Determination of uniaxial large-strain workhardening of high-strength steel sheets from in-plane stretch-bending testing

This paper proposes a novel approach to determining the large-strain uniaxial stress-strain curves of sheet metals by using in-plane stretch-bending test data. In this approach, a stress-strain curve given by uniaxial tension is extrapolated by a constitutive equation of the combined Swift and Voce laws. The weighting coefficient in the combined Swift-Voce equation is determined by minimizing the difference in the stretch-bending mechanical responses, i.e., punch stroke (or load) vs. maximum bending strain, between the finite element (FE) simulation and the corresponding experimental result. This method is verified by performing experiments on four types of high-strength steel (HSS) sheets (the dual-phase type 590Y, 780Y, and 980Y and the precipitation type 590R). For all sheets, stress-strain curves are determined for strain levels of more than twice the achieved uniaxial-tension uniform elongations. The effects of sheet anisotropy and the Bauschinger effect in this method are discussed. The determined workhardening characteristic by this method for 590R at a large strain range is found to differ from that under an equi-biaxial stress state, as obtained from the bulge biaxial test.