Abstract

The way cracks in brittle crystals initiate and propagate is a fundamental problem in fracture, not yet elucidated. The main complication is the fact that two opposing mechanisms dictate crack path; the tendency of the crack to propagate on the preferred low energy cleavage plane and the requirement to reduce the shear stresses following propagation. Another complication is attributed to the anisotropy of the elastic properties. Among the materials properties, the cleavage energies of the preferred planes play a major role in the fracture processes. We study the way cracks initiate and propagate in brittle crystals using a special designed experimental methods. Specimens are fractured under three point bending and under combined tensile and shear modes in controllable manner. We fracture silicon as the most studied, nearly ideal brittle crystal. As the two low energy cleavage planes of silicon, {111} and {110}, have comparable cleavage energies, complex occurrences during fracture are assured. The phenomenon of crack deflection from {110} plane of silicon to {111} plane from energy consideration was identified. It was found that the deflection is velocity and crystallographic orientation dependent. The generation of new type of surface instabilities at low crack velocity on the {111} crack system of silicon were revealed recently under bending. Same new surface instabilities were revealed under tension. This is in contrast to the assumption that slow cracks are stable. We showed that contrary, fast cracks in crystals are stable. The path and surface instability occurring in {111} and {110} cleavage systems of silicon under tensile and tensile and shear modes were studied. We show that cracks propagating at low speed maintain propagating on the cleavage system even when the shear to tensile modes ratio, GII /GI, is relatively high. Faster cracks do not propagate on the cleavage plane. Instead, they deflect to the path that maximizes GI. This maximum is crack velocity dependent. This is attributed to the atomistic arrangement and atomistic vibrations at the crack tip.