Molecular dynamics simulations were used to study the shock-induced collapse of cylindrical pores in oriented single crystals of the energetic material a-1,3,5-trinitroperhydro-1,3,5-triazine (a-RDX). The shock propagation direction was parallel to the [100] crystal direction and the cylinder axis of the initially 35.0 nm diameter pore was parallel to [010]. Features of the collapse were studied for Rankine-Hugoniot shock pressures P s = 9.71, 24.00, and 42.48 GPa. Pore collapse for the weak shock is dominated by visco-plastic deformation in which the pore pinches shut without jet formation and with little penetration of the upstream material into the downstream pore wall. For the strong shock the collapse is hydrodynamiclike and results in the formation of a jet that penetrates significantly into the downstream pore wall. Material flow during collapse was characterized by examining the spread and mixing of sets of initially contiguous molecules and evolution of local velocity fields. Local disorder during collapse was assessed using time autocorrelation functions for molecular rotation. Energy deposition and localization was studied using spatial maps of temperature and pressure calculated as functions of time.
Molecular dynamics simulations were used to study the mechanisms of shock-induced inelastic deformation in oriented single crystals of the energetic material pentaerythritol tetranitrate (PETN). Supported planar shock waves with Rankine–Hugoniot shock pressures P
R–H ∼ 9 GPa were propagated along two different crystal directions: one that is sensitive to initiation ([001]) and another that is relatively insensitive to initiation ([100]). Qualitatively, it was observed that for the sensitive orientation only elastic compression occurred, leading to the propagation of a single wave through the material, whereas for the insensitive direction elastic compression at and immediately behind the shock front was followed by inelastic deformation, leading to a two-wave structure in which the sharp elastic front moves through the crystal at a higher speed than the broader plastic wave. The detailed responses were characterized by calculating several structural and thermal properties including: relative center-of-mass molecular displacements (RMDs), classification of molecules behind the shock front as either elastically compressed or inelastically displaced, spatially resolved intermolecular and intramolecular temperatures (kinetic energies), and pre- and postshock intramolecular dihedral angle distributions. A quasi-2D system was studied for the [100] shock to further characterize the inelastic deformation mechanisms. Subregions exhibiting differing types of deformation were identified and examined in greater detail; specifically, time histories of the total kinetic energy (expressed in temperature units) and the rotational order parameter were calculated separately for elastically compressed and inelastically displaced molecules in a given subregion. The times required for re-establishment of the Maxwell–Boltzmann distribution of atomic kinetic energies and molecular center-of-mass kinetic energies in the shocked material were determined.
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