A forming limit framework accounting for various failure mechanisms : Localization, ductile and cleavage fracture

The forming limits and failure properties of three distinct advanced high-strength steels (AHSS) have been investigated under various stress states in tensile tests with optimized specimen geometries. In addition to the commonly observed failure patterns governed by localized necking and ductile fracture for two of the AHSS, after substantial plastic deformation at room temperature, cleavage fracture occurs for a large range of stress states in a laboratory quenching and partitioning steel with superior tensile properties. The competition between failure patterns, encompassing ductile and cleavage fractures with and without necking, is governed by the mechanical properties of materials and the stress states, as a transition of failure mechanisms occurs with increasing triaxiality. The forming limit framework is, therefore, further extended to seamlessly integrate cleavage fracture in this study, where the competition between various failure mechanisms is demonstrated using three AHSS in the space of critical strain and principal stress. These findings shed light on the importance of considering cleavage fracture strength as a parameter besides the strength-ductility synergy in advanced high-strength metallic materials, and the proposed framework also gives a more comprehensive guide in designing and conducting the sheet metal forming processes.