Yield Surface Modeling of Dual Phase Steels Subjected to Non-Linear Strain Path Loading

Abstract

Non-linear strain path experiments give a unique outlook on how the material deforms. This thesis discusses the effect of a prior loading history for a dual phase steel sheet metal, DP780, on its mechanical response, namely, the flow curve, Lankford coefficient or r-values, and yield surface. Performed in a medium-scale uniaxial specimen with the longitudinal axis in the rolling direction (RD), an initial pre-strain is induced in the material before machining smaller specimens for various stress states at different orientations and reloading them. The orientations for uniaxial reloading comprised of 150, 300, 450, 600, 750 and 900 from the RD. Stress-strain ratios and r-values are extracted from these subsequent experiments. Compression enacted a different observation on the material behavior in non-linear strain-path change. A distortional hardening model, the so-called Homogenous Anisotropic Hardening (HAH) model, was selected for the analysis of the experimental data. The purpose of all these experiments is to compare the predicted and experimental yield surfaces of uniaxial tension and uniaxial compression of a pre-strained material. According to the model, the yield surface is affected by a combination of different amount of cross-loading and Bauschinger-like effects depending on the severity of the strain path change. The particular goal of this work is to check how the yield surface behaves for changes close to orthogonal loading stress-state and if the HAH model can replicate similar behavior.