Dynamic crack path selection in brittle crystals under mixed mode loading

Abstract

We studied, experimentally, the conditions required to deflect a dynamic crack from the low energy cleavage planes of propagation under in-plane mixed mode loading, and the path selected by the crack afterwards, where the shear stress, crack speed, and crystal structure were the governing variables. Silicon crystal, which shows cleavage along the {110} and {111} families of planes, was used as a model material. The experiments were carried out by our recently developed coefficient of thermal expansion mismatch (CTEM) method. Finite element analysis was used to compute the quasi-static mode-mixity and energies release rate, and Freund equation of motion was used to estimate crack speed.Deflection occurs due to the competition between the propensity of the crack to propagate on the low energy plane and the requirement to reduce the shear energy. This energy balance depends on crack speed and crystal structure. It is shown that when the crack speed increases, higher shear mode stresses are required to deflect the crack macroscopically from the low energy cleavage plane. Our work indicates that crack path selection in single crystal silicon at the macro-scale can vary between nearly perfect cleavage on the {111} low energy cleavage planes and nearly isotropic path selection on the {110} planes. Finally, we postulate that in mixed mode loading, the tensile mode is the driving force for crack speed while the shear mode is responsible for crack deflection.