Explanation of edge defect influence on sapphire bending strength scatter using the coupled criterion

Sapphire fracture is studied by means of four-point bending tests on mirror-polished millimetric specimens having their crystallographic $\overrightarrow{a}$-axis oriented along the specimen length. The scattering of bending strengths between 500 MPa and 750 MPa is mainly due to edge defects of some tens of microns resulting from the specimen cutting process. Crack initiation occurs from an edge defect along a a-plane, perpendicular to the direction of maximum tensile stress, and further deviates along weaker m-planes. Numerical simulations of edge defect-induced crack initiation based on the coupled criterion reveal that the material sensitivity to edge defect-induced crack initiation mainly depends on Irwin’s length. For Irwin’s lengths larger than twice the defect depth, the bending strength is the same as the one obtained without defect. By retrieving the bending strength variation as a function of the defect depth measured experimentally, the proposed approach enables the identification of sapphire a-plane $850\pm 90MPa$ tensile strength for a $G_c=15J{m}^{-2}$ critical energy release rate.