It has been widely observed that the loading/unloading behavior of metals which have previously undergone plastic deformation is nonlinear. Furthermore it shows a hysteresis behavior upon further unloading/reloading cycles. The origin of this nonlinearity is attributed to additional dislocation based micro-mechanics which contribute to the total reversible strain, referred to as anelastic strain. Compared to a FE model prediction using only elastic contribution to reversible strain the actual springback will be larger. In this work the unloading behavior of DP800 AHSS is analyzed in detail and a mixed physical-phenomenological model is proposed to describe the observed nonlinearity for different levels of pre-strain. This one dimensional uniaxial model is generalized to a 3D constitutive model incorporating elastic, anelastic and plastic strains. The performance of the model is evaluated by comparing the predicted cyclic unloading/reloading stress-strain curves with the experimental ones. It is shown that by incorporating anelastic behavior in the model the prediction of the cyclic behavior of the material is significantly improved.
- Nonlinear unloading