Yield Models Considering Strength Difference Effect And Anisotropy

Sheet metal stamping is an important production method in the automobile industry.The quality of stamped parts has a great impact on the production cost and production efficiency of the automobile.The effect of anisotropy and tension-compression strength difference of sheet metal plays an important role in sheet metal forming process.Therefore,how to accurately describe the plastic constitutive relation and mechanical properties during sheet forming is of great significance to the development of plastic forming theory and will have great engineering application value for actual production.In this paper,based on the forming of sheet metal in automobile lightweighting,and based on the principle of metal plastic forming,anisotropic yield models considering tension-compression asymmetry were introduced and studied in detail.The theoretical formulas of the yield criteria of Yld2000-SD and CPB06ex2 were deduced,and the anisotropy parameters were calibrated with different metal materials.Using the finite element software development platform of ABAQUS,the user subroutine of CPB06ex2 yield model was written.The mechanical properties of ZEK100-O magnesium alloy were characterized by simulation,and the accuracy and reliability of the yield model were verified.Firstly,an overview of the research progress and achievements on the yield models by scholars at home and abroad,several classic anisotropic yield models were introduced in detail,and their applications,advantages and disadvantages were summarized.Secondly,the coefficients of each model were calibrated with different materials,and fitting accuracy was quantitatively analyzed.In order to study the effect of anisotropy and tension-compression strength difference(SD effect)in sheet metal forming,the Yld2000-SD yield model was established.The yield stresses of tensile/compressive and equal-biaxial stress were derived.Four metal materials were selected,and the anisotropy coefficients of Yld2000-SD yield function were calibrated.The fitting effects of Yld2000-2D and Yld2000-SD yield criteria were compared from three aspects:yield surface,yield stress,Lankford coefficient,and compared with experimental values.Through verification,it was found that the Yld2000-SD yield criterion can predict the yield stress and thickness anisotropy coefficient more accurately overly than Yld2000,especially for two magnesium alloys with strong SD effect.The CPB06ex2 yield model was used to establish the equivalent plastic strain expression The theoretical formula of the CPB06ex2 yield model was systematically derived.The mechanical properties of HCP metal materials were characterized.Using the experimental values of ZEK100-O magnesium alloy,the deformation history was considered.The coefficients of the model were calibrated under the non-AFR flow law,and the evolution equations of the coefficients with respect to the cumulative plastic strain were summarized..Dynamic explicit algorithms were used,VUMAT subroutines were written and embedded in the finite element software,and samples of different shapes were simulated.The accuracy and reliability of the CPB06ex2 yield model were verified by comparison with experimental data and quantitative analysis.The achievements of this thesis have important effect on the theoretical research for anisotropy and SD effect of metal materials,and they are beneficial to improve the simulation accuracy of metal forming of materials with strength difference effect and anisotropy.