Practical Evaluation and Prediction of Edge Cracking in Forming Advanced High Strength Steels (AHSS)

Practical evaluation and accurate prediction of edge cracking are challenging issues in stamping AHSS for automotive body structures. This paper introduces a new hole-expansion testing method that could be more relevant to the edge cracking problem observed in stamping AHSS. A new testing method adopted a large hole diameter of 75 mm compared to the ISO standard hole diameter of 10 mm. A larger hole diameter was determined to be sensitive to edge cracking using the finite element method (FEM) based sensitivity analyses with various hole sizes. A die punching tool was developed to replicate typical production blanking conditions. An inline monitoring system was developed to visually monitor the hole edge cracking during the test and synchronize with the load-displacement data. Two AHSS materials, DP980 and TRIP780, and an aluminum alloy, A1 5182-O, were experimentally evaluated. Results of the new testing method were compared to the results of the ISO standard hole-expansion testing as well as the half-specimen dome testing that have been developed and used by the automotive industry. This new testing method showed good correlations of results with the half-specimen dome testing. It showed a much larger difference (up to 11%) of the hole expansion ratio (HER) between DP980 and TRIP780 compared to the standard HER testing method giving less than 1% difference. Test results showed several interesting findings. The water-jet cut hole samples showed about 2× larger HER than the punched hole samples for DP980 and TRIP780. However, the Al 5182-O samples showed less sensitivity to edge cracking. A maximum thinning based failure criterion was developed to predict edge cracking in FEM simulation. Experimental data from both new hole expansion testing and half-specimen dome testing were used to develop the edge cracking failure criteria for DP980 and TRIP780. Finally, the developed failure criteria was applied to predict edge cracking in stamping of TRIP780 channel parts with various bending angles and the side flange widths of the parts in industry field testing at KTH. The FEM predictions of edge cracking and conventional necking failure showed good correlations with the stamping trials.