where ∂2σ/∂ɛ2 tends to zero and (iii) region III, View the MathML source increases again (see Fig. 5). Fig. 5 illustrates a schematic representation of the true stress-strain curve and the slope of this curve, in which ɛ∗ is defined as the strain value associated with the transition between region II and region III. Experimentally, it is observed that ɛ∗ is not very sensitive to the strain rate and, thus, it can be assumed as a material constant.