Study On Fatigue Life Prediction Method Under Multiaxial Random Loading

Engineering components and structures in service are mostly subjected to the multiaxial random load, the traditional fatigue theory is not satisfied with the requirement of the strength design and life design of high-tech products in modern society, so in recent years analysis of fatigue strength and life estimation of structures and components under multiaxial random load is the main target of fatigue research at home and abroad. If the multiaxial fatigue damage and the life prediction method are researched systematically, the accuracy of fatigue life prediction will be I increased effectively. Thus the mechanical structures are ensured to run safely, and the competitive ability of the mechanical product will increase. This study not only has important theoretical significance, but also has the great application value and economic benefit.In this paper, the development of the domestic and international multiaxial fatigue research were overviