An improved yield criterion characterizing the anisotropic and tension-compression asymmetric behavior of magnesium alloy

A novel yield criterion based on CPB06 considering anisotropic and tension-compression asymmetric behaviors of magnesium alloys was derived and proposed (called M_CPB06). This yield criterion can simultaneously predict the yield stresses and the Lankford ratios at different angles (if any) under uniaxial tension, compression, equal-biaxial and equal-compression conditions. Then, in order to further describe the anisotropic strain-hardening characteristics of magnesium alloy, the proposed M_CPB06 criterion was further evolved to the M_CPB06ev model by expressing the parameters of the M_CPB06 model as functions of the plastic strain. As the model was developed, the stresses and Lankford ratios of AZ31B and ZK61M magnesium alloys at different angles under tensile, compressive and through-thickness compressive conditions were used to calibrate the M_CPB06/M_CPB06ev and the existing CPB06ex2 model. Calibration results reveal that compared with the CPB06ex2 yield criterion with equal quantity of coefficients, the M_CPB06 criterion exhibits certain advancement, and meanwhile the M_CPB06ev model can relatively accurately predict the change of the yield locus with increase of the plastic strain. Finally, the M_CPB06ev model was developed through UMAT in LS-DYNA. Finite element simulations using the subroutine were conducted on the specimens of different angles to the rolling direction under tension and compression. Simulation results were highly consistent with the experimental results, demonstrating a good reliability and accuracy of the developed subroutine.