Effect of initial void shape on ductile failure in a shear field

For voids in a shear field unit cell model analyses have been used to show that ductile failure is predicted even though the stress triaxiality is low or perhaps negative, so that the void volume fraction does not grow during deformation. Here, the effect of the void shape is studied by analyzing materials where the voids have initially ellipsoidal shapes. The cell models are in plane strain, so that the voids are modeled as cylindrical holes. Periodic boundary conditions are used to represent a material with a periodic distribution of voids having different spacings in the two in-plane coordinate directions, and subjected to different average stress states. Depending on the initial orientation of the ellipsoidal voids, the principal axes of the elongated voids rotate initially in different directions relative to the shear field. After some deformation the behavior is much like that found for voids with circular cross-section, i.e. the voids in shear flatten out to micro-cracks, which rotate and elongate until interaction with neighboring micro-cracks gives coalescence. Even though the mechanism of ductile failure is the same, the load carrying capacity predicted, for the same initial void volume fraction, is rather different for different initial orientations of the ellipsoidal voids.