Damage evolution around shear loaded intervoid ligaments in plane strain and plane stress

Ductile fracture is a process involving microvoids nucleation and growth, ending abruptly by void coalescence. The nature of the local mechanical fields is of prime importance to model the fracture process, and those have been widely studied, generally assuming plane strain or axisymmetric conditions. We suggest that the void surroundings and the intervoid ductile ligament undergo a transition from plane strain to plane stress, as extreme conditions, as the ligament gets increasingly thinner. This transition is evident from fracture surfaces of ductile materials when considering two, commonly observed features: thin intervoid ridges and rather equiaxed ligaments containing smaller voids. This work considers dominant shear loading as the driving force for plastic deformation, using periodic cell calculations and the shear extended GTN model. We systematically investigate plane strain and plane stress configurations. Whereas the initial void shape evolution is similar under both assumptions, the plane-strain shear loading terminates rapidly at low strains while the plane stress confers additional ductility and void shape change to the sheared ligament. Careful consideration of the local stress triaxiality near the void's tips shows that the high triaxiality values in plane strain decrease under plane stress, which expectedly delays both void growth and damage nucleation so that the intervoid ligaments become more ductile under plane stress conditions.