Strength and Toughness in Ceramic Systems

There has been a steady development and use of fracture mechanics, for describing the strength of brittle materials, ever since its inception by Griffith (1920). Materials considered include glass, ceramics, cement, brittle polymers, brittle metals, and a wide range of composite materials such as fibrous composites. The complexity of the fracture process generally increases as one passes along this list. A basic concept of fracture mechanics is a fracture surface energy or fracture toughness. Ideally this is a material constant but this is rarely true in practice. This paper summarizes the current state of understanding for ceramics and discusses a number of specific problem areas. This information should thus act as a signal to workers using fracture mechanics on cementitious materials. Problems considered include environmental sensitivity, statistical variations in strength, crack length effects, the concept of a flaw for porous materials, plastic effects, and transformation toughening.