Effect of residual stress on the bending of aluminium

Skin passing or temper rolling introduces a strain gradient through the thickness of the sheet which, in turn, results in a residual stress profile through the thickness. These residual stresses can have a major effect on the elastic–plastic transition in bending operations, but the effect of residual stresses on downstream forming has yet to be determined. In this work, a commercial aluminium alloy, AA6063, was reduced in thickness by rolling in a laboratory mill to introduce residual stresses into the material. The strips were then tested in tension and pure bending and the material behaviour close to the elastic–plastic transition investigated. Substantial lowering of the elastic–plastic transition in the bending moment was observed while the opposite trend was found in the tensile test. The rolling process was analyzed using finite element analysis and the distribution of residual stresses determined; theoretical moment curvature characteristics were obtained using the output of this analysis as the input to the modelling for pure bending. The results show qualitatively how the material is softened in bending after rolling and demonstrate that this is partly due to the residual stress gradients in the material.