Prediction of Stretch Flangeability Limits of Advanced High Strength Steels using the Hole Expansion Test 2007-01-1693

More and more advanced high strength steels (AHSS) such as dual phase steels and TRIP steels are implemented in automotive components due to their superior crash performance and vehicle weight reduction capabilities. Recent trends show increased applications of higher strength grades such as 780/800 MPa and 980/1000 MPa tensile strength for crash sensitive components to meet more stringent safety regulations in front crash, side impact and roll-over situations. Several issues related to AHSS stamping have been raised during implementation such as springback, stretch bending fracture with a small radius to thickness ratio, edge cracking, etc. It has been shown that the failure strains in the stretch bending fracture and edge cracking can be significantly lower than the predicted forming limits, and no failure criteria are currently available to predict these failures. In this study, the forming mode of edge cracking is evaluated, and a failure criterion is proposed based on hole expansion experimental data. The new failure criterion is validated in a lab simulative test and is successfully applied to two example industrial automotive components.