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] or ientations using velocity interferometer system for any reflector inst rumentation for samples 3-6 mm thick. Input shock strength was 1.14 GP a. 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 ori entation was computed. Details of the elastic and plastic wave profile s are discussed. Molecular mechanics modeling of the shear induced by the uniaxial strain of a plane shock wave in this molecular crystal wa s also performed using the AMBER Code. This may be the first applicati on of molecular mechanics computation to a shear problem. The modeling correctly predicts the dependence of the precursor amplitude on cryst al orientation for the cases considered. The results confirm the impor tance of steric hindrance to shear in controlling the orientation-depe ndent strength in molecular crystals and sensitivity to shock initiati on of detonation in molecular explosive crystals. Details of the molec ular 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 quasistat ic deformation in spite of having the longest Burgers vector. The dyna mics of sterically hindered, shock-induced shear is considered.