Damage and failure at negative stress triaxialities: Experiments, modeling and numerical simulations

The paper deals with an anisotropic continuum damage and failure model for ductile metals with focus on negative stress triaxialities. The continuum framework has been generalized to take into account the effect of stress state on damage criteria as well as on evolution equations of damage strains. Different branches of the criteria are considered corresponding to various micro-mechanisms like growth of voids, formation of micro-shear-cracks and their combination which depend on stress intensity, stress triaxiality and the Lode parameter. A series of shear-compression experiments with biaxially loaded specimens and corresponding numerical simulations have been performed to validate the proposed phenomenological approach. Digital image correlation technique has been used to analyze formation of strain fields in critical regions of the specimens for different loading conditions especially in the compression range. Microscopic failure mechanisms are revealed by scanning electron microscopy of fracture surfaces. Based on experimental data and corresponding numerical results the existence of the cut-off value of stress triaxiality is discussed below which fracture never occurs and a stress-state-dependent function of this parameter is proposed.