3D study of plastic flow localization at a void-sheet

A single layer of voids in a ductile material is considered to study the effect of this material imperfection on the onset of shear band localization, and to study the failure evolution in the void-sheet mechanism. The layer is inclined relative to the main tensile direction, and the initially spherical voids are taken to be uniformly spaced both along the inclined direction and transverse to it. The material is elastic-viscoplastic with isotropic hardening. When the solid is subjected to a prescribed strain-rate and the shear band width is negligible compared to specimen dimensions, an infinite ratio of the strain-rate in the band relative to the overall prescribed strain-rate defines the onset of localization. For each stress-state analyzed a limited number of initial inclinations of the void layer are considered, with an angular spacing of 5 degrees, and the angle of inclination giving the smallest critical strain is used as a good approximation of the critical band orientation. Different initial void sizes, relative to the void spacing, are considered. It is shown that increased void spacing in the transverse direction significantly delays localization. Also the effects of different levels of strain hardening are compared. The evolution of the void shape during shear localization is studied, to be able to follow the void-sheet failure mechanism. The critical strain for the onset of void coalescence is compared with the critical strain for shear bands.