The interaction of cleavage cracks with inhomogeneities in sodium chloride crystals

Abstract

Single crystals of sodium chloride have been treated by electrodiffusion so that they contain small voids and particles of gold. Cleavage cracks of known velocities have been introduced into these crystals, and the effect of the inhomogeneities on crack propagation has been studied. Measurements of the velocity of cracks have indicated that the inhomogeneities impede crack propagation. Subsequent examination of the cleavage surfaces by optical microscopy and, in greater detail, by electron microscopy of surface replicas, has revealed characteristic patterns of steps at the inhomogeneities. These have been explained in terms of cleavage steps and slip steps which result from dislocations that are generated at the front of a crack when it undergoes changes of velocity and shape at the inhomogeneities. These observations may be applied to the problem of ‘crack-stopping’ in certain ceramic materials.