Fracture mechanics. The quantitative analytical prediction of the ability of a material to resist fracture based on the relationship amongst the applied stress state, a preexisting flaw, and the fracture toughness of a material.
The ability to resist fracture may be expressed as a critical value of the stress-intensity factor (K), the strain energy release rate, the J-integral, or the crack tip opening displacement (CTOD).
Stress-intensity factor. A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. Various subscripts are used to denote different loading conditions or fracture toughness.
Fracture toughness. A generic term for measures of resistance to extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or pre-cracked specimens not based on fracture-mechanics analysis. Results from the tests of the latter type are often useful for fracture control, based on either service experience or empirical co-relations with fracture mechanics tests.