Fracture‐Strength Anisotropy of Sapphire

At 22°C, the bend strength of as‐ground sapphire bars with different orientations of tensile surface and tensile axis varies from 4×10 4 to 12×10 4 psi as a result of the anisotropies of machining damage and fracture energy. Generally, the strength‐controlling surface flaws caused by grinding are of constant size; failure from such flaws is affected by grinding‐induced slip and twinning. Surface striations, which occur when there is negligible subsurface slip, can lead to stress concentrations, whereas twins can lead to increases in flaw size; each of these effects reduces strength. In addition, the large anisotropy in fracture strength, which is not predicted by the elastic anisotropy, is controlled by the anisotropic fracture energy. Approximation of the attractive forces across the fracture plane based on elastic moduli and interplanar distance can be used to understand the fracture energy anisotropy.