Crack Tip Blunting and Crack Initiation in a Ductile Material under Mixed Mode Loading with Large Scale Yielding

Crack tip blunting and crack initiation subjected to mixed mode loading of Mode I and Mode II were examined under the plane strain condition with large scale yielding. Fracture toughness tests were performed for tension specimens with double-edge cracks inclined to the loading axis. Elastic-plastic finite element analysis, based on finite deformation theory, was also carried out in order to understand the development of the crack tip blunting and the plastic strain concentration in the vicinity of the crack tip due to the blunting. The generalized COD, δ_r, was newly introduced for a ductile crack under mixed mode loading. It was shown that δ_r dominates the deformation in the vicinity of the blunted crack tip, independently of the crack angle, so that δ_r is useful as a fracture parameter. It was verified that the critical values for the crack initiation of δ_r and of the plastic work done, W_p, in the vicinity of the crack tip, were independent of crack angle, in contrast to the J-integral and the usual COD, δ.