Characterizing the fracture forming limit curve from shear to uniaxial tension in ultrathin sheet metals using specimens with cutouts

The fracture forming limit curves of ultrathin sheets from pure shear to uniaxial tension were characterized in this study under uniaxial tension test (UTT) and Nakajima test (NT) conditions by using tension-shear specimens with different cutout offsets. Considering deformation concentration at the edge of the tension-shear deformation region in conventional tension-shear specimens with cutouts, a new tension-shear specimen with deliberately designed cutoff contour in terms of cutout offset was proposed. It was numerically validated that a more pronounced strain localization can be obtained at the center of the tension-shear deformation region. On this basis, tension-shear deformation tests were experimentally conducted under UTT and NT conditions on three ultrathin sheets (0.25 mm interstitial-free steel, 0.255 mm aluminum killed steel, and 0.3 mm pre-plated nickel aluminum killed steel). The onset of fracture in the tension-shear specimens was determined based on the strain evolution in different regions. The results prove that the fracture forming limit for any strain path between pure shear and uniaxial tension can be correctly obtained by combining the optimized tension-shear specimens with the proposed fracture onset identification method. The prediction accuracies of three ductile fracture criteria (MMC, DF2014, and Hu-DFC) from pure shear to uniaxial tension were evaluated using the determined shear fracture forming limits.