Instability prediction during stretch bending of AHSS sheet metal

Under stretch-bending conditions where the curvature is relatively large, the plane stress assumption leads to a significant inaccuracy in forming limit prediction. In processes where a contact pressure acts on the material combined with a small punch radius it is observed by FE simulations that a significant thickness stress gradient develops. This gradient influences the materials plastic flow behavior through thickness and hence affects the cross-sectional stability of the sheet. Based on this observation a theoretical study has been performed in order to quantify the effects of different process and material parameters on the formability of the sheet. Furthermore, it is observed numerically and experimentally that during stretch-bending the cross-sectional stability is not lost instantaneously but gradually over time. Therefore, the concept of FLC proves to be insufficient as to judge acceptability of the deformed product. Another approach is proposed where an instability criterion is developed that also accounts for the partial loss of stability of the cross-section. The criterion is implemented in FE environment to be used in Shell type elements. The results are compared with stretch-bending experiments on selected HSS and AHSS.