Plastic behaviour of the steel 20 MnMoNi 5 5 at transient high velocity loading

The dynamic deformation behaviour of the steel 20 MnMoNi 5 5 has been investigated using the Split Hopkinson Pressure Bar technique. Contrary to the usual practice short loading pulses were used. In this way highly transient loading conditions were attained with maximum strain rates in the range of 104 s-1, which simulates the situation near rapidly moving crack tips. One result is that the often used viscoplastic material models of Bodner-Partom and Perzyna are not suited to describe the transient response behaviour of the investigated steel. A model developed in the present work describes the observed response quite well. The model takes into account delayed yielding. Furthermore it is assumed that the density of mobile dislocations depends on the loading history. (orig.)