A finite element analysis of mixed-mode fracture initiation by ductile failure mechanisms

Abstract

Ductile crack initiation from a notch under mixed-mode loading involving Modes I and II is studied within the context of plane strain, small-scale yielding conditions. A finite element procedure is employed along with the finite strain version of the Gurson constitutive model that accounts for the ductile failure mechanisms of micro-void nucleation. growth and coalescence. Attention is focussed on two issues. Firstly, the competition between two different failure mechanisms, involving micro-void coalescence and localized plastic deformation in the form of an intense band, which are simultaneously operating near the notch under mixed-mode loading, is examined. Secondly, the effect of mixed-mode loading on the critical value of the J-integral at incipient material failure is investigated. The results show that for Mode I predominant loading micro-void coalescence near the blunted portion of the notch is clearly the preponderant failure mechanism. On the other hand, for mixed-mode loading with a high Mode II component, a band of intense plastic strain concentration begins to form near the sharpened part of the notch before failure by micro-void coalescence can occur. Also, it is found that the critical value of J decreases as the loading changes from Mode I to Mode II. A local fracture parameter based on notch tip deformation is identified for characterizing mixed-mode failure due to micro-void coalescence.