Toughness, extrinsic intrinsic

In the Irwin approach, as with the Griffith approach, strength is found to depend on a combination of a material property (intrinsic) and a flaw size (extrinsic). In the linear elastic fracture mechanics approach, however, the material property is T or R and it has a component that depends on the microstructure of the material. Thus, if the mechanisms that increase T for a material can be identified, an approach is available to increase the reliability of brittle materials. It is this philosophy that has been a major driving force in the recent production of ceramics with higher strengths and toughnesses than had previously been considered possible. 

In the above relations, Tq is intrinsic toughness, is the extrinsic toughness mechanism (crack-tip shielding), Rq is the fracture resistance energy and R is the crack resistance energy contribution. The critical condition for crack extension is then given by ... 

The low value of fracture toughness makes ceramic cells sensitive to material defects and unsuspected flaws. The consequence is that the ultimate strength of ceramic cells deviates from their nominal mean value, and significant safety factors (maximum tolerated intrinsic load/applied extrinsic load) have to be used in order to... 

---