast and to alternate immersion in salt water and synthetic seawater. Maximum exposure periods of one year at the seacoast, 0.3 or 0.7 of a month for alternate immersion in salt water, and three months for synthetic seawater were indicated for aluminum alloys to avoid false indications of stress corrosion cracking failure resulting from pitting. Correlation of the results was very good among the three test media using the selected exposure periods. It is concluded that either of the laboratory test media is suitable for evaluating the stress corrosion cracking performance of aluminum alloys in seacoast atmosphere.

The effect of pre-stress cycles on fatigue crack growth - An analysis of crack growth mechanism. [in Al alloy plates

Science.gov (United States)

Kang, T. S.; Liu, H. W.

1974-01-01

Cyclic prestress increases subsequent fatigue crack growth rate in 2024-T351 aluminum alloy. This increase in growth rate, caused by the prestress, and the increased rate, caused by temper embrittlement as observed by Ritchie and Knott (1973), cannot be explained by the crack tip blunting model alone. Each fatigue crack increment consists of two components, a brittle and a ductile component. They are controlled by the ductility of the material and its cyclic yield strength, respectively.

Hydrogen induced crack propagation in metal under plain-strain deformation

International Nuclear Information System (INIS)

Fishgojt, A.V.; Kolachev, B.A.

1981-01-01

A model of subcritical crack propagation conditioned by the effect of dissolved hydrogen in the case of plane-strain deformation of high-strength materials, is suggested. It is supposed that diffusion takes place in the isotropic material and hydrogen diffuses in the region of tensile stress maximum before crack tip under the effect of the stress gradient. When hydrogen achieves the critical concentration, microcrack growth takes place. Values of crack growth rates experimentally obtained agree with values calculated according to the suggested formula. Calculation and experimental data on the Ti-6Al-4V alloy, are presented [ru

Prediction of PWSCC in nickel base alloys using crack growth rate models

International Nuclear Information System (INIS)

Thompson, C.D.

1995-01-01

The Ford/Andresen slip dissolution SCC model, originally developed for stainless steel components in BWR environments, has been applied to Alloy 600 and Alloy X-750 tested in deaerated pure water chemistry. A method is described whereby the crack growth rates measured in compact tension specimens can be used to estimate crack growth in a component. Good agreement was found between model prediction and measured SCC in X-750 threaded fasteners over a wide range of temperatures, stresses, and material condition. Most data support the basic assumption of this model that cracks initiate early in life. The evidence supporting a particular SCC mechanism is mixed. Electrochemical repassivation data and estimates of oxide fracture strain indicate that the slip dissolution model can account for the observed crack growth rates, provided primary rather than secondary creep rates are used. However, approximately 100 cross-sectional TEM foils of SCC cracks including crack tips reveal no evidence of enhanced plasticity or unique dislocation patterns at the crack tip or along the crack to support a classic slip dissolution mechanism. No voids, hydrides,, or microcracks are found in the vicinity of the crack tips creating doubt about classic hydrogen related mechanisms. The bulk oxide films exhibit a surface oxide which is often different than the oxide found within a crack. Although bulk chromium concentration affects the rate of SCC, analytical data indicates the mechanism does not result from chromium depletion at the grain boundaries. The overall findings support a corrosion/dissolution mechanism but not one necessarily related to slip at the crack tip. (author). 12 refs, 27 figs

Hydride precipitation crack propagation in zircaloy cladding during a decreasing temperature history

International Nuclear Information System (INIS)

Stout, R.B.

2001-01-01

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)

Hydride precipitation crack propagation in zircaloy cladding during a decreasing temperature history

Energy Technology Data Exchange (ETDEWEB)

Stout, R.B. [California Univ., Livermore, CA (United States). Lawrence Livermore National Lab

2001-07-01

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)

Main factors causing intergranular and quasi-cleavage fractures at hydrogen-induced cracking in tempered martensitic steels

Science.gov (United States)

Kurokawa, Ami; Doshida, Tomoki; Hagihara, Yukito; Suzuki, Hiroshi; Takai, Kenichi

