Molecular dynamics simulations of shock-induced pore collapse in single crystal [beta]-HMX

Abstract

This thesis presents all-atom classical molecular dynamics (MD) simulations of shock-wave passage across a pore-type defect in single-crystal [beta]-1,3,5,7-tetranitro-1,3,5,7-tetraocane ([beta]-HMX). The purpose of this study is to understand the anisotropic thermo-mechanical response of HMX during and following shock-induced pore collapse. This thesis is comprised of three studies. The first study is focused on understanding the sensitivity of overall results to: sample thickness in a quasi-2D cell, transverse orientation of the crystal relative to the shock direction, and run-to-run variability. The second and third studies are focused on comparing MD predictions head-to-head against continuum predictions for a shock wave passing through, respectively, a cylindrical pore and an elliptical pore. The first sub-study in chapter 2 focuses on shock-induced pore collapse for two different crystal orientations and two different cell thicknesses, to validate decisions on suitable cell orientations and cell dimensions for the subsequent studies. The second sub-study compares predictions from three different independent realizations of the same simulation scenario, to assess run-to-run variability among results for simulations initiated from different but statistically equivalent initial conditions. This study revealed that cell orientation seems to have a moderate effect on the collapse and that transverse thickness has almost no effect on overall system response. The repeatability study provided confidence that a single simulation for a particular scenario can be interpreted as representative. The conclusions from Chapter 2 were then used to guide the design of the MD systems in the second and third study (Chapter 3 4), where MD is used as "ground truth" against continuum model that can capture the circular pore collapse phenomena. Head-to-head comparisons were done between MD predictions and a systematic hierarchy of five different continuum models, each incorporating increasing physical fidelity and amount of information from MD. Analyzing the simulation data qualitatively and quantitatively revealed that the hydrodynamic pore collapse induced by strong shocks was captured closer to MD predictions using a model that was incorporated with most MD-derived parameters. However, for weaker shocks, differences were still evident even for the "best" continuum model. The "best" continuum model from Chapter 3 was then tested for the case of an elongated pore in Chapter 4, where both the MD and continuum models were identical in cell dimensions and impact conditions. This work confirmed that, for non-trivial pore shapes at intermediate-strength shock speed, isotropic rate-dependent Johnson-Cook-type elastoplastic continuum models predict pore-collapse mechanisms that are consistent with predictions from MD.