Molecular Dynamics Predictions of Shock-Induced Pore Collapse in (010)-Oriented beta-HMX: Effects of Sample Thickness and Transverse Orientation, and Run-To-Run Variability among Statistically Equivalent Samples

PH Zhao and D Perera and T Sewell, PROPELLANTS EXPLOSIVES PYROTECHNICS, 47, e202200030 (2022).

DOI: 10.1002/prep.202200030

Using shock-induced quasi-2D pore collapse in beta-HMX as a specific case study, we address three practical questions that arise when designing and interpreting molecular dynamics (MD) simulations of explicit shock wave propagation in crystals. How sensitive are the overall results to sample thickness perpendicular to the (quasi) plane of the problem? For impacts on a given crystal surface, how much does the transverse orientation of the sample matter? And, for a given sample size and orientation, how much run-to-run variability exists among results for independent but statistically equivalent realizations of the shock? The first and second questions are interrelated but pertain individually to assessing the roles of finite-size and crystal anisotropy effects, respectively, on the simulated collapse mechanisms and associated local thermo-mechanical states in the sample during and after collapse. The third addresses the confidence with which the results of individual simulations can be regarded as representative. All three questions become particularly important if the MD predictions are intended to serve as "ground truth" for validation of continuum mesoscale models. Here, quasi-2D samples of (010)-oriented single- crystal beta-HMX (beta-1,3,5,7-tetranitro-1,3,5,7-tetrazocane) containing a cylindrical pore were subjected to reverse-ballistic impacts, resulting in explicit, supported shock propagation initially along 010, for impact speeds up =1.0 and 0.5 km s(-1). The individual samples differed in either the thickness perpendicular to the sample plane or the transverse crystal orientation normal to 010 in the quasi-2D cell. Three independent realizations of the shock were performed for a selected case to assess run-to-run variability. Comparisons of qualitative features of the collapse, temperature and pressure distributions in the samples, and time scales for pore collapse suggest that there is little sensitivity to sample thickness for the same crystal orientation and moderate sensitivity to transverse crystal orientation for samples of the same thickness. Run-to-run variability is evident to the eye in side-by-side system observations. However, overall mechanisms of collapse, distributions for temperature and pressure in the samples, and time scales for collapse are in near-quantitative agreement among the realizations.

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