Frequency Domain Method For Multi Axial Random Fatigue Research And Application

The paper has a research on the multi axial random vibration fatigue in the time domain and frequency domain basing on the critical damage plane method. Combined with the stress tenser components’ covariance matrix, and then locating the critical plane of the fatigue weak points by the program using the maximum variance method. Then the impact of the direction cosine vector step which is used to describe the critical plane and the threshold of the critical plane has been explored. The results show that the smaller of the step, the more accurate location of the critical plane, but it will seriously affect the computational efficiency when the value is too small. Meanwhile, the larger of the threshold, the greater deviation between the maximum variance and the actual value is, thus the location of the critical plane also can be affected. After comparing and analyzing, the result indicates when the direction cosine step reaches 0.01 and the threshold is 1e-6, an accurate location of the critical plane can be found. According to the determined location of the critical plane and combining with the multi axial stress equivalent criteria, the equivalent stress and equivalent stress power spectral density can be obtained.For the cantilever model, the results of the fatigue life are discussed respectively in the time domain and the frequency domain with four different cases. In the time domain, basing on the equivalent stress and the fatigue properties of the materials, after cycle counting and then the fatigue life can be estimated with the linear damage accumulation method.In the frequency domain, the parameters which are used to calculate the fatigue life with the frequency domain formula can be determined by the equivalent stress power spectral density. On the one hand, the paper bases on auto spectrum density and the cross spectrum of the time domain response, then use both to structure the stress components’ power spectral density and the auto spectrum only to structure the stress components’ power spectral density to calculate the multi -axial fatigue life. On the other hand, it is the random vibration frequency response analysis that be used to calculate the multi axial fatigue life. Meanwhile, after comparing the time domain method with the three kinds of frequency domain methods, found that the influence of the cross spectrum is quite small. So in the range of the frequency domain, the random vibration frequency response analysis basing on the critical plane method can be used to analyze the multi axial fatigue life.The paper also discusses the influence of the bandwidth on the fatigue life calculated in the time domain and the frequency domain. The result shows the larger difference occurred in the narrowband excitation. But in the broadband frequency excitation, both match well.The paper separately uses uniaxial damage accumulation and multi axial frequency domain method to check the vibration fatigue strength of the internal combustion engine power pack frame. The results show that the methods both are able to predict the fatigue life of the structure. But the extraordinary phase and non-proportional changes of the stresses caused by the various excitations due to the multi axial loading process, which leads to the fatigue life of the former is always smaller than the latter at the same site of the structure.