Biaxial Specimens Research Articles

Fracture parameters at bi-material ceramic interfaces under bi-axial state of stress

The stress intensity factors at the tip of a crack approaching an interface are determined numerically using finite element analysis. In order to investigate the effect of mixed mode on the fracture parameters a bi-axial specimen made half of alumina and half of zirconia was considered with a 45° inclined crack. The effects of crack length, shielding and anti-shielding were also addressed.

Asymptotic stress field at the tip of an inclined crack terminating to an interface

This paper presents the numerical results for the asymptotic stress field and the fracture parameters at the tip of an inclined cracks terminating to a bi-material ceramic interface. The numerical analysis was carried out using FRANC2D/L fracture analysis code. A biaxial specimen was modeled for producing different mixed mode loads and two materials combinations of Al2O3 and ZrO2 were considered. The influence of the material combination and applied mixed mode load on the singularity orders, stress distributions and stress intensity factors is highlighted.

Study on the Interface Characteristic of Superplastic Diffusion Bonding Joint of Dissimilar Steels

Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 μm) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Biaxial Tensile Deformation Behavior and Microstructural Evolutions of Superplasticity in AZ31 Magnesium Alloy

Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 )m) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

A Study on the Cracked Specimen for Biaxial Tensile Loading Test

In the recent years, the studies on the mechanical behaviors of various materials subjected to biaxial loading have been worked since they are more complicated and intrinsically different from those under the simple uniaxial condition. The cruciform specimen without any slots has been commonly used for the goal so far. We prepare improved biaxial specimen with slots and make sure its validity by means of finite element analysis and photoelastic experiment. Even though the equal load biaxiality was applied to the specimens, as the results, we found that the stress biaxiality ratios in central region of specimen differ according to the position from the center of them, the specimen with slots in the arms is more effective to make state stress uniform than the specimen without slot.

Characterisation of material properties for draping of dry woven composite material

Three test methods, uniaxial bias extension, biaxial and picture frame tests are used to characterise the shear behaviour of dry woven fabric during draping. The deformation of the bias extension and biaxial specimens is measured from images of a central gauge section. The forces applied to the material are resolved into forces along and parallel to the tow directions. The deformation of the material in the bias extension and biaxial tests is found to behave in a manner which is reasonably well described by a pin-jointed net analysis. There is negligible change in the shear resistance of the material during biaxial loading, while a slight increase in shear resistance is observed in the picture frame tests. Microscopic examination of the tow architecture [Compos Sci Technol 63 (2003) 99], which shows a significantly smaller crimp amplitude for picture frame tests than for the bias extension and biaxial tests, supports the suggestion raised by Harrison et al. [Proceedings of the 10th European Conference on Composite Materials, 2002], that the increase in resistance in the picture frame tests is associated with an increase in tow cross-over force generated by large loads along the tows.

Experimental approach to determine dynamic plasticity of micro-materials

For the dynamic yield stress estimation of micro scale material under low velocity impact loading, a technique has been developed which combines experimental measurements, of both load and deflexion of micro biaxial flexure Specimen, with stochastic finite element analysis for the stress level assessment. The experimental impact set-up, designed for the micro scale world with a needle as striker accelerated by way of a voice coil motor, allows to accomplish numerous tests easily, that is fundamental when mechanical characteristics depends highly of the material processing. An application with a 10 μm thickness aluminium foil at 99.5% of purity is proposed.

Tensile fracture of HTPB based propellant specimens

The failure of a viscoelastic material, such as solid propellant, is a complex phenomenon with a requirement to address the following two fundamental and complementary aspects in the failure criteria. First, what is the failure criterion for a solid propellant in stress–strain space, and what are the variations in failure data when failure is influenced by time and temperature effects? Second, how are conditions for failure influenced by the microstructure of the propellant? In fact, the usual practice is to present failure data in the form of two failure loci: one in the stress–strain plane where failure points are generated from tests at different strain rates from very low to very high values, and in others these failure stresses and failure strains are plotted against the corresponding strain rates. Uniaxial tensile specimens are generally used to determine the standard failure properties such as tensile strength and the percentage of elongation under constant strain rate and temperature. In order to understand the failure behaviour of propellant materials, the fracture data of a HTPB (hydroxyl terminated polybutadiene) based propellant generated from the uniaxial tensile specimens, is considered. Following the work of Smith and Stredry, the fracture data are represented by the curve of strain at maximum stress versus temperature reduced strain rate, and the failure boundary curve of maximum stress with the corresponding strain. Uniaxial and strip biaxial tension specimens are analysed here for various loading rates. Failure analysis has been carried out by considering the master stress relaxation modulus data as well as the failure data of the propellant.

