Mixed‐Mode Fracture of Ceramics in Asymmetric Four‐Point Bending: Effect of Crack‐Face Grain Interlocking/Bridging

The mixed‐mode fracture of a large‐grain‐size alumina ceramic and a soda‐lime glass is investigated. These ceramics are tested using straight‐through precracked or notched specimens. The straight‐through precrack is introduced by the single‐edge‐precracked beam method. Precracked or notched specimens are subjected to combined mode I/II or pure mode II fracture, under asymmetric four‐point bending, and pure mode I fracture, under symmetric four‐point bending. A pure mode II fracture is never achieved in the precracked polycrystalline alumina by the crack‐face friction inevitably induced by grain interlocking/bridging. The crack‐face friction in sliding mode reduces the local mode II stress intensity factor in the crack‐tip region and produces a sizable amount of mode I deformation. Accounting for the contribution of the crack‐face friction to the crack‐tip local stress intensity factors, K I and K II , in mixed‐mode fracture tests, the experimental results of the K I / K I c versus K II / K I c envelope and the initial angle of noncoplanar crack extension are in good agreement with the theoretical predictions of the maximum hoop‐stress theory.