A New Testing Method to Evaluate Edge Cracking with Considerations of the Shear Clearance and Press Speed 2020-01-0758

As the automotive industry increasingly adopts Advanced High Strength Steel (AHSS) for the vehicle light-weighting and crashworthiness, the edge cracking significantly increases in stamping AHSS. Different lab-scale test methods such as the ISO standard hole-expansion test and the half specimen dome test are available to evaluate edge formability. However, none of these lab-scale testing methods emulates production conditions such as various shear clearances, part complexity, and shearing speed associated with the mechanical or hydraulic press operation. To address these limitations of the available testing methods, a new punching and stamping test was developed.

This paper introduces the simulation and experimental approach in developing this unique testing method to design the peanut-shaped hole that is sensitive to edge cracking in stamping. Three different sheet materials, DP780, 980 GEN3, and aluminum 6016-T4 were tested to validate the reliability of the newly developed testing method. Selected materials were punched with two different shear clearances between 10~20% that are commonly used for industrial applications. Also, two different press motions, conventional mechanical press motion and variable-speed servo press motion, were used for punching the peanut-shaped holes. The punched-hole blank was subsequently formed with the stamping tool. During the forming test, the die stroke corresponding to the edge cracking was detected by the load analyzer from the servo press. Strain distribution on the tested parts was measured and compared using the optical strain measurement tool, ARGUS. The work-hardening on the sheared edge was quantified with the hardness measurements. The edge formability was evaluated with the measured die stroke corresponding to the edge cracking and strain measurements.