Characterization On The Strain-Rate And Tension-Compression Asymmetric Mechanical Behavior Of Magnesium Alloys And Study On The Energy Absorption Characteristics Of Thin-Walled Tubes

With the popularization of automobile lightweight,more and more magnesium alloy materials are used in automobile parts.Meanwhile the passive safety performance of the automobile can not be ignored.On the one hand,the safety of the structure depends on the mechanical properties of the materials used,on the other hand,it is also related to the crashworthiness of the structure.In the automotive frame,the metal is generally used in thin-walled tube structure,thus it is of great significance to study the mechanical properties of magnesium alloy materials and the crashworthiness of magnesium alloy thin-walled tube.In this study,quasi-static uniaxial tensile,circular notched tensile(R20,R5 and R2.4),right-angle notched tensile,shear,compression and uniaxial tensile tests at strain rates from 1/s to 1000/s were carried out on extruded thin-walled tubes made of AZ31B magnesium alloy.The results show that the hardening curves of AZ31B magnesium alloy exhibit obvious tension-compression asymmetry and strain rate dependent characteristics,and the fracture strain of AZ31B magnesium alloy also show obvious strain rate effect.Moreover,the mechanical properties of AZ31B magnesium alloy vary distinctly with different stress states.The Johnson-Cook hardening model and damage models were modified based on the mechanical properties of AZ31B magnesium alloy.According to the tensioncompression asymmetry of AZ31B magnesium alloy,the hardening term function of the Johnson-Cook hardening model is expressed as two functional forms which can describe the tensile hardening and compression hardening respectively on the basis of the stress triaxiality.Based on the original Johnson-Cook damage model,the fracture strain of the model is expressed by piecewise function.And the strain rate term of the hardening and damage models were also modified.The modified models can significantly improve the prediction accuracy of mechanical behavior of AZ31B materials.Then the modified constitutive model is coded into the UMAT user subroutine of LS-DYNA software,and the experimental results were used to verify the developed model.The results show that the simulation results are consistent with the experimental results,which indicates that the developed subroutine is reliable.The axial compression experiments of thin-walled square and round tubes made of AZ31B magnesium alloy with different aspect ratios(1 to 4)at different compression speeds(3mm/min to 5m/s),and the quasi-static three-point bending experiments were carried out.The buckling and fracture laws of thin-walled tubes with different specifications were analyzed.According to the experimental conditions,the developed UMAT subroutine was used to carry out the finite element simulation and verification of thin-walled tubes,and the verification results show that the developed subroutine model can well simulate the buckling and fracture behavior of magnesium alloy thin-walled tubes under complex conditions.Finally,according to the fracture features of AZ31B magnesium alloy,three types of new thin-walled magnesium alloy structures,longitudinal thickness gradient thin-walled magnesium alloy tubes,aluminum foam-filled thin-walled magnesium alloy tubes,magnesium and aluminum alloy composite double-layer/triple-layer tubes,were designed for the purpose of improving the crashworthiness and energy absorption capacity of magnesium alloy tubes.The finite element simulations of the new structure were carried out,and the crashworthiness indexes such as SEA and SCF(specific crush force)were used for evaluating the crashworthiness of new structures.It is found that the magnesium and aluminum alloy composite tubes exhibit better crashworthiness.There are 53 figures,15 tables and 81 references in this thesis.