Characterizing springback stress behavior in VPB by experimental-numerical hybrid method

Abstract

Springback is a stress-driven phenomenon during unloading, so the stress behavior during unloading is of great significance for exploring and understanding springback. However, the accuracy of stress field after loading procedure in FEM, as the basis of springback simulation, is greatly affected by the assumptions of material model and boundary condition. Moreover, the selection of constraints in springback simulation is crucial and highly subjective, thus affecting the authenticity of results. In this paper, the characteristic of springback stress is studied in viscous pressure bulging (VPB) using an experimental-numerical hybrid method, combining the DIC method and numerical calculation. The springback results under different viscous mediums are obtained and their laws are discussed. The measured springback data containing the information of material properties and boundary conditions are used to calculate springback stress by the presented numerical calculation program. The springback stresses of sections along rolling direction and transverse direction are obtained. The variation law of the magnitude and direction of springback stress vector with springback is revealed. Then the mechanism of springback stress variation and the springback reduction are analyzed. Results show that the springback stress conforming to objective and practical conditions can be obtained, and it changes regularly with springback. The viscous medium affects the shape and stress of sheet, causing the change of springback stress. Furthermore, the change of springback stress reduces the reverse bending moment to restrain the resulting springback.