Analysis of the necking inception in the quasi-static tests at different temperatures
In order to assess the influence of the thermal softening on the necking inception, the quasi-static curves at different temperatures have been analysed. The obtained quasi-static experimental true curves are shown in Fig. 1 while the mean yield stress and necking strains, obtained for each group of tests by the maximum load condition, are reported in Table 2.
Such results clearly demonstrate that despite the temperature is maintained constant during each test, it has a great effect in reducing the necking inception strain, the latter varying from 0.44 at room temperature to 0.18 at 300 °C. This is in contrast with eq. (5), predicting that whatever temperature, if constant, cannot affect the necking onset.
Such fact means that the thermal softening cannot only depend on the temperature as postulated in eq. (5), but it must also include a direct dependence on the strain.
In other words, different constant temperatures do not simply scale the quasi-static flow curves by constant factors leaving them homothetic to each other, but such different temperatures also change the shapes of the stress-strain curves during the straining history thanks to a certain degree of strain-dependency; otherwise the necking inception strain would not have changed.
Table 2. Quasi-static Yield Stress and Necking Strain at different temperatures