A ductile fracture criterion in sheet metal forming process

Forming limit curves at neck (FLCN) and forming limit curves at fracture (FLCF) for various steel sheets were determined from a hemispherical punch stretching. The measured FLCF of the sheets with higher ductility limit had approximately linear shape like one of the bulk forming process. Whereas, the FLCF of the sheets with relatively lower ductility limit had a rather complex shape approaching the FLCN towards the equi-biaxial strain paths. Some experimental FLCFs of aluminum sheets and steel sheets were compared with the fracture strains predicted from some ductile fracture criteria reported in the literature. To determine the constants in the various ductile fracture criteria, a numerical procedure, which is based on plane stress condition and Barlat’s nonquadratic anisotropic yield criterion, was used. To predict precisely the experimental FLCFs with both the linear shape and the complex shape approaching the FLCN towards the equi-biaxial strain paths, from a combination of the Cockroft–Latham ductile fracture criterion and the maximum shear stress criterion, an empirical ductile fracture criterion in sheet metal forming process was newly suggested. The empirical criterion could be used to predict the FLCFs with the rather complex shape as well as the linear shape.