Shock-Induced Inelastic Deformation in Oriented Crystalline Pentaerythritol Tetranitrate
RM Eason and TD Sewell, JOURNAL OF PHYSICAL CHEMISTRY C, 116, 2226-2239 (2012).
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.
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