2018-05-01

Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under various hydrogen contents. While the crack initiation of the L-Si steel change from QC fracture at the notch tip to QC and IG fractures from approximately 10 µm ahead of the notch tip as increasing in hydrogen content. For L-Si steels, two possibilities are considered that the QC or IG fracture occurred firstly, or the QC and IG fractures occurred simultaneously. Furthermore, the principal stress and equivalent plastic strain distributions near the notch tip were calculated with FEM (Finite Element Method) analysis. The plastic strain was the maximum at the notch tip and the principle stress was the maximum at approximately 10 µm from the notch tip. The position of the initiation of QC and IG fracture observed using FE-SEM corresponds to the position of maximum strain and stress obtained with FEM, respectively. These findings indicate that the main factors causing hydrogen-induced cracking are different between QC and IG fractures.

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

Science.gov (United States)

Huang, J. H.; Altstetter, C. J.

1991-11-01

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocity vs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip.

Fatigue crack threshold relevant to stress ratio, crack wake and loading histories

International Nuclear Information System (INIS)

Okazaki, Masakazu; Iwasaki, Akira; Kasahara, Naoto

2013-01-01

Fatigue crack propagation behavior was investigated in a low alloy steel which experienced several kind of loading histories. Both the effects of stress ratio, test temperature on the fatigue crack threshold, and the change in the threshold depending on the thermo-mechanical loading histories, were experimentally investigated. It was shown that the thermo-mechanical loading history left its effect along the prior fatigue crack wake resulting in the change of fatigue crack threshold. Some discussions are made on how this type of loading history effect should be treated from engineering point of view. (author)

Application of acoustic emission to hydride cracking

International Nuclear Information System (INIS)

Sagat, S.; Ambler, J.F.R.; Coleman, C.E.

1986-07-01

Acoustic emission has been used for over a decade to study delayed hydride cracking (DHC) in zirconium alloys. At first acoustic emission was used primarily to detect the onset of DHC. This was possible because DHC was accompanied by very little plastic deformation of the material and furthermore the amplitudes of the acoustic pulses produced during cracking of the brittle hydride phase were much larger than those from dislocation motion and twinning. Acoustic emission was also used for measuring crack growth when it was found that for a suitable amplitude threshold, the total number of acoustic emission counts was linearly related to the cracked area. Once the proportionality constant was established, the acoustic counts could be converted to the crack length. Now the proportionality between the count rate and the crack growth rate is used to provide feedback between the crack length and the applied load, using computer technology. In such a system, the stress at the crack tip can be maintained constant during the test by adjusting the applied load as the crack progresses, or it can be changed in a predetermined manner, for example, to measure the threshold stress for cracking

Stress intensity factors of corner cracks in two nozzle-cylinder intersections

International Nuclear Information System (INIS)

Kobayashi, A.S.; Polvanich, N.; Emery, A.F.; Love, W.J.

1977-01-01

In a recent paper, the authors presented the stress-intensity-magnification factors of a quarter-elliptical surface crack in a quarter-infinite solid and a circular crack approaching a reentry corner in a three-quarter infinite solid. These stress-intensity-magnification factors were used together with a curvature-correction factor to estimate the stress-intensity factor of a corner crack at a nozzle-cylinder intersection. Through appropriate superposition of the above stress-intensity-magnification factors, stress-intensity factors for hypothetical corner cracks at a nozzle-cylinder intersection subjected to internal pressure and transient thermal-stress loadings can be obtained. A description of a computer code based on this procedure as well as its applications in analyzing two corner-crack problems at a nozzle-cylinder intersection are discussed in this paper

Stress intensity factors of corner cracks in two nozzle-cylinder interactions

International Nuclear Information System (INIS)

Kobayashi, A.S.; Polvanich, N.; Emery, A.F.; Love, W.J.

1977-01-01

In a recent paper, the authors presented the stress-intensity-magnification factors of a quarter-elliptical surface crack in a quarter-infinite solid and a circular crack approaching a reentry corner in a three-quarter infinite solid. These stress-intensity-magnification factors were used together with a curvature-correction factor to estimate the stress-intensity factor of a corner crack at a nozzle-cylinder interaction. Through appropriate superposition of the above stress-intensity-magnification factors, stress-intensity factors for hypothetical corner cracks at a nozzle-cylinder intersection subjected to internal pressure and transient thermal-stress loadings can be obtained. A description of a computer code based on this procedure as well as its applications in analyzing two corner-crack probems at a nozzle-cylinder intersection are discussed in this paper. (Auth.)

