3D modelling of sheet forming operations and prediction of forming limits

Abstract : Numerical modeling of processes and experimental measurements now make it possible to optimise the sheet forming parameters. In this paper, two criteria for the detection of localised necking during stamping of sheets are introduced and used for orthotropic and Barlat's anisotropic laws. The first approach uses perturbation techniques to calculate how the stability of the local mechanical equilibrium can be violated locally. The second criterion is based on the study of diffuse necking. It assumes that the material in the localisation band tends to a plane strain deformation mode and the maximum force for this local configuration is calculated. Both criteria are used with 3D finite element analysis of Nakazima and cross die stamping tests. Numerical predictions are compared to experimental results. A 3D strain measurement system based on correlation of images is used to determine the strain field on sheets, specifically in the area of localisation or fracture.