Confirmation of machine-induced load oscillations during superplastic tensile testing
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have suggested that it is the combination of the varying crosshead speed coupled with the strain-rate sensitivity of the flow stress that gives rise to this phenomenon. They report that whereas the overall average crosshead speed may be constant (or uniformly varying in the case of a constant strain-rate test), there are, nonetheless, short period variations in the crosshead speed that result in the imposition of Varying strain rate imposed on the test specimen, and for a strain-rate-sensitive material, such strain-rate variations will cause corresponding load variations. They provide supporting evidence of this possibility by measuring loads resulting from a dashpot having a high strain-rate sensitivity; an m value of 0.9, for which load variations were shown; and the amplitude correspond to the strain-rate variations imposed by the test machine. While the results clearly show that the load oscillations can occur when a rate-sensitive load train (i.e., dashpot) is exposed to crosshead speed variations, there is not yet evidence definitive that this is the sole mechanism during deformation of a superplastic material. It is felt that a one-to-one correspondence between crosshead velocity and load variations during testing of a superplastic material would provide additional support for this mechanism. The purpose of this study is to explore this correlation and provide data further confirming or questioning that the mechanism suggested by Li and Blackwell[4] is the cause of the load oscillations observed in tensile testing superplastic materials. The material used in this study is the AA5083 A1 alloy processed to a grain size of about 8 ~m in diameter and is modestly superplastic, having a maximum m of 0.6 and a total elongation of 300 to 400 pct at a strain rate of 5 x 10-4 s -~. This alloy exhibited load oscillations when tested at 520 ~ on a modified Instron screw machine (model "Iq'E). This machine was modified by having the input shaft turned by a computer-controlled stepper motor, which permits testing under constant strain rate rather than just constant extension rate. No change was made to the Instron crosshead control system [that is, no change in the proportional-integral-derivative (PID) parameters or in the tuning of this analog system]. The computer-controlled stepper motor (Superior M061-FD02) is used in an open-loop system. An input instantaneous crosshead velocity value is converted to angular velocity and sent via IEEE488 to the stepper motor controller (Superior SP153A) such that constant strain rate is imposed. In the high-temperature superplastic tests of the 5083 alloy, load and crosshead displacement measurements were acquired through an HP* 3497A data acquisition unit. Data was stored on the *HP is a trademark of Hewlett-Packard Company, Colorado Springs, CO.
486/33 computer at a collection rate of 1 Hz. Figure 1 shows a portion of the load vs displacement of this "constant" strain-rate test (0.0005/s with velocity updates at 1 Hz). Load oscillations are observed to be about ---0.5 kg in magni
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