The effect of single overloading on stress corrosion cracking

International Nuclear Information System (INIS)

Ito, Yuzuru; Saito, Masahiro

2008-01-01

In the normal course of nuclear power plant operation in Japan, proof testing has been performed after periodic plant inspections. In this proof test procedure, the reactor pressure vessel and pipes of the primary coolant loop are subjected to a specified overload with a slightly higher hydraulic pressure than during normal operation. This specified overload is so called a single overload' in material testing. It is well known that the fatigue crack growth rate is decreased after a single overload has been applied to the specimen. However, it is not clear whether the stress corrosion cracking rate is also decreased after a single overload. In this study, the effect of a single overload on the stress corrosion cracking rate under simulated boiling water reactor environment was evaluated by examining a singly overloaded WOL (wedge opening load) specimen. The WOL specimen for the stress corrosion cracking test was machined from sensitized 304 type austenitic stainless steel. Since the crack extension length was 3.2% longer in the case of a more severely overloaded specimen, it was observed than the stress corrosion cracking rate is also decreased after the single overload has been applied to the specimen. (author)

Influence of corrosion environment composition on crack propagation in high-strength martensitic steel

International Nuclear Information System (INIS)

Romaniv, O.N.; Nikiforchin, G.N.; Tsirul'nik, A.T.

1984-01-01

The 40 Kh steel is taken as an example to investigate the dependence of electrochemical parameters in the crack tip and characteristics of corrosion static cracking resistance of martensitic steel on the composition of environment. The tests are performed in acidic and alkaline solutions prepared by adding HC or NaOH in distilled water. It is established that growth of pH value of initial solutions trom 0 to 13 brings about linear increase of a threshold stress intensity factor. It is found that acidic medium in the crack tip preserves up to pH 13 of initial medium. The possibility of corrosion crack propagation in alkaline solutions according to the mechanism of hydrogen embrittlement is proved

Iodine-induced stress corrosion cracking of fixed deflection stressed slotted rings of Zircaloy fuel cladding

International Nuclear Information System (INIS)

Sejnoha, R.; Wood, J.C.

1978-01-01

Stress corrosion cracking of Zircaloy fuel cladding by fission products is thought to be an important mechanism influencing power ramping defects of water-reactor fuels. We have used the fixed-deflection stressed slotted-ring technique to demonstrate cracking. The results show both the sensitivity and limitations of the stressed slotted-ring method in determining the responses of tubing to stress corrosion cracking. They are interpreted in terms of stress relaxation behavior, both on a microscopic scale for hydrogen-induced stress-relief and on a macroscopic scale for stress-time characteristics. Analysis also takes account of nonuniform plastic deformation during loading and residual stress buildup on unloading. 27 refs

Influence of fatigue crack wake length and state of stress on crack closure

Science.gov (United States)

Telesman, Jack; Fisher, Douglas M.

1988-01-01

The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of K sub CL in the crack wake was found to be significantly higher for plasticity induced closure in comparison to roughness induced closure. Roughness induced closure was uniform throughout the thickness of the specimen while plasticity induced closure levels were 50 percent higher in the near surface region than in the midthickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of delta K sub eff.

Inhibition of stress corrosion cracking of alloy 600 in 10% NaOH solutions with and with lead oxide at 315 C

International Nuclear Information System (INIS)

Hur, D.H.; Kim, J.S.; Baek, J.S.; Kim, J.G.

