Instability in bending-under-tension of aged steel sheet

In combined bending and stretching of aged steel sheet, an instability of an unusual kind is identified. Analysis of the behaviour of a sheet using a simplified material model shows that under certain conditions the maximum tension is less than the fully yielding tension and springback in the opposite direction to that normally observed could occur. f the sheet used industrially for panels of small curvature may age and there is considerable current interest in the prediction of springback in panels formed by combined bending and stretching.