The analysis of forming limit in re-penetration process of the hole-flanging of sheet metal

A methodology for formulating an elasto-plastic finite element model was developed to analyze the direct penetration and re-penetration in the hole-flanging process. The model was based on the updated lagrangian formulation, Prandtl-Reuss flow rule, and Hill's yield criterion. Simulation results included the relationship between punch load and punch stroke, the variation of the workpiece thickness, and the forming limit. Two sets of tools with different dimensions were designed for experiments to verify the reliability of the program. By using the designed tools for re-penetration process, this paper studied the effect of the direct penetration and re-penetration on the workpiece. Besides, this paper also discussed the comparison of experiments and numerical analysis between penetration and re-penetration processes. According to the comparison between simulation and experiment, the punch load of re-penetration was less than the punch load of direct penetration. The maximum punch load increased remarkably upon decreasing the profile radius of cylindrical punch and the initial diameter of the inner hole of blank. Moreover, the re-penetration had better formability than the direct penetration in the same condition. The forming limit ratio of the re-penetration process was improved from 2.70 to 2.97. The simulation results showed good agreement with the experiments. (C) 2007 Elsevier B.V. All rights reserved.