Shock-Induced Inelastic Deformation in Oriented Crystalline Pentaerythritol Tetranitrate

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 PR-H similar to 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.