Modeling of shear ductile fracture considering a changeable cut-off value for stress triaxiality

• The cut-off value for stress triaxiality is discussed based on experimental observation.
• A shear-controlled ductile fracture criterion is proposed with a changeable cut-off value for stress triaxiality.
• A cut-off value function for stress triaxiality is suggested as the zero maximum principal stress with a constant offset.
• The proposed criterion accurately predicts ductile fracture behavior of 2024-T351 from compression to tension.

A macroscopic ductile fracture criterion is proposed based on micro-mechanism analysis of nucleation, growth and shear coalescence of voids from experimental observation of fracture surfaces. The proposed ductile fracture model endows a changeable cut-off value for the stress triaxiality to represent effect of micro-structures, the Lode parameter, temperature, and strain rate on ductility of metals. The proposed model is used to construct fracture loci of AA 2024-T351. The constructed fracture loci are compared with experimental data covering wide stress triaxiality ranging between −0.5 and 1.0. The fracture loci are constructed in full stress spaces and plane stress conditions to analyze characteristics of the proposed fracture loci. Errors of the equivalent stress to fracture are calculated and compared with those predicted by the MSV model (Khan and Liu, 2012a) and series of the modified Mohr–Coulomb criteria. The comparison suggests that the proposed model can provide a satisfactory prediction of ductile fracture for metals from compressive upsetting tests to plane strain tension with slanted fracture surfaces. Moreover, it is expected that the proposed model reasonably describes ductile fracture behavior in high velocity perforation simulation since a reasonable cut-off value for the stress triaxiality is coupled with the proposed ductile fracture criterion.