Study On Deformation Damage And Fracture Mechanism And Materials Characterization For Aluminum Automotive With FEM Simulation

The light-weight automobile meets the requirement of saving energy,protecting environment and developing economy.The aluminum and its alloys have been used to manufacture the light-weight automobile due to their low density,good plasticity and high strength/weight.Good crashworthiness is one of the desired qualities for a vehicle.During the impacting process,the stress states and the strain rates of the automobile components are different at different locations.Moreover,the stress state and strain rates change with the passing of time.The FEM simulation is an important method used to study the aluminum components' crashworthiness.The investigation on the deformation,energy absorption and failure modes of the space frame structure by FEM simulation can shorten exploitation cycle and reduce exploitation expenditure.However,a lot of problems come into being in the process of using the FEM software.For example,there is a great of difference between the results of simulation and experimental results.The reasons don't lie in the FEM software itself but in the shortage of exact constitutive model used to describe the deformation behavior of materials and damage model used to describe the failure and fracture of materials. Through a lot of experiments and measure of mechanical parameters,this dissertation tries to establish the constitutive relationship and its evolvement law of damage under different stress states and strain rates thus lay the foundation for correct FEM simulation.Focusing on the above problems,the study has centered on the following several aspects:Considering the anisotropy effect of extrusion 6063 aluminum alloy(T5)having influence on the results of simulation,the characterization on mechanical property at an angle of 0,45,90 to the extrusion direction of material has been implemented in this paper.At the same time,the characterization provides a necessary theory foundation for sampling modes in the following experiments.Butterfly specimen and Arcan fixture have been devised in this study.The damage and fracture behaviors of aluminum alloy were investigated by different angles tensile tests with the modified Arcan fixture on the butterfly specimens.The results of study show:the mechanical properties of aluminum alloy are different under different stress states.With stress triaxiality decreasing,the maximum loading decreases gradually,but the fracture strain increase gradually.With stress triaxiality decreasing,the damage modes are microcracks and microvoids to microcracks to localization shear bands and microcracks in turn.With stress triaxiality decreasing,the mechanism of dimple fracture decreases and the mechanism of shear fracture increases.The uniform tensile fracture law can be used to describe the fracture modes of specimens under different stress states.The mechanical properties and deformation fracture behaviors of aluminum alloy were investigated by flat tensile tests under different strain rates in this study.The results of study show:the aluminum alloy is the materials with insensitive strain rate.With strain rate increasing,the yield strength increases slightly and the fracture strain slightly decreases,but the fracture strength and ultimate strength remain constant basically under the moderate strain rates.However,the sensitivity of material strain rate appears under high strain rates.With strain rate increasing,the yield strength and ultimate strength and the fracture strain all increase.The negative strain weakening effect appears when the strain rate gets to a critical value.With strain rate increasing,the damage modes among specimens were microcracks.But, the mechanism and forms of producing microcracks are different.A lot of microcracks are produced along grain boundaries due to the deformation inconsistence between adjacent grains under moderate strain rates.The mechanism of rapid shear is dominant and the localized shear deformation bands are produced firstly among specimens and the microcracks are produced in the localized shear deformation bands subsequently.The microcracks are long under moderate strain rates and they are short cracks along grain boundaries under high strain rates.With strain rates increasing,the direction of dimples on the fracture surfaces becomes more and more obvious and the proportion of shear plane on the fracture surfaces becomes larger and larger.Namely,with strain rate increasing,the trend of shear fracture mechanism increases.The modified power-law of strain rate hardening model can be used to describe the strain rate hardening behaviors of the yield strength and ultimate strength under high strain rates.The pure shear specimen by tensile has been devised in this paper.The mechanical properties and deformation fracture behaviors of aluminum alloy were investigated by tensile shear tests under different shear strain rates.The results of study show:the shear strain rates have little influence on the strength of aluminum alloy under moderate shear strain rate.It further proved that the aluminum alloy is rate-irrespective materials.With shear strain rate increasing,the shear fracture strain decreases according to power-exponent law.The shear strain rate has no influence on fracture surfaces of tensile shear specimens.The in-situ tensile specimens designed to produce different stress states have been devised.The whole deformation processes from tensile to fracture were investigated by in-situ tensile under different stress states.The results of study show:The weakest links in this special 6063 aluminum alloy are the grain boundaries.Most cracks initiate at the grain boundaries.The distribution of stress triaxaility among the least section is different for the specimens with different shapes.Therefore,the driving force and the mode of cracks initiation and their propagation are different.In the 0°in-situ tensile specimen,the normal tensile stress dominates the crack initiation and propagation.While,in the 45°and the 90°specimens,the shear stress dominates the crack initiation and propagation.With stress triaxiality decreasing, the angles between the slipping deformation bands and the loading direction decrease gradually and the fracture modes changed from normal fracture to shear fracture,and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode.The FEM simulation was carried out to simulate the behaviors of damage and fracture under different stress states and strain rates with G-T-N damage model and Johnson-cook damage model based on the above experiments.The results of FEM simulation show:the shear fracture is dominant for the fracture modes of specimens and the FEM simulation was preformed with Johnson-cook damage model when the stress triaxiality is 0～1/3.The microvoids-coalescence fracture is dominant for the fracture modes of specimens and the FEM simulation with G-T-N damage model results in satisfactory result when the stress triaxiality exceeds 1/3.The G-T-N damage model can be used to simulate the damage behaviors of the fiat tensile tests under different middle-low strain rates.The results of simulation show that the critical damage parameter f_c has no insensitivity to mesh size and strain rates under different middle-low strain rates.But the damage parameters f_c and f_F among G-T-N damage model depend on the mesh size of FEM model under high strain rates: the damage parameters f_c and f_F are small when the mesh size is big.Under the same mesh size,the relationship between the damage parameters f_c and f_F and stress triaxiality was gained by fitting the damage parameters of G-T-N model under different stress states.At the same time,the results of simulation also show that the original void volume fraction f₀ has different influence on the critical damage parameter f_c:the original void volume fraction f₀ has on influence on the critical damage parameter f_c when the f₀ is very small(f₀≤0.001), while the original void volume fraction f₀ has influence on the critical damage parameter f_c when the f₀ is relatively big(f₀＞0.001).In order to verify the correctness of materials model, damage model and the method of materials characterization,the FEM simulation was carried out in extrusion tests of thin-walled tube of aluminum alloy with G-T-N damage model and Johnson-cook damage model.The results show that:compared to Johnson-cook damage model,the G-T-N damage model used to simulate the thin-walled tube results in very close simulation results with experimental results in both the loading-displacement curves and the configuration of failure element,which indicated that the G-T-N damage model can be used to simulate the deformation and fracture behaviors during the extrusion process of thin-walled tube.