A new miniature mechanical testing procedure: Application to intermetallics
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I.
INTRODUCTION
THE mechanical properties of materials at elevated temperatures are usually determined by testing a series of specimens fabricated to a specific shape and size. The two simplest and most common procedures are tension and compression testing. However, care must be exercised for both procedures in determining the values of the true stress and true strain at any selected instant, because the gage length in tension and the cross section in compression both change continuously with increasing deformation. Furthermore, compression testing provides no information on specimen ductility, and the use of tensile testing invariably leads to difficulties when attempts are made to fabricate specimens with reduced gauge lengths from hard or semibrittle materials. The alternative procedure of double-shear testing was introduced several years ago as a simple method for obtaining information on the high-temperature deformation of pure metals and metal alloys.[1,2] For tests in double shear, a symmetrical specimen is machined such that there are two outer gripping sections, an inner section for application of the load and two small reduced sections that are subjected to conditions of pure shear during testing. This procedure has been used successfully to date in numerous experiments, including the testing of large-grained metals under creep conditions,[3–6] small-grained metals under superplastic conditions,[7,8] and metal matrix composites in high-temperature creep.[9,10] There are three important advantages in using doubleshear testing. First, fabrication of the test specimens is rel-
atively easy, so the procedure is attractive for use with semibrittle materials such as intermetallics. Second, since the specimens experience no change in cross-sectional area during testing, the experiments provide a direct measure of true stress and true strain. Third, when using the doubleshear configuration in creep testing, the tests are necessarily conducted under conditions of true constant stress without the requirement of incorporating a constant stress lever arm into the testing facility. The present study was motivated by the need to establish a simple and consistent testing procedure in order to determine the high-temperature mechanical properties of selected intermetallics. Since intermetallic materials are usually available only in very limited quantities, it was deemed advisable not only to test under double-shear conditions but also to decrease the overall length of the specimens from the standard value of ;3 to 5 cm to a reduced value of ;1.0 to 1.5 cm. Therefore, a series of careful experiments was conducted initially in order to examine the feasibility of performing satisfactory tests using a miniature configuration of this type. The purposes of this paper are twofold. First, experimental data are presented on pure Al and an Al-5 pct Mg solid solution alloy using the miniature double-shear configuration, and it is demonstrated that the results serve to establish the viability and reproducibility of this testing p
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