Influences of the evolving plastic behavior of sheet metal on V-bending and springback analysis considering different stress states

Bending is a common forming process that covers various stress states during sheet metal forming, mainly from uniaxial to plane strain states of both tension and compression depending on the inner or outer sides of the sheet. To model the tension-compression asymmetry (TCA) at different stress states, a five stress states-sensitive (Five-SSS) yield function is proposed. The non -linear effect of stress triaxiality eta on plasticity is considered. Multiple analytical calibration methods are obtained for the Five-SSS function by considering different groups of stress states from equibiaxial compression to equi-biaxial tension. One of the methods considers the uniaxial tension, uniaxial compression, plane strain tension, plane strain compression and equi-biaxial tension, which is used to simulate the V-bending process of an advanced high-strength steel QP1180 and a magnesium alloy AZ31. A modified Lode-dependent anisotropic-asymmetric (LAA) function is adopted as plastic potential to model both the TCA and evolution of r-value. The Five-SSS + modified LAA functions can predict the double shift of neutral layer during V-bending for the two investigated materials due to multiple reverse loading in the neutral layer shift zone. For the springback prediction of V-bending, it is observed that the Five-SSS yield function can highly improve the prediction accuracy compared to other functions because it models the TCA under both uniaxial and plane strain states. In addition, the TCA of unloading modulus should be considered for AZ31 to significantly improve the accuracy of springback prediction.