Theoretical predictions of fracture and springback for high tensile strength steel sheets under stretch bending

For the last two decades, strong demands for the light weighted structures of automobiles have been increasing in all over the world. In order to satisfy the demands, the high strength steel sheets (hereafter HSSs) have been widely utilized in the various automobile related companies. However, for the companies, it is still one of the hard tasks to apply HSSs for the automobile parts with complex shapes, due to low ductility and large springback of the HSSs. In the present research, we assumed fracture limits in stretch bending are theoretically obtained with the assumption that fracture occurs when stretching force reaches its maximum value. The authors proposed an explicitly theoretical analysis method based on the maximum load criterion that can easily and rapidly allow us to predict the fracture and springback of HSSs by using two no dimensional parameters (limited wall stretch L/L₀ (L: limit wall length of a sheet, L₀: initial wall length) and bending radius t/R (t: sheet thickness, R: bending radius)). For the evaluations of this method, the comparisons between stretching bending experiments of HSSs and the corresponding theoretical calculations were conducted. From the comparisons, the calculated results by our proposed method have good agreements with the experimental observations.