Formation mechanisms and geometry of the crack assemble formed under uniaxial cyclic compression of NaCl single crystals

Abstract

Using light-scattering method, the cracks formation was experimentally studied in NaCI single crystals under uniaxial cyclic compression (10⁷ N/m²) during ~10 c at room temperature and following annealing at T = 500°C for ~10 min. The observed cracks were sorted (conventionally, in comparison with wavelength of the light used) into small ones showing the pattern of isotropic scattering, and big ones showing the scattering pattern with sharp maxima. It was established, that both small and big cracks were positioned in {100} planes and formed by Cottrell mechanism. In late stages of deformation, a little quantity of cracks was observed in {110} planes. Among possible mechanisms of the cracks formation, Stroll mechanism is considered as the most probable in our case of small deformations and stress. As the cracks are accumulated, the formation of ordered compact groups of cracks is found which are analogous to ordered groups of solid foreign-phase precipitations under decomposition of supersaturated solid solutions. The volume fraction of small cracks is by two orders of value higher than the volume fraction of big ones. The volume fractions of both kinds of cracks increase with the cycle number, but the big crack volume fraction increases much faster.