Numerical and experimental investigation of formability enhancement during continuous-bending-under-tension (CBT) of AA6022-T4

Abstract

Abstract Continuous-bending-under-tension (CBT) is a stabilized tension test, whereby a strip is bent around three reciprocating rollers while being loaded remotely with an axial force. This subjects the strip to plastic bending while under tension, and has the beneficial effect of suppressing the instability that, in a conventional tension test, leads to localization of deformation and rupture. CBT experiments were performed on strips extracted from cold-rolled, automotive-grade AA6022-T4 sheet using a custom CBT testing machine. The CBT experiments revealed that the remote axial force necessary to plastically deform the materials was lower than the yield force under uniaxial tension, due to the synergy between tension and plastic bending. Furthermore, the force-displacement curves exhibited a saw-tooth like pattern, which can be explained from the kinematics of the process. A parametric investigation identified the combination of bending depth, crosshead velocity and roller velocity that yielded the maximum elongation-to-fracture. However, the experiments also revealed that the elongation-to-fracture was different between the rolling and transverse directions of the sheet, and significantly more than the uniaxial testing would indicate. The CBT experiments were then simulated using the non-linear finite element code Abaqus. Solid, linear, full-integration elements were used in the simulations, which replicated only half of the strip width, due to the symmetries present. The key for successful simulations is the use of an appropriate material model. Cyclic tension-compression tests on the AA6022-T4 were performed, and the results were captured accurately by a combined isotropic/non-linear kinematic hardening model, using a simple, exponentially-decaying, expansion of the yield surface and a 4-term Chaboche model, respectively. The force-displacement response from the simulations matched the experimental results well. In the future, the simulations will be used to probe the stress and strain development during each CBT cycle, to improve the understanding of the stabilization of deformation that the CBT loading mode provides.