Effect of crystal orientation on dynamic strength of LiF

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Abstract
Shock propagation data along 〈100〉, 〈110〉, and 〈111〉 directions in LiF crystals are presented. A marked anisotropy in wave profiles and in dynamic compressive strengths is observed. The variation in dynamic compressive strengths is in excellent agreement with predictions of slip systems based on quasistatic and microscopic studies. The 〈110〉 crystals show a two-wave structure only at a large propagation distance. Also, the 〈110〉 results confirm the presence of a shear-stress threshold for rapid elastic wave attenuation, as determined from earlier studies, along the 〈100〉 direction. For the 〈111〉 orientation, an elastic response to 42-kbar compressive stress is observed. This value corresponds to a resolved shear stress of C′44/27 along the {100} planes, where C′44 is the effective shear modulus for the 〈111〉 direction. This shear stress is approaching the theoretical estimates of ideal lattice strength in pure shear. In contrast to the 〈100〉 crystals, the 〈111〉 crystals are not influenced by heat treatment. The upper limit of the elastic response along 〈111〉 cannot be much higher, and measurements of this response may provide a measure of lattice strength. Present results suggest the use of shock experiments to examine mechanical lattice behavior at stresses beyond the range of quasistatic experiments.