Mixed‐Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I‐mode II fracture toughness values of ceramic materials can be determined using four‐point bend specimens containing sharp, through‐thickness precracks is discussed. In this method, notched and fatigue‐precracked specimens of brittle solids are subjected to combined mode I‐mode II and pure mode II fracture under asymmetric four‐point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I‐mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K_Ic, the mode III fracture initiation toughness is 2.3 times higher than K_Ic. The predictions of fracture toughness and crack path based on various mixed‐mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed‐mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.