Prediction of edge fracture during hole-flanging of advanced high-strength steel considering blanking pre-damage

The stretch-flangeability of advanced high-strength steel (AHSS) sheets after blanking highly depends on the stress states as well as the edge pre-damage inherited from the initial blanking process. It cannot be predicted by using conventional forming limit diagram methods. To solve this issue, in this study, a numerical simulation model of the two-step hole cutting-hole expansion tests for the TRIP800 sheet was established by considering the initial surface irregularities induced by hole fabrication into the hole expansion process. Two methods, punching and wire-cut electrical discharge machining with low speed were adopted to cut holes. The modified Mohr-Coulomb fracture model together with post-failure softening was utilized to predict the fracture initiation and crack propagation during two forming processes. The results manifested that the WEDM-LS cutting method brought negligible damage to the hole edge and the sheet edge fabricated by this method can be assumed without pre-damage. Multiple finer notches were detected at the surface of the punched hole. These features triggered stress states with higher stress triaxiality up to 0.6 and thus accelerated the edge fracture occurrence during the hole expansion deformation. The measured hole expansion ratio of the punched hole was 26.1%, which was correctly predicted by the proposed model by considering edge pre-damage. As a comparison, the ideal model without pre-damage overestimated the value by about 100%.