Numerical and experimental determination of the forming limit curves of AA6061 using the modified GTN damage model

The current research presents an experimental and numerical investigation of the forming limit of ECARed (Equal Channel Angular Rolling) 6061 aluminum alloy using a modified Gurson–Tvergaard–Needleman (GTN) failure model. Additionally, to achieve more precise results, Hill's second-degree anisotropic yield criterion with shear loading terms was employed instead of the Von Mises isotropic yield criterion, representing another innovative aspect of this research. The parameters associated with Hill's yield criterion were calculated using Lankford's coefficients (r0, r45, and r90), which can be derived from uniaxial tensile tests conducted at angles of 0, 45, and 90 degrees relative to the rolling direction. The GTN parameters were determined by minimizing the difference between experimental and numerical results from standard tensile tests. Following the calibration of the modified GTN model, spherical head expansion tests were conducted to derive forming limit curves. Finite element method (FEM) was also performed using the developed GTN failure model, and the results were compared with experimental data. The findings demonstrate a favorable agreement with an error of less than 5% between the experimental results and the numerical predictions obtained from the modified GTN failure model. Also, the results show that the use of Hill's second-degree anisotropic yield criterion compared to Von Mises isotropic in the GTN failure model is more consistent with the experimental results and has higher accuracy.