Effects of lattice orientation and crack tip constraint on ductile fracture initiation in Mg single crystals

In this work, plane strain finite element simulations of cylindrical void growth ahead of a notch tip in Mg single crystals are conducted to understand the effects of lattice orientation and crack tip constraint on the ductile fracture processes. The constitutive response is represented through crystal plasticity theory accounting for both slip and twinning. Three specimens - shallow, deeply cracked single edge notch tension and deeply cracked four point bend specimens, which display increasing levels of elastic biaxiality ratio (or constraint) are considered. Also two lattice orientations, with c-axis along the thickness direction and normal to the flat surfaces of notch (referred to as A and B) are studied. It is found that void growth and coalescence in orientation A (where predominantly prismatic slip occurs) take place directly ahead of the tip and fracture resistance enhances with reduction in constraint. By contrast, orientation B shows void coalescence by inclined shear bands, displays retarded void growth and higher J at crack initiation. The ductile fracture behavior is analyzed by examining slip, porosity and mean stress development in the voided cells ahead of the notch tip.