On the Modeling of the Unloading Modulus for Metal Sheets

The springback phenomenon is defined as the elastic recovery of the residual stresses produced during the forming of a material. An accurate prediction of springback puts high demands on the constitutive modeling. A constitutive model for springback prediction should of course be able to accurately predict the stress state after the forming phase. However, it should also be able to predict the material behavior during the unloading phase. In classical plasticity theory, the unloading of a material after plastic deformation is assumed to be linearly elastic with the stiffness constantly equal to Young’s modulus. However, several experimental investigations have revealed that this is an incorrect assumption.