Characteristics Of Fatigue Crack Propagation In Welded Joint Of Aluminum Alloy Used In Veicle Chasis Of High Speed Strain

The research on materials and manufacture technology for high speed train has been carried out in our country in recent years. Vehicle chassis is the main load-bearing part when the train travels. As raising the train speed it requires higher properties of load-bearing part to meet the demand of stronger carrying capacity, especially the improvement of its fatigue failure performance. In this thesis, the welding residual stress and its effect on fatigue crack propagation of A7N01 aluminum alloy utilized in high speed train chassis were studied by numerical simulation method and experimental analysis. Influences of microstructure in different zones of the welded joint on fatigue crack propagation were researched as well.To accurately simulate the welding residual stress in vehicle chassis of high speed train, the influence of thermo-physical and mechanical parameters on welding residual stress was investigated by numerical simulation. The results show that the peak value of longitudinal welding residual stress increases with the decrease of the heat conductivity and increase of the such parameters as specific heat capacity, density, coefficient of thermal expansion, and Young's modulus as well. The peak shape of longitudinal welding residual stress changes from single peak to double peaks as decreasing the thermal conductivity or increasing either the density or Young's modulus. For most of aluminum alloys, yield strength and thermal conductivity approve more effects on the welding residual stress, whereas, the specific heat capacity, Young's modulus, coefficient of thermal expansion and density show little influence on it. Moreover, the influence of Poisson ratio is so little that it can be ignored.The welding residual stress in vehicle chassis can be relaxed to a certain degree under dynamic loading condition. In this thesis, the relaxation behavior of welding residual stress in the welded plate of A7N01 aluminum alloy under a dynamic loading was researched. The results show that relaxation of welding residual stress occurs when the total stress of welding residual stress and working load is greater than the instant yield strength under either dynamic or static loading condition. The magnitude of stress relaxation is related with peak value of cyclical load, i.e. the higher peak value of cyclical load is applied, and the more relaxation of stress is occurred. For cyclical hardening or neutral materials, the relaxation of welding residual stress is independent with load cycles and it occurs only in the first load cycle. These can be explained that the welding residual stress decreases after the relaxation at the first load cycle. Consequently, the total stress of welding residual stress and peak value of cyclical working load is hard to exceed the instant yield strength of materials again because their yield strength maintain constant or rise as further increasing the load cycles. However, for cyclical softening materials, the instant yield stress decreases continuously because the softening behavior happened all the time during the cyclical loading process. More and more stress will be relaxed with increasing the load cycles because the total stress of welding residual stress and peak value of cyclical working load will exceed their instant yield strength.Influence of softening behavior of welded joints on the longitudinal welding residual stress was analyzed by finite element method. The results show that relaxation of longitudinal welding residual stress occurs when softening effect produces in the welded joint. Further more, the welding residual stress may change from tensile tress to compressive stress and leaves compressive stress in the welded joint after general yield. However, the welding residual stress will not change from tensile stress to compressive stress, though relaxation of longitudinal welding residual stress may happen when there shows no softening effect in the welded joint. Eventually, welding residual stress is removed after general yield of welded joint.For accuracy of the simulation analysis, the yield stress changed with temperature and plastic strain in different hardening rules must been considered in the numerical simulation model for strain hardening behavior of aluminum alloy occurs below 250℃, while strain softening behavior appears above 250℃. The according simulation results obtained from the true yield-stress model in mixed hardening rule are more accurate than the one from ideal elastic-plastic yield-stress model. The simulation results show that welding residual stress in such these zones as inside holes in towing beam, welded seam close to holes in towing beam, welded seam at interface of reinforcing plate and buffer beam, and welded seam at interface of buffer beam and cross beam are higher than that in other zones, and these zones should be the dangerous positions for the vehicle chassis. The stress level and effective stress ratio in these zones have also been achieved.Influence of the dimension and shape of secondary phase particles on fatigue crack propagation behavior of A7N01 aluminum alloy was studied by simulation method. The results show that the big hard secondary phase particles affect the fatigue crack propagation by two ways, which are promoting or suppressing the propagation of fatigue crack. The stress level in plastic zone is increased when the secondary phase particles is in the plastic zone of crack tip, which lead to the increase of maximal stress intensity factor of crack tip and promoting the propagation of fatigue crack. The secondary phase particles may, in a certain degree, suppress the propagation of fatigue crack by following means. First, the propagation path of fatigue crack may be changed by the secondary phase particles, and this may improve the roughness of crack surface. Moreover, the roughness of crack surface is increased with increasing the dimension of secondary phase particles, which may enhance the closure effect of the crack tip. Second, when the crack rounds the second particles, the insert effect of second phase particles in the rear of crack tip during unloading process due to the incompatible deformation between the second phase particles and the parent metal can also increase the closure effect of crack tip. Otherwise, plastic deformation around the sharp corner of second phase particles is larger than that where there is no second phase particle. Thus the larger plastic deformation induced by second phase particles can further cause closure effect. The scope of effective stress intensity factor of fatigue crack propagation may decrease caused by the crack closure effect, thus the propagation of fatigue crack may in a certain extent be suppressed.The evolution of stress in fatigue crack tip under elastic and plastic conditions was investigated by finite element analysis, respectively. It is suggested that the maximal stress intensity factor K_max^* and the effective stress intensity factor range K_max^* can be gained by using the residual stress and the working stress in the effective crack length tip. The expression of fatigue crack propagation rate curve considering the effect of residual stress is proposed asnd dNa=C K_max^*, and the formula exhibits good condensed effect.