An engineering approach to remove the specimen geometry constraint dependence of elastic-plastic fracture toughness

Constraint effects on fracture have been the subject of a number of recent studies. In this paper, the effects of constraint changes on the elastic-plastic fracture toughness values J_lc are investigated and an engineering approach for constraint correction is proposed. A relationship between the conventional elastic-plastic fracture toughness, J_lc, and crack tip constraint characterized by crack tip stress triaxiality, T, is derived from a modified damage criterion for ductile fracture. Then, a new parameter J_sc and associated J-like criterion, J_s = J_sc, for ductile fracture are proposed, in which both the crack tip deformation characterized by the J-integral and the crack tip constraint characterized by stress triaxiality are included. Several experiments show that the toughness variation with specimen geometry constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper and that the new parameter J_sc is a material constant independent of specimen geometry (constraint). This parameter can serve as a new parameter to differentiate the elastic-plastic fracture toughness of engineering materials, which provides a new approach to the fracture assessments of structures. It is not necessary to determine which laboratory specimen matches the structural constraint; rather, any specimen geometry can be tested to measure the size independent fracture toughness J_sc. The potential advantage is clear and the results are very encouraging.