Characterization of surface defects after flanging of metallic sheets

This paper deals with surface defects of automobile outer panels, which alter significantly the vehicle quality. Such defects occur during springback, after forming or flanging steps, and are characterized by concave depression of small amplitude over the convex shape of the part. The aim of this work is to reproduce at the laboratory scale a surface defect that occurs after flanging on a geometry similar to a door upper corner. A dedicated device has been designed in order to generate small size surface defects during flanging of metallic thin sheets. The outer surface of the sample was digitalized and the spatial geometry of the defect was evaluated from curvature change along 2D profiles. This study shows that the flanging height does not influence significantly the surface defect geometry whereas the flanging radius and the corner radius modify the defect depth. Finite element simulations were also performed to compare numerical depth of the defect with experiments and also to investigate the stress and strain states in the defect area. It was shown that the spatial location of the defect is well predicted but its depth was either over- or under-estimated. Moreover, strains remained very small in the defect area, and a compression state prevailed after flanging, whereas a stress gradient developed through the thickness after springback.