Effects of hydrostatic stress and strain hardening exponent on near-tip fields under mixed mode loading

A large deformation finite element analysis has been performed to study the stress and strain fields near the crack-tip under mixed mode conditions. The effects of microvoids on plastic flow are taken into account by using the continuum constitutive model introduced by Gurson. The displacements characterized by the stress intensity factors are imposed at the nodes on a peripheral circle far from a crack tip. Computations are carried out for the case of KII/KI=0 and 1.732 with the plane strain or plane stress condition. In the mixed mode loading, contrastive deformations appear at the upper and lower regions of the crack-tip, i.e., sharpening and blunting. The deformed shape of the crack-tip is influenced by strain hardening exponent. The distributions of the equivalent plastic strain and the void volume fraction near the crack-tip are also influenced by the stress state (plane strain or plane stress) as well as the strain hardening exponent. Based on these numerical results, discussions are made on the experimental observations that under mixed mode loading conditions cracks due to shear fracture initiated at the sharpened corner of the crack tip near the surfaces of the specimen while another crack due to dimple fracture occurred at the blunted corner of the crack tip near the midthickness of the specimen.