The next step is to estimate the parameters: σf,β,KIandn from the first part of Eq. (3) and View the MathML source from the second part for each engineering strain rate. In order to do this, all parameters were estimated using the Levenberg–Marquardt method [27], [28] and [29], which is a well-known and powerful iterative method for solving nonlinear least squares problems of parameter estimation. These parameters, for different engineering strain rates, are displayed in Table 1 and Table 2.