Fracture Toughness and Frontal Process Zone Size of Ceramics

Our previous technique for estimating critical sizes of the frontal process zone (FPZ) in ceramics was improved using exact solutions of stress distributions around crack and elliptical hole tips in an infinite plate for calculating local fracture stresses. The local fracture stress was calculated at the characteristic distance from the notch tip on the basis of a local fracture criterion. A three-point flexure test for alumina was carried out using V-notched specimens, and a formula for the relation between the strength and the notch depth including the short notch depth region was successfully established. The critical FPZ size of alumina was estimated from the fracture toughness and strength of the material. The critical FPZ sizes for alumina-based nanocomposites were also estimated. A concept of material design for toughened ceramics was discussed on the basis of these data. It was proved that there was a linear relationship between the fracture toughness and the product of strength and square-root of the FPZ size for these materials. This relation suggests that both the strength and the critical FPZ size must be increased to enhance the fracture toughness of ceramics.