Analysis of the failure of H240LA steel sheets subjected to stretch-bending conditions

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Industrial sheet metal forming processes with certain complexity involve intrinsically non-linear strain paths, abrupt changes in the local strain ratio (β) due to successive stages during the manufacturing operation and/or the existence of strain/stress gradients through the sheet thickness due to the simultaneous action of stretching and bending. Under these situations, both the use of classical failure criteria assuming a uniform strain/stress distribution across the sheet thickness and the use of strain-path dependent metrics, such as the conventional FLC (Forming Limit Curve), for assessing sheet formability are highly questionable. In this work a failure model for stretch-bending which combines both the use of a path-independent formability diagram, based on the polar equivalent plastic strain curve (epFLC), along with different rules to account for the influence of the stress/strain gradient across the sheet thickness, particularly the Mid-Plane Rule (MPR), Concave-Side Rule (CSR) and Convex-Side Rule (CxSR), is explored. Their capabilities to predict failure by necking in a series of stretch-bending tests over H204LA steel sheets of 1.2 mm thickness are discussed in the light of experimental results. To this end, a 3D calibrated numerical model was used to precisely describe the realistic distribution of stresses/strains across the sheet thickness.