Mechanism of fracture in sheet metal cutting processes and its effect on sheared edge stretchability

In recent years, implementation of aluminum alloys, the advanced high strength steels and ultra high strength steels (UHSS) is quickly increasing in automotive industry. However, these materials are often sensitive to sheared edge cracking if stretching along the sheared edge occurs in such processes as drawing of panels with blanked windows, stretch flanging and stretch hemming of edges of the panel. This study is dedicated to development of experimental techniques necessary to account for sheared edge condition on material formability as well as reporting the experimental results and general trends. Analysis of the hole punching process indicated that uniformity of the cutting clearance is rather difficult to maintain, especially for UHSS material where cutting forces are substantially higher than for mild steels or aluminum alloys, and stiffness of the tool starts playing critical role. Therefore, the majority of experimental studies were performed as tensile tests of samples sheared along a straight line in a dedicated trim tool where special measures were taken to achieve consistency of the die clearance. Experimental results on sheared edge stretchability of aluminum alloys similar to 6111-T4 and UHSS steel DP980 are reported. The mechanism of fracture propagation in trimming and hole punching processes is discussed in conjunction with sheared edge stretchability. The rather unique mechanism of fracture observed for trimming of UHSS DP980 steel leads to burr breaking at the final stage of the shearing process.