Influence of the Loading Apparatus on the Stresses within Biaxial Specimens

Abstract The design and construction of a biaxial loading machine is presented in this paper. Two major concerns are addressed in designing the machine: alignment between the loading axes and the specimen, and shear stresses (friction) that may develop between the loading platens and the specimen. The alignment requirement is solved by constructing a multi-frame machine in which the horizontal frame hangs from the top of a 890 kN (200 Kip) Baldwin machine by cables and springs, thus allowing displacements and rotations of the horizontal loading axis with respect to the vertical axis. Shear stresses between the platens and the specimen are reduced by using brush platens. These platens are made of independent plates flexible enough to bend and thus reduce the transmitted friction. Numerical modeling of the interaction between the machine, platens, and the specimen shows the advantage of the multi-frame machine with brush platens over other combinations. With a multi-frame machine with brush platens, the stress field is uniform at the center of the specimen; differences between the applied and the obtained stresses are mostly within a 10% error, and are produced at the top boundary of the specimen. A single-frame machine with brush platens produces a nonsymmetric stress field and increases the confinement in the specimen as much as 5%. Rigid platens produce unloading within the specimen, which will cause an overprediction in the strength of the material.

Biaxial direct tensile tests in a large range of strain rates. Results on a ferritic nuclear steel

Constitutive equations are usually calibrated only trough the experimental results obtained by means of uniaxial tests because of the lack of adequate biaxial experimental data especially at high strain rate conditions. These data are however important for the validation of analytical models and also for the predictions of mechanical behaviour of real structures subjected to multiaxial loading by numerical simulations. In this paper some developments are shown concerning biaxial cruciform specimens and different experimental machines allowing biaxial tests in a large range of strain rates. This experimental campaign has also allowed study of the influence of changing the strain paths. Diagrams of equivalent stress versus straining direction and also equivalent plastic fracture strain versus straining direction are shown.

Failure Investigation of Graphite/Polyimide Fabric Composites at Room and Elevated Temperatures Using the Biaxial Iosipescu Test

The biaxial and modified biaxial Iosipescu shear test methods were applied to determine the shear dominated, biaxial mechanical response of graphite/PMR-15 and graphite/Avimid-R woven fabric composites at room and elevated temperatures. Three different composite architectures were examined: T650-35 warp-aligned, 8-harness satin (8HS) fabric in a PMR-15 matrix, T650-35 warp-aligned, 8HS fabric in an Avimid-R matrix and T650-35 0°/90°, 8HS fabric in an Avimid-R matrix. Several biaxial Iosipescu tests were performed at room temperature under shear, shear-tension, and shear-compression loading conditions to characterize damage and obtain biaxial, shear dominated failure properties. Shear tests were also conducted at elevated temperatures approaching 316 °C to determine the effects of temperature on the shear strengths of the composites investigated. A nonlinear finite element analysis is briefly introduced to evaluate the effects of specimen sliding and geometric nonlinearities on the stress and strain distributions in the biaxial Iosipescu specimens. In addition, the effects of different loading block geometries on the stress distribution in the Iosipescu specimens subjected to biaxial loads were also investigated. Within this investigation, it was found that graphite/Avimid-R was more resistant to biaxial, shear dominated failure at room temperature in comparison to graphite/PMR-15. However, the graphite/PMR-15 composite system exhibited better shear strength properties at elevated temperatures above 232 °C. It was also found that the effect of compression along the notch root axis generated by the loading blocks did not affect the loads at failure.

