Dynamic crack path selection in brittle crystals under mixed mode loading

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 {1 1 1} low energy cleavage planes and nearly isotropic path selection on the {1 1 0} 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.