Void growth versus void collapse in a creeping single crystal

Three dimensional finite deformation finite element analyses of unit cells containing a single initially spherical void are carried out. The materials are characterized by a rate power law viscous crystal plasticity constitutive relation and three values of the power law creep exponent are considered. The evolution of porosity is calculated for a fixed value of tensile nominal stress. The overall true stress components on the unit cell are adjusted so that a fixed value of stress triaxiality and a fixed value of the Lode parameter are maintained. Results are presented for a stress triaxiality value of 1/3 and Lode parameter values of −1−1, 0 and 1. Two initial void volume fractions are considered: 1% and 10%. For each initial void volume fraction calculations are carried out for various initial void spacings. For sufficiently closely spaced voids, void coalescence occurs while for widely spaced voids void collapse can occur. Whether void coalescence, void collapse or neither occurs mainly depends on the value of the Lode parameter.