2002-01-01

Alloy 600 steam generator tube materials have experienced various degradations by corrosion such as stress corrosion cracking (SCC) on the inner and outer diameter surface of tube, intergranular attack and pitting, and by mechanical damage such as fretting-wear and fatigue. These tube degradations not only increase the costs for tube inspection, maintenance and repair but also reduce the operation safety and the efficiency of plants. Therefore, the methodologies have been extensively developed to mitigate them. The addition of inhibitors to the coolant is a feasible method to mitigate tube degradations in operating plants. In this paper, a new inhibitor is proposed to mitigate the secondary side stress corrosion cracking of alloy 600 tubes. The effect of inhibitors on the electrochemical behavior and the stress corrosion cracking resistance of alloy 600 was evaluated in 10% sodium hydroxide solution with and without lead oxide at 315 C. The specimens of a C-ring type for stress corrosion cracking test were polarized at 150 mV above the corrosion potential for 120 hours without and with inhibitors such as titanium oxide, titanium boride, cerium boride. The chemical compositions of the films formed on the crack tip in the C-ring specimens were analyzed using a scanning Auger electron spectroscopy. The cerium boride, the most effective inhibitors, was observed to decrease the crack propagation rate more than a factor of three compared with that obtained in pure 10% NaOH solution. Furthermore, no SCC was observed in lead contaminated 10% NaOH solution by the addition of the cerium boride. (authors)

Prediction of cleavage crack propagation and arrest in a nuclear pressure vessel steel (16MND5) under thermal shock

International Nuclear Information System (INIS)

Yang, Xiaoyu

2015-01-01

The purpose of this PhD study is to predict the propagation and arrest of cleavage cracks in a French PWR vessel steel (16MND5). This is accomplished through use of a local criterion based on the critical stress calculated ahead of crack tip. Previous work has shown that fracture mechanism was cleavage associated with the ductile shear zone between the different planes of cracking. Thus, the critical stress at crack tip depends on stain rate. This thesis consists of numerical optimization, identification and validation of a local criterion based on experiments which have complex thermomechanical loads. The criterion accounts for various crack paths, deepening the knowledge about micro mechanisms during crack propagation in order to justify the established criterion. Criterion identification was carried out by using numerical simulations of tension tests performed on CT (Compact Tension) specimens at four different temperatures (-150 C, -125 C, -100 C and -75 C). The eXtended Finite Element Method (X-FEM) was used in CAST3M FE software to model dynamic crack propagation and arrest. The analysis results in 2D and 3D showed that the critical stress at crack tip increased with the inelastic strain rate. Therefore, a criterion based on the critical stress was established. An analytical model was developed to justify the identified criterion. The critical stress given by the local criterion was considered as the sum of the critical cleavage stress and the stress generated by the deformation of ligaments behind the crack tip. In order to quantify this phenomenon, measurements of ligaments' characteristics have been performed on fracture surfaces and on cross-sections of the specimens. The stress profile of the crack lips generated by ligaments was calculated by modeling of multi-cracks on specimen's cross-section. The contribution of stress generated by ligaments to the critical stress at crack tip was obtained with this method, and then the analytical model of

Biaxial nominal state of stress at the crack front

International Nuclear Information System (INIS)

Dietmann, H.; Kussmaul, K.

1979-01-01

In fracture toughness testing with CT-specimens there is an unaxial nominal stress state caused by the nominal stress psub(y) perpendicular to the crack surface. This paper investigates the question whether the fracture toughness, or generally speaking, the fracture load, is influenced by additional nominal stresses psub(x) and psub(z) in the crack surface, i.e. by a multiaxial stress state. (orig.)

Criterion of cleavage crack propagation and arrest in a nuclear PWR vessel steel

International Nuclear Information System (INIS)

Bousquet, Amaury

2013-01-01

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

Correlation between oxidation and stress corrosion cracking of U-4.5 wt.% Nb

International Nuclear Information System (INIS)

Magnani, N.J.; Holloway, P.H.

1976-01-01

To investigate the mechanisms causing stress corrosion cracking on uranium alloys, the kinetics of crack propagation and oxide film growth for U-4.5 percent Nb were investigated at temperatures between 0 0 C and 200 0 C in oxygen, water vapor and oxygen-water vapor mixtures. Three regions of crack velocity rate versus stress intensity were observed in laboratory air. At low stress intensities (but above an effective K/sub ISCC/ of 22 MN/m/sup 3 / 2 /) crack velocity varied approximately as K 70 . In an intermediate stress intensity region (region II) the crack velocity was dependent upon K 4 . In the high stress intensity region, mechanical overloading was observed and crack velocities varied approximately as K 12 . Both cracking (region II) and oxidation rates were characterized by an activation energy of 7 kcal/mole. For stress corrosion cracking it was shown that oxygen was the primary stress corrodent, but a synergistic effect upon crack propagation rates was observed for oxygen-water vapor mixtures. Crack velocities were dependent upon the pressure of oxygen (P/sub O 2 //sup 1 / 3 /) and water vapor, while the oxidation rate was essentially independent of the pressure of these species. Stress sorption and oxide film formation stress corrosion cracking mechanisms were considered and reconciled with the stress corrosion and oxidation data

