Fracture

Abstract

In ceramics it is essential to consider all kinds of fractures that a material might experience during its service life time as a consequence of deformation. Fracture propensity is critical in ceramics which does not show elongation (plasticity) because failure can set in at deformations which basically are elastic (brittle ceramics). It is important to understand the theories of fracture, and relate them to the theoretical strength of materials. Among the important theories one can mention Griffith's theory on fracture, Orowan's fracture theory, and the dislocation theory of brittle fracture including the Stroh model of fracture. One of the most important parameters regarding fracture is toughness. Fracture toughness is the property that describes the ability of a material containing a crack to resist fracture and is one of the most important properties of any material for design applications. Related to fracture toughness is the term R-curve, which refers to fracture toughness that increases as a crack grows. Prediction of the effect of existing flaws in ceramics on fracture strength is the R-curve. Fracture toughness is an indicator for failure in ceramics and the R-curve expresses ceramic crack resistance. Another way to characterize a ceramic is by the energy absorption concept which is related to its fracture toughness. The J-integral as a fracture criterion is used to express the energy absorbed during crack extension. Fracture may occur in ceramics under static load, time dependent and cyclic deformation. Toughness can be improved by changing the course of crack, by crack tip shielding, crack bridging and crack healing. In ceramics undergoing transformation, transformation-toughening can improve the toughness.