Optimum design of flattening process for roll-formed door impact beam with GPa-grade steel

Lightweight door impact beams utilise roll forming with high-strength steels and require an additional roll-flattening process for direct bolt joints. However, conventional sheet designs fabricated using this approach are susceptible to various defects, including twisting, edge wrinkling, and cracking. This study proposes two notch designs for coil sheets: one featuring an internal hole, and the other having a dog-bone shape. The stress and strain states experienced by the sheet during the roll pass were analysed using finite element analysis. The results demonstrated that each notch design effectively altered the internal stress and strain distributions, leading to a reduction in the twist or edge defects. The internal hole design achieved a 77% reduction in edge compressive strain, mitigating wrinkling, and a 65% reduction in waviness height. However, it suffers from high longitudinal strain and strain concentration at the edge, potentially causing bow and fracture defects, respectively. The dog-bone design achieved a 78% reduction in residual longitudinal stress, preventing bow, and a 41% reduction in plastic strain at the edge. While this design leads to a two-fold increase in waviness height due to underdeformation, this remains within acceptable limits as subsequent press machine stages can address insufficient flattening.