Containment concrete specimens(4000, 5000psi) were tested under biaxial stress and presented basic physical properties and biaxial failure envelops for the concrete specimens. Failure behaviors of concrete under biaxial stress were assessed with stress-strain responses and failure modes. Here provided real test data to develop nonlinear finite element concrete models. (author). 15 refs., 46 figs., 4 tabs.

Biaxial failure criteria and stress-strain response for plain concrete of containment structure on nuclear power plants are studied under uniaxial and biaxial stress(compression-compression, compression-tension, and tension-tension combined stress). The concrete specimens of a square plate type are used for uniaxial and biaxial loading. The experimental data indicate that the strength of concrete under biaxial compression, f{sub 2}/f{sub 1}=-1/-1, is 17 percent larger than under uniaxial compression and the poisson's ratio of concrete is 0.1745. On the base of the results, a biaxial failure envelope for plain concrete that the uniaxial strength is 5660 psi are provided, and the biaxial failure behaviors for three biaxial loading areas are plotted respectively. And, various analytical equations having the reliability are proposed for representations of the biaxial failure criteria and stress-strain response curves of concrete.

Effects of biaxial stress in steel on magnetization in a direction normal to the stress plane were investigated both theoretically and experimentally. The two results, which agreed qualitatively, showed that the magnetization in the normal direction generally decreased with the absolute value of the sum of the two principal stresses. The implication to nondestructive measurements of biaxial stress is discussed.

Research was done on the biaxial stress problem accomplished in the first half of the second year. All of the work done was preparatory to magnetic measurements. Issues addressed were: construction of a model for extracting changes in the magnetic properties of a specimen from the readings of an indirect sensor; initial development of a model for how biaxial stress alters the intrinsic magnetic properties of thespecimen; use of finite element stress analysis modeling to determine a detailed shape for the cruciform biaxial stress specimen; and construction of the biaxial stress loading apparatus.

TE/TM polarization switching in ridge-waveguide InGaAsP lasers is analyzed with due consideration to biaxial stress effects on both the waveguiding and gain properties of the device. One establishes the conditions of switching for several models. In particular, the gain expression for uniaxial stress is extended to include biaxial stress, and the differences between the different models are presented.

A micromagnetic formulation has been developed for modeling the effect of biaxial stress on magnetoelastic processes in polycrystalline steels. In particular, the formulation employs the Schneider--Cannell--Watts model and involves substitution of an effective stress equal to one of the deviatoric (i.e., distortional) normal stress components, depending on whether the field is parallel to a tensile or compressive axis or to the third axis perpendicular to the plane of biaxial stress. Computer results are compared to experimental results on the effects of biaxial stress on magnetic properties in mild steel and in SAE-4130 steel. Good qualitative agreement is found in almost all cases, in that in going from one biaxial stress case to the next, the same kinds of changes are seen magnetically. It is also shown from the model and the data that a method can be formulated to nondestructively determine the difference in biaxial stresses.

Acoustic emission (AE) testing has been carried out with uniaxial and biaxial (2:1 stress ratio) stressing of smooth samples of 21-6-9 and 304 stainless steel (SS). Uniaxial testing was done with simple tensile and compression samples as well as with the special biaxial specimens. Biaxial tensile stressing was accomplished with a specially designed specimen, which had been used previously to characterize AE in 7075 aluminum under biaxial stressing. Results were obtained for air-melt and for vacuum-melt samples of 21-6-9 SS. The air-melt samples contain considerably more inclusion particles than the vacuum-melt samples. For the 304 SS, as received material was examined. To allow AE correlations with microstructure, extensive characterization of the 21-6-9 microstructure was carried out. Significant differences in AE occur in biaxially stressed specimens as compared to uniaxially stressed samples. 15 figures, 3 tables.

Using the George Washington University biaxial test system, a static fracture toughness study of two polymers (PMMA and PVC) and three aluminum alloys was performed for several variations in specimen geometry. Photoelastic experiments indicate that the applied load biaxiality has a very strong influence on the size and shape of the crack-tip stress field, and fracture toughness values for both polymers were seen to decrease with increasing load biaxiality. The load biaxiality was also found to have a strong influence on the crack growth direction in PMMA and a negligible influence on the PVC. The 7075-T6 aluminum toughness values increased with biaxiality, while intermediate peak toughness values were noted at a 0.5 biaxiality ratio for the more ductile 2024-T3 and 6061-T4 alloys. Fracture toughnesses at the highest biaxiality ratios were found to be equal to the uniaxial results.

The biaxial creep damage of type 304 stainless steel is studied at 973K. Biaxial tension creep tests were carried out using cruciform specimens under the principal stress ratio (l) of 0l1, where the principal stress ratio is the ratio of y-directional principal stress to x-directional stress in the gage part of the specimen (l=y/x). Creep rupture times under biaxial stress conditions were shorter than those in uniaxial conditions at the same von Mises equivalent stress. Earlier void nucleation and faster void growth were observed in creep tests at larger principal stress ratio tests. Creep rupture times in biaxial stress states were discussed in relation to the void observations.

The effect of temperature changes on the estimation of biaxial stresses in pipelines by the acoustic method is considered. A method of using the acoustoelastic effect for the estimation of the uniaxial and biaxial stressed states using the changes in the propagation times of longitudinal and shear elastic waves along the normal to the surface of a pipe before and after changes in stress is described. Uniaxial stresses or the difference between biaxial stresses may be estimated by measuring mutually perpendicularly polarized shear wave delays, whose relative difference proportional to the stress value is temperature-independent. The algorithms for estimating the biaxial stresses incorporate parameters that are sensitive to the difference between the temperature dependences of the velocities...

To design more effective tissue-engineered heart valve replacements, the replacement tissue may need to mimic the biaxial stress–strain behavior of native heart valve tissue. This study characterized the planar biaxial properties of tissue-engineered valve leaflets and native aortic valve leaflets. ...

To provide biaxial failure behavior characteristics of concrete of a standard Korean nuclear containment building, the concrete specimens with the dimensions of 200 mmx200 mmx60 mm were tested under different biaxial load combinations. The specimens were subjected to biaxial load combinations covering the three regions of compression-compression, compression-tension, nd tension-tension. To avoid a confining effect due to friction in the boundary surface between the concrete specimen and the loading platen, the loading platens with Teflon pads were used. The principal deformations in the specimens were recorded, and the failure modes along with each stress ratio were examined. Based on the strength data, the biaxial ultimate strength envelopes were developed and the biaxial stress-strain responses in three different biaxial loading regions were plotted. The test results indicated hat the concrete strength under equal biaxial compression, f{sub 1}=f{sub 2}, is higher by about 17% on the average than that under the uniaxial compression and the concrete strength under biaxial tension is almost independent of the stress ratio and is similar to that under the uniaxial tension.

Irradiation creep tests were performed in fast reactors using the stress states of uniaxial tension, biaxial tension, bending and torsion. In order to compare the saturated transient strain irradiation creep component, the test data were converted to equivalent strain and equivalent stress. The saturated transient irradiation creep component was observed to depend on the stress state. The highest value was exhibited by the uniaxial tension stress state, and the lowest by the torsion stress state. The biaxial tension and bending stress state transient component values were intermediate. This behavior appears to be related to the dislocation or microscopic substructure resulting from fabrication processing and the applied stress direction. (orig.) 9 refs.

Creep-fatigue damage in high temperature structural components in a FBR progress under multiaxial stress condition depending on their operating conditions and configuration. Therefore, multiaxial stress effects on creep-fatigue damage evolution must be clarified to make precise creep-fatigue damage evaluation of these components. In this study, creep-fatigue tests in FBR high temperature materials such as SUS304, 316FR stainless steels and a modified 9Cr steel were conducted under biaxial stress subjecting tension-compression and torsion loading, in order to examine biaxial stress effects on failure mechanism and life property, and to discuss creep-fatigue life evaluation methods under biaxial stress. Main results obtained in this study are summarized as follows: 1. The main cracks under cyclic torsion loading propagated by shear mode in three materials. But intergranular failure was occurred in SUS304 and 316FR, and transgranular failure was observed in Mod.9Cr steel. 2. Nonlinear damage accumulation model proposed based on uniaxial creep-fatigue test results was extended to apply for creep-fatigue damage evaluation under biaxial stress state by considering the biaxial stress effects on fatigue and creep damage evolution. 3. It was confirmed that creep-fatigue life under biaxial stress could be predicted by the extended evaluation method with higher accuracy than existing methods. (author).

The effect of heat-pressing and subsequent pre-cementation (acid-etching) and resin-cementation operative techniques on the development of transient and residual stresses in different thicknesses of a lithium disilicate glass-ceramic were characterised using profilometry prior to biaxial ...

A cruciform biaxial test specimen was designed and seven biaxial tensile tests were conducted on 2219-T87 aluminum alloy. An elastic-plastic finite element analysis was used to simulate each tests and predict the yield stresses. The elastic-plastic finite analysis accurately simulated the measured load-strain behavior for each test. The yield stresses predicted by the finite element analyses indicated that the yield behavior of the 2219-T87 aluminum alloy agrees with the von Mises yield criterion.

Materials which obey a maximum tensile stress failure criteria can fall at strains in biaxial tension which are significantly below the uniaxial failure strain. First, a failure strain envelope is developed which accounts for the degree of biaxiality and the work hardening of the material. A material test which would account for the various states of biaxiality is then proposed. By deforming a flat plate into a prescribed surface, different states of biaxial strain can be achieved. The development of such a biaxial test as a cost-effective and reliable means of screening certain materials is presented. The test is compatible with most drop weight test facilities and can be used to screen materials over a wide variety of temperature and strain rates.

The method to determine the biaxial strength in ball-on-3-ball test was studied. The biaxial strength of alumina specimen was measured by ball-on-3-ball test and piston-on-3-ball test. As the results of ANOVA(Analysis of Variance) for the biaxial strength by piston-on-3-ball test and the biaxial strength by ball-on-3-ball test which was calculated using the equation of the strength by piston-on-3-ball test and equivalent radius, the mean values of the both methods were almost same. Consequently, the biaxial strength by the ball-on-3-ball test can be calculated using the equation of the strength in piston-on-3-ball test and equivalent radius. The stress distribution of the specimen in ball-on-3-ball test was calculated by FEM(finite element method). 17 refs., 10 figs.

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws. Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow, surface flaws. The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a far-field, out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure-temperature loading of an RPV. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for an RPV material. The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness. Initial emphasis was placed on assessment of stress-based methodologies, namely, the J-Q formulation, the Dodds-Anderson toughness scaling model, and the Weibull approach. Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness; the conventional maximum principal stress criterion indicated no effect. A three-parameter Weibull model based on the hydrostatic stress criterion is shown to correlate with the experimentally observed biaxial effect on cleavage fracture toughness by providing a scaling mechanism between uniaxial and biaxial loading states. (orig.) 31 refs.

A series of tests for creep, stress relaxation, and biaxial ratchetting of type 304 stainless steel after cyclic preloading were carried out to investigate their interaction. The interesting fact was pointed out that back stress in cyclic plasticity played an important role to describe creep, relaxation, and biaxial ratchetting following cyclic preloading. Then, the test results showed that the material behavior due to creep after cyclic preloading could be represented by the modified Bailey-Norton law with stress levels evaluated from the current center of the yield surface, i.e., back stress which was determined by the hybrid constitutive model for cyclic plasticity proposed by the authors. In addition, biaxial ratchetting of axial strain induced by cyclic shear straining after cyclic preloading was expressed by the shear stress amplitude, the number of cycle and the axial stress level from the current center.

Transverse creep equations have been developed, from uniaxial creep tests at essentially constant temperature, to predict the ballooning of Zr-2.5 wt % Nb pressure tubes during a hypothetical loss-of-coolant accident in a CANDU reactor. Numerous biaxial creep tests were done in which the temperature of internally pressurized sections of pressure tube was ramped to check the ability of these equations to predict the transverse creep strain under biaxial stress conditions with varying temperature. As the biaxial tests could not cover the complete range of temperature ramp rates that might occur during a loss-of-coolant accident, uniaxial creep tests were also done with ramp rates from 1/sup 0/C/s to 50/sup 0/C/s and transverse stresses from 5 MPa to 130 MPa. The equations were successful in predicting the transverse creep strains observed in all the biaxial and uniaxial creep tests.

Casein was treated with triethanolamine and made into films. The mechanical properties of the alkali treated casein films were studied under uniaxial and biaxial stress conditions. A reduction in longitudinal stress and elongation at break was observed with the simultaneous application of lateral stress.   

In this paper the biaxial low cycle fatigue behavior under proportional loading of a recently developed metastable austenitic stainless cast steel is presented. Total strain controlled tests were carried out on a 250 kN biaxial servohydraulic tension-compression testing machine equipped with a biaxial orthogonal extensometer to measure the principal strains in the gauge area of the used cruciform specimens. The principal stresses were determined based on the compliance after the load reversals. The low cycle fatigue behavior under biaxial synchronous loading is compared to the uniaxial behavior. Therefore, biaxial single step tests and a biaxial multiple step load increase test were carried out. The dependence of the stress state on the cyclic deformation curves, cyclic stress-strain curves and the formation of martensite are described. Finally, the fatigue life relationship according to Basquin and Manson-Coffin was determined and compared to the Smith, Watson and Topper damage parameter, which provides a satisfactory fatigue life prediction. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

This paper describes the development of multiaxial creep testing machine using cruciform specimens in a wide range of biaxial stress conditions of 0 {le} {lambda} {le} 1 and the multiaxial creep rupture tests at 923 K, where {lambda} denotes the ratio of the minimum principal stress to the maximum principal stress. The developed test machine has a loading capacity of 98 KN and the maximum temperature is 923 K. Three dimensional finite element (FE) creep analyses were made to determine the shape and dimensions of the cruciform specimen having uniform stress distribution along the gage length. The specimen shape determined by the FE analyses has a 32 mm {times} 32 mm parallel part with 5 mm thickness. In the biaxial Mises` stress constant creep tests using type 304 stainless steel cruciform specimens, creep rupture time increased with increasing {lambda}, so that the biaxial stress has an effect on rupture time. The rupture data did not depend on {lambda} when correlated with the equivalent stress based on crack opening displacement, but they depended on {lambda} with Huddleston`s stress. Huddleston`s stress gave more conservative equi-biaxial tension creep rupture time than the Mises` stress. The Mises` equivalent strain rate decreased with increasing {lambda} in multiaxial creep tests. 9 refs., 16 figs., 2 tabs.

Cruciform beam fracture mechanics specimensl have been developed in the Heavy Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL) to introduce a prototypic, far- field, out-of-plane biaxird bending stress component in the test section that approximates the nonlinear biaxial stresses resulting from pressurized-thernxd-shock or pressure-temperature loading of a nuclear reactor pressure vessel (RPV). Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shtdlow, surface flaws. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. Two and three- parameter Weibull models have been calibrated using a new scheme (developed at the University of Illinois) that maps toughness data from test specimens with distinctly different levels of crack-tip constraint to a small scale yielding (SSY) Weibull stress space. These models, using the new hydrostatic stress criterion in place of the more commonly used maximum principal stress in the kernel of the OW integral definition, have been shown to correlate the experimentally observed biaxiaI effect in cruciform specimens, thereby providing a scaling mechanism between uniaxial and biaxial loading states.

