Synergistic effects of plastic anisotropy and void coalescence on fracture mode in plane strain

The macroscopic fracture in plane strain is known to be shear-like in ductile materials. In most structural materials, fracture starts after diffuse necking, at the Centre of the specimen, by micro-void coalescence giving rise afterwards to the macroscopic shear fracture mode. In this paper, the effect of coalescence on shear band development and on associated fracture mode in plane strain is analysed numerically. The calculations are performed using a recent elastic-viscoplastic Gurson-like model that accounts for void shape evolution, coalescence and post-coalescence micromechanics along with isotropic hardening and orthotropic plasticity for the matrix behaviour. The latter is introduced to represent the actual flow properties of hot-worked materials. No kinematic hardening or nucleation formulation is used in order to focus attention on coalescence effects and to discuss, with respect to experiments, published results based on kinematic hardening and nucleation effects. The most important finding is the synergistic effect of plastic anisotropy and post-coalescence yield surface curvature upon the onset of a shear band after the fracture sets in at the Centre of the specimen.