Effect of Skin Passing on Bending and Tension: An Experimental Investigation

Skin passing is a common processing step used to control yield phenomena and improve surface qualities in sheet metal. The process leaves residual stresses within the sheet that may have a significant effect on material behaviour in forming processes where the major deformation mode is that of bending combined with unloading and reverse bending. Previous studies have revealed that the effect of residual stresses is not well indicated by the tensile test. This has prompted the development of a new bend test arrangement to obtain bending and springback data in heavy gauge and high strength sheet. In this paper, an experimental study on the effect of skin passing on the material behaviour of aluminium strip in pure bending and tension is presented. The aluminium strip was given a recrystallisation anneal to eliminate all pre-existing deformation effects, and then skin passed to different final thicknesses. Subsequently the material behaviour of the skin passed strip and the annealed starting material were determined by bending and tensile testing. The experimental results indicate that skin passing has a softening effect on the material behaviour in bending in the early stages of elastic-plastic bending. In contrast to that, the material properties derived from the tensile test indicated an increasing material strength with skin pass level.