Analysis of interacting cracks due to sequential indentations on sapphire

Abstract Crack interaction behavior due to two sequential static Vickers indentations on a-plane sapphire was investigated. Secondary indentations were performed at various radial separations and angular positions around a primary indentation. The results indicate that interaction sensitivity for sapphire is highly dependent on the spatial relationship of the indentation patterns relative to the crystal orientation. Heightened crack interaction sensitivity was observed when sequential indentations were aligned along the m-axis while the lowest interaction sensitivity occurred when indentations were aligned along the c-axis. Cracks propagating parallel to the rhombohedral planes (R1 and R2) interacted first for most of the indentation orientations tested, while the basal plane (B) cracks interacted first only when indentations were performed along the c-axis. A new bulk material separation mode was identified when two sequential indentations were aligned along the m-axis. Two new terms are defined to characterize the sensitivity of the material to crack interactions due to sequential indentations: (i) the “interaction limit” (IL), which refers to the maximum distance between indentations that results in an extension of initial crack systems for a given orientation; and (ii) the “unification limit” (UL), which refers to the maximum distance between two indentations that results in complete unification or joining of crack systems originating from each indentation. The implications of these results for design of impact-resistant transparent windows and for establishing guidelines for the multi-hit resistance of sapphire are discussed.