Stress state dependency of unloading behavior in high strength steels

Accuracy of springback prediction strongly depends on whether the unloading behavior of the material is properly considered in the material model or not. It is known that the stress-strain relationship for steel sheet during unloading is nonlinear. For accurate springback prediction, the nonlinear unloading behaviors should be observed not only under uniaxial, but also under multi-axial stress state, and properly considered in FEsimulations. In this study, unloading stress-strain curves of high strength steels under four stress states: uniaxial tension, plane strain tension, biaxial tension and shear, were experimentally obtained in several material tests. From the obtained curves with different prestrains, the average elastic moduli were calculated and converted into the equivalent modulus using the isotropic Hooke's law. Moreover, the nonlinearity of the unloading stress-strain curve was evaluated by the instantaneous stress-strain slope. The average elastic modulus and the nonlinearity obviously differ by stress states, prestrains and types of steel. The investigated steels show the stress state dependency of the unloading behavior.