DUCTILE FRACTURE IN HY100 STEEL UNDER MIXED MODE-I MODE-II LOADING

A number of criteria have been proposed which predict the direction of cracking under mixed Mode I/Mode II loading. All have been evaluated for brittle materials, in which a crack subjected to tension and shear propagates normal to the maximum tensile stress (i.e. fracture is of the Mode I type). In a ductile material, however, a notch subjected to mixed Mode I/Mode II loading may initiate a crack in the direction of maximum shear. This paper shows that the profile of the notch tip changes with increasing mixed mode load in such a way that one side of the tip blunts while the other sharpens. Various specimens, subjected to the same mixed mode ratio, were unloaded from different points on the load-displacement curves to study the change in notch-tip profile. Studies under the Scanning Electron Microscope (SEM) have shown that cracks initiate at the sharpened end, along a microscopic shear band. Using a dislocation pile-up model for decohesion of the carbide-matrix interface, a micromechanical model has been proposed for crack initiation in the shear band. It is shown that a theoretical prediction of the shear strain required for decohesion gives a result that is, of magnitude, similar to that of the shear strain at crack initiation measured in the experiments.