On the role of shape and distribution of secondary voids in the mechanism of coalescence

Ductile fracture of metals at room temperature has been studied extensively. Since localization of plastic flow is often viewed as a precursor to ductile fracture, several computational studies analyzing the influence of stress triaxiality and the Lode parameter on the critical strain for onset of localization have been reported. The implications of strain localization in materials containing randomly distributed voids for the mode of failure, that is, internal necking or shear banding have also been discussed. Apart from the influence of stress state, it is also of interest to model the effect of fine-scale microstructural attributes on the mode of failure and, hence, on material's ductility. Therefore, in the present work, the mechanism of ductile fracture of an elastic–plastic solid containing two populations of pre-existing voids is modeled numerically. In the undeformed state, all the voids belonging to each family are assumed to have identical shape and size. Two different periodic spatial arrangements of primary voids are considered such that, in the absence of secondary voids, one of the configurations leads to internal necking whereas the other one exhibits a tendency for shear localization as the mode of coalescence for the larger voids. Small size secondary voids are then introduced explicitly and the role of shape and distribution of these sub-microscopic voids in the mechanism of coalescence and final failure is illustrated under plane strain as well as axisymmetric tension with a superposed hydrostatic stress. Depending on the microstructural attributes of the secondary voids, the mode of primary void coalescence is observed to shift from internal necking to void-sheeting and vice-versa. The implications of this shift in the mode of void coalescence on mesoscopic ductility is examined and a comparison of numerical predictions is made with the available experimental results.