Strain Rate Effects in Shock and Quasi-Isentropic Compression of Solids*

We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock and quasi-isentropic compression (QIC) in defective copper crystals. The atomic interactions were modeled employing a well-tested embedded-atom method (EAM) potential for Cu. We examined the deformation mechanisms and material strength for strain rates in the range of $10^9$ - 10$^{12}$ s$^{-1}$, a regime relevant to the validation of material strength models. For samples with a relatively low density of pre-existing defects, the strain rate dependence of the flow stress follows a power law with an exponent of 0.40. On the other hand, for samples with a higher density of pre-existing defects, the flow stress exhibits a narrow linear regime at strain rates above $10^9$ s$^{-1}$ and then bends over at higher strain rates in a manner reminiscent of shear thinning in fluids. Both the NEMD shocks and QIC show behavior similar to sheared fluids, which can be described by the Ree-Eyring theory of non-Newtonian viscous flow.