A diaphragm-type film specimen was used to study in vitro degradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading. This geometry allowed both uniaxial and biaxial loading in a single experiment. During testing, the film was exposed to a H(2)O(2)/CoCl(2) solution that simulated in vivo oxidation of PEUU. The combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation. The effects of biaxial strain magnitude and strain rate were examined separately by increasing the frequency of fatigue loading from 0 to 1 Hz with constant maximum biaxial strain and by changing the maximum biaxial strain while maintaining constant strain rate. In the ranges of biaxial strain energy (0.17 to 0.55 MPa) and strain rate (0 to 46% s(-1)) tested, the rate of degradation increased with increasing strain rate whereas strain magnitude had essentially no effect on degradation rate. Although loading conditions affected the rate of oxidative degradation, ATR-FTIR analysis suggested that in all cases the mechanism of degradation did not change. Chemical degradation produced a brittle crosslinked surface layer marked by dimpling and pitting, as observed with scanning electron microscopy. Pits served as stress concentrators and initiated environmental stress cracks under dynamic loading but not under static (creep) loading. Small pits were sufficient to initiate cracks at higher strain rates whereas only large pits initiated cracks at lower strain rates. Consequently, a higher strain rate produced more profuse cracking. PMID:12918028

Published yield and ultimate biaxial stress and strain data for two grades of beryllium are correlated with a more complete method of characterizing macroscopic strain at fracture initiation in ductile materials. Results are compared with those obtained from an exponential, mean stress dependent, model. Simple statistical methods are employed to illustrate the degree of correlation for each method with the experimental data.

The biaxial stress rupture behavior for two of the alloys, Type 310 stainless steel and Haynes 188, is shown in figures. The other two alloys show similar behavior. The rupture parameter, P, is an empirical quantity which reflects the simultaneous effects of both temperature and duration of applied stress on stress rupture. Based on these results, several trends are apparent: (1) the biaxial stress rupture tests show the same trends and approximately the same stress rupture values as uniaxial data obtained from the literature for each alloy tested; (2) only for the Haynes 188 may the stress rupture strength/life in CGA have been significantly less than in air. But further testing has indicated there is probably no reduction of biaxial stress rupture strength/life in CGA even for this alloy; (3) the biaxial strain at rupture was small, typically only a few percent. It is appropriate to mention that in the uniaxial stress rupture testing at SwRI, exposure to CGA generally resulted in a shorter rupture life than with testing in air. No explanation is yet available for the observed difference in behavior between SwRI and INEL test specimens.

This paper describes the creep-fatigue damage evaluation of SUS 304 stainless steel in wide ranges of multiaxial stress and strain states. Low cycle fatigue and creep-fatigue tests were carried out using a cruciform specimen in a full range of strain biaxiality including equi-biaxial tension/compression at 823K and 873K. Mises` equivalent strain, maximum principal strain, the equivalent strain based on crack opening displacement (COD) and {Gamma}{sup *}-parameter were applied to the experimental low cycle fatigue data. The latter two strain parameters gave a better correlation than the former two strain parameters. The discussion on the correlation of the biaxial creep rupture data was briefly made and the equivalent stress based on COD was the most suitable stress parameter correlating the biaxial creep rupture data. Creep-fatigue damage was evaluated based on the equivalent strain and stress based on COD. The creep-fatigue damage evaluated fell around a line of unity in value, which indicates that the linear damage rule holds in the biaxial creep-fatigue. (author)

The elastic constants are approximately determined using a multi-step linear approximation method for more accurate material nonlinear analysis of coated fabric membrane structures. The approximate determination is performed within each quadrilateral or triangular surface defined on the surfaces of the biaxial elongation property formed with the data of stress-strain curves measured by biaxial extension testings. Two types of 40 × 40 cm square flat membrane model of a PTFE coated plain-weave glass fiber fabric are analyzed applying finite element method adapting a 20-steps linear approximation method. One of the models is an inplane deformation model subjected to biaxial extension under the condition of stress ratio of 1:1. The stress-strain response in the central region of the membrane is analyzed. And the other model is a membrane deformed by the application of air pressure. The relation between the central deflection and the pressure is analyzed. The results of analysis showed excellent coincidences with experimental results.   

A technique and apparatus for estimating in situ stresses by measuring stress-induced velocity anisotropy around a borehole. Two sets each of radially and tangentially polarized transducers are placed inside the hole with displacement directions either parallel or perpendicular to the principal stress directions. With this configuration, relative travel times are measured by both a pulsed phase-locked loop technique and a cross correlation of digitized waveforms. The biaxial velocity data is used to back-calculate the applied stress.

Fatigue degradation and life prediction for a plain woven glass fabric reinforced polyester under tension/torsion biaxial loadings were investigated. Typical S-N diagrams were given at several biaxial ratios when the biaxial cyclic loads were proportionally applied to the specimens. A fatigue damage accumulation model based on the continuum damage mechanics theory was developed, where modulus decay ratios in tension and shear were used as indicators for damage variables (D). In the model, the damage variables are considered to be second-order tensors. Then, the maximum principal damage variable, D* is introduced. According to the similarity to the principal stress, D* is obtained as the maximum eigen value of damage tensor [D{prime}]. Under proportional tension/torsion loadings, fatigue lives were satisfactorily predicted at any biaxial stress ratios using the present model in which the fatigue characteristics only under uniaxial tension and pure torsion loadings were needed. For a certain biaxial stress ratio, the effect of loading path on the fatigue strength was examined. The experimental result does not show a strong effect of loading path on the fatigue life.

The classical criteria for yield and fracture strengths of materials in biaxial stutes of stress must be applied selectively. The proper selection depends upon the particular metallurgical mcde of failure involved, which, even for a given material, depends upon the particular biaxial stute of stress as well as other factors. In the case of beryllium, added complications can arise from directionalities that are the result of preferred orientution of the grain lattice structure. The above considerations are discussed and mcdificd criteria for yield and fracture are intrcduced to account for preferred orientutions. Recommended testing procedures are presented along with a brief bibliography. (auth)

We deposit zirconium nitride films on silicon (100) substrates using direct current reactive magnetron sputtering, and investigate the effects of the substrate bias voltage on the preferred orientation, phase transition and hardness for the obtained films via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscope (HRTEM) and nanoindentation measurement. It is our finding that the preferred orientation for the films changes significantly with increasing the substrate bias voltage, which can be attributed to an increase in the biaxial compressive stress. With a further increase the substrate bias voltage, leading to a high biaxial stress, a phase transition from substoichiometric to overstoichiomet...

Biaxial creep-fatigue test data for Type 316 stainless steel tubes at 1100/sup 0/F for use in high-temperature components in solar central receivers are presented. The specimens were subjected to constant internal pressure and fluctuating axial strain with and without hold times in tension as well as compression. The results show that internal pressure significantly affects diametral ratchetting and axial stress range. Axial tensile hold is found to be more damaging than axial compressive hold even under a biaxial state of stress.

Despite continued progress in the treatment of aortic valve (AV) disease, current treatments continue to be challenged to consistently restore AV function for extended durations. Improved approaches for AV repair and replacement rests upon our ability to more fully comprehend and simulate AV function. While the elastic behavior the AV leaflet (AVL) has been previously investigated, time-dependent behaviors under physiological biaxial loading states have yet to be quantified. In the current study, we performed strain rate, creep, and stress-relaxation experiments using porcine AVL under planar biaxial stretch and loaded to physiological levels (60N/m equi-biaxial tension), with strain rates ranging from quasi-static to physiologic. The resulting stress-strain responses were found to be inde...

The high-temperature rupture behavior of the 5083-Al alloy was tested to failure at 548?K under multiaxial stress states of uniaxial tension using smooth bar specimens, biaxial shearing using double shear bar specimens, and triaxial tension using notched bar specimens. Rupture times were compared for uniaxial, biaxial, and triaxial stress states with respect to the maximum principal stress, the von-Mises effective stress, and the principal facet stress. The results indicate that the von Mises effective and principal facet stresses show a good correlation for the investigated material. The success with two parameters implies that the creep rupture of the 5083-Al alloy is dominated by grain boundary cavitation that is constrained by the creep deformation of the surroundings. The experimental...

The maximum principal stresses, von Mises effective stresses and principal facet stresses at the time of creep rupture were compared in uniaxial, biaxial, and triaxial stress states for AZ31 magnesium alloy. The creep rupture of this alloy was experimentally controlled by cavitation, which was the result of a low damage tolerance, ?. Creep deformation could be correlated with the von Mises effective stress parameter. The failure-mechanism control parameter governing the stress state coincided with the experimental results of the rupture of the materials under multiaxial stress states. Finally, the theoretical prediction based on constrained cavity growth and continuous nucleation agreed with the experimental rupture data to within a factor of three.

Defining the stability criteria for biaxially strained Ni, we show that the body-centred tetragonal (bct) Ni structure is not stable on an Au(001) substrate and changes to a face centred cubic (fcc) (110) structure with many stacking faults. Nevertheless, for a thin film, the bct Ni structure can be stabilized by the interface stresses. Using the stress at atomic level, the profile of the internal stresses is given as a function of Ni film thickness.

Biaxial creep specimens will be included in the HFIR RB-12J experiment to study in-reactor creep of the V-4Cr-4Ti alloy at {approx}500{degrees}C and 5 dpa. These specimens were fabricated with the 500-kg, heat (832665) material and pressurized to attain 0, 50, 100, 150, and 200 MPa mid-wall hoop stresses during the irradiation.

Abstract A series of biaxial High Cycle Fatigue tests at room temperature is performed to build up an extensive and well-documented database. The testing specimen is a maltese cross thinned in its centre with non homogeneous strain/stress fields. The experimental protocol uses exclusively fu...

A method is developed for analyzing in a unified manner both uniaxial and uniform biaxial strain data obtained from nearly isotropic tissues. The formulation is a direct application of nonlinear elasticity theory pertaining to large deformations. The general relation between Eulerian stress (?) and ...

filled with plugs of the ablator and sealed with an elastomeric silicone rubber compound. Reference ..... always the short dimension of a panel with an aspect ratio of 1.5. Table 10 shows .... Poisson effects that exist in a biaxial stress field. ..... RTV 560. Faces bonded to core using. HT- 424. Bonded to panel face with. HT-424 ...

One seamless and one welded and reworked biaxial creep test capsules of T-111 alloy tubing were tested at 2200 exp 0 F for 5000 hours each in a high-vacuum environment. The sealed capsules were half filled with liquid potassium. The stress in the thin wal...

In this paper, we present a new approach for the bi-axial characterization of in vitro human arteries and we prove its feasibility on an example. The specificity of the approach is that it can handle heterogeneous strain and stress distributions in arterial segments. From the full-field experimental...

We study the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, the emission blueshifts and the binding energies of the positive trion (X+) and biexciton (XX) relative to the neutral exc...

Biaxial fatigue is often encountered in the complex thermo-mechanical loadings present in gas turbine engines. Engine strain histories can involve non-constant temperature, mean stress, creep, environmental effects, both isotropic and anisotropic materials and non-proportional loading. Life prediction for the general case involving all the above factors is not a practicable research project. The current research program is limited to isothermal fatigue at room temperature and 1200 F of Hastalloy-X for both proportional and non-proportional loading. An improved method for predicting the fatigue life and deformation response under biaxial cycle loading is sought.

Planar solid oxide fuel cells are a leading candidate for future power generation. Integral to their utility is the robustness of the component materials and their ability to tolerate the mechanical stresses expected during the lifetime, including fabrication, of the cell. Most current planar designs rely on either the anode or cathode as the structural-supporting member. To gain a fundamental basis of data on which to compare future cell designs, a series of half-cell (electrolyte/anode) specimens was fabricated in which processes variables and additives typical of current cells were varied. These samples were tested in unreduced state in a biaxial flexure apparatus to determine the biaxial flexure strength. The samples were oriented with the electrolyte on the tensile surface. Results were compared to a concurrent study in which similar specimens were tested by a uniaxial tensile method with statistical distribution parameters used to adjust for volumetric differences to validate the biaxial flexure data.

By thermally cycling through their transformation temperature range, coarse-grained, polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism is investigated under biaxial stress conditions during thermal cycling of unalloyed titanium, Ti-6Al-4V, and their composites (Ti/10 vol.% TiC{sub p}, Ti-6Al-4V/10 vol% TiC{sub p} and Ti-6Al-4V/5 vol.% TiB{sub w}). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify the operation of this deformation mechanism in all experimental materials, and compare the biaxial results to uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with experimentally measured profiles.

In Part I, a novel hypoelastic framework for soft-tissues was presented. One of the hallmarks of this new theory is that the well-known exponential behavior of soft-tissues arises consistently and spontaneously from the integration of a rate based formulation. In Part II, we examine the application of this framework to the problem of biaxial kinematics, which are common in experimental soft-tissue characterization. We confine our attention to an isotropic formulation in order to highlight the distinction between non-linearity and anisotropy. In order to provide a sound foundation for the membrane extension of our earlier hypoelastic framework, the kinematics and kinetics of in-plane biaxial extension are revisited, and some enhancements are provided. Specifically, the conventional stress-to-traction mapping for this boundary value problem is shown to violate the conservation of angular momentum. In response, we provide a corrected mapping. In addition, a novel means for applying loads to in-plane biaxial experiments is proposed. An isotropic, isochoric, hypoelastic, constitutive model is applied to an in-plane biaxial experiment done on glutaraldehyde treated bovine pericardium. The experiment is comprised of eight protocols that radially probe the biaxial plane. Considering its simplicity (two adjustable parameters) the model does a reasonably good job of describing the non-linear normal responses observed in these experimental data, which are more prevalent than are the anisotropic responses exhibited by this tissue. PMID:21394222

Silicon oxynitride films were deposited by plasma-enhanced chemical-vapor deposition. The elemental composition was varied between silicon nitride and silicon dioxide: SiO(0.3)N(1.0), SiO(0.7)N(1.6), SiO(0.7)N(1.1), and SiO(1.7)N(0.%). These films were annealed in air, at temperatures of 40-240 C above the deposition temperature (260 C), to determine the stability and behavior or each composition. the biaxial modulus, biaxial intrinsic stress, and elemental composition were measured at discrete intervals within the annealing cycle. Films deposited from primarily ammonia possessed considerable hydrogen (up to 38 at.%) and lost nitrogen and hydrogen at anneal temperatures (260-300 C) only marginally higher than the deposition temperature. As the initial oxygen content increased a different mechanism controlled the behavior or the film: The temperature threshold for change rose to approximately equal to 350 C and the loss of nitrogen was compensated by an equivalent rise in the oxygen content. The transformation from silicon oxynitride to silica was completed after 50 h at 400 C. The initial biaxial modulus of all compositions was 21-3- GPa and the intrinsic stress was -30 to 85 MPa. Increasing the oxygen content raised the temperature threshold where cracking first occurred; the two film compositions with the highest initial oxygen content did not crack, even at the highest temperature (450 C) investigated. At 450 C the biaxial modulus increased to approximately equal to 100 GPa and the intrinsic stress was approximately equal to 200 MPa. These increases could be correlated with the observed change in the film's composition. When nitrogen was replaced by oxygen, the induced stress remained lower than the biaxial strength of the material, but, when nitrogen and hydrogen were lost, stress-relieving microcracking occurred.

