Bending effect in the failure of stretch-bend metal sheets

A study of the strain/stress gradient effect in the formability of stretch-bend metal sheets is presented. Depending on the severity of these gradients, two independent kinds of failure can be defined: Necking- and Fracture- controlled Failure. In a previous work the authors assumed that the former failure mode takes place when the inner fibre necks, and the latter mode arises when the outer fibre of the sheet fractures. Obviously, this assumption could seem a priori too much restrictive. Both types of failure are remodelled in the present work, but now it assumes that failure is controlled by the evolution of the strains and stresses in a certain material volume at the inner and the outer surface of the sheet. The size of this critical volume and their effects in the failure predictions are discussed.