Extension of a shear-controlled ductile fracture model considering the stress triaxiality and the Lode parameter

Abstract

This paper is concerned with the extension of a shear-controlled ductile fracture criterion for accurate prediction of fracture forming limit diagrams (FFLD) in sheet metal forming processes. A shear-controlled ductile fracture criterion is extended to a general three-dimensional stress space with dependence on the stress triaxiality and the Lode parameter. The underlying mechanisms of Lode parameter dependence of ductile fracture are first correlated to the effect of the maximum shear stress on shear-coalescence of voids. The effect of the stress triaxiality and the Lode parameter on the equivalent plastic strain to fracture is investigated in the space of (η,L,ε¯). For the purpose of comparison, the Mohr–Coulomb criterion is also transformed into the space of (η,L,ε¯) using the technique of the Mohr’s circles. Both criteria are applied to construct fracture loci of Al 2024-T351. Fracture loci constructed are compared to experimental data points to validate the performance of two criteria. The comparison demonstrates that fracture loci constructed by two criteria are close to experimental results except for two data points in the high stress triaxiality. The big difference between two criteria is that a cut-off value for the stress triaxiality is extremely small for the Mohr–Coulomb criterion while the new ductile fracture criterion endows a constant cut-off value of −1/3 which is reasonable for ductile materials. Due to this limitation of the Mohr–Coulomb criterion, the new criterion is more suitable to model ductile fracture in metal forming processes.