Fracture of Ge and Al₂O₃

The fracture strength σ_F of germanium at constant loading rate has been determined over a temperature range from −196°C to 500°C. σ_F decreases slowly up to about 200°C, then increases to about 400°C, followed by subsequent decrease. σ_F is found to be essentially independent of ambient for a wide variety of chemical species. These observations are analyzed in terms of fracture nucleation at a flaw near, but not exposed to, the surface, by a presently unknown kinetic process. The role of dislocations apparently is to inhibit fracture by relaxation of stress concentrations up to about 400°C, above which general plastic deformation precedes fracture, suggesting the possibility of fracture nucleation by dislocation interactions. The applicability of this analysis to another brittle system, Al₂O₃, is considered in the light of its reported fracture, deformation, wear and friction characteristics. Striking similarities are found in the behavior of these two crystals, suggesting that the underlying mechanisms must have much in common.