Influence Of Vibration Frequency On Fatigue Strength Of 6082-t6 Aluminum Alloy Fillet Welds

With the lightweight requirement of the rail vehicle structures and equipments,aluminum alloy welded structures are widely used.Vehicle components are often used in broadband random vibration environment,and the influence of vibration frequency on structures fatigue strength is worthy of attention as an important load parameter.Therefore,the frequency effect of 6082-T6 aluminum alloy fillet welds was analyzed based on vibration fatigue test and finite element simulation in this thesis.Firstly,the tooling and specimen were designed according to the test conditions.The first natural frequency 67.5 Hz of the specimen was determined by the frequency sweep test,and the P-S-N curves at the resonance and non-resonant frequencies were tested and the data were processed based on the group vibration fatigue test.Besides,the relationship of the strain and acceleration responses among different vibration frequencies was analyzed.The results show that the two response signals continue to decrease with the increase of time in the resonance state,but the constant amplitude are maintained in the non-resonant state.The strain and acceleration responses at 57.5 Hz are about three times and twice as much as those at 77.5 Hz respectively,but the frequencies of the response signals are always consistent with the excitation frequencies.The peaks of power spectral density of response signals are the highest at the resonance frequency,followed by 57.5 Hz,and the minimum at 77.5 Hz.Secondly,the vibration fatigue life of specimens was analyzed statistically,and the applicability of the five fatigue life probability distribution models was compared,including normal distribution,lognormal distribution,two-parameter and three-parameter weibull distribution,and exponential distribution.Furthermore,the P-S-N curves at the three vibration frequencies were fitted by power function model,and the frequency effects were analyzed.The results show that the three-parameter weibull model has the best overall description effect on the fatigue life distribution.In addition,resonance and non-resonant frequencies have a great influence on the P-S-N curves of the nominal stress.The fitting parameters m and lg C of the P-S-N curves at the resonance frequency are significantly smaller than those at the non-resonant frequency,and the fatigue strength of two million cycles at the resonance frequency is about 50% to 55% of that at the non-resonant frequency.Relatively,the differences of the P-S-N curves are smaller at the two non-resonant frequencies.Finally,the fatigue strength of aluminum alloy fillet joints under different tensionbending ratios was studied based on the finite element simulation.The stresses and fatigue life of the weld toe were calculated by the nominal stress and the hot spot stress methods based on transient dynamic analysis respectively,and the results were compared with the test results.In addition,the influence of P-S-N curves at different vibration frequencies on random vibration fatigue life of fillet welds was analyzed based on an air-conditioning cabinet on the EMU.The results show that the fatigue strength of fillet welds decreases with the increase of tension-bending ratio,and the stress equivalent extraction locations are farther away from the weld toe with the increase of tension-bending ratio.Besides,the relative errors of the stresses and fatigue life between simulation and test based on the nominal stress are less than those of the hot spot stress method.The fatigue damage of the air-conditioning cabinet fillet welds at the resonance frequency is much greater than that at the non-resonant frequencies.Therefore,the P-S-N curves in the resonance state should be selected in the random vibration fatigue analysis of the aluminum alloy welded structures in order to obtain more conservative fatigue life.