Strength and elastic deformation of natural and synthetic diamond crystals shock compressed along [100] J.A series of magnesium single crystals, from 0.2 to 3 mm thick, were shock loaded in directions parallel and perpendicular to the c-axis of the hexagonal closed packed (hcp) structure and at 45 to the c-axis. Shock compression along the c-axis is associated with the largest Hugoniot elastic limit (HEL) for this material. Microscopic observation of recovered c-cut samples demonstrated intense twinning with a greater density of twins near the impact surface. The low-energy basal slip was activated by shock loading along the inclined direction and has the smallest HEL. In all cases, we observe the decay of the elastic precursor wave and growth of the HEL with increasing temperature. For the inclined shock compression after the HEL, two plastic waves were found where the stress level of the first plastic wave depends on the peak shock stress. Finally, the largest spall strength was along the transversal direction and the smallest in the off-axis direction. The fracture surface of the sample of transversal orientation contains numerous groves oriented along the base planes of the crystals. V C 2014 AIP Publishing LLC. [http://dx.

Shock-wave profiles of sapphire (single-crystal Al 2 O 3 ) with seven crystallographic orientations (c, d, r, n, s, g, and m-cut) were measured with time-resolved VISAR interferometry at shock stresses in the range 16 to 86 GPa. Shock propagation was in the direction normal to the surface of each cut. The angle between the c-axis of the hexagonal representation of the sapphire crystal structure and the direction of shock propagation varied from 0 for c-cut up to 90 degrees for m-cut in the basal plane. Based on published shock-induced transparencies for 3 directions of shock propagation, shock-induced optical transparency correlates with the smoothness of the mechanical shock-wave profile. The ultimate goal was to find the direction of shock propagation for which shock-compressed sapphire is most transparent as a window material. In the experiments particle velocity histories were recorded at the interface between a sapphire crystal and a LiF window. In most cases measured wave profiles are noisy as a result of heterogeneity of deformation. Measured values of Hugoniot Elastic Limits (HELs) depend on direction of shock compression and peak shock stress. The largest HEL values (24 GPa) were recorded for shock loading along the c-axis and perpendicular to c along the m-direction. Shock compression along the m-and s-directions is accompanied by the smallest heterogeneity of deformation and the smallest rise time of the plastic shock wave. m-and s-cut sapphire most closely approach ideal elastic-plastic flow, which suggests that m-and s-cut sapphire are probably the sapphire orientations that remains the most transparent to the highest shock pressures. Under purely elastic deformation sapphire demonstrates very high spall strength, which depends on both load duration and peak stress. Plastic deformation of sapphire causes loss of its tensile strength.2

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Elastic–plastic shock compression, unloading, and the stepwise shock compression of copper were investigated at room temperature, 710 °C, and 850 °C to expand the measurement range of high-rate deformations. The dependences of the dynamic yield stress on the temperature and pressure of shock compression were determined from an analysis of the free-surface velocity histories. Although the initial resistance to high-rate deformation increases anomalously with increasing temperature, even a small strain in the shock wave can change the sign of the temperature dependence of the flow stress. Using these data, the dependence of the plastic strain rate on the shear stress in shock waves and temperature was obtained in the range 105–107 s−1. It was found that at room temperature, the ratio between the shear stress and the plastic shear strain rate in a shock wave practically does not depend on the loading history, whereas at 850 °C, the parameters of the plastic flow in the second shock wave deviates significantly from the initial dependence for lower stresses and higher strain rates.