A three-dimensional viscoelastic constitutive model for particulate composites with growing damage and its experimental validation

A relatively simple nonlinear viscoelastic constitutive model for particle-filled rubber under three-dimensional stress states is developed from an existing axisymmetric constitutive equation and then experimentally verified. In extending the existing model to three dimensions, it is assumed that the damage leads to transverse isotropy with the axis of isotropy coinciding with the local, instantaneous maximum principal stress direction for monotonic loading. Rate-type evolution laws are used to account for the time-dependent changes in damage, and viscoelasticity of the rubber matrix is explicitly taken into account by using so-called pseudo variables; although strains may be large, in this theory rotations must be small. The model has been implemented in a finite element analysis to account for various geometry and loading conditions. Experiments were used to check the model. Various biaxial specimens of different aspect ratios, with and without holes or cracks, were tested with different cross-head rates. Load-deformation information reveal good agreement between theory and experiments, which is far better than using linear theory. A computer code based on the digital image correlation method has been developed and refined for accurate experimental displacement data extraction. The displacement field was further processed with a smoothing program for noise reduction. The specimen surface strain distribution is compared to predictions from theory. Nonlinear theory shows better agreement with experimental strain fields than linear theory.

Silicon Strength Testing for Mesoscale Structural Applications

AbstractThe development of power MEMS, such as the Microengine being developed at MIT, requires highly stressed structures to achieve high power densities. Material strength is, therefore, a critical issue for the design of such devices. Due to the stochastic nature of the strength of brittle materials, the length scales of the test specimens should be close to those of the structure in order to avoid excessive extrapolation of the test data. In this paper, strength characterization and supporting analysis of mesoscale biaxial flexure and radiused hub flexure single crystal silicon specimens are presented. The Weibull reference strength' of planar biaxial flexure specimens was found to lie in the range 1.2 to 4.6 GPa, depending on the surface quality. The local strength at stress concentrations was obtained by testing radiused hub flexure specimens. For the case of deep reactive ion etched (DRIE) specimens, the strength at fillet radii was found to be significantly lower than that measured from planar biaxial flexure specimens due to the inferior surface quality in such regions. It was found that strength could be significantly increased by the introduction of an additional isotropic etch after the DRIE step. The test results reported herein have important implications for the development of highly stressed microfabricated structures. The sensitivity of the mechanical strength to surface processing and etching techniques must be accounted for in the design cycle, particular with regard to the selection of the appropriate fabrication route. Furthermore, in the design of highly stressed MEMS devices, it is important to account for the stochastic nature of the material strength.

F.E. computation of a triaxial specimen using a polycrystalline model

The crystallographic approach provides an improved framework with respect to the classical macroscopic models to predict the stress-strain behavior of polycrystalline material. The model consists in a set of equations to represent the phase behavior, and a concentration rule. The present paper shows a numerical implementation of such a model, written in the framework of viscoplasticity with a threshold, in a parallel version of the F.E. code ZéBuLoN. Heavy computations can then be made. The system is used to simulate the mechanical response of a biaxial specimen developed at Laboratoire de Mécanique et Technologie de Cachan. This type of specimen is specially designed to support three-dimensional non-proportional loading paths, which are badly represented by classical models. The response obtained with the polycrystalline model is shown, and compared with the experimental data at the global scale. Local information concerning the stress and strain heterogeneity and the slip system activity are also available from the computation.

Life Prediction of Notched Specimens Using Multiaxial Surface and Subsurface Strain Analyses

Multiaxial fatigue life prediction analyses are applied to notched specimens subjected to high strain constant amplitude loading. The specimens are made from isotropic and anisotropic batches of the structural steel EN15R. Two sets of experimental data are used for the analyses; data from the biaxial fatigue of thin walled specimens and data from the uniaxial fatigue of hourglass specimens. The maximum strain parameter and two multiaxial fatigue approaches, the Brown-Miller and the Lohr-Ellison theories, are used to predict the fatigue life. A simple subsurface strain model is developed to overcome the geometry difference between the thin walled biaxial specimens and the solid bar notched specimens. Fairly good life prediction is obtained with the multiaxial fatigue parameters and the subsurface model using elastic-plastic finite element simulations.

