Dynamic hardness, deformation and fracture of simple ionic crystals at very high rates of strain

Single crystals of NaCl, LiF and MgO were deformed at strain rates of 105 s−1 by impacting them with glass and tungsten carbide spherical particles of diameter in the range of 0·4-G l·0 mm. By measuring the incident and rebound velocities and the size of the residual indentation, the dynamic hardnesses of these materials were determined and found to be several times the quasi-static values. At these strain rates the crystals behaved in a brittle manner. The fracture phenomena during the particle impact were investigated with a high-speed framing camera and from the photographic sequences velocities of cracks and impact times were also obtained. An attempt has been made to explain these results in terms of the dislocation densities, their velocities and the applied stress. Finally a brief comment is also made on the mechanism of material removal.