Experimental and numerical investigation of failure during bending of AA6061 aluminum alloy sheet using the modified Mohr-Coulomb fracture criterion

In this paper, the modified Mohr-Coulomb (MMC) ductile fracture criterion was adopted to analyze the fracture behavior of the AA6061-T6 aluminum alloy sheet during the U-bending process. Appropriate calibration procedures were employed using various tension tests. A finite element (FE) model was built using the commercial FE code Abaqus/Explicit incorporating the calibrated MMC fracture criterion. It was found that the MMC criterion calibrated by the uniaxial, plane strain, notched, and modified in-plane shear tension tests can predict the onset of fracture in the U-bending process with reasonable accuracy. The error of the predicted fracture displacement was about 2% as compared to experiments. Due to the dependency of the fracture prediction accuracy on the stress state, the effects of process parameters on stress triaxiality and normalized Lode angle parameter were investigated. The negligible effect of process parameters on the stress state confirmed that the calibrated fracture criterion (MMC) was able to predict fracture in different forming conditions with equal accuracy. Also, the proportional stress and strain state in the U-bending process indicate that, besides the high fracture prediction accuracy of the MMC's damage accumulation function, the fracture forming limit diagram (under proportional loading assumption) can serve as a reliable tool for estimating the onset of fracture in the U-bending process. Therefore, a series of studies were also conducted on damage evolution, crack propagation, principal strains, and fracture displacement during the U-bending process under various forming conditions.