Unbiased estimates of the parameter rate sensitivity of superplastic materials

The procedure for determining the rheological superplasticity parameters K and m entering the standard power relation sigma = K xi (m) by the input data set describing the dependence of the flow stress (sigma) on the strain rate (xi) established by the results of standard uniaxial tests is suggested. The procedure is based on minimizing the deviations of the computed values of sigma from the corresponding experimental data. In contrast with the standard procedure, the suggested approach does not involve the linearization of the material model sigma = K xi (m) . The procedure is approved by the experimental data for alloys Al-12Si, Al-33Cu, Al-Cu-0.4Zr, and SURPAL known from the references. It is established that the linearization of the nonlinear superplasticity model sigma = K xi (m) can cause a shift in the estimates of parameter m by 5-10%, which in turn leads to an overestimation of the duration of superplastic forming by 10-30%.