Void Coalescence With and Without Prestrain History

In this study, void coalescence with and without a plastic prestrain history is studied using stress-controlled axisymmetric unit cell models. In addition to spherical voids, both oblate and prolate voids are considered. In the case with prestrain history a uniaxial prestrain up to 10% was applied and the material is thereafter subjected to loadings with constant stress triaxiality. It is found that the microscopic position of the maximum axial stress in void ligament can be taken as an indicator for void coalescence. In the beginning of plastic loading the maximum axial stress occurs at the edge close to the void. With the increase of plastic deformation, the position of maximum axial stress shifts from the void edge to cell boundary and coalescence starts when the position appears at the boundary. It is shown that a prestrain history significantly reduces the void coalescence strain. The prestrain effect on void coalescence depends strongly on the initial void shape. Prestrain history induces both strain hardening and void shape change. The effect of prestrain-induced void shape change on coalescence strain is relatively small while the effect of prestrain-induced local hardening is significant.