  Shock-wave profiles of sapphire (single-crystal Al2O3) with seven
crystallographic orientations were measured with time-resolved VISAR
interferometry at shock stresses in the range 16 to 86 GPa. Shock propagation
was normal to the surface of each cut. The angle between the c-axis of the
hexagonal 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, shock-induced optical transparency correlates
with the smoothness of the shock-wave profile. The ultimate goal was to find
the direction of shock propagation in sapphire that is most transparent as a
window. 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 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 orientations that remains most transparent to
highest shock pressures. Under purely elastic deformation sapphire has very
high spall strength, which depends on load duration and peak stress. Plastic
deformation of sapphire causes loss of its tensile strength.
