A Novel Mechanical Method to Measure Shear Strength in Specimens Under Pressure
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0929-II03-01
A Novel Mechanical Method to Measure Shear Strength in Specimens Under Pressure Juan Pablo Escobedo1, David Field1, David Lassila2, and Mary Leblanc2 1 Mechanical and Materials Engineering, Washington State University, Spokane and College, Pullman, WA, 99164 2 Lawrence Livermore National Laboratory, Livermore, Ca, 94550
ABSTRACT A new experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (on the order of 4 GPa). This testing procedure experimentally determines the pressure-dependent shear strength of thin foil specimens. The experiments provide calibration data for models of materials subjected to extreme pressures such as the Steinberg-Guinan hardening model and can assist in model validation for discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 1 to 4 GPa. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure. INTRODUCTION Plastic deformation in metallic systems occurs primarily via dislocation generation and movement due to shear stresses. In the case of hydrostatic loading no shear stresses are present to provide a driving force for dislocation motion, therefore structural properties are not changed due to hydrostatic pressure alone. Nevertheless it is found in the literature that properties such as hardening and ductility of metals are sensitive to superimposed pressures. Even at relatively low pressures of 0.7-3.0 GPa a remarkable increase in ductility of some materials has been reported [1]. Bridgman’s results [2,3,4,5] suggest that pressure hardening occurs in Mo, Ta and Ni. Another property of interest is the shock-induced phase transformation. Numerous cases of phase changes have been reported when materials were subjected to pressures exceeding 30 GPa [6,7,8]. Finally from the physics perspective, the relationship between material strength, elastic constants, and microstructure is of great interest, and can lead to new insights of mechanisms of plastic flow under conditions of imposed pressure. Most high-pressure research has been carried out under static conditions. For pressures in the range of 0-3 GPa testing has been conducted using a variety of media including solid, liquids and gases [1]. To achieve higher pressures, experiments have been conducted using the diamond anvil cell, [6], where the specimen is loaded to high pressures between the diamond anvils. Although this device allows ultrahigh pressures to be reached readily, it has the deficiency that the hydrostatic, frictional and deviatoric stresses increase in an uncontrolled manner as the load increases, making it difficult or impossible to determine strength and work hardening from experimental results. Another disadvantage of this method is that typically the volume of material tested in these kinds of systems is s
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