BIAXIAL HIGH CYCLE FATIGUE TESTS ON A GAS TRANSMISSION PIPELINE STEEL

Abstract— Gas transmission pipes are sometimes subject to external damage due to interference by excavators. Optimized grinding of gouges may offer a solution in repairing pipes. Since pipelines may be subjected to internal pressure variations, the Dang Van criterion has been used to size the allowable grinding depth. The criterion's boundary has been determined from uniaxial fatigue testing and extrapolated to the higher hydrostatic pressure which occurs in practice. The aim of this paper is to check by means of biaxial fatigue tests on specimens, that this extrapolation ensures a lifetime of 105 cycles for the ground pipes. The state of stresses in the central area of the biaxial specimen was calculated from an elastic finite element simulation. The test machine had independent motions of the two perpendicular axes of loading. The fatigue testing was limited to 105 cycles and was carried out at a hydrostatic pressure of 200 MPa. The results validated the extrapolation technique for the Dang Van criterion.

A biaxial test specimen for crack arrest studies

A biaxially loaded, single edge notched (SEN) fracture specimen, with mixed modes I and II loading, was used to study the crack arrest capability of a bonded and riveted tear strap without and with simulated multiple site damage (MSD). MSD was modeled by a 50-percent groove without which the running crack would inevitably kink due to KII loading. A total of thirty-one 2024-T3 aluminum specimens with various crack and MSD configurations were tested. The fracture parameters associated with straight and curved crack paths were determined by using the experimenta results to drive a dynamic finite element model of the specimen in its generation mode. The crack kinking and extension criteria were verified by the excellent agreement between the prediction based on these fracture parameters and the measured crack kinking angles. Comparison between the test results generated by the biaxial stress specimens and by those generated by small- and full-scale pressurized fuselage rupture experiments showed that this specimen can be used to prescreen the effectiveness of tear straps and crack arrestors in an airplane fuselage.

K0-Compression of Reconstituted Loess and Sand with Stress Perturbations

A simple stress path - for uniaxial confined (K0-) loading - has been chosen to demonstrate the effect of stress probing on the value of the coefficient of earth pressure at rest K0. Two materials were used - reconstituted loess and medium sand - and two levels of hydrostatic stress probing (about 20 and 100 kPa). In the first case, with loess, no structural changes were observed. In the latter case, with sand, the soil structure changed partially. Various apparatuses were used : common and true triaxial and biaxial, and the results compared. Triaxial test results were identical and more or less confirmed Jaky's relation and the relationship proposed by the senior author, as well as biaxial tests with dense sand. Cam-clay overpredicted the K0-values. K0-coefficients measured on loose sand in the biaxial apparatus were lower than those of the triaxial apparatus. This difference was considered to be due to the stiffer response of biaxial specimens resulting from kinematic confinement.

Prévision de la durée de vie d’une structure précontrainte

The proposed model of calculation determines the duration of life of a structure with initial state; this is introduced in the finite element code through simulation of an imaginary thermal charge. The coefficients of the criteria of Crossland fatigue are calculated for each cycle of failure only with the help of the data obtened from the S-N curves of machined test tubes (unpeened test tubes).Shot peened aluminium alloy structures, cylindrical (traction and torsion) and biaxial specimens, have been used. The fatigue tests on shot peened cylindrical specimens revealed that the model correlates to the experiment. Besides, the application of the model has permitted to determine the S-N curves of the biaxial specimen (complete structure) peenned and unpeened.

Observations on the effects of particle rotations on the failure of idealized granular materials

Based on the computer simulations of biaxial tests, particle rotations are shown to have little influence on the elastic properties of idealized granular materials, but significant effects on their shear strength. The overall peak and residual friction angles, ø p and ø r, are smaller than the interparticle friction angle ø μ because of the concentration of rotations in shear bands. When particle rotations are artificially prevented, ø p and ø r, become larger than ø μ and the localized failure patterns become different from those observed experimentally in granular media. The average particle rotation remains small in the complete biaxial specimen, but increases in shear bands. The particle rotations follow an exponential distribution independent of the axial strain. Particle rotations induce twice as many rolling contacts as sliding contacts. Compared to sliding contacts, rolling contacts are oriented differently, generate different fabric tensors, and are less capable of dissipating energy and supporting inclined contact forces.