Effects of Blanking Conditions to Edge Cracking in Stamping of Advanced-High Strength Steels (AHSS)

Practical evaluation and reduction of edge cracking are two challenging issues in stamping AHSS for automotive body structures. In this paper, the effects of the shear clearance and shear rake angle on edge cracking were investigated with three different grades of AHSS; TRIP780, DP 980, and DP 1180. Five different shear clearances, between 5% and 25% of material thickness, were applied to the flexible shearing machine to generate samples for the half specimen dome test (HSDT). The shear loads and the shear edge quality were thoroughly characterized and compared. The HSDT created the edge forming limits as compared to the base material forming limit diagram. The load-displacement curve was acquired by the load-cell and the strain distribution was measured using a digital image correlation (DIC) system during the dome test. The best shear clearance was determined for each test material based on the maximum punch stroke onset of edge cracking as well as the DIC measured major strain and thinning values. The results showed that TRIP780 showed the maximum edge stretching with 15% shear clearance while both DP 980 and DP 1180 were with 20% shear clearance. The DIC measured onset thinning value of edge cracking was used as the major input data for edge cracking prediction using commercial finite element (FE) software, PAM-STAMP. Simulation results showed decent correlations with experimental observations. The shear peak load was found to be a good indicator for representing shear edge quality. TRIP 780 and DP 980 showed the most sensitivity of shearing-induced hardness associated with edge cracking. However, DP 1180 showed relatively lower sensitivity of hardness increase to edge cracking, because DP 1180 was observed to relatively small hardness variations between the sheared edge and base metal compared to TRIP 780 and DP 980. For industrial application, both the shear load and burr height can be monitored in the shearing process to improve sheared edge quality and edge stretchability.