Development continues on the technology used to assess the safety of irradiation-embrittled nuclear reactor pressure vessels (RPVs) containing flaws. Fracture mechanics tests on RPV steel, coupled with detailed elastic-plastic finite-element analyses of the crack-tip stress fields, have shown that (1) constraint relaxation at the crack tip of shallow surface flaws results in increased data scatter but no increase in the lower-bound fracture toughness, (2) the nil ductility temperature (NDT) performs better than the reference temperature for nil ductility transition (RT{sub NDT}) as a normalizing parameter for shallow-flaw fracture toughness data, (3) biaxial loading can reduce the shallow-flaw fracture toughness, (4) stress-based dual-parameter fracture toughness correlations cannot predict the effect of biaxial loading on shallow-flaw fracture toughness because in-plane stresses at the crack tip are not influenced by biaxial loading, and (5) an implicit strain-based dual-parameter fracture toughness correlation can predict the effect of biaxial loading on shallow-flaw fracture toughness. Experimental irradiation investigations have shown that (1) the irradiation-induced shift in Charpy V-notch vs temperature behavior may not be adequate to conservatively assess fracture toughness shifts due to embrittlement, and (2) the wide global variations of initial chemistry and fracture properties of a nominally uniform material within a pressure vessel may confound accurate integrity assessments that require baseline properties.

Shallow-flaw fracture technology is being developed for application to the safety assessment of radiation-embrittled nuclear reactor pressure vessels (RPVS) containing flaws. Fracture mechanics tests on RPV steel, coupled with detailed elastic-plastic finite-element analyses of the crack-tip stress fields, have shown that (1) constraint relaxation at the crack tip of shallow surface flaws results in increased data scatter but no increase in the lower-bound fracture toughness, (2) the nil ductility temperature (NDT) performs better than the reference temperature for nil ductility transition (RT{sub NDT}) as a normalizing parameter for shallow-flaw fracture toughness data, (3) biaxial loading can reduce the shallow-flaw fracture toughness, (4) stress-based dual-parameter fracture toughness correlations cannot predict the effect of biaxial loading on shallow-flaw fracture toughness because in-plane stresses at the crack tip are not influenced by biaxial loading, and (5) a strain-based dual-parameter fracture toughness correlation can predict the effect of biaxial loading on shallow-flaw fracture toughness.

A finite structure of Ge under tensile stress was investigated theoretically and experimentally focusing on applications to near-infrared photodetectors on (001) Si. We calculated the direct band gap energy of strained Ge between the conduction band and the heavy/light-hole valence band via the k·p theory. Three types of in-plane stresses were considered, i.e., a biaxial stress and uniaxial stresses along the ‹100› and ‹110› directions. On the basis of the direct band gap change, absorption spectra due to the direct transitions were calculated. The calculated absorption spectra showed that the biaxial stress is more effective than the uniaxial stresses in terms of the absorption red-shift, which increases the detection wavelength range. Localized strain measurements revealed that a selectively grown Ge mesa on (001) Si maintains a biaxial strain caused by the thermal expansion mismatch when its width is larger than 1 ?m. A uniaxial stress probably develops owing to the strain relaxation in a finite Ge structure smaller than 1 ?m. The application of Ge finite structures to waveguide photodetectors is discussed.   

Sheet specimens of aluminum alloy 5754 were deformed along a series of bi-linear, equal-biaxial and uniaxial, strain paths while simultaneously measuring stress-strain behavior. Using the measured crystallographic texture before and after deformation, the VPSC model that incorporates texture evolution was used to simulate the flow stress and hardening behavior. Including latent hardening of multiple slip planes allowed the model to explain the decrease in flow stress when changing from equal-biaxial to uniaxial deformation. However, the model did not capture the details of the drop in flow stress nor the magnitude of the plastic hardening after the change in deformation mode. This is likely due to room temperature recovery between the two steps of testing.

A dependence of elastic response on the stress-state of a thin film has been demonstrated using the interfacial force microscope (IFM). Indentation response was measured as a function of the applied biaxial stress-state for 100 nm thick Au films. An increase in measured elastic modulus with applied compressive stress, and a decrease with applied tensile stress was observed. Measurements of elastic modulus before and after applying stress were identical indicating that the observed change in response is not due to a permanent change in film properties.

High-density polyethylene is subjected to biaxial states of stress to examine the yield behavior of the semicrystalline thermoplastic under constant octahedral shear-stress rates. Combinations of internal pressures and axial loads are applied to thin-walled tubes of polyethylene, and the strain response in the axial and hoop directions are measured. The polyethylene specimens are found to be anisotropic, and the experimental measurements are compared to yield criteria that are applicable to isotropic and anisotropic materials.

Martensite reorientation via twin boundary motion in NiMnGa single crystals was experimentally studied under biaxial loadings. The threshold driving force (related to the energy dissipation) of the twin boundary motion, and the transformation strain due to martensite reorientation are found to be constant in all tested 2D stress states. These findings imply that the materials can work at high levels of multi-axial stresses while keeping their advantages — low intrinsic dissipation and large reversible strain.

Investigating material behavior under complex stress states is often done using in-plane biaxial loading approach. Utilizing such techniques requires using cruciform type specimens fabricated from plate material tested by gripping the specimen at four locations and loaded along two orthogonal axes. Servohydraulic systems are generally used in this application which is similar to those used for uniaxial testing. These kind of testing capabilities are currently being conducted at NASA Glenn Research Center via a new in-house testing facility. This is in support of the development of major components for the Stirling Radioisotope Generator (SRG). It is also used to assist in the generation of an analytical life prediction methodology [1] and to experimentally verify the flight-design component's life. Further, this work is intended to carry the immediate goal of developing a specimen design that is fully compatible with the in-plane biaxial testing systems installed at NASA Glenn Research Center [2]. Thus, details of the specimen design and its applicability to the ongoing experimental activities are being reported and discussed. Finite element analyses were carried out to optimize the geometry of specimen and to evaluate the stress response under biaxial loading conditions [3, 4]. The material of interest used in this research is nickel based superalloy. The data presented concluded that the specimen can be used to investigate the deformation behavior under general forms of biaxial loading. The provided measurement and observation are limited to 1-in [2.54 cm] diameter circular region at the specimen center.

The typical parallel fractures of spalling or slabbing is often encountered in deep underground excavation. In recent years, another type of parallel fractures in large scale called zonal disintegration is captured by borehole camera in deep surrounding rock mass. The mechanism of these two types of failure and their correlations are discussed through 3D numerical testing. Results show that the biaxial stress parallel to the free surface confined by zero or low confinement stress contributes to the parallel fracturing of spalling or slabbing. Zonal disintegration can be motivated from spalling or slabbing failure to deeper area under high multi-axial stress. Parallel fracturing can be taken as an inherent character of rock like material of heterogeneity under high biaxial or multi-axial st...

In optically birefringent uniaxial and biaxial crystals, analyzed by plane polariscope, isochromate interference fringes can be observed. By means of the classical electromagnetic theory a Cassini-like analytical equation of the isochromate fringes, depending on the refraction indexes, has been obtained. The proposed analytical equation is a useful tool to evaluate the internal stress state, as it is related to the isochromate shapes owing to the induced variation of the refraction indexes. Uniaxial crystals can assume complex biaxial behaviour due to particular stress configurations. PbWO4 (PWO) uniaxial scintillating crystals have been studied. The Cassini-like curves fit well experimental measurements in the case of uniaxial stress. In this research work, a simple model has been proved ...

Biaxial tensile tests of austenitic stainless steel sheet (SUS304) 0.2mm thick have been carried out using cruciform specimens. The specimens are loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. The successive contours of plastic work in biaxial stress space changed their shapes progressively, exemplifying differential work hardening. The geometry of the entire family of the work contours and the directions of plastic strain rates have been precisely measured and compared with those calculated using conventional yield functions. Yld2000-2d [Barlat, F., Brem, J.C., Yoon, J.W., Chung, K., Dick, R.E., Lege, D.J., Pourboghrat, F., Choi, S.H. and Chu, E., International Journal of Plasticity, Vol. 19, (2003), pp. 1297-1319.] with an exponent of 6 was capable of reproducing the general trends of the work contours and the directions of plastic strain rates with good accuracy. Furthermore, in order to quantitatively evaluate the Bauschinger effect of the test material, in-plane tension/compression tests are conducted. It was found that the non-dimensional (? /?u) - ?? /(?u/ E) curves measured during unloading almost fall on a single curve and are not affected by the amount of pre-strain, where ? is the current stress during unloading, ?u is the stress immediately before unloading, ?? (< 0) is the total strain increment during unloading.   

Biaxial tensile tests of austenitic stainless steel sheet (SUS304) 0.2mm thick have been carried out using cruciform specimens. The specimens are loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. The successive contours of plastic work in biaxial stress space changed their shapes progressively, exemplifying differential work hardening. The geometry of the entire family of the work contours and the directions of plastic strain rates have been precisely measured and compared with those calculated using conventional yield functions. Yld2000-2d [Barlat, F., Brem, J.C., Yoon, J.W., Chung, K., Dick, R.E., Lege, D.J., Pourboghrat, F., Choi, S.H. and Chu, E., International Journal of Plasticity, Vol. 19, (2003), pp. 1297-1319.] with an exponent of 6 was capable of reproducing the general trends of the work contours and the directions of plastic strain rates with good accuracy. Furthermore, in order to quantitatively evaluate the Bauschinger effect of the test material, in-plane tension/compression tests are conducted. It was found that the non-dimensional (? /?u) - ?? /(?u/ E) curves measured during unloading almost fall on a single curve and are not affected by the amount of pre-strain, where ? is the current stress during unloading, ?u is the stress immediately before unloading, ?? (< 0) is the total strain increment during unloading.

In this paper, we studied the viscoelastic behaviors of isolated aortic elastin using combined modeling and experimental approaches. Biaxial stress relaxation and creep experiments were performed to study the time-dependent behavior of elastin. Experimental results reveal that stress relaxation preconditioning is necessary in order to obtain repeatable stress relaxation responses. Elastin exhibits less stress relaxation than intact or decellularized aorta. The rate of stress relaxation of intact and decellularized aorta is linearly dependent on the initial stress levels. The rate of stress relaxation for elastin increases linearly at stress levels below about 60 kPa; however, the rate changes very slightly at higher initial stress levels. Experimental results also show that creep response ...

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.

A method utilizing expansion of a diaphragm-type film specimen was developed to study in vitro biodegradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading (fatigue). A finite element model was used to describe the strain state, which ranged from uniaxial at the edges of the film to balanced biaxial tensile strain at the center. During testing, the film was exposed to a H(2)O(2)/CoCl(2) solution, which simulated in vivo oxidative biodegradation of PEUU. The extent of chemical degradation was determined by infrared analysis. Physical damage of the film surface was characterized by optical microscopy and scanning electron microscopy. Dynamic loading did not affect the rate of degradation relative to unstressed and constant stress (creep) controls in regions of the film that experienced primarily uniaxial fatigue; however, degradation was accelerated in regions that experienced balanced biaxial or almost balanced biaxial fatigue. It was concluded that the combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation in this system. Chemical degradation produced a brittle surface layer that was marked by numerous pits and dimples. Physical damage of the surface in the form of cracking occurred only in fatigue experiments. Cracking was not observed in unstressed or creep tests. Cracks initiated at the dimples produced by chemical degradation, and propagated in a direction that was determined by the strain state. PMID:12761843

In this paper the J-Q two-parameter characterization of elastic-plastic crack front fields is examined for surface cracked plates under static uniaxial and biaxial bending. Extensive three-dimensional elastic-plastic finite element analyses are performed for semi-elliptical surface cracks in a finite thickness plate, under remote uniaxial and biaxial bending conditions, covering from small-scale to large-scale yielding. Surface cracks with aspect ratios a/c=0.2, 1.0 and relative depths a/t=0.2, 0.6, corresponding to four specimen configurations, are investigated. The full stress field solutions are obtained in topological planes perpendicular to the crack fronts. The question of J-Q characterization is addressed by comparing these elastic-plastic crack-front stress fields with J-Q family o...

The growth of dominant crack is sometimes affected by distributed small cracks in multiaxial low cycle fatigue. Considering such a situation, crack initiation in biaxial fatigue was first modeled in this work. In modeled grains, normal directions of slip planes and slip directions on respective planes were independently given at random. A slip-band crack was presumed to be initiated along the given slip direction on the specified slip plane when the resolved shear stress in the slip direction exceeded a critical shear stress. The crack initiation life was also evaluated using a dislocation pile-up model. Simulations on crack initiation were conducted under the same loading conditions as experiments. Simulated directional distribution of initiated cracks was compared with experimental results obtained in biaxial fatigue tests using tubular specimens of pure copper, and it showed a good agreement with the experimental observation.   

One seamless and one welded and reworked biaxial creep test capsules of T-111 alloy tubing were tested at 2200/sup 0/F for 5000 hours each in a high-vacuum environment. The sealed capsules were half filled with liquid potassium. The stress in the thin wall gauge sections was generated by the potassium vapor pressure generated within the refluxing capsule. Diametral measurements were made in the thin wall gauge sections during the uninterrupted test and were subsequently converted to equivalent uniaxial creep values. The creep data obtained was then compared with the results of a standard uniaxial creep test conducted on the same material at the same temperature, time and stress level. The agreement between the biaxial creep results and the uniaxial creep results in the 1 to 2% creep range was found to be good and the creep results were also found to agree with previously reported uniaxial creep data for T-111 alloy.

In this article, we report biaxial and hydrostatic stress in Ga(++) implanted Gallium Nitride epitaxial layers estimated via Raman measurements. We have calculated deformation potential constants for E2(high) and A1(TO) modes in these epi-layers. With increase in implantation fluence a new peak appears at 290 cm(-1), which reflects the formation of the cubic phase in this system. The behavior of polar Raman modes has also been presented.

Zircaloy is commonly used as a cladding material for nuclear fuel elements. The cladding is subject to time-varying multiaxial stresses in service and the ability to accurately predict cladding behavior is necessary to maintain fuel integrity. This work investigates the biaxial creep behavior of recrystallized zircaloy at three temperatures and with four different crystallographic textures. In addition to measuring the creep behavior, the crystallographic texture is used to independently predict the creep behavior. 48 refs.

An analytical method for the solution of two-dimensional nonlinear creep problems is developed using as an example the biaxial extension of a plane from a stochastically inhomogeneous material with damage accumulation and the third stage of creep taken into account. The governing creep relation is adopted in accordance with the energetic version of the nonlinear theory of viscous flow. The stochasticity of the material is defined by two random functions of coordinates. Formulas for calculating the stress variance are obtained.

