| The light-weight automobile meets the requirement of saving energy,protecting environment and developing economy.The aluminum 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 pasting 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,to analyze the evolvement law of damage of 6061 aluminum alloy under different stress states and strain rates and lay the foundation for establishing the constitutive relationship and correct FEM simulation.Based on the above,the study has centered on the following several aspects:First,through comparative analysis of a variety of aluminum alloy,6061 aluminum alloy can be achieved the requirements of vehicle profile with the size deviation,mechanical properties,corrosion resistance and appearance of quality by optimization process,moreover,it has reasonable prices.So at the end we make sure of our study object is 6061 aluminum alloy.Second,for realizing different stress state,different gap tensile specimens have been designed.Through tensile the specimens with quasi-static,the mechanics property and fracture behavior of 6061 aluminum alloy under different stress states were investigated.Study indicated that the bigger the radius of notch is,the lower the stress triaxiality is.With stress triaxiality decreasing,the maximum loading decease,but the shrinkage of cross-section and fracture strain all increase.At the same time,the area of shear lip on fracture surface are gradually increasing,the amount of the sawtooth-shaped on the fiber zone decrease,but the depth decrease.Third,Fracture processes of 6061 aluminum alloy specimens under different stress states are investigated with in-situ tensile experiments on the SEM-520.The results indicate that a lot of slip bands are produced in all specimens' surfaces under different stress states,but the fracture mechanism is different.With stress triaxiality decreasing,the fracture modes vary from dimple-coalescence fracture and dimple-coalescence mixed with shear fracture to pure shear fracture and the fracture surfaces also vary from dimple fracture mode to shear fracture mode.The grain boundaries of 6061 aluminum alloy are the weakest area and microcracks initiate at the grain boundaries.With loading increasing,the microcracks extend and coalesce. The specimens fracture due to coalescence or shearing between the microcracks.The less the stress traixiality is,the more obvious the orientation of the dimple of fracture surface tends to be,and the smoother the fracture surfaces becomes.Forth,the deformation and fracture behaviors of aluminum alloy were investigated by flat tensile tests under different strain rates in this study.The results of study show:With strain rate increasing,the yield strength increases slightly but the fracture strain and the fracture strength and max ultimate strength remain constant basically.However,the sensitively of material strain rate appears under very high strain rates:With stain rates increasing,the yield strength and max strength and the fracture strain all increase.The negative strain rate weak effect appears when the strain rate gets to a critical value.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 surfaces becomes larger and larger.Fifth,using of a common drop hammer impact test machine,oscillographic impact with the instrument,fixed the sample on the devices design by us,carry out the different speeds impact of pull off tensile test and tensile unloading test.By observe and analysis the fracture surfance and the unloading samples with the optical microscopy and scanning electron microscopy,study the mechanical behavior of 6061 aluminum alloy materials under impact loading.The experimental results show that:The fracture behavior of 6061 aluminum alloy under impact load at the time of quasi-static tensile compared with no significant change:The form of damage is still the micro-holes on the grain boundary to connect and then to take shape of micro-cracks. On the macro level,still emerge significant necking,the fracture surfaces of the specimen with the tensile axis approximately as 45°angle.With the height of the bob different,the deformation band on the sample surface different.When the height of bob is 141°,a "white belt deformation"emerge,The white belt deformation is not the result of phase transition,but is a corrosive effect as strain rate reaches a certain critical value.From the fracture point of view is a typical example of tensile broken,Slip is the main deformation mode.Micro-fracture formed by dimples,and with the increase of impact velocity,a slight change in morphology of dimples.Impact speed of 141°at the time of dimples is smaller than other velocitys,the reason is that the material was deformated too fast,holes have not yet fully developed,the specimen has been broken.In short,the 6061 aluminum alloy has good plasticity material.the impact of the speed of drop hammer was not enough for changing the fracture behavior of 6061 aluminum alloy. |
PDF Full Text Request