1 Basic Concepts

In order for a crack to initiate and propagate in a structure, the stresses and strains in the vicinity of the crack tip must be high enough to separate the material and there must be sufficient energy available to form the new surfaces and drive the crack forward. Under linear elastic conditions, the stress intensity factor, K, provides a single parameter that describes the stress and strain fields in the vicinity of the crack tip, outside the crack tip plastic zone and is related to the energy release rate, G, through the relationship: G = K ²/E. A crack will initiate when the applied stress intensity equals the critical value for the material, termed the fracture toughness, Kc.

Once initiated at the onset of a fast fracture event, a rapidly propagating crack experiences a sudden increase in crack velocity. The crack will continue to propagate as long...