Study On Constitutive Model Of Magnesium Alloy Considering Anisotropy And Strain Rate

In this study,the mechanical properties of AZ31B magnesium alloy,especially the yield behavior,were first studied by conducting quasi-static uniaxial tensile,compressive and through-thickness compressive(equi-biaxial tension)tests.The yield stress and lankford ratio curves with five different angles from the rolling direction were obtained.The anisotropic characteristics of the material were analyzed,and the tension-compression asymmetry of the material are studied by comparing the yield stress and Lankford ratios under tensile and compressive conditions.In addition,dynamic uniaxial tensile tests with different strain rates were carried out to analyze the strain rate effect of the material and the experimental results show that AZ31B magnesium alloy has obvious positive strain rate effect.Based on the CPB06 yield criterion,a new yield criterion was proposed,which can accurately predict the yield stress and lankford ratios at different angles to the rolling direction under tensile and compression conditions at the same time.Compared with the CPB06ex2 yield criterion with the same number of coefficients,the accuracy of the new yield criterion was significantly improved.In order to describe the anisotropic hardening characteristic of AZ31B magnesium alloy that the yield surface changes shape with the increase of plastic strain,based on the idea of interpolation,the coefficients of the proposed yield criterion was expressed by the equation of plastic strain to capture the shape change of yield surface with plastic strain.The calibration results show that the improved model can well predict the yield surface under different plastic strain.Hill 1948,Barlat1989 and YLD2000-2D yield criteria were calibrated under different strain to verify the applicability of the non-associated flow criterion to the present study magnesium alloy.Based on the above conclusion,non-associated CPB06 yield criterion with asymmetric yield surface was proposed by applying the non associated folw rule to it.In order to improve the prediction accuracy of material strain rate effect,the Johnson-cook hardening model was improved.Compared with the original Johnson-cook hardening model,the modified hardening criterion can accurately characterize the hardening curve of the material under different strain rates and the prediction accuracy of strain rate effect of the studied AZ31B magnesium alloy was significantly improved.Finally,the new plastic constitutive model was implemented into the finite element software LS-DYNA by writing a user material subroutine.and that was used to simulate uniaxial tension at different strain rates and quasi-static uniaxial tension and compression at different degrees to rolling direction.The correctness and efficiency of material subroutine were verified by comparing the results between the simluation and experiment.