The problems commonly encountered in the numerical analysis of reinforced structures are often related to biaxial stress states in the structure. In this study, this problem is solved with the formulation of a composite plasticity model, which describes both cracking and crushing of concrete within the framework of plasticity theory. The other issue which is treated in this study is the rational modeling of the interaction between concrete and reinforcement.

This paper presents a simple and effective model for the behavior and capacity of circular concrete-filled tube (CFT) short columns under extreme loading conditions. Firstly, efficient methods are presented to predict the complete stress–strain curve of concrete and steel, which are subjected to triaxial and biaxial stresses, respectively. Empirical expressions are proposed for the confinement effect, which increases both the ductility and strength of concrete but decreases the yield stress of steel. Subsequently, the fiber model is adopted to describe the complete behavior of the CFT columns under axial force and bending moment. The accuracy and the applicability of the proposed method are examined in a companion paper.

High-temperature rupture behavior of 5083-A1 alloy was tested for failure at 548K under multiaxial stress conditions: uniaxial tension using smooth bar specimens, biaxial shearing using double shear bar specimens, and triaxial tension using notched bar specimens. Rupture times were compared for uniaxial, biaxial, and triaxial stress conditions with respect to the maximum principal stress, the von Mises effective stress, and the principal facet stress. The results indicate that the von Mises effective and principal facet stresses give good correlation for the material investigated, and these parameters can predict creep life data under the multiaxial stress states with the rupture data obtained from specimens under the uniaxial stress. The results suggest that the creep rupture of this alloy under the testing condition is controlled by cavitation coupled with highly localized deformation process, such as grain boundary sliding. It is also conceivable that strain softening controls the highly localized deformation modes which result in cavitation damage in controlling rupture time of this alloy.

To study the effect of stress within the thin amorphous film generated atop Si irradiated by Ar+, we model the film as a viscoelastic medium into which the ion beam continually injects biaxial compressive stress. We find that at normal incidence, the model predicts a steady compressive stress of a magnitude comparable to experiment. However, linear stability analysis at normal incidence reveals that this mechanism of stress generation is unconditionally stabilizing due to a purely kinematic material flow, depending on none of the material parameters. Thus, despite plausible conjectures in the literature as to its potential role in pattern formation, we conclude that beam stress at normal incidence is unlikely to be a source of instability at any energy, supporting recent theories attributing hexagonal ordered dots to the effects of composition. In addition, we find that the elastic moduli appear in neither the steady film stress nor the leading order smoothening, suggesting that the primary effects of stress ...

The thin films of titanium nitride(TiN)with the thickness of 0.5, 1.0, 2.0, 4.0µm were coated on a steel substrate by the ion beam mixing method. The film had a strong fiber texture with <110> axis perpendicular to the film surface. The initial residual stress was equi-biaxial compression between -4.4 to -5.6GPa. For all thickness cases, the initial part of the changes of the in-plane stresses in the film due to external tensile loading agreed well with the prediction based on elasticity. While the substrate was under an uniaxial stress, the film was in the biaxial state of stress because of the mismatch of Poisson's ratio. When the measured stress in the film exceeded a certain value, the stress departed from the linear relation and leveled off. The onset of nonlinearity was nearly coincident with the first appearance of cracks. The stresses at the onset of nonlinearity and leveling-off decreased with increasing film thickness. The ratio of Young's modulus between loading and unloading decreased as the film thickness increased.   

The relationships between applied stress, deformation and internal pressure of water filled inclusions embedded in soft soils were studied to improve understanding of pore-scale mechanical behavior and towards development of methods for in situ measurement of key mechanical properties. Analytical expressions for inclusion internal fluid pressure as a function of biaxial remote stresses and material properties were developed and tested with Finite Element calculations and experimental results. We found that the applied mean stress, matrix yield stress and Poisson's ratio influence pressure in a water filled inclusion. Internal pressure tends to be higher than applied mean stress, in contrast with established effective stress theory for saturated soils. Changes in inclusion shape is mainly controlled by matrix shear properties (changes in volume are negligible due to water incompressibility). The results of this study provide the framework for development of sensors for in-situ measurement of soil mechanical and rheological properties required for linking mechanical and hydraulic properties.

The biaxial material behaviour of case-hardening steel 20MoCrS4 is investigated under combined tension/compression torsion loading by using thinwalled tubes in order to determine the strain rate dependence of the yield locus. A method is introduced to determine the yield locus at a given tension-to-torsion ratio within the four quadrants. Enhanced strain rates lead to an extension of the yield locus curve and to a nonsymmetric stress increase. The known strength differential (SD) effect is proved under biaxial loading, too. The resulting yield loci are described by a modified ellipse equation, which takes the extending and the centre shift of the yield loci into account. As a consequence, in order to perform precise numerical simulations of plastic deformations in finite element method, the influence of the strain rate on the yield locus curve has to be considered. (orig.)

The first and second normal stress differences, N 1 and N2 in steady shear flow are calculated using differential constitutive equations proposed by Leonov and Giesekus. At low shear rates, the Leonov model gives -N2/ N1=0.25 for both single and multiple relaxation modes. In the Giesekus model, ?N2/ N1 increases with increasing anisotropy mobility parameter ?. Both models predict that ?N2/N1 is a decreasing function of the shear rate at high shear rates. The shear rate dependence of ?N2/N1 becomes weaker with increasing width of relaxation time distribution. The BKZ type integral constitutive equation is employed to investigate the effect of a model parameter b (=N2/N 1) on steady planar, uniaxial and biaxial extensional flows. It is found that the strain rate dependences of planar2 and biaxial extensional viscosities are very sensitive to the parameter b, where 2 in planar extension denotes the direction of constant width.   

In Part I, a novel hypoelastic framework for soft tissues was presented. One of the hallmarks of this new theory is that the well-known exponential behavior of soft tissues arises consistently and spontaneously from the integration of a rate based formulation. In Part II, we examine the application of this framework to the problems of biaxial kinematics, which are common in experimental soft-tissue characterization. We confine our attention to an isotropic formulation in order to highlight the distinction between nonlinearity and anisotropy. In order to provide a sound foundation for the membrane extension of our earlier hypoelastic framework, the kinematics and kinetics of in-plane biaxial extension are revisited, and some enhancements are provided. Specifically, the conventional stress-t...

In this study, the electron effective masses, including longitudinal, transverse, density-of-states and conductivity effective masses, have been systematically investigated in (001), (101) and (111) biaxially strained Si and Si1- x Ge x . It is found that the effect of strain on the longitudinal and transverse masses can be neglected, that the density-of-states masses in (001) and (110) biaxially strained Si and Si1- x Ge x materials decrease significantly with increasing Ge fraction ( x), and that the conductivity masses along typical orientations in (001) and (110) strained Si and Si1- x Ge x .are obviously different from those in relaxed Si. The quantitative results obtained from this work may provide valuable theoretical references to understanding strained materials physics and studying conduction channel design related to stress and orientations in the strained devices.

The ductile forming limit in nonlinear strain paths is investigated for a 11% Cr steel which displays a high r-value and is used practically in difficult forming applications. Experimental measurements were performed to examine the forming limit in strain paths which involve plane strain loading in first-stage loading and loading at various strain ratios in the second stage. Because the forming limit diagram (FLD) is not applicable to nonlinear loading paths, the forming limit stress diagram (FLSD) theory is investigated. In order to evaluate the stress at the forming limit from measured strain, the anisotropic yield criterion of the material is investigated by uniaxial tension testing in three directions and biaxial stretching using hydraulic bulging. Hill quadratic yield criterion gives the best approximation for biaxial tension, whereas Hosford criterion is more appropriate for uniaxial tension. Yld2000-2d is the ideal criterion for both stress states. Considering the possible effect of dependency of r-values on measured strain, the evolution of r-values is measured. Under sufficiently large strain, the r-value becomes virtually constant. Based on the preliminary investigation outlined above, it was found that the evaluated forming limit stresses in nonlinear strain paths lie on one consistent line in the principal stress field. The limit stress line is not affected by changes in the amount of strain applied in the first loading stage, demonstrating that the FLSD theory of ductile fracture is adequate for this particular material.   

The series of equilibrium states reached by disordered packings of rigid, frictionless discs in two dimensions, under gradually varying stress, are studied by numerical simulations. Statistical properties of trajectories in configuration space are found to be independent of specific assumptions ruling granular dynamics, and determined by geometry only. A monotonic increase in some macroscopic loading parameter causes a discrete sequence of rearrangements. For a biaxial compression, we show that, due to the statistical importance of such events of large magnitudes, the dependence of the resulting strain on stress direction is a Levy flight in the thermodynamic limit.

Tests performed on various steels (A42 mild steel, 304 and 316 L stainless steels) show that a new overload cycles have a favorable effect on the fatigue life in push-pull, in stress control, but a detrimental effect in strain-control, and that biaxial non-proportional loadings (90 deg out-of-phase tension and torsion) also enhance the fatigue life in stress control but reduce it in strain control. A method to estimate the influence of cyclic overloading and non-proportional loadings yields conservative predictions of the fatigue life. (authors)

Creep flow rules of 316L stainless steel are studied in tensile and axial-torsion experiments. Through tensile and biaxial proportional loadings it is shown that at low creep values of epsilonkT/DGb a single kinematical variable: the internal stress takes a part in these laws. This is confirmed in non-proportional experiments. The power law with the power of nsup(*)approx.=2 relates applied and internal stresses. At higher creep rates a second scalar internal variable must be introduced and the power law no longer applies. Limiting functions in steady creep are determined for hardening and recovery.

Samples of fully stabilized zirconia made by the tape-casting process were broken at room temperature and 950 C using a biaxial flexure test. The fracture stress values were corrected to allow for the stress distortion which occurs as a result of the test geometry, and the mean strength, Weibull modulus, and fracture microstructure compared at each temperature. It was found that there was only a slight decrease in strength at 950 C, yet the Weibull modulus decreased significantly. Also there was a change in the appearance of the fracture surface at the higher temperature, indicating that a change in the fracture mechanism could be occurring.

The strength and fracture criteria at the biaxial stress state were examined for two grades of HTGR graphites, i.e., a fine-grained isotropic IG-11 and a medium-grained semi-isotropic PGX. The biaxial stress state was realized using a servohydraulic testing machine in combination with an apparatus for applying the internal pressure or the torque force to a tubular specimen. Three kinds of specimens were tested at room temperature: (1) the larger specimens with different wall thicknesses (2 to 20 mm) to make clear the effect of wall thickness on the biaxial strength, (2) the smaller ones with wall thickness of 2 mm to obtain data for the statistical analysis, and (3) specimens tested at ORNL to examine if there is any discrepancy in the strength data which may result from the differences in the rig and specimen size. Main results are: (1) As for the failure surface no significant effect of wall thickness was observed though the number of the specimens tested was not large enough to evaluate the data statistically. (2) On the basis of the statistical analysis of the data on the smaller IG-11 specimens, the modified maximum strain energy criterion gave the best fit to the data points both in the first and fourth quadrants of the fracture surface. (3) The data obtained at ORNL fell on the scatter band of the JAERI data, which indicated that no appreciable difference in the biaxial strength of IG-11 graphite was found despite the difference in the test fixture and specimen dimensions. (4) The maximum strain energy criterion was also believed to be most appropriate for PGX graphite for the first quadrant of the failure surface. (author).

In optically birefringent uniaxial and biaxial crystals, analyzed by plane polariscope, isochromate interference fringes can be observed. By means of the classical electromagnetic theory a Cassini-like analytical equation of the isochromate fringes, depending on the refraction indexes, has been obtained. The proposed analytical equation is a useful tool to evaluate the internal stress state, as it is related to the isochromate shapes owing to the induced variation of the refraction indexes. Uniaxial crystals can assume complex biaxial behaviour due to particular stress configurations. PbWO4 (PWO) uniaxial scintillating crystals have been studied. The Cassini-like curves fit well experimental measurements in the case of uniaxial stress. In this research work, a simple model has been proved in the case of strong isochromate fringes distortion due to a stress gradient induced by the bending load. The model fits well the interference pattern, acquired experimentally. This study can pave the way for the quality control on scintillating crystals, used in the fields of high-energy detectors, security and biomedical applications, with complex internal stress state.

Abstract in english The results of an experimental investigation on the microcracking of high-performance concrete subjected to biaxial tension-compression stresses are presented. Short-term static tests and microcracking mapping were performed on 12.5 cm square by 1.25 cm thick plates. Strain controlled tests were executed in a biaxial testing machine constructed at the University of Texas. The primary variables studied were the deformations and the ultimate stress level at each stress rati (more) o as well as the microcracking patterns and total crack lengths. For the microcracking study, the plates, after straining, were impregnated by an epoxy and then examined under a microscope. Microcracks were classified into simple and combined cracks, since this distinction allows for a much better representation of the microcracking process. A simple crack is either a bond or mortar crack where a combined crack contains both of these. For all stress ratios tested, the stress-strain behavior was directly related to the internal microcracking pattern. In all cases, the failure was directly related to the formation and propagation of the combined cracks.

Changes in the shape and stress distributions during free bulging in axisymmetrically blow-formed superplastic sheets are analyzed. Generally, the shape is assumed to be spherical and the stress distribution is a function satisfying only boundary conditions without any theoretical bases, because of the nonlinear constitutive equations of superplasticity. An assumed stress distribution function gives a particular pressure distribution on the workpiece. If the pressure distribution is uniform, the function represents a true distribution of stress. Such a function is proposed in this paper and is verified by the analysis without these additional assumptions. With progress in forming, all areas except those in the neighborhood of the periphery approach the equi-biaxial tensile stress state and the entire shape approaches that of a hemisphere.   

