A planar simple shear test and flow behavior in a superplastic Al-Mg alloy
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10/7/03
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A Planar Simple Shear Test and Flow Behavior in a Superplastic Al-Mg Alloy D.H. BAE and A.K. GHOSH Superplasticity is generally studied by performing tensile and gas-pressure-bulge tests. In formed parts, however, a variety of strain states, including in-plane shear, are encountered. The understanding of the mechanical response in shear is helpful in the study of superplastic metal forming. In this study, a device for a planar simple shear test was designed and used to perform tests on a superplastic Al-Mg alloy sheet at the elevated temperatures of 500 °C (773K) and 550 °C (823K). In such a test, the incremental rotation of the principal strain axes and specimen-end effects during deformation can complicate the determination of true mechanical response. The possible approximations regarding the strain state in the specimen gage have been investigated. The e-e curves developed based on a simple-shear assumption show a lower flow stress than that under uniaxial tension, and strain hardening # is related to dynamic grain growth. The rate of strain hardening at a fixed e level is essentially the same for both uniaxial tension and shear, but the difference in the effective stress between uniaxial tension and shear depends upon strain rate and temperature. This study marks the first known attempt to characterize large strain response for superplastic metals under conditions of simple shear.
I. INTRODUCTION
A deformation mode of shear is an important stress state in many sheetmetal forming operations. In order to study the mechanical response and microstructural evolution characteristics such as dynamic grain growth and cavitation during superplastic deformation, the shear test on a superplastic sheet specimen is worth investigating. Typical shear test methods for metallic materials are summarized in Reference 1. Except for the torsion test method, which imposes a pure shear state in tube or bar specimens, most other test methods provide a mixed stress state in the plane of the sheet. The torsion test has certain limitations. For solid bars in torsion, stress and strain are nonuniformly distributed in the specimen. For hollow tubes in torsion, plastic instability occurs at low strain, thus limiting its usefulness for large plastic strains. Furthermore, these methods are unsatisfactory for sheet materials, since producing a tube from a sheet would change its properties. A planar simple shear test method has been used here to learn about the mechanical behavior of sheet specimens. This test has shown some promise for determining the flow stress at large strains,[2–5] which is not always possible in the uniaxial tension test due to the effects of strain localization, failure, and cavitation. The simple shear deformation of a parallelepiped solid can be defined, as illustrated in Figure 1, by the relative displacement of parallel long-transverse (T) planes along the shear direction (longitudinal (L) axis). The engineering shear strain is expressed by the ratio /w, where is the relat
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