Anisotropic fracture forming limit curve and its applications for sheet metal forming with complex strain paths of aluminum sheet

In this work, formabilities of aluminum sheet alloy grade AA5052-H32 were investigated under consideration of the anisotropic behavior of material. Experimental tensile tests and modified Marciniak in-plane stretch-forming tests of sheet samples with varying shapes were performed for different sample orientations. The critical fracture strains at various states of stress were gathered by means of a digital image correlation (DIC) technique. Then, the Lou-Huh ductile fracture criterion was applied in combination with the Hill'48 and Yld2000-2d yield criteria for generating the fracture loci (FLs) of examined sheet. Hereby, material parameters of the fracture model were calibrated by the tensile tests of pure shear, uniaxial tension and plane strain samples. It was found that the Yld2000-2d-based fracture model could more accurately predict the experimental limit strains at all stress states. In addition, the representative anisotropic FL was proposed and transformed to fracture forming limit curve (FFLC), which was afterwards verified by a cross die forming test and a square cup drawing test. The predicted force-displacement curves, moment of fracture onsets, and fracture sites of samples positioned in different directions were well in accordance with the experimental results. The state of stress and material anisotropy strongly affected the fracture occurrences of tested samples. The representative FFLC by the Lou-Huh model and the Yld2000-2d yield function more accurately described the forming limits of investigated aluminum sheet.