Effect Of Loading Frequency On Vibration Fatigue Life

The components of aircraft must be in service in harsh vibration environment,and there is serious fatigue failure in the service process.Some components need to work under the excitation of broadband.The traditional S-N curve is generally obtained under the condition of low frequency test.However,under the action of high frequency excitation,the damage effect of high frequency response and low frequency response on the structure cannot be ignored.The traditional S-N analysis of broadband vibration will lead to large errors,so it is necessary to consider the influence of frequency in the analysis.To solve this problem,the frequency-dependent fatigue life model is proposed based on the frequency effect through the constant frequency vibration fatigue tests with different loading frequencies,and the applicability of the proposed model is verified by the broadband random vibration test.Firstly,three groups of 7075-T6 aluminum alloy specimens with different natural frequencies were designed,and the vibration fatigue test with constant frequency was carried out by using the vibration test bench.With the decrease ratio of the first-order natural frequency of 5 % as the failure criterion,the fatigue life of the specimens under different loading frequencies and stress conditions was extracted.Secondly,the fatigue S-N curves at different frequencies are obtained by fitting the constant frequency vibration fatigue test data.The law of the parameters in the traditional S-N curve changing with the loading frequency is analyzed.On the basis of the traditional S-N curve,the frequency influence item is added,and the frequency-dependent fatigue life model N(S,f)is proposed.The specific mathematical expression of the frequency-dependent fatigue life model of 7075-T6 aluminum alloy is obtained by fitting the test data.Finally,on the basis of the broadband random vibration test,the fatigue life of the test specimen is predicted by using the fatigue life models with and without frequency,respectively,and the life prediction results of the two are compared with the test life.The results show that the SN curves obtained under constant frequency vibration tests with different loading frequencies are different,and the slope and intercept have a good linear relationship with the logarithm of the loading frequency.In the broadband random vibration life prediction,the prediction accuracy of the fatigue life model considering the influence of the loading frequency is higher than that of the fatigue life model without considering the influence of the loading frequency,which is in good agreement with the test results,verifying that the frequency-dependent fatigue life model N(S,f)proposed in this paper is more applicable.This paper studies the effect of frequency on vibration fatigue life,and provides a more accurate model for broadband vibration life prediction.