An analytical model for predicting springback and side wall curl of sheet after U-bending

An analytical model for predicting sheet springback after U-bending is proposed in this paper based on Hill48 yielding criterion and plane strain condition. The model takes into account of the effects of deformation history, thickness thinning and neutral surface shift on the sheet springback of U-bending. Three rules for material hardening – kinematic, isotropic and combined hardening – have been used to consider the effect of complex deformation history that has undergone stretching, bending, and unbending deformations on the sheet springback. The model is applied to the benchmark of NUMISHEET’93 2-D draw bending problem. It indicates that the springback is overestimated when isotropic hardening is applied, while is underestimated when kinematic hardening is applied. For reverse loading problem, the combined hardening is a good approach to the practical material. In addition to that, the effects of blank holding force, friction coefficient between sheet and tools, sheet thickness and anisotropy have been investigated. When the shifting distance of neutral surface exceeds one-fourth of sheet thickness, the springback can be reduced effectively by increasing the blank holding force and friction between sheet and die. And the springback increases with anisotropy and friction between sheet and punch, and decreases with the sheet thickness.