Analysis of surface deflection of sheet metals in press forming

摘要

Hill's criterion of uniqueness to analyze the deflection of sheet metals in press forming cannot be applied to most cases since it could contact the die. Therefore, the following method is proposed to characterize the deflection. Deflections ofsheet metal are caused by shear stress in the out-of-plane direction, such that the development of surface deflections can be easily detected when shear stress is observed using deformation analysis during press forming. In this paper, the relationshipbetween the results of deformation and bifurcation analyses is represented in numerical examples by using Mindlin's shell theory in terms of axial compression of sheet metal and embossing of circular sheet metal by punches. It is revealed that theproposed method can determine the approximate bifurcation limit and the mode of the deflection of sheet metal, where initial imperfection of the configuration with a reasonable value should be assigned to sheet metal. Criteria to evaluate the deflectioncaused by a lack of bifurcation must also be established.