In allowing compression along the femoral shaft (uniaxial dynamization) and optional compression along the femoral neck (biaxial dynamization), the Medoff sliding plate (MSP) represents a new principle in the fixation of trochanteric hip fractures. The Twin hook with 2 apical hooks was designed as an alternative to the lag screw. In 3 prospective consecutive case series and 1 prospective randomized study together comprising 342 trochanteric fractures, these alternative techniques were investigated. 3 postoperative fixation failures occurred in the unstable intertrochanteric fractures treated with biaxial dynamization with the MSP (n = 194), and 5 in those treated with the sliding hip screw (n = 62) (p = 0.04). A mean femoral shortening of 15 mm with the MSP and 11 mm with the sliding hip screw was found (p = 0.03). More medialization of the femoral shaft occurred with the sliding hip screw (26%) than with the MSP (12%) in patients with marked femoral shortening (p = 0.03). 3 postoperative fixation failures occurred in subtrochanteric fractures treated with uniaxial dynamization (n = 29) and 2 in those treated with biaxial dynamization (n = 19). Medialization of the femoral shaft occurred in 9 of the 19 biaxially dynamized fractures. The Twin hook was used in 50 patients and appeared to provide similar fixation stability as the lag screw. Biomechanical tests confirmed improved stress transmission over the fracture area with the MSP compared to the sliding hip screw in intertrochanteric fractures, and similar fixation stability with the MSP and the Intramedullary Hip Screw in subtrochanteric fractures. In axial and torsional loading, the Twin hook demonstrated gradually increasing resistance to migration. With the lag screw, the peak load was higher, but after migration with failure of the support by the threads, the loads were similar. Biaxial dynamization with the MSP appears to control fracture impaction effectively and minimizes the rate of postoperative fixation failure in intertrochanteric fractures. In subtrochanteric fractures, uniaxial dynamization prevents medialization of the femoral shaft and is therefore preferred to biaxial dynamization. The Twin hook appears to provide adequate fixation stability, and with potential for simplified intraoperative handling and reduced dissection, the Twin hook may pose advantages compared to the lag screw. PMID:11116961

The deep drawing formability of a material is established as a function of standard indexes, as strength coefficient and anisotropy coefficient. But these indexes are determined in different conditions to those that take place in the forming process. The simulative assays do not separate the actions due to the different variables that work in the process, as for example, the rolling direction. In the present work a test that uses a wedge shape die is considered in order to obtain the strength and anisotropy coefficients as a function of rolling direction. This way, the assays are carried out under a tensile-biaxial compression stress state similar to that one taking place in the flange zone in deep drawing. The experimented material is a deep drawing quality stainless steel AISI 304. The influence of initial strengthened states, rolling and uniaxial tensile on the steel behaviour are also studied. The results permits the authors establish the validity of the assay from the point of view of the strains produced in the sheet. The initial strain has a higher effect on the material than that one obtained from the tensile-biaxial of the state than the tensile-biaxial compression causes. The anisotropy coefficient changes with the strain for the sheet rolling direction. (Author).

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880-1020 C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000 C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted with existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known. (orig.)

A new in-house test capability has been developed at the NASA Glenn Research Center to conduct highly critical tests in support of major and significant components of the Stirling Radioisotope Generator (SRG). It is to aid the development of analytical life prediction methodology and to experimentally assist in verifying the flight-design component's life. Components within the SRG such as the heater head pressure vessel endure a very high temperature environment for a long period of time. Such conditions impose life-limiting failure by means of material creep, a slow gradual increase in strain which leads to an eventual failure of the pressure vessel. To properly evaluate the performance and assist in the design of this component, testing under multiaxial loading setting is essential, since the heater head is subjected to a biaxial state of stress. Thus, the current work undertakes conducting analytical studies under equibiaxial and non-equi-biaxial loadings situations at various temperatures emulating creep environment. These analytical activities will utilize the finite element method to analyze cruciform type specimens both, under linear elastic and creep conditions. And further to calibrate the in-plane biaxial-test system. The specimen finite element model is generated with MSC/Patran [1] and analytical calculations are conducted with MARC and ANSYS finite element codes [2-3]. Complementing these calculations will undertake conducting experimental tests. However, only results pertaining to the analytical studies are reported and their impact on estimating the life of the component is evaluated.

We report results of a systematic computational study of the electromigration-driven complex surface dynamics of voids in mechanically stressed thin films of face-centered cubic metals with -oriented film planes. The films are subjected to an external electric field simultaneously with biaxial mechanical loading, which can be either purely compressive, ranging from purely isotropic to strongly anisotropic including uniaxial, or a mixed type of loading with both tensile and compressive stress components in the applied stress tensor. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological evolution following a well validated, two-dimensional, and fully nonlinear model. We find that depending on the electromechanical conditions, void size, and surface diffusional anisotropy, two types of asymptotic states can be stabilized in the void surface dynamical response, namely, morphologically steady or time-periodic traveling voids, and film failure can be caused by void tip extension. The loading mode as well as the loading anisotropy are found to be the significant factors in determining the void morphological stability domains and can be tailored to stabilize steady or time-periodic states and to increase the film's resistance to failure. Under a mixed (tensile + compressive) loading mode, we find that it is impossible to stabilize steady states in the void morphological response and that the stress levels that the film can sustain prior to failure are much lower than those under purely tensile or purely compressive biaxial loading.

Stress-strain curves of PTFE-coated glass fiber plain-weave fabric (A) and PVC-coated polyester fiber plain-weave fabric (B) were obtained by maintaining the ratio of warp stress/fill stress at (1/1), (2/1), and {1/2} in the biaxial tensile test. All extension curves, especially in the fill tension of (A), showed high nonlinearity. For the purpose of realizing the analysis which is applicable to the biaxial deformation condition of the actual membrane structure, a method of multi-step-linear approximation to the extension curves at various stress ratios was proposed. The least square method was used to minimize the error which results from the approximation of elastic constants in the constitutive equation for each secant line through the extension curve. Then, the three-step-linear approximation was conducted on (A) and (B) to examine the errors. The accuracy of the approximation was good for the practical use, but was not sufficient for the fill extension curve of (A). Nevertheless, it can be said that this approximation method is practically effective for the elastic analysis of the membrane structure. 9 refs., 9 figs., 3 tabs.

A fundamental assumption in mitral valve (MV) therapies is that a repaired or replaced valve should mimic the functionality of the native valve as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the function and mechanical properties of the native normal MV. In a recent study [Grashow et al. ABME 34(2), 2006] we demonstrated that the planar biaxial stress-strain relationship of the MV anterior leaflet (MVAL) exhibited minimal hysteresis and a stress-strain response independent of strain rate, suggesting that MVAL could be modeled as a "quasi-elastic" material. The objective of our current study was to expand these results to provide a more complete picture of the time-dependent mechanical properties of the MVAL. To accomplish this, biaxial stress-relaxation and creep studies were performed on porcine MVAL specimens. Our primary finding was that while the MVAL leaflet exhibited significant stress relaxation, it exhibited negligible creep over the 3-h test. These results furthered our assertion that the MVAL functionally behaves not as a linear or non-linear viscoelastic material, but as an anisotropic quasi-elastic material. These results appear to be unique in the soft tissue literature; suggesting that valvular tissues are unequalled in their ability to withstand significant loading without time-dependent material effects. Moreover, insight into these specialized characteristics can help guide and inform efforts directed toward surgical repair and engineered valvular tissue replacements. PMID:17016761

Rubbers and soft biological tissues may undergo large deformations and are also viscoelastic. The formulation of constitutive models for these materials poses special challenges. In several applications, especially in biomechanics, these materials are also relatively thin, implying that in-plane stresses dominate and that plane stress may therefore be assumed. In the present paper, a constitutive model for viscoelastic materials in the finite strain regime and under the assumption of plane stress is proposed. It is assumed that the relaxation behaviour in the direction of plane stress can be treated separately, which makes it possible to formulate evolution laws for the plastic strains on explicit form at the same time as incompressibility is fulfilled. Experimental results from biomechanics (dynamic inflation of dog aorta) and rubber mechanics (biaxial stretching of rubber sheets) were used to assess the proposed model. The assessment clearly indicates that the model is fully able to predict the experimental outcome for these types of material.

The effects of compressive stress on the TO phonon frequencies of hexagonal boron nitride (hBN) in cubic BN (cBN) films were investigated using infrared absorption spectroscopy, showing that the B-N stretching vibration of hBN at 1380 cm-1 shifted to high wavenumbers under biaxial compressive stress with the rate 2.65 cm-1 per GPa, while the B-N-B bending vibration near 780 cm-1 shifted to low wavenumbers with the rate -3.45 cm-1/GPa. The density functional perturbation theoretical calculation was carried out to check the above phonon frequencies under stress for two typical orientations of hBN crystallite. The results are shown to be in fair agreement with the experimental data. Our results suggest that the residual compressive stress accumulated in cBN films can be evaluated from the IR peak position near 780 cm-1.

This paper focuses on the development of biaxial stress in Cu{sub 0.57}Ni{sub 0.42}Mn{sub 0.01} thin films during annealing in Ar and, for comparison, in vacuum. Besides stress-temperature measurements also resistance-temperature investigations as well as chemical and microstructural characterization by Auger electron spectroscopy, scanning and transmission electron microscopy, and X-ray diffraction were carried out. To explain the stress evolution, atomic rearrangement (excess-vacancy annihilation, grain-boundary relaxation, and shrinkage of grain-boundary voids) and oxidation were considered. Up to 250--300 C grain-boundary relaxation was found to be the dominating process. A sharp transition from compressive to tensile stress between 300 C and 380 C is explained by the formation of a NiO surface layer.

Constitutive equations for the multiaxial stress-strain behavior of aluminum alloy 5754 sheets were developed, based on crystal plasticity. A Taylor-based polycrystal plasticity model, a tangent formulation of the self-consistent viscoplastic model (VPSC), and an N-site viscoplastic model based on the fast Fourier transform (VPFFT) were used to fit a single slip system hardening law to the available data for tension, plane strain, and biaxial stretching. The fitting procedure yields good agreement with the monotonic stress-strain data, with similar parameter values for each model. When simulating multiaxial tests using the developed hardening law, models that allow both stress and strain variations in grains give better predictions of the stress-strain curves. Furthermore, generally, the s...

An analytical model is developed to estimate the effect of creep deformation on the stress relaxation and distribution in the film/substrate bilayer structure during prolonged exposure to high temperature. In the model, either the film or the substrate subjecting to high-temperature creep is considered. Closed-form solutions for the residual stresses in the film and the substrate are derived. A relationship between the stress relaxation rate in the film/substrate system and the relaxation time is obtained. Case studies show that the ratios of the biaxial modulus and the thickness of the film to those of the substrate, the creep parameters, and the exposure temperature have significant influence on the residual stress relaxation rate.

Hypervelocity impacts were performed on six unstressed and six stressed titanium coupons with aluminium shielding in order to assess the effects of the partial penetration damage on the post impact micromechanical properties of titanium and on the residual strength after impact. This work is performed in support of the definition of the penetration criteria of the propellant tanks surfaces for the service module of the crew exploration vehicle where such a criterion is based on testing and analyses rather than on historical precedence. The objective of this work is to assess the effects of applied biaxial stress on the damage dynamics and morphology. The crater statistics revealed minute differences between stressed and unstressed coupon damage. The post impact residual stress analyses showed that the titanium strength properties were generally unchanged for the unstressed coupons when compared with undamaged titanium. However, high localized strains were shown near the craters during the tensile tests.

Monolithic Al{sub 2}O{sub 3} ceramic laminates were fabricated via a tape casting process. The strength of the single tapes was compared with that of laminates, using biaxial flexure tests. The fracture stress was similar. However, the laminates presented a lower Weibull modulus. The feasibility of eliminating or diminishing void-type flaws present in the green tapes was also assessed. To this end, tapes were first punctured, then laminated and sintered, and the effect of these known flaws in the final ceramic was assessed in four-point flexure tests. The thermocompression of green tapes during laminate fabrication was found to modify the flaws to a more forgiving morphology.

Elasto-plasticity behavior of type A5052-O and AA6016-T4 aluminum alloy sheets was examined by performing several experiments of uniaxial tension, biaxial stretching and in-plane cyclic tension-compression. Both sheets exhibit apparent r-value planar anisotropy, especially for AA6016-T4 it is extremely strong, while their flow stress directionality under uniaxial tension is not so significant. Both the sheets show strong cyclic hardening with weak Bauschinger effect. Such material behavior is well described by Yoshida-Uemori kinematic hardening model combined with an appropriate choice of anisotropic yield function.   

The reliability of alumina disks subjected to biaxial flexure is predicted on the basis of statistical fracture theory using a critical strain energy release rate fracture criterion. Results on a sintered silicon nitride are consistent with reliability predictions based on pore-initiated penny-shaped cracks with preferred orientation normal to the maximum principal stress. Assumptions with regard to flaw types and their orientations in each ceramic can be justified by fractography. It is shown that there are no universal guidelines for selecting fracture criteria or assuming flaw orientations in reliability analyses.

We present two representations of the Doi-Edwards model without Independent Alignment explicitly expressed in terms of the Finger strain tensor, its inverse and its invariants. The two representations provide explicit expressions for the stress prior to and after Rouse relaxation of chain stretch, respectively. The maximum deviations from the exact representations in simple shear, biaxial extension and uniaxial extension are of order 2%. Based on these two representations, we propose a framework for Doi-Edwards models including chain stretch in the memory integral form.

Low-carbon, nitrogen-controlled 316 stainless steel called 316FR was developed and is regarded as a principal candidate for a main structural material of liquid metal-cooled fast breeder reactor plants in Japan. To develop a creep-fatigue evaluation method suitable for this steel, a number of uniaxial creep-fatigue tests have been conducted for three products of this steel. Long-term data up to about 35,000h were obtained and applicability of failure life prediction methods was studied based upon their results. Cruciform shaped specimens were also tested under biaxial loading conditions to examine the effect of stress multiaxiality on failure life under creep-fatigue condition.

A model for predicting the crack growth rate of an initially angled crack under biaxial loads of arbitrary direction is suggested. The model is based on a combination of both the Manson-Coffin equation for low cycle fatigue and the Paris equation for fatigue crack propagation. The model takes into consideration the change in material plastic properties in the region around the crack tip due to the stress state, together with the initial orientation of the crack and also its trajectory of growth. Predictions of crack growth rate for any mixed mode fracture is based on the results of uniaxial tension experiments.

Based on biaxial shear creep tests conducted on rock samples with different water contents, we present the results of our study on the regularities of electromagnetic and acoustic emission during the process of creep experiments in which we have analyzed the contribution of water to the occurrence of electromagnetic radiation. The result shows that in the creep-fracturing course of rock samples, when the water content increases, the initial frequency and amplitude of electromagnetic and acoustic emission also increases, but at a decreasing growth rate caused by loading stress. This can be used as a criterion for the long-term stability of rock masses under conditions of repeated inundation and discharge of water.

We discuss which rheological material functions of wheat flour dough are most relevant for structure development in baked products under common processing conditions. We consider the growth of gas cells during dough proofing (driven by yeast) and during baking, where the growth is driven by a combination of CO2 desorption, water and ethanol evaporation, and thermal expansion of gas. Attention is given to upper limits on biaxial extension rate and stress and the consequences for the required rheological material functions. The applicability of the ``Considère criterion'' to predict the probability of coalescence between gas cells and its effect on loaf aeration is briefly discussed.

