Stress threshold for precursor decay in LiF

Single crystals of LiF have been mechanically shocked by projectile impact so as to produce shocks propagating in a 〈100〉 direction. Velocities of projectiles have been varied to produce shock pressures from 4.9 to 28.6 kbar in the LiF. Pressures were measured with thick quartz gauges after shock travel distances of approximately 3 mm. The 4.9-kbar wave was perfectly elastic. The precursor of an 8.3-kbar wave showed no attenuation, but stress relaxation occurred between precursor and plastic shock. A 10.4-kbar precursor was measurably attenuated from its impact value. These results are taken to indicate a threshold shear stress between 2.4 and 3.0 kbar for nucleation of dislocations in the shock front.