Initiation model for intergranular stress corrosion cracking in BWR pipes

International Nuclear Information System (INIS)

Hishida, Mamoru; Kawakubo, Takashi; Nakagawa, Yuji; Arii, Mitsuru.

1981-01-01

Discussions were made on the keys of intergranular stress corrosion cracking of austenitic stainless steel in high-temperature water in laboratories and stress corrosion cracking incidents in operating plants. Based on these discussions, a model was set up of intergranular stress corrosion cracking initiation in BWR pipes. Regarding the model, it was presumed that the intergranular stress corrosion cracking initiates during start up periods whenever heat-affected zones in welded pipes are highly sensitized and suffer dynamic strain in transient water containing dissolved oxygen. A series of BWR start up simulation tests were made by using a flowing autoclave system with slow strain rate test equipment. Validity of the model was confirmed through the test results. (author)

The relationship between X-ray residual stress near the crack and crack opening/closing behavior controlling fatigue crack propagation in Ti-6Al-4V alloy

International Nuclear Information System (INIS)

Torii, Tashiyuki; Toi, Norihiko; Nakano, Kohji; Honda, Kazuo

1998-01-01

Using the X-ray method of stress measurement for Ti-6Al-4V alloys, the residual stress near the crack was measured for annealed (AN) and solution treated and aged (STA) titanium alloys, under the condition that the measured X-ray stress was in satisfactory agreement with the applied stress under tension. The residual stress measured in the wake of the propagating fatigue crack, σ r , was compressive, resulting in a smaller crack opening displacement, COD, than theorized. The measured σ r and COD-values let us understand the fatigue crack propagation rate da/dN in terms of the effective stress intensity factor K eff . As a result, the da/dN under the same K eff -value was smaller in the AN specimen with zigzag crack growth than in the STA specimen with straight crack growth, although the da/dN-K eff relationship under various stress amplitudes was represented by a straight line in a log-log scale separately for the AN and STA specimens. (author)

The effects of location, thermal stress, and residual stress on corner cracks in nozzles with cladding

International Nuclear Information System (INIS)

Besuner, P.M.; Cohen, L.M.; McLean, J.L.

1977-01-01

The stress intensity factors (Ksub(I)) for corner cracks in a boiling water reactor feedwater nozzle with stainless steel cladding are obtained for loading by internal pressure, and a fluid quench in the nozzle. Conditions with and without residual stress in the component are considered. The residual stress is simulated by means of a reference temperature change. The stress distribution for the uncracked structure is obtained from a three-dimensional finite element model. A three-dimensional influence function (IF) method, in conjunction with the boundary-integral equation method for structural analysis, is employed to compute Ksub(I) values from the uncracked structure's stress distribution. For each type of loading Ksub(I) values are given for cracks at 15 nozzle locations and for six crack depths. Reasonable agreement is noted between calculated and previously published pressure-induced Ksub(I) values. Comparisons are made to determine the effect on Ksub(I) of crack location, thermal stress, and residual stress as compared to pressure stress. For the thermal transient it is shown that Ksub(I) for small crack depths is maximized early in the transient while Ksub(I) for large cracks is maximized later, under steady state conditions. Ksub(I) computations should, therefore, be made for several transient time points and the maximum Ksub(I) for a given crack depth should be used for design analysis. It is concluded that the effects on Ksub(I) of location, thermal stresses, and residual stresses are significant and generally too complex to evalute without advanced numerical procedures. The utilized combination of finite element analysis of the uncracked structure and three-dimensional influence function analysis of the cracked structure is demonstrated

Control of welding residual stress for ensuring integrity against fatigue and stress-corrosion cracking

International Nuclear Information System (INIS)

