Description of shear failure in ductile metals via back stress concept linked to damage-microporosity softening

Starting from experimental observations on hat shape samples of a Titanium alloy of Ti6Al4V family showing that the failure of ductile materials under low triaxiality may result (at least partially) from void growth, the present work develops an approach based on a damage related softening mechanism acting as a kinematic mean stress drop causing a shift of the yield locus centre. For micro-porous viscoplastic material described via micro-mechanics based elliptic potentials (and obeying the normality rule) this ‘back mean stress’ concept allows reproducing the inelastic dilatancy due to void growth, in addition to the isochoric plastic deformation due to dislocation glide, at zero and low negative stress triaxiality. The approach is applied to the well-known GTN model and the 3D constitutive equations are implemented as user material in the engineering finite element computation code Abaqus®. Numerical simulations are conducted considering a single finite element under simple shear loading and a hat shape specimen under shear–compression loading. Numerical results are compared with experimental ones.