The influence of strain history on ductile failure of steel

By comparing the micromechanisms of ductile fracture of metallic materials (void nucleation, void growth and void coalescence) and the loading conditi ons in terms of local stresses and strains, the fracture behaviour may be d escribed in terms of local properties as, for example, void volume fraction , to find out the damage development during plastic deformation and the con ditions of crack initiation. For the damage development due to void growth the stress triaxiality during deformation is thought to be an important par ameter. There are different models for void coalescence considering a criti cal combination of stress triaxiality and plastic strain, a critical void v olume fraction and a critical amount of energy added. Experiments on notche d tensile specimens had been done to determine the plastic strain at crack initiation. FEM-calculations were used to calculate the parameters stress t riaxiality 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 tw o-step experiments with tensile bars of different notch radii. To find a cr iterion for void coalescence the following two models are applied: (1) Void growth model: By using this equation to model the damage process a critica l size to describe crack initiation by void coalescence is sought. However, it is found that based on this model void coalescence may be described onl y by a two-parameter-criterion, which in this case is geometrical. (2) Dama ge work model: To find a one-parameter-criterion the ''damage work model" i s applied. Tt 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 pa rameter which in this case is an accumulated local energy. (C) 1998 Elsevie r Science B.V. AU rights reserved.