Mochizuki, Masahito

2007-01-01

The availability of several techniques for residual stress control is discussed in this paper. The effectiveness of these techniques in protecting from fatigue and stress-corrosion cracking is verified by numerical analysis and actual experiment. In-process control during welding for residual stress reduction is easier to apply than using post-weld treatment. As an example, control of the welding pass sequence for multi-pass welding is applied to cruciform joints and butt-joints with an X-shaped groove. However, residual stress improvement is confirmed for post-weld processes. Water jet peening is useful for obtaining a compressive residual stress on the surface, and the tolerance against both fatigue and stress-corrosion cracking is verified. Because cladding with a corrosion-resistant material is also effective for preventing stress-corrosion cracking from a metallurgical perspective, the residual stress at the interface of the base metal is carefully considered. The residual stress of the base metal near the clad edge is confirmed to be within the tolerance of crack generation. Controlling methods both during and after welding processes are found to be effective for ensuring the integrity of welded components

Corrosion and stress corrosion cracking in supercritical water

Science.gov (United States)

Was, G. S.; Ampornrat, P.; Gupta, G.; Teysseyre, S.; West, E. A.; Allen, T. R.; Sridharan, K.; Tan, L.; Chen, Y.; Ren, X.; Pister, C.

2007-09-01

Supercritical water (SCW) has attracted increasing attention since SCW boiler power plants were implemented to increase the efficiency of fossil-based power plants. The SCW reactor (SCWR) design has been selected as one of the Generation IV reactor concepts because of its higher thermal efficiency and plant simplification as compared to current light water reactors (LWRs). Reactor operating conditions call for a core coolant temperature between 280 °C and 620 °C at a pressure of 25 MPa and maximum expected neutron damage levels to any replaceable or permanent core component of 15 dpa (thermal reactor design) and 100 dpa (fast reactor design). Irradiation-induced changes in microstructure (swelling, radiation-induced segregation (RIS), hardening, phase stability) and mechanical properties (strength, thermal and irradiation-induced creep, fatigue) are also major concerns. Throughout the core, corrosion, stress corrosion cracking, and the effect of irradiation on these degradation modes are critical issues. This paper reviews the current understanding of the response of candidate materials for SCWR systems, focusing on the corrosion and stress corrosion cracking response, and highlights the design trade-offs associated with certain alloy systems. Ferritic-martensitic steels generally have the best resistance to stress corrosion cracking, but suffer from the worst oxidation. Austenitic stainless steels and Ni-base alloys have better oxidation resistance but are more susceptible to stress corrosion cracking. The promise of grain boundary engineering and surface modification in addressing corrosion and stress corrosion cracking performance is discussed.

A fracture- mechanics calculation of crack growth rate for a gas turbine blade

International Nuclear Information System (INIS)

Mirzaei, M.; Karimi, R.

2002-01-01

The existence of thermo-mechanical stresses, due to the frequent start-ups and shutdowns of gas turbines. Combined with high working temperatures may cause creep and fatigue failure of the blades. This paper describes a fracture-mechanics life assessment of a gas turbine blade. Initially, the distributions of thermal and mechanical stresses were obtained by using the finite element method. Accordingly; the crack modeling was performed in a high stress region at the suction side surface of the blade. Several crack growth increments were observed and the related crack tip parameters were calculated. Finally; the creep-fatigue crack growth in each cycle was calculated and the total number of start-stop cycles was determined

A small angle X-ray scattering method to investigate the crack tip in metals. Final report of the Marie Curie individual fellowship project

International Nuclear Information System (INIS)

Ouytsel, K. van; Boehmert, J.; Mueller, G.

