Molecular modeling of the time-dependent shock response in polyurea with Hugoniostat and explicit impact simulations

Molecular dynamics simulations are applied to study the shock response of the hard and soft phases of nanostructured polyureas. Shocks are modeled using both a nonequilibrium explicit impact method and a quasistatic Hugoniostat method. These methods subject systems to different strain rate histories, resulting in dissimilarities in the transient evolution of the thermodynamic state of the material during shock loading. Once materials are compressed into glassy states, both Hugoniostat and impact simulations display similar Eyring-like relaxation of the residual shear stress corresponding to a logarithmic slowdown in plastic deformation post-shock. The slowly relaxing shear stress makes the prediction of post-shock densities and shock speeds sensitive to total simulation time. While Hugoniostats can efficiently resolve the steady-state limiting Hugoniot, our results indicate that this is unlikely to be realized in experiments over the timescales during which shocks transit real microstructures. Instead, real shocks will travel at transient shock speeds governed by the dynamic stress relaxation of the relevant phases, which can be predicted by explicit impact simulations.