Stress in the diaphragm, transdiaphragmatic pressure, and diaphragm shape are interrelated by a balance of forces. Using precise in vivo measurements of diaphragm shape and transdiaphragmatic pressure distribution in combination with finite-element analysis (ANSYS), we determined the direction and magnitude of stress in the passive diaphragm at relaxation volume. Lead spheres sutured along muscle bundles identified muscle bundle location and orientation in vivo. The x, y, and z coordinates of the lead spheres and entire surface of the diaphragm, excluding the zone of apposition, were determined to within 1.4 mm. Thin shell elements were used to construct a finite-element model of the diaphragm with a 2.1- to 4.2-mm internodal spacing. The diaphragm was assumed to have a uniform thickness of 2.5 mm, and magnitude and direction of the principal stresses were computed. The results show that 1) diaphragm stress is nonuniform and anisotropic (i.e., varies both with location on diaphragm surface and direction examined), 2) largest stress (sigma 1) is aligned with muscle bundles and is two to four times larger than sigma 2 (perpendicular to sigma 1 in diaphragm plane), and 3) stress along the muscle bundles is larger in vivo under conditions of biaxial stress than at same length in vitro under uniaxial stress. Although diaphragm stress and tension have often been assumed to be uniform, our finding that stress is oriented primarily along the muscle fibers should be considered in future models of the diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS) PMID:8063670

A major phase of structural benchmark testing began at the NASA Glenn Research Center (http://www.grc.nasa.gov) for a critical component of the 110-W Stirling Radioisotope Generator (SRG110). Durability testing of the heater head component was initiated on two test articles under prototypical environmental conditions: Glenn s special-purpose test rigs subject the heater head specimens to the design operating temperatures and pressure. Under these conditions, the primary life-limiting damage mechanism is creep deformation. The experimental data produced support the development of an analytical life-prediction methodology (ref. 1). The testing will be terminated after approximately 1 year of operation. The SRG110 is being developed to provide electric power for multimission uses, including possible future long-duration NASA space science missions such as deep-space missions or lunar applications (ref. 2). For these uses, the heater head component must endure high temperature at low stress for a long time. The heater head is designed to minimize the effects of material creep a slow, gradual increase in the pressure vessel diameter that could result in reduced system performance if not properly designed. Although creep-limited components have been designed satisfactorily using material properties generated from traditional uniaxial tests, the heater head is subjected to a biaxial state of stress. To supplement the ongoing uniaxial creep tests of flight heat Inconel 718 material (ref. 3), Glenn researchers developed benchmark testing to experimentally evaluate the response to this specific biaxial stress condition.

Gallium nitride epitaxial layers were grown on sapphire by molecular-beam epitaxy using nitridated gallium metal films as buffer layers. The mechanical properties of the buffer layers were investigated and correlated with their chemical composition as determined by synchrotron radiation photoelectron spectroscopy. Biaxial tension experiments were performed by bending the substrates in a pressure cell designed for simultaneous photoluminescence measurements. The shift of the excitonic luminescence peak was used to determine the stress induced in the main GaN epilayer. The fraction of stress transferred from substrate to main layer was as low as 27% for samples grown on nitridated metal buffer layers, compared to nearly 100% for samples on conventional low-temperature GaN buffer layers. The efficiency of stress relief increased in proportion to the fraction of metallic Ga in the nitridated metal buffer layers. These findings suggest GaN films containing residual metallic Ga may serve as compliant buffer layers for heteroepitaxy.

The evolution of the domain structure of a Ho0.6Y2.4Fe5O12 single crystal under the action of biaxial mechanical stresses was investigated using the magneto-optical method. The investigations were performed on a specimen in the form of a plane-parallel plate that was cut parallel to the (110) crystallographic plane. The mechanical stresses in the specimen were induced by the compressive forces acting on it and oriented in the (110) plane along the directions and . It was found that, under stresses induced in the specimen, the reorientation of the easy magnetization axis occurs through a first-order phase transition. The obtained results were discussed in terms of the thermodynamic theory of magnetic orientational phase transitions.

A plethora of phenomenological and structure-motivated constitutive models have thus far been used as pseudoelastic descriptors in arterial biomechanics, but their parameters have not been explicitly correlated with histology. This study associated biaxial histological data with strain-energy function (SEF) parameters derived from uniaxial tension data of arteries from different topographical sites (carotid artery vs. thoracic aorta vs. femoral artery). A two-term SEF fitted the passive stress-strain data of healthy porcine tissue, justified by the biphasic response characterizing elastin-rich tissues. Selection of a quadratic (orthotropic) over the neo-Hookean (isotropic) term was dictated by the directional dissimilarities in low-stress mechanical response, consistent with our histological data indicating orthotropic symmetry for unstressed elastin. Use of the exponential term was dictated by mechanical dissimilarities at high stresses and variations in unstressed collagen composition and orientation. Accurate fits were attained; topographical variations and anisotropy in material parameters were accounted by respective variations in histomorphometrical data. PMID:20390462

Abstract in english Disks of a commercial alumina were fabricated by slip casting, calcination and sintering. The surfaces were machined using SiC papers (120 and 320 grit) and characterized by residual stresses measurements. The mechanical strength was determined in biaxial flexure (ball on discontinuous ring). The specimens were subjected to thermal shock conditions (cooling using a high-velocity air jet) and the critical temperature differential for crack propagation was determined. The t (more) emperature and stress distributions during air impinging were calculated using a finite element method. The value of the heat transfer coefficient was estimated by fitting the calculated temperature profiles with those measured during each test. The calculated tension for the thermal shock fracture was compared with the mechanical strength, together with the fracture features in each case. The differences were explained on the basis of the calculated stress distributions.

Confocal piezo-spectroscopy was applied to analyze the residual stress distribution developed in a CaMoO4-film/Si-substrate system, based on the spectroscopic shift upon stress of the 879 cm-1 Ag Raman mode of CaMoO4. In the stress characterization, use was made of spectral line scans, as well as laser defocusing measurements. A preliminary spectroscopic calibration of the Raman band was performed with using a ball-on-ring flexural bending jig. As a result, a value of -1.4 cm-1/GPa was found for the biaxial piezo-spectroscopic coefficient of the Ag Raman mode of CaMoO4. A probe deconvolution procedure was introduced in the effort to correct the convoluting effect due to the finite size of the laser probe, taking advantage of the measurement of the probe response function. A theoretical ana...

The constant length crack steadily propagating in an anisotropic body, acted on at infinity by uniform biaxial and shear loading, is investigated. The analytical solutions to the dynamic crack tip stress fields and displacement fields are derived. The obtained crack tip fields contain second order term, and are unified and applicable to the analysis of the crack tip fields of anisotropic material, orthotropic material and isotropic material under dynamic or static load. Crack propagation characteristics are represented by crack propagation velocity M and fiber direction a. The faster the crack velocity is, the greater the dynamic stress components and dynamic displacement components around crack tip is. Distributions of the stress fields and displacement fields vary with M and a but the in...

Equal biaxial residual stresses (of up to about 175 MPa) have been generated in thin copper foils via differential thermal contraction. These foils were subsequently indented, under displacement control, and the load-displacement-time characteristics were measured. The applied load required for penetration to a given depth (in a given time) was found to decrease with increasing (tensile) residual stress, in accordance with predictions from a finite-element model (incorporating both plasticity and creep). The main thrust of this paper concerns sensitivities. Relatively small changes in residual stress (of the order of a few tens of MPa) were observed to generate effects that should be detectable via their influence on the nanoindentation response. This is encouraging in terms of the potenti...

Creep of Zircaloy-4 was studied using biaxially loaded tubes at 673 K. Pronounced creep anisotropy is attributed to the presence of strong preferred orientations (texture). Stress and temperature dependences for steady state creep were determined from close-end internal pressurization tests. The derived activation energy and area suggest that the climb of edge dislocations is the rate controlling mechanism for creep in Zircaloy. A single relationship is derived which correlates the temperature and stress dependence of primary creep behavior over the range of experimental investigation. Anisotropic creep behavior as a function of loading state is modelled using Hill's anisotropic formulation modified for constant levels of creep potential. This approach is successful for both cold-worked stress-relieved materials and partially recrystallized materials.

Biaxial creep tests were performed on fine-grained Ti-3Al-2.5V tubing at 823 and 873 K in the stress range ?/E = 1.7 × 10-4 to ?/E = 5.9 × 10-4. Subsequently, the creep data were analysed to determine the stress exponent and activation energy. A stress exponent value of 1 and an activation energy equal to that for grain boundary diffusion were suggestive of a Coble creep-controlled deformation regime. However, discrepancy between the experimental creep rates and Coble creep model predictions along with subsequent observation of deformed microstructures decorated with slip bands implied the operation of a different viscous creep mechanism. A slip band model proposed by Spingarn and Nix was found to provide a better description of the experimental strain rates rather th...

In this paper, we studied the viscoelastic behaviors of isolated aortic elastin using combined modeling and experimental approaches. Biaxial stress relaxation and creep experiments were performed to study the time-dependent behavior of elastin. Experimental results reveal that stress relaxation preconditioning is necessary in order to obtain repeatable stress relaxation responses. Elastin exhibits less stress relaxation than intact or decellularized aorta. The rate of stress relaxation of intact and decellularized aorta is linearly dependent on the initial stress levels. The rate of stress relaxation for elastin increases linearly at stress levels below about 60 kPa; however, the rate changes very slightly at higher initial stress levels. Experimental results also show that creep response is negligible for elastin, and the intact or decellularized aorta. A quasi-linear viscoelasticity model was incorporated into a statistical mechanics based eight-chain microstructural model at the fiber level to simulate the orthotropic viscoelastic behavior of elastin. A user material subroutine was developed for finite element analysis. Results demonstrate that this model is suitable to capture both the orthotropic hyperelasticity and viscoelasticity of elastin. PMID:20963623

The structural integrity of the abdominal aorta is maintained by elastin, collagen, and vascular smooth muscle cells. Changes with age in the structure can lead to develop-ment of aneurysms. This paper presents initial work to capture these changes in a finite element model (FEM) of a structural-ly-motivated anisotropic constitutive relation for the “four fiber family” arterial model. First a 2D implementation is used for benchmarking the FEM implementation to fitted biaxial stress-strain data obtained experimentally from four different groups of persons; 19-29 years, 30-60 years, 61-79 years and abdominal aortic aneurysm (AAA) patients. Next the constitu-tive model is implemented in an anisotropic 3D FEM formula-tion for future simulation of intact aortic geometries. The 2D simulations of the biaxial test experiment show good agree-ment with experimental data with a standard deviation below 0.5% in all cases. The maximum axial and hoop stress in the group of AAA patients was 94.9 kPa (±0.283 kPa) and 94.3 kPa (±0.224 kPa) at maximum stretch ratios of 1.043 and 1.037, respectively. In the 3D simulations, the maximum stress is also found to occur in the AAA patient group, with the highest stress in the circumferential direction (275 kPa). Comparison with an already published isotropic model indicates that the latter underestimates the peak stress significantly. Based on these results it is concluded that the four fiber family model has been successfully implemented into a 3D anisotropic finite element model and that this model can provide more accurate insight into the stress conditions in aortic aneurysms.

Most failures of ductile materials in metal forming processes occurred due to material damage evolution- void nucleation, growth and coalescence of neighboring voids. Recently, Gologanu-Leblond-Devaux (J. Mech. Phys. Solids, Vol.41 (1993), pp.1723-1754) extended the classical Gurson model (ASME J. Engng. Mater. Technology, Vol.99 (1997), pp.2-15) to a ductile material containing an oblate ellipsoidal cavity. And, they proposed a new approximate yield function incorporating the initial void shape effects, which is significant especially at low stress triaxiality. In the present work, the Gologanu-Leblond-Devaux's yield function for anisotropic sheet materials containing axisymmetric prolate ellipsoidal cavities is adopted in evaluating analytically forming limits of sheet metals under biaxial stretching by Marciniak and Kuczynski (M-K) model. The effect of a void shape and growth on the forming limits of sheet metals under biaxial tensile loading is introduced and examined within the framework of the M-K model, along with the effect of including a first-order strain gradient term in the flow stress. To confirm the validity of the proposed model, the predicted FLDs were compared with experimental results for steel sheets. The predicted forming limits for the voided sheets were found to agree well with the experimental data.   

The structural properties of chalcopyrite single crystals and monocrystalline epitaxial layers of ternary (CuGaSe2, CuInSe2, CuGaS2, CuInS2) and quaternary (CuIn1-xGaxSe2) chalcopyrite absorbers with applications in solar-cell device technology are analysed by optical modulation techniques. Photoreflectance (PR) Spectroscopy is applied at room and low temperatures to quantify elastic strain effects. With respect to bulk chalcopyrites, epitaxially grown layers exhibit band energy shifts due to mismatch and thermal strain evolving in semiconductor heterostructures. In the uppermost 100 nm of the 500 nm thick layers, the magnitude of the respective stress measures 100 MPa, at 300 K, and 400 MPa, at 20 K, and is reduced by up to 50% compared with the stress at the chalcopyrite/GaAs-substrate interface. The overall strain calculated from the energy shift of the PR spectra is compared with the strain calculated in terms of elasticity theory. The coexistence of biaxial and hydrostatic strain due to partial anion/cation substitution in ternary to form quaternary chalcopyrite layers is also discussed. Based on the evaluation of effective strain as a result of both hydrostatic and biaxial strain in quaternary chalcopyrite layers CuIn1-xGaxSe2 and the evaluation of the PR spectra of the layers, the band energies of the respective non-strained quaternary alloy with x = 0.19 are determined to be Ea = 1.09 eV and Eb = 1.22 eV.

A numerical study of the creeping flow of an UCM fluid is carried out in a three-dimensional cross-slot geometry with inlets and outlets in all three orthogonal directions. Using two different inlet flow rate configurations, Io = 4:2 and 2:4, representing uniaxial extension and biaxial extension, respectively, it was possible to assess the importance of different types of extensional flow near the stagnation point. Two different methods of calculation of the polymer stress were used, the standard approach and the log-conformation approach. The uniaxial extension flow configuration is prone to the onset of steady flow asymmetries, at a rather small Deborah number (Decrit = 0.21) and regardless of the stress computation method. However, for the biaxial extension flow configuration a perfectly symmetric flow has so far been observed up to De = 0.5 using the log-conformation approach. The use of these two configurations allowed the variation of the amount of stretch and compression near the stagnation point, providing new insights into the viscoelastic flow instability mechanisms in cross-slot flows.

Recently, elastic stress has been among several mechanisms hypothesized to induce the formation of ordered structures in Si irradiated at normal incidence by energetic ions. To test this hypothesis, we model the thin amorphous film atop ion-irradiated Si as a viscoelastic continuum into which the ion beam continually injects biaxial compressive stress. We find that at normal incidence, the model predicts a steady compressive stress of a magnitude comparable to experiment and molecular dynamics simulation. However, linear stability analysis at normal incidence reveals that this mechanism of stress generation is unconditionally stabilizing due to a purely kinematic material flow, depending on none of the material parameters. Thus, despite plausible conjectures in the literature as to its potential role in pattern formation, we conclude that compressive stress induced by normal-incidence ion bombardment is unlikely to be a source of instability at any energy. In fact, with this result, all hypothesized mechanisms suggested to explain structures on pure materials under normal incidence irradiation have now been overturned, supporting recent theories attributing hexagonal ordered dots to the effects of composition. In addition to this result, we find that the elastic moduli appear in neither the steady film stress nor the leading-order smoothening, suggesting that the primary effects of stress can be captured even if elasticity is neglected. This supports the basic framework recently adopted by other authors and should allow future analytical studies of highly nonplanar surface evolution, in which the beam-injected stress is considered to be an important effect.

