Fracture prediction of AA6061-T6 sheet in bending process using Gurson-Tvergaard-Needleman model

Fracture is one of the limiting factors of sheet metal forming processes. For this reason, finite element simulations are widely used for providing suitable forming approaches. However, the success of these approaches depends on the correct modeling of fracture in the simulations. In this paper, the fracture behavior of the Aluminum 6061-T6 sheet in the U-bending process is investigated by applying the Gurson-Tvergaard-Needleman model. For this purpose, the Gurson-Tvergaard-Needleman model is defined in the ABAQUS software by means of a proper subroutine. The Gurson-Tvergaard-Needleman model is also modified to take into account the anisotropy effect on the yield surface and the damage evolution due to shear stresses. Then, the proposed models are calibrated using the inverse analysis method based on tension tests with different stress states. The deformation mechanics analysis reveals that considering the normalized Lode angle parameter in the damage function increases the accuracy of fracture prediction, especially in low-stress triaxiality. Comparing the predicted fracture displacements with the experimental value in the U-bending process shows that the modified Gurson-Tvergaard-Needleman model calibrated by the uniaxial tension and in-plane shear tension tests can predict the onset of fracture with acceptable accuracy in the bending process while the original Gurson-Tvergaard-Needleman should be calibrated by a test with stress state close to that in the bending area to have such a result.