Identification of ductile fracture design curve for hardened quasi-brittle AISI-D2 tool steel to predict shearing tool failure

Abstract

Tool deformation and fracture during the shearing of ultra-high-strength steel has become a severe problem. Finite element (FE) simulations can help to solve this problem by evaluating improved tool edge designs, but they require the strain/stress range in which no fracture occurs. This study accordingly identified these properties for conventional and improved AISI-D2 tool steels by shearing them in sheet form using harder tools. First, the work-hardening parameters were fitted so that the FE simulation agreed with the measured punching force vs. stroke curves fitted using the Swift and Voce laws for the conventional and improved steels, respectively, at plastic strain levels exceeding 0.1. The plastic strain ε¯p vs. historically averaged stress triaxiality η¯ plots were derived from each element in the FE simulation at the punch stroke corresponding to fracture. The safe zone against ductile fracture was evaluated as a set of ε¯p — η¯ plots, and the exponential curves through the realms of these safe zones were drawn as the ductile fracture design curves. This evaluation suggested a drastically large fracture strain in the negative η¯ region compared with the fracture strain obtained using high-pressure tensile tests also conducted in this study. The effectiveness of the proposed method was confirmed as it clearly indicated the superiority of the improved AISI-D2 steel material.