The influence of crack size on the fracture behaviour of short cracks

Crack size effects are investigated by applying local fracture criteria at a specific distance ahead of a crack tip, related to the material's microstructure. For sufficiently long cracks, the criteria are applied at points within the singularity-dominated region and then failure corresponds to the attainment of a critical intensity of the singularity but for shorter cracks this may not be the case and size effects may be anticipated.Comparisons of stress and strain fields around crack tips are made between cracked specimens and a boundary layer model which allows for a finite T-stress parallel to the line of the crack; these demonstrate that size effects can be characterized, for a useful range of crack sizes, in terms of the elasticT-term.The variation of the ductile-brittle transition temperature with crack size is studied on the basis of model calculations based on a power-law hardening material with temperature-dependent yield stress, coupled with two competing fracture criteria, chosen to model brittle or ductile crack extensions respectively. Generally, shorter cracks display lower transition temperatures and higher toughness. This potentially beneficial effect may, however, be nullified by stress triaxiality.