2003-08-01

Structural materials, such as ferritic and austenitic steels or aluminium alloys used in the nuclear and aircraft industry, are subjected to external operational loads in different environments. Adopting a damage tolerant design principle, understanding the growth of preexisting or newly formed cracks under these conditions is of prime relevance to prevent extensive crack propagation and failure of the component. Within this framework, the characterization of early stages of the damage processes, as nucleation, growth and coalescence of micro-voids and the evolution of the spatial dislocation distribution (dislocation patterning) is a particularly challenging aspect. It was the objective of the work performed to investigate the damage structure near a crack tip by means of small angle X-ray scattering (SAXS). Pre-cracked fracture mechanics standard specimens from different aluminium alloys and steels were loaded up to different amounts of crack growth. From the crack tip range samples of 100 to 200 μm thickness were prepared and a small region around the crack tip was scanned using a microfocused Synchrotron beam. The SAXS experiments were performed at different Synchrotron sources and equipments with different beam cross section, scan step width and X-ray energy. Additionally, the investigation was completed by other methods like X-ray diffraction, X-ray imaging diffraction technique (MAXIM), transmission electron microscopy, scanning electron microscopy, and positron annihilation spectroscopy. The SAXS intensity pattern shows location-related effects. Potential SAXS parameters to characterize the damage are the integral intensity, a fractal dimension parameter and a value determined from the ratio of the intensity vertical and horizontal to the direction of crack growth. Above all, the last parameter is suitable to depict the damage zone around the crack tip. It is robust and applicable even for a material which exhibits an anisotropic SAXS pattern in the

Remote detection of stress corrosion cracking: Surface composition and crack detection

Science.gov (United States)

Lissenden, Cliff J.; Jovanovic, Igor; Motta, Arthur T.; Xiao, Xuan; Le Berre, Samuel; Fobar, David; Cho, Hwanjeong; Choi, Sungho

2018-04-01

Chloride induced stress corrosion cracking (SCC) of austenitic stainless steel is a potential issue in long term dry storage of spent nuclear fuel canisters. In order for SCC to occur there must be a corrosive environment, a susceptible material, and a driving force. Because it is likely that the material in the heat affected zone (HAZ) of welded stainless steel structures has been sensitized as a result of chromium depletion at the grain boundaries and a thermal residual stress driving force is likely present if solution annealing is not performed, two issues are critical. Is the environment corrosive, i.e., are chlorides present in solution on the surface? And then, are there cracks that could propagate? Remote detection of chlorides on the surface can be accomplished by laser induced breakdown spectroscopy (LIBS), while cracks can be detected by shear horizontal guided waves generated by electromagnetic acoustic transducers (EMATs). Both are noncontact methods that are amenable to robotic delivery systems and harsh environments. The sensitivity to chlorine on stainless steel of a LIBS system that employs optical fiber for pulse delivery is demonstrated. Likewise, the ability of the EMAT system to detect cracks of a prescribed size and orientation is shown. These results show the potential for remote detection of Cl and cracks in dry storage spent fuel canisters.

Effect of micromorphology at the fatigue crack tip on the crack growth in electron beam welded Ti-6Al-4V joint

International Nuclear Information System (INIS)

Tao, Junhui; Hu, Shubing; Ji, Longbo

2016-01-01

In this paper, we describe experiments on welded joints of Ti-6Al-4V alloy specimens exhibiting fatigue characteristics in the base metal (BM), hot affected zone (HAZ) and fuse zone (FZ). The effect of micromorphology on crack propagation at the tip of the fatigue crack in joints formed by electron beam welding was investigated using an optical microscope, transmission electron microscope and other methodologies. The results demonstrated that the fatigue crack originated in and propagated along α/β boundaries in the BM. In the HAZ, the fatigue crack occurred at the boundary between martensite laths, and propagated through most irregular-equiaxed α phases and a few martensite laths. In the FZ, the fatigue crack originated at the boundaries between the fine crushing phases among martensite laths, and propagated along a majority of α/β boundaries and several narrow martensite laths. The electron beam welded joint of Ti-6Al-4V alloy showed instances of zigzag fatigue cracks that increased in degree from lowest in the HAZ, moderate in the FZ to greatest in the BM. Conversely, fatigue crack growth rate (FCGR) was greatest in the HAZ, less in the FZ and slowest in the BM. - Highlights: •Ti-6Al-4V welded joint exhibits different fatigue characteristics. •The fatigue crack propagates along α/β boundaries in the BM. •The fatigue crack propagates through α phases and martensite laths in the HAZ. •The fatigue crack propagates along α/β boundaries and martensite laths in the FZ. •Fatigue crack growth rate is fastest in the HAZ, less in the FZ, slowest in the BM.

Transient thermal stress problem for a circumferentially cracked hollow cylinder

Science.gov (United States)

Nied, H. F.; Erdogan, F.

1982-01-01