It is known that the rapid loss of the stress carrying capacity with increasing strain is a very complex microstructural phenomenon, where a macroscopic approach cannot be used. The present paper describes a microstructural computer approach in which the damage process is characterized by the evolution of the microvoid volume fraction due to the growth of existing microvoids, the nucleation of new microvoids by inclusion fractures or decohesions, and the coalescence of neighbouring microvoids. The ductile fracture is predicted with the complete loss of the stress carrying capacity.
First, the constitutive damage model and its numerical implementation inside a finite element code are developed. This model is based on the assumption about plastic strain controlled microvoid nucleation and the prediction of the porous material flow by the Gurson yield surface. In computer realization of the constitutive damage model for porous elasto-viscoplastic model, the Gurson criterion modified by Tvergaard potential is used as plastic criterion, and the stress tensor is corrected by using this potential.
As numerical examples the authors consider: (i) the uniaxial tensile test of a thin square unit specimen with porous material under dynamic loading, and (ii) the dynamic bending test of a cantilever porous beam with a \(U\)-cross-section and with microvoid nucleation controlled by plastic strain or by stress. In the first case, the damage evolution depends strongly on the nucleation description type. As the plastic strain controls a nucleation, then the damage evolution is independent of the plastic strain rate. As the stress controls a nucleation, then the damage evolution is strongly modified with varying plastic strain rate. In the case of the bending tests, there are modification of the microvoid volume fraction distribution and differences in the deformed shapes for both nucleation description types.