Contact effects in bending affecting stress and formability

If a strip is pulled over a curved tool there is a contact stress acting on the strip. This contact stress changes the stress state in the material, which is analysed with a simple model. One effect is that the yield stress in tension is reduced. Predictions by the model agree with observation from a 90-degree bending test found in literature, and indirectly with observation from a stretch-bend test also found in literature. Another effect is that a 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. There may be other mechanisms at work, and one candidate is presented.