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