Basic studies of ductile failure processes and implications for fracture prediction

Fracture of ductile materials has frequently been observed to result from the nucleation, growth and coalescence of microscopic voids. Experimental and analytical studies have shown that both the stress constraint factor and the effective plastic strain play a significant role in the ductile failure process. Experimental results also suggest that these two parameters are not independent of each other at failure initiation. In this study, a methodology for characterizing the effect of stress constraint A_m (which is defined to be the ratio of the mean stress and the effective stress A_m=σ_m/σ_e ) on ductile failure is proposed. This methodology is based on experimental evidence that shows the effective plastic strain at failure initiation has a one‐to‐one relationship with stress constraint. Numerical analyses based on plane strain and three‐dimensional unit‐cell models have been carried out to investigate failure initiation of the unit cell under different constraint conditions. Results from the numerical studies indicate (a) for each void volume fraction, there exists a local failure locus in terms of mesoscopic quantities, σ_m and σ_e, that adequately predict incipient local micro‐void link‐up, (b) the results are fully consistent with a failure criterion that maximizes mesoscopic effective stress for a constant level of stress constraint A_m, (c) for high to moderate constraint A_m, the link‐up envelope values for σ_m and σ_e are consistent with limit load conditions where the critical principal stress σ_1c corresponds to the maximum principal stress in the loading history and (d) for low constraint, the link‐up envelope values for σ_m and σ_e correspond to link‐up conditions having high levels of plastic strain and a principal stress σ₁ that is lower than the maximum value for this loading history. Thus, the results suggest that a two‐parameter ductile fracture criterion is plausible, such as critical crack opening displacement (COD) and stress constraint A_m, for predicting the process of stable tearing in materials undergoing ductile void growth during the fracture process.