Fatigue failures are typically caused by unstable crack growth from some geometrical stress raiser such as a notch. The fatal flaw usually initiates at the stress raiser's surface, near the mid-thickness plane, where geometrical constraint is the largest. Unfortunately, the fatigue characteristics at this location are not completely known due to the inherent limitations of classical and experimental techniques. The purpose of this study is to independently characterize both the local biaxial strain/stress behavior and the onset of fatigue crack formation at a notch root's center over a range of constraints. This information is then used to examine the influence of the deformation state on the qualitative and quantitative characteristics of grain-sized microcrack initiation. Three separate approaches were used in this research to study the overall fatigue characteristics of notched specimens. First, experiments were conducted on fine-grained, isotropic, HY-80 steel specimens to +/- 1 percent axial strain. Four different levels of constraint were produced in the double-notched specimens simply by varying the thickness-to-notch-radius ratio. Experimentally, the biaxial strains at the notch root were then determined using an interferometric based technique which accurately measured real-time strains over small gage lengths (150-200 micron). Next, a three-dimensional, elastoplastic, finite element study simulated the actual tests so that the results can be compared with the experimental data. The finite element study also obtains full-field stress-strain information and is used to evaluate the underlying assumptions which constitute two approximate notch root models: the Neuber and Glinka relations. Finally, microcrack initiation was examined on identical double-notched specimens using an acetylcellulose replication technique. These results were then compared with predictions from three different crack initiation models: the Coffin-Manson relation, a plastic work theory, and a micromechanical model. The results of this study indicate that while constraint does greatly influence notch fatigue behavior, the notch shape and specimen thickness are also important. The notch root strain experiments show that the severity of the biaxial, maximum and residual strains decreases as the amount of constraint increases. The three-dimensional finite element results confirm these trends for both the local strains and stresses. These results also show that constraint and the notch shape define the full-field distribution and delineate the regions where the Neuber and Glinka models are most effective. The microcrack initiation tests indicate that initiation is governed not only by the geometrical constraint and notch shape, but by the specimen thickness. Coffin-Manson and plastic work crack initiation theories do not significantly account for constraint effects and consequently do not predict the differences in microcrack initiation evident in these results. &The micromechanical model, however, does predict initiation more accurately.

Constitutive equations are reviewed and presented for low alloy ferritic steels which undergo creep deformation and damage at high temperatures; and, a thermodynamic framework is provided for the deformation rate potentials used in the equations. Finite element continuum damage mechanics studies have been carried out using these constitutive equations on butt-welded low alloy ferritic steel pipes subjected to combined internal pressure and axial loads at 590 and 620 degrees C. Two dominant modes of failure have been identified: firstly, fusion boundary failure at high stresses; and, secondly, Type IV failure at low stresses. The stress level at which the switch in failure mechanism takes place has been found to be associated with the relative creep resistance and lifetimes, over a wide range of uniaxial stresses, for parent, heat affected zone, Type IV and weld materials. The equi-biaxial stress loading condition (mean diameter stress equal to the axial stress) has been confirmed to be the worst loading condition. For this condition, simple design formulae are proposed for both 590 and 620 degrees C. PMID:16243708

A tubular specimen for acoustic emission testing of unflawed metals under uniaxial or biaxial loading was designed. The test technique and specimen design using 7075-T651 aluminium were evaluated. Biaxial result differed significantly from uniaxial results.

Biaxially. Oriented. PET Films. Tensile Strength Properties of Simultaneously. Biaxially. Oriented PET ..... acetate, various lead salts and various titanium salts have all been reported as suitable for ..... in complete degradation of the film into a ...

This paper presents an experimental investigation of the behaviour of filament wound glass fibre reinforced epoxy (GRE) composite pipe under hydrostatic and biaxial load conditions at temperatures up to 95^oC. The format of the experiments has been chosen to be compatible with the Future Pipe Industries (FPI) procedure using the ultimate elastic wall stress (UEWS) concept in the qualification and production control of GRE. The test appears to provide an attractive alternative to the current 1000hour test procedure detailed in ASTM D2992 for the detection of manufacturing changes and reconfirmation of the design basis of the pipe. Six different stress ratios ranging from pure axial loading 0:1, 0.5:1, 1:1, 2:1, 4:1 and pure hoop 1:0 loading were tested. Three distinct failure modes were obs...

The damage tolerance behavior of internally pressurized, axially slit, graphite/epoxy tape cylinders was investigated. Specifically, the effects of axial stress, structural anisotropy, and subcritical damage were considered. In addition, the limitations of a methodology which uses coupon fracture data to predict cylinder failure were explored. This predictive methodology was previously shown to be valid for quasi-isotropic fabric and tape cylinders but invalid for structurally anisotropic (+/-45/90)(sub s) and (+/-45/0)(sub s) cylinders. The effects of axial stress and structural anisotropy were assessed by testing tape cylinders with (90/0/+/-45)(sub s), (+/-45/90)(sub s), and (+/-45/0)(sub s) layups in a uniaxial test apparatus, specially designed and built for this work, and comparing the results to previous tests conducted in biaxial loading. Structural anisotropy effects were also investigated by testing cylinders with the quasi-isotropic (0/+/-45/90)(sub s) layup which is a stacking sequence variation of the previously tested (90/0/+/-45)(sub s) layup with higher D(sub 16) and D(sub 26) terms but comparable D(sub 16) and D(sub 26) to D(sub 11) ratios. All cylinders tested and used for comparison are made from AS4/3501-6 graphite/epoxy tape and have a diameter of 305 mm. Cylinder slit lengths range from 12.7 to 50.8 mm. Failure pressures are lower for the uniaxially loaded cylinders in all cases. The smallest percent failure pressure decreases are observed for the (+/-45/90)(sub s) cylinders, while the greatest such decreases are observed for the (+/-45/0)(sub s) cylinders. The relative effects of the axial stress on the cylinder failure pressures do not correlate with the degree of structural coupling. The predictive methodology is not applicable for uniaxially loaded (+/-45/90)(sub s) and (+/-45/0)(sub s) cylinders, may be applicable for uniaxially loaded (90/0/+/-45)(sub s) cylinders, and is applicable for the biaxially loaded (90/0/+/-45)(sub s) and (0/+/-45/90)(sub s) cylinders. This indicates that the ratios of D(sub 16) and D(sub 26) to D(sub 11), as opposed to the absolute magnitudes of D(sub 16) and D(sub 26), may be important in the failure of these cylinders and in the applicability of the methodology. Discontinuities observed in the slit tip hoop strains for all the cylinders tested indicate that subcritical damage can play an important role in the failure of tape cylinders. This role varies with layup and loading condition and is likely coupled to the effects of structural anisotropy. Biaxial failure pressures may exceed the uniaxial values because the axial stress contributes to the formation of 0 deg ply splitting (accompanied by delamination) or similar stress-mitigating subcritical damage. The failure behavior of similar cylinders can also vary as a result of differences in the role of subcritical damage as observed for the case of a biaxially loaded (90/0/+/-45)(sub s) cylinder with a 12.7 mm slit. For this case, the methodology is valid when the initial coupon and cylinder fracture modes agree. However, the methodology underpredicts the failure pressure of the cylinder when a circumferential fracture path, suggestive of a 0 deg ply split, occurs at one slit tip. Thus, the failure behavior of some tape cylinders may be highly sensitive to the initial subcritical damage mechanism. Finite element analyses are recommended to determine how structural anisotropy and axial stress modify the slit tip stress states in cylinders from those found in flat plates since similarity of these stress states is a fundamental assumption of the current predictive methodology.

The mechanical stability of porous Ba"0"."5Sr"0"."5Co"0"."8Fe"0"."2O"3"-"d (BSCF) material was investigated using depth-sensitive microindentation and ring-on-ring biaxial bending tests. The porous BSCF was characterized as potential substrate material for the deposition of a dense membrane layer. Indentation tests yielded values for hardness and fracture toughness up to a temperature of 400^oC, while bending tests permitted an assessment of elastic modulus and fracture stress up to 800^oC. In addition the fracture toughness was evaluated up to 800^oC measuring in bending tests the fracture stress of pre-indented specimens. The results proof that the indentation-strength method can be applied for the determination of the fracture toughness of this porous material. In comparison to dense ma...

An explicit, direct approach is proposed to obtain multiaxial elastic potentials that exactly match finite strain data of four benchmark tests for incompressible rubberlike materials, including uniaxial, biaxial, plane-strain compression and simple shear tests. This approach is composed of three direct procedures. At first, we utilize a usual interpolating method for single-variable functions to obtain one-dimensional stress?strain relations matching data of uniaxial test and simple shear test, separately, and then obtain two one-dimensional elastic potentials via directly integrating the stress power. After that, we introduce a multiaxial bridging method based on certain invariants of Hencky strain and extend the foregoing two one-dimensional potentials to two potentials for multiaxial ca...

While modelling single crystal deformation, often the hardening behaviour is approximated using a power law type hardening equation, the coefficients of which are determined by fitting predicted stress–strain curve to experimental stress–strain curve in the uniaxial tension. Also, in the literature, for BCC material either 24 or 48 slip systems have been considered with mixed conclusions. In this work, the suitability of such phenomenologically determined coefficients and the effect of using 24 and 48 slip systems on prediction capability for anisotropic hardening of an ultra low carbon interstitial free high strength steel observed during in-plane biaxial tension tests were assessed. It was found that, though there was some difference between the results for 24 and 48 slip...

Lead zirconate titanate (PZT) is an important piezoelectric material which has wide range of applications as sensors, actuators and transducers. Various forms are required for different devices applications. In this work, extrusion and press forming of PZT ceramic rods and thick films produced via a viscous polymer processing (VPP) route have been investigated. The relationships between the rheology, microstructure and formability of both an aqueous and a non-aqueous polymer binder system have been compared. The non-aqueous PVB system exhibits substantially higher bulk yield stress during plug die flow and higher biaxial extensional stress during squeeze film flow compared to the aqueous PVA system. Discussion of the results is based on differences in the adsorption of the polymer onto the...

A series of mechanical ratcheting tests under tension-torsion biaxial conditions has been conducted with an advanced 316 stainless steel at 923 K. Accumulation of torsional ratcheting strain was measured with a cyclic axial strain ranges of 0.005-0.02, cyclic axial strain rate of 10{sup -5} to 10{sup -3} s{sup -1} and steady torsional stresses of 24.5/{radical}(3) to 73.5/{radical}(3)MPa. The accumulation of ratcheting shear strain is mainly affected by the cyclic axial strain range and the steady shear stress, and increases with an increase of both these parameters. A simple evaluation of the accumulation of ratchet strain is proposed. Although this procedure is based on the separation of creep and plasticity and uses experimental data to skip the plasticity analysis, the obtained results show good agreement with the experimental results. (orig.)

Biaxial creep-fatigue data for Incoloy 800 and Type 316H stainless steel at elevated temperatures are presented. Tubular specimens were subjected to constant internal pressure and strain-controlled axial cycling with and without hold times in tension as well as in compression. The results show that the internal pressure affects diametral ratchetting and axial stress range significantly. However, the effect of a relatively small and steady hoop stress on the cyclic life of the materials is minimal. A 1-min compressive hold per cycle does not seriously reduce the fatigue life of either material; a tensile hold of equal duration causes a significant reduction in life for Type 316H stainless steel, but none for Incoloy 800. Fracture surfaces of specimens made of both materials were studied by scanning electron microscopy to determine the reason for the difference in behavior.

The rheological characteristics of twenty wheat flour samples obtained from four organic flour blends and a non-organic control were compared in relation to their ability to predict subsequent loaf volume in the baked bread. The flour samples considered had protein contents that varied between 11-14g/100g. Four different rheological methods were employed. Oscillatory stress rheometry on the protein gel extracted from the wheat flour, oscillatory stress rheometry and creep measurement on undeveloped dough samples and biaxial extensional measurements on simple flour-water doughs. None of the fundamental rheological parameters correlated with loaf volume. There was a correlation between the storage modulus of the gel protein and storage modulus for the undeveloped dough (r=0.85). There was a ...

Combined experimental and theoretical investigations into the inelastic deformation and damage behavior of engineering alloys at elevated temperatures are being pursued. Modeling of effects of recovery of state observed in modified 9Cr-lMo steel has been completed. Finite deformation formulations of viscoplasticity theory based on overstress (VBO) include a modified growth law for the equilibrium stress and a rationale for choosing objective derivatives of stress-like state variables. Numerical simulations are in progress. Seven biaxial low-cycle fatigue tests at 538C have been completed with the reversing DC potential drop apparatus attached. A new method of data analysis and smoothing was developed which showed a significant increase in voltage drop in the area of crack formation. Correlation with solutions of Laplace`s Equation for a semi-elliptical crack showed similar shapes for the voltage drop.

Nematic liquid crystals possess three different phases: isotropic, uniaxial, and biaxial. The ground state of most nematics is either isotropic or uniaxial, depending on the external temperature. Nevertheless, biaxial domains have been frequently identified, especially close to defects or external surfaces. In this paper we show that any spatially-varying director pattern may be a source of biaxiality. We prove that biaxiality arises naturally whenever the symmetric tensor $\\Sb=(\\grad \

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10{sup 7}/sec to 10{sup 10}/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.

Macroscopic response and microscopic dislocation structures of Zr-4 subjected to biaxial fatigue under different phase angles of 30{degree}, 60{degree}, 90{degree}, and different equivalent strain ranges of 0.8%, 0.6%, 0.4% were studied. The testing results show that the delay angle between the stress deviators and strain increment tensors is strongly dependent on phase angle and the equivalent strain range. When phase angle equals 60{degree}, the delay angle has the minimum variation range for all specimens. The mean value of the delay angle decreases with increasing phase angle or the equivalent strain range. The variation range and average value of the Mises equivalent stress have the maximum in S3 with the phase angle of 90{degree}. They decrease as the equivalent strain range decreases. Zr-4 displays a pronounced initial hardening followed by a continuous softening for all specimens during out-of-phase cycling. The stabilized saturation stresses of Zr-4 under out-of-phase cycling are much higher than that under uniaxial cycling. It indicates that Zr-4 displays an obvious additional hardening. As the phase angle increases, the typical dislocation structure changes from dislocation cells to tangles. The dislocation-dislocation interactions increase resulting in an additional hardening. In essence, the degree of additional hardening depends, among other factors, on the maximum shear stress ratio of resolved shear stresses and critical resolved shear stresses (RSS/CRSS).

Abstract in spanish Los modelos reológicos de un alimento procesado permiten simular la respuesta del material a un esfuerzo o deformación aplicada, al igual que predecir el comportamiento del material de acuerdo a su composición y su forma de preparación. Su aplicación se puede llevar a cabo cuando se tienen datos experimentales expresados en unidades fundamentales. Este artículo describe dos modelos reológicos empleados en el estudio de masas de trigo y maíz, el modelo extensional (more) biaxial y el modelo dinámico oscilatorio. Además, muestra los resultados de algunas investigaciones sobre este tema Abstract in english Rheological models of a food system are useful for simulating a material?s response to an applied stress or strain and for predicting the effect of composition and processing conditions. Rheological models can be applied when experimental data is expressed in fundamental units. This article describes two rheological models used for studying wheat and corn dough: the biaxial extensional model and the oscillatory dynamic model. The results of research related to this topic are also reported.

