Differential hardening in IF steel - Experimental results and a crystal plasticity based model

Work hardening in metals is commonly described by isotropic hardening, especially for monotonically increasing proportional loading. The relation between different stress states in this case is determined by equivalent stress and strain definitions, based on equal plastic dissipation. However, experiments for IF steel under uniaxial and equibiaxial conditions show that this is not an accurate description. In this work, the determination of the equibiaxial stress–strain relation with 3 different tests will be elaborated: a stack compression test, a cruciform tensile test and a bulge test. A consistent shape of the hardening curve is obtained which deviates from that of a uniaxial tensile test. Several physical explanations based on crystal plasticity are considered, including texture evolution, strain inhomogeneity and glide system hardening models. Texture evolution changes the shape of the yield surface and hence causes differential hardening, however, the observed differences at low strains cannot be explained by texture evolution. Accounting for the strain heterogeneity in the polycrystal, with equilibrium of forces over grain boundaries, improves the prediction of differential hardening considerably, even with a simplified interaction model (Alamel) and simple hardening laws for the glide systems. The presentation is based on a recently published paper by the authors [1]