On the origin of the stress spike decay in the elastic precursor in shocked metals

Abstract

High-strain rate experiments are commonly employed to study the dynamic strength of metals, by generating a plane shock wave and measuring the amplitude of the elastic precursor. In some cases, the shock wave is rapidly relaxed after the elastic precursor, leading to a spike in the stress wave. We propose that the observed spike and the following relaxation arise from the interplay between the rate by which dislocations are nucleated and the mobility of the existing ones. In addition, we suggest that the elastic precursor decays since glide takes a larger role in the plastic deformation as the plastic strain rate decreases. The interplay is demonstrated in a physically, dislocation-based dynamic strength model, using dislocation mobility rules from molecular dynamics simulations, as well as a dislocation nucleation model which is fitted using a metamodel optimization technique. Our results show that the stress spike and its decay in annealed body-centered cubic specimens arise from the need to nucleate dislocations to generate a plastic deformation when the mobility of existing dislocations is insufficient to accommodate plastic strain. Cold-rolled targets have sufficient amount of initial dislocations, so glide, rather than nucleation, can accommodate the plastic relaxation, and as such do not exhibit a spike. These insights shed light on the experimentally observed differences between dynamic and static strength of materials, and, in particular, on the anomalous dependence of the dynamic strength on temperature and pretreatment of materials at high-strain rates.