From the results of our ab initio pseudopotential total-energy calculation on Cu in the body-centered-tetragonal (bct) crystal system, a first-principles phase diagram for bulk bct Cu is derived as a function of externally applied stresses. An interesting fcc-to-bcc structural transition is predicted, induced by the application of a biaxial tension and a hydrostatic pressure. The biaxial tension needed to initiate the transformation is found, however, to be too large to attain macroscopically without inducing plastic flow (i.e., dislocation motion) in single crystal copper. An explanation for the existence of a metastable (albeit rather disordered) phase of bcc Cu on an iron substrate, recently observed by Wang et al., is put forth by considering the energy of tetragonal distortions on the cubic phases of Cu. The results of our bulk bct calculations suggest that the disorder observed on Cu/Fe/100/ is due to a nonoptimal match of lattice constants. The growth of a stable bct phase of Cu on a suitable cubic substrate with lattice constant 2.76 A may be possible.

Commercially produced Zr--2.5Nb fuel cladding was biaxially creep tested in- and out-of-reactor to generate data for fuel modeling studies. A creep equation was developed describing the steady-state hoop-creep rate at temperatures between 300 and 500/sup 0/C. The equation assumes that two mechanisms of creep operate at low and high stresses and that the rates of these are additive. The results show little effect of a fast neutron flux of 5 x 10/sup 17/ neutrons (n)/m/sup 2//s on creep rate at 400/sup 0/C and above but an enhancement of about two in the creep rate at 320/sup 0/C. The biaxial creep of Zr--2.5Nb fuel cladding is about ten times more rapid than that of pressure-tube materials of the same composition. Texture and second-phase distribution are considered to be the causes of the differences in behavior. Some measurements also have been made on irradition growth of fuel cladding in the longitudinal direction. These are discussed.

Numerous scaffold materials have been developed for tissue engineering and regenerative medicine applications to replace or repair damaged tissues and organs. Naturally occurring scaffold materials derived from acellular xenogeneic and autologous extracellular matrix (ECM) are currently in clinical use. These biological scaffold materials possess inherent variations in mechanical properties. Spherical indentation or ball burst testing has commonly been used to evaluate ECM and harvested tissue due to its ease of use and simulation of physiological biaxial loading, but has been limited by complex material deformation profiles. An analytical methodology has been developed and applied to experimental load-deflection data of a model hyperelastic material and lyophilized ECM scaffolds. An optimum rehydration protocol was developed based on water absorption, hydration relaxation and dynamic mechanical analysis. The analytical methodology was compared with finite element simulations of the tests and excellent correlation was seen between the computed biaxial stress resultants and geometry deformations. A minimum rehydration period of 5 min at 37°C was sufficient for the evaluated multilaminated ECM materials. The proposed approach may be implemented for convenient comparative analysis of ECM materials and source tissues, process optimization or during lot release testing. PMID:21864728

We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function. PMID:17227101

The work described in this thesis has been focussed on the search of an elusive liquid crystal phase, known as the biaxial nematic phase. Indeed, despite nearly thirty years of intense research, no-one has been able to characterise unambiguously a biaxial nematic phase in a low-molar-mass thermotropic system. Our research is based on the concept of molecular biaxiality as distinct from shape biaxiality. Thus, we are seeking to design palladium complexes where specific intermolecular interactions could exist. Therefore, a few original synthetic strategies were developed to tackle the challenge of discovering the biaxial nematic phase.

Order parameters and phenomenological theory for both high- and low-symmetry biaxial nematic phases are presented and it is predicted that the chiral low-symmetry biaxial phase must be ferroelectric. This conclusion is based on general symmetry arguments and on the results of the Landau-de Gennes theory. The microscopic mechanism of the ferroelectric ordering in this chiral biaxial phase is illustrated using a simple molecular model based on dispersion interactions between biaxial molecules of low symmetry. Similar to the chiral smectic C* phase, the ferroelectricity in the chiral biaxial nematic phase is improper, i.e., polarization is not a primary order parameter and is not determined by dipolar interactions. Ferroelectric ordering in biaxial nematics may be found, in principle, in materials composed of chiral analogues of the tetrapod molecules which are known to exhibit biaxial phases. (fast track communication)

A conventional rigid plastic finite element method based on mixed formulation sometimes shows false collapse mechanisms containing hourglass modes and locking modes. These false mechanisms are mainly due to the spacial discretization of stresses within an element; a constant stress state is assumed within an element. To overcome these difficulties, a new spacial discretization of stresses is proposed in this paper; two additional variables per element are introduced to express stress distribution in an element. The validity of a newly proposed method is numerically scrutinized by solving the following four typical examples whose analytical solutions have been available; 1) Limit load of a rigid flat punching, 2) Limit load of a hollow cylinder subject to inner pressure, 3) Limit bending of a cantilever beam and 4) Limit load of a plate with a hole subject to biaxial stress. Numerical solutions show good agreement to analytical solutions for all the examples. It can be concluded that the proposed method is able to control both hourglass and locking modes successfully. It is also able to obtain rational plastic solutions with respect to both limit loads and collapse mechanisms.

A multiaxial test program is to be conducted by Oak Ridge National Laboratory (ORNL) on the core component graphite. The objectives of the tests are to obtain failure data under uniaxial and biaxial states of stress in order to construct a failure surface in a two-dimensional stress space. These data will be used in verifying the accuracy of the maximum stress failure theory being proposed for use in designing the core graphite components. Tubular specimens are proposed to be used and are either loaded axially and/or subjected to internal pressure. This report includes a study on three specimen configurations. The conclusions of that study indicate that an elliptical transition geometry procedures the smallest discontinuity effects. Several loading combustions were studied using the elliptical transition specimen. The primary purpose is to establish the location of the highest stress state and its relation to the gage section for all of the loading conditions. The tension/internal pres sure loading condition (1:1) indicated that the high stress area is just outside the gage section but still should be acceptable. 5 refs., 18 figs.

The main objective of this research is to study the residual macroscopic stress in titanium-nitride, TiN, coatings deposited onto a tool steer substrate. The measurements were performed with a theta-theta decoupled X-ray diffractometer. The coatings were manufactured using an industrial pulsed-DC plasma-enhanced chemical vapour deposition (PECVD) technique. The coatings were characterized in terms of microstructure, mechanical and tribological properties. A parametric study of the deposition parameters was performed. Process pressure, bias voltage, temperature and partial gas flows (argon, hydrogen, nitrogen and titanium tetra chloride) were varied in an effort to obtain optimal coating properties. Besides the bi-axial stress, the stress-free lattice constant, d(0), are presented as well as an indication of the changes in texture as a function of process parameter. Total macroscopic stress values were found to range from -1.5 to 1.5 GPa. The intrinsic stresses for the major part of the coatings were close to zero lending to low intrinsic strain energies favouring a preferred orientation of the coating corresponding to the plane with the lowest surface energy which is (200). Other properties are also discussed, e.g. microstructure, composition and hardness.

We report on laboratory friction experiments in which simulated faults are exposed to shear velocity oscillations at different amplitudes and frequencies. Granular layers are sheared in a servo-controlled biaxial apparatus at constant normal stress and background shear velocity, with a shear velocity sinusoid superimposed. Correlation between oscillations and stick-slip events is determined by the timing of dynamic failure with respect to the oscillation phase. Schuster's test is used to calculate the statistical likelihood of phase recurrence. We find that correlation of failure with the shear stress oscillation depends on oscillation frequency and amplitude at low frequencies and solely on oscillation amplitude at high frequencies. The frequency boundary between these two regimes is proportional to the inverse of the time needed to displace the frictional critical slip distance. We evaluate changes in failure characteristics, including failure strength, recurrence time between events, creep, and phase of the oscillation at failure, to assess the effects of stressing rate oscillations. Failure occurs at maximum shear stressing rate at low frequencies and lags peak stressing rate in the high-frequency regime. Friction at the onset of dynamic failure decreases with increasing frequency. The distribution of events through time depends on the frequency of the shear oscillation; low-frequency oscillations produce bimodal distributions and high-frequency oscillations produce unimodal distributions. If the transition between the failure regimes depends on the critical displacement length as our experiments imply, the critical frequency will vary for faults with different gouge layer thicknesses and total displacement.

Abstract The response of epitaxial CoFe2O4 thin films to biaxial compressive stress imposed by MgAl2O4 and SrTiO3 single crystalline substrates is studied using X-ray diffraction and Raman spectroscopy. It is found that the Poisson ratio signals a non-auxetic behavior and depends on the substrate used. The Raman modes show an increase in frequency when increasing compressive strain by reducing film thickness; this is due to the shrinking of the unit cell volume. Such behavior is in qualitative agreement with recent ab initio calculations, although the measured values are significantly smaller than predictions. In contrast, the measured Poisson ratio is found to be in good agreement with expectations based on general arguments of atomic packing density. Possible guidelines for searching aux...

In this study, numerical simulations using the discrete element method (DEM) were performed to examine the evolution of pore characteristics in dense and loose samples subjected to a biaxial creep test. Sliding creep between particle contacts was incorporated in the DEM simulations, which displayed similar creep behavior found in experiments. The irregularly shaped pore geometry in the soil packing was quantified with a best-fitting ellipse with the aid of the region-based method. It has been found that the initial density of soils and the deviatoric stress values under which creep starts determine unique evolution of pore space. In addition, the weak pore structures, elongated along the horizontal direction (or perpendicular to the axial loading), collapse first and ultimately only those ...

We study the influence of inherent anisotropy, i.e. bedding angle on stress-strain behavior and shear band formation in quasi-static granular media. Plane strain biaxial tests are carried out using two-dimensional distinct element method (DEM). Oval/elliptical-shaped particles are generated by overlapping the discrete circular elements. Particle assemblies with four different bedding angles are tested. Evolution of the microstructure inside and outside the shear band and effect of bedding angle on the microstructure are investigated. Influence of bedding angle on fabric and force anisotropy is studied. It is found that by using non-circular particles, generation of large voids and excess particle rotations inside the shear band are reproduced in a quite similar manner to those of the natur...

Consideration is given to characteristics of micro-level fracture in notched unidirectional graphite-epoxy, progressive matrix cracking of crossply composite laminates under biaxial loading, the use of microcrack analysis in performance simulation for composite material systems, modeling of the flexural behavior of the ceramic-matrix composites, the effects of microcracking on thermal expansion and cyclic stress-strain relations of composites, and analysis of damage growth in particulate composites using a work potential. Attention is also given to modeling of cracking induced damage in particulate and fiber-reinforced composites, interaction of cracks in anisotropic matrix and related problems, micromechanics of brittle composites exposed to chemically aggressive ambients, and effects of interface on tribological properties of graphite/aluminum composites.

Stress-strain and durability information is often desirable for situations in which strain and temperature are changing simultaneously. To obtain such information, strain controlled uniaxial push-pull tests have typically been done. In order to control the mechanical strain, it is necessary in such tests to compute the mechanical strain from the total measured strain using measured temperature and the thermal expansion properties of the specimen. A system for conducting torsional thermomechanical tests is described which has the great advantage that the torsional strain is unaffected by the changing temperature and thus real time computations of quantities is not required for control of the test and the mechanical strain need not be determined from the subtraction of two measured qnantities as is the case in the uniaxial test. In addition to describing torsional thermomechanical tests, guidelines for software to be used in running biaxial thermomechanical tests will also be presented.

Fracture, toughness values for A533-B reactor pressure vessel (RPV) steel obtained from test programs at Oak Ridge National Laboratory (ORNL) and University of Kansas (KU) are interpreted using the J-A{sub 2} analytical model. The analytical model is based on the critical stress concept and takes into consideration the constraint effect using the second parameter A{sub 2} in addition to the generally accepted first parameter J which represents the loading level. It is demonstrated that with the constraint level included in the model effects of crack depth (shallow vs deep), specimen size (small vs. large), and loading type (uniaxial vs biaxial) on the fracture toughness from the test programs can be interpreted and predicted.

A description is given of the multi-term, finite strip analysis of the free vibration and buckling, under a system of applied biaxial direct and shear stresses, of thin, prismatic shell structures. The walls of the structure may be composite laminates with a general lay-up. The analysis is based on the use of Koiter-Sanders thin shell theory. Combinations of diaphragm, clamped and free conditions at the two ends of a structure are incorporated. The displacement field of a transversely-curved finite strip utilizes Bernoulli-Euler beam functions in the longitudinal direction and quintic polynomial representations in the circumferential direction. The superstrip concept is used in conjunction with the modified Sturm sequence-bisection approach to provide an efficient analysis capability. Several applications involving flat plates, curved plates and complete cylinders are detailed.

A nonlinear and time-dependent fibre beam element model able to simulate the response of existing reinforced concrete (RC) frame structures subjected to repair and strengthening interventions is presented in this paper. The relevant attributes of the proposed formulation are: (i) its capability for considering shear effects in both service and ultimate levels and (ii) the step-by-step nonlinear sequential type of analysis, which allows capturing the strengthening effects, accounting for the state of the structure prior to the intervention. The 2D fibre beam element developed is based on the Timoshenko theory and a hybrid (kinematic/force) formulation is used to simulate the response of RC sections under combined normal and shear stresses. Biaxial constitutive equations assuming smeared rot...

Boron nitride (BN) thin films were deposited on monocrystalline Si (100) wafers using electron beam evaporation of boron with simultaneous bombardment by nitrogen and argon ions. The effect of film thickness on the resultant BN phase was investigated using Fourier transform infrared (FTIR) spectroscopy and high resolution transmission electron microscopy (HRTEM). These techniques revealed the consecutive deposition of an initial 20 A thick layer of amorphous BN, 20--50 A of hexagonal BN having a layered structure, and a final layer of the cubic phase. The growth sequence of the layers is believed to result primarily from increasing biaxial compressive stresses. Favorable surface and interface energy and crystallographic relationships may also assist in the nucleation of the cubic and the hexagonal phases, respectively. The presence of the amorphous and hexagonal regions explains why there have been no reports of the growth of 100% cubic boron nitride on Si.

The uniaxial drawing of PA6/tie/PE multilayer blown films is investigated in relation to the behavior of the pure components. The tensile stress-strain behavior of the multilayers obeys a simple additive mixture law of the one of the components which is consistent with the parallel mechanical coupling of the layer structure of the films. No significant difference is observed in the crystallographic evolution of each layer in the multilayer films as compared with the parent monolayer films. The rupture of the multilayer films yet appears to be directly governed by the PA6 layer, as it triggers an early rupture of the PE layer without decohesion. This contrasts with the rupture behavior upon biaxial drawing that was previously shown to strongly depend on the layer thickness ratio: the thicke...

Afilament stretching rheometer (FSR) was used for measuring the start-up of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational rate. A narrow molecular mass distribution linear polystyrene with a molecular weight of 145 kg / mole wis subjected to the start-up of elongation for three Hencky strain units and subsequently the reversed flow. The integral molecular stress function formulation within the 'interchain pressure' concept agrees with the experiments. In the experiments the Hencky strain at which the str~ss becomes zero (the recovery strain) in the reversed flow has been identified. The recovery strain is found to increase with elongational rate, and has a maximum value of approximately 1.45. The Doi Edwards model using any stretch evolution equation is not able to predict the correct level of the recovery strain.