3.3. Parameters identification
As mentioned in the previous section, the proposed model (see Eq. (3)), can be divided into two parts, where the parameter ɛ∗ must be determined first. For estimating this parameter, the last transition point (transition between region II and III) from the slope of true stress-strain curve was taken into account. Fig. 7(a–d) show the experimental data of true stress-strain curves and the relative slopes for different constant engineering strain rates. The value of ɛ∗ is assumed approximately equal to 0.8 for all cases. Note that in Fig. 7(b) and (c), the rupture occurs immediately after this value.