Control of superplastic deformation rate during uniaxial tensile tests
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I.
INTRODUCTION
THE ability to control and predict the rate of deformation during superplastic deformation is essential for the success of superplastic forming processes. For widespread application of this technology it is necessary to predict the development of the strain state, strain rate, and the necessary forming parameters for the deforming sheet during forming operation. Control of strain rate is also an essential element in the pursuit of accurate experimental data required for developing reliable constitutive equations which form the foundation for mechanistic models. In the past, superplastic tensile tests have been conducted under constant crosshead speed conditions in which the sample experiences a decreasing strain rate during a test. The large elongations (300 to 1000 pct) inherent in superplasticity make the associated decreases in strain rate both significant and unpredictable. Also, because of typically high test temperatures and softness of test samples, monitoring of strain rate with extensometry has not been possible. This inability to control strain rate within the uniform gauge section during tensile tests makes it difficult to determine stresses accurately in these highly rate-sensitive materials. A prior approach to correcting for strain-rate variation involved continuously changing the crosshead speed (CHS) during tensile test to achieve a nearly constant strain rate. v] This simple procedure assumed uniform deformation within the gauge length and no end effects. The resulting exponential CHS schedule (Eq. [1]) has been successfully used for a Ti-6A1-4V alloy.[1] CHS = efo e ~'
[1]
where ~ = desired test strain rate, ~?o = initial gauge length, and t = time from the start of the test.
P.A. FRIEDMAN, Graduate Student, and A.K. GHOSH, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted August 10, 1995. 3030~VOLUME 27A, OCTOBER 1996
Tensile tests at constant CHS as well as constant strain rate (using Eq. [1]) have been conducted on superplastic 5083 A1 test specimens.t2] The typical superplastic specimen geometry and grip design used in this study are depicted in Figures l(a) and (b). A comparison of stress-strain plots for these cases is shown in Figure l(c). The differences in flow stress and hardening behavior observed between these two experimental techniques are significant. It has been discovered that the "so-called" constant strain-rate test actually produced an increasing strain rate in the specimen, thereby causing excessive apparent strain hardening (Figure 1(c)). Typically, the gauge length of superplastic specimens is limited to 12 mm because, with extensive deformation seen in superplastic materials, there is a possibility that the extended sample length might exceed the length of uniform temperature zone of conventional laboratory tube furnaces. End effects such as material flow from the grip region into the gauge section have been found to create strain gradients along gauge lengths
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