Effect of Thickness Stress in Stretch-Bending

In any situation where a strip is pulled over a curved tool, locally a contact stress acts on the strip in thickness direction. This contact stress changes the stress state in the material, which will influence the deformation. One effect is that the yield stress in the plane of the strip is reduced. Predictions by a simple model agree with observation from a 90-degree bending test found in literature, and indirectly with observation from a stretch-bend test found in literature. The change in stress state also affects the formability. This is analyzed by applying the maximum force condition on this situation. The predictions agree with a more thorough analysis of the effect of thickness stress in general, but the predictions of both methods are lower than actually observed in tests.