Elastic-plastic and inelastic characteristics of high strength steel sheets under biaxial loading and unloading

Abstract

Biaxial tensile tests followed by biaxial unloading and reloading are carried out for BH340 and DP590 steel alloys. The contours of plastic work and the directions of plastic strain rates were measured at different levels of plastic work in the first and second quadrants of stress space. The applicability of conventional anisotropic yield functions, Hill's quadratic function and the Yld2000-2d function to the accurate prediction of the plastic deformation behavior of these steel alloys is discussed using the measured data. The measured work contours and directions of plastic strain rates were in good agreement with those calculated using the Yld2000-2d yield function with an exponent of four. The initial and subsequent elastic moduli after prestraining and the instantaneous tangent moduli during subsequent unloading after an equivalent plastic prestrain of ε0p=0.02 are measured from the biaxial loading, unloading and reloading experiments. The moduli of elasticity at reloading were lower by 9 to 17% than those at initial loading. The amount of strain recovery along the rolling direction (RD) is more than that along the transverse direction (TD) for uniaxial unloading, as well as for biaxial unloading. An exponential decay model is proposed that provides good reproduction of the unloading stress-strain relations, [σ/σ u)-Δε/(σu/E2), of both materials under different stress ratios. © 2010 ISIJ.