MOLECULAR MECHANICS MODELING OF SHEAR AND THE CRYSTAL ORIENTATION DEPENDENCE OF THE ELASTIC PRECURSOR SHOCK STRENGTH IN PENTAERYTHRITOL TETRANITRATE

The elastic precursor shock strengths of pentaerythritol tetranitrate explosive crystals were measured for [100], [101], [110], and [001] orientations using velocity interferometer system for any reflector instrumentation for samples 3-6 mm thick. Input shock strength was 1.14 GPa. Measured precursor amplitudes were 0.38, 0.58, 0.98, and 1.22 GPa, respectively, for the four orientations. Critical shear stress for the slip system with the maximum resolved shear stress for each shock orientation was computed. Details of the elastic and plastic wave profiles are discussed. Molecular mechanics modeling of the shear induced by the uniaxial strain of a plane shock wave in this molecular crystal was also performed using the AMBER Code. This may be the first application of molecular mechanics computation to a shear problem. The modeling correctly predicts the dependence of the precursor amplitude on crystal orientation for the cases considered. The results confirm the importance of steric hindrance to shear in controlling the orientation-dependent strength in molecular crystals and sensitivity to shock initiation of detonation in molecular explosive crystals. Details of the molecular deformations and contributions to the energy barrier to inelastic shear for different orientations are given. The computational results also explain why the {110} [111] slip system is observed in quasistatic deformation in spite of having the longest Burgers vector. The dynamics of sterically hindered, shock-induced shear is considered.