The influence of strain history on ductile failure of steel

Abstract

By comparing the micromechanisms of ductile fracture of metallic materials (void nucleation, void growth and void coalescence) and the loading conditions in terms of local stresses and strains, the fracture behaviour may be described in terms of local properties as, for example, void volume fraction, to find out the damage development during plastic deformation and the conditions of crack initiation. For the damage development due to void growth the stress triaxiality during deformation is thought to be an important parameter. There are different models for void coalescence considering a critical combination of stress triaxiality and plastic strain, a critical void volume fraction and a critical amount of energy added. Experiments on notched tensile specimens had been done to determine the plastic strain at crack initiation. FEM-calculations were used to calculate the parameters stress triaxiality and equivalent plastic strain during deformation at the point of void coalescence taking into regard the loading history. Strongly varying stress triaxiality during deformation was achieved by employing one- and two-step experiments with tensile bars of different notch radii. To find a criterion for void coalescence the following two models are applied: (1) Void growth model: By using this equation to model the damage process a critical size to describe crack initiation by void coalescence is sought. However, it is found that based on this model void coalescence may be described only by a two-parameter-criterion, which in this case is geometrical. (2) Damage work model: To find a one-parameter-criterion the “damage work model” is applied. It takes into account the major influence of stress triaxiality on damage development and the stress-strain-curve of the material. Based on this model crack initiation by void coalescence may be described by one parameter which in this case is an accumulated local energy.