Determination of flow stress for sheet metal forming using the viscous pressure bulge (VPB) test

In sheet metal forming operations the mechanical properties of the sheet material (i.e. flow stress or stress–strain curve) greatly influence metal flow and product quality. Therefore, accurate determination of the flow stress is of paramount importance in process simulation via finite element method (FEM). In this paper the use of the viscous pressure bulge (VPB) test for determination of flow stress under biaxial state of stress is discussed. With the VPB test, larger strains, which are relevant for stamping operations, can be achieved compared to the standard tensile test used to-date. In this study, FEM simulations and experiments have been performed in order to study the interrelationship of the geometric and material variables such as dome wall thinning, dome radius, dome height, strain hardening index, material strength coefficient, and anisotropy. From the study a robust method to determine the flow stress under biaxial deformation conditions using a viscous material as pressure medium has been developed.