The Influence of Thermal Activation on Plastic Deformation Instability in Iron and Steels

Abstract

Several phenomenological criteria have been proposed to determine the conditions of appearance of the tensile plastic instability. Generally the critical conditions are given by relations including the work hardening coefficient γ and the rate sensitivity m. In order to show the effect of physical parameters on the criteria, we have taken into account the following points: plastic deformation is thermally activated, density of mobile dislocations varies as deformation proceeds and internal stress depends on grain size. This analysis leads to an instability criterion which describes qualitatively the experimental results obtained at low and very high strain rates in iron and steels. So we have noticed that the instability appears earlier during testing if test temperatures and grain size decrease and if strain rate and activation volume increase.