Ductile fracture by void growth to coalescence usually occurs at moderately high stress triaxiality. However, here the focus will be on two extreme cases, very high stress triaxiality where cavitation instabilities can occur, or low stress triaxiality as occurs in simple shear.
At high stress triaxiality and with very small void volume fractions the phenomenon of a cavitation instability can occur. In practice this mechanism is important when plastic flow occurs under highly constrained conditions, such as in metal-ceramic systems. At the tip of a blunting crack in a homogeneous metal the hydrostatic tension is not sufficiently high for this mechanism. The discussion will include effects of strain gradient plasticity in the metal surrounding the cavity, and the effect of neighbouring voids.
When voids are present in a ductile material subject to a shear dominated stress state under low stress triaxiality the voids collapse to micro-cracks, which can subsequently result in ductile failure. Traditional Gurson type constitutive models are not able to describe ductile fracture in the absence of hydrostatic tension in the material. However, micro-mechanical studies have demonstrated how the voids are flattened out to micro-cracks, which coalescence with neighbouring micro-cracks due to their continued rotation and elongation, so that the failure mechanism in shear is very different from that under tensile loading. Some comparisons with numerical studies based on a shear-extended Gurson model will be shown. Also, significant differences between predictions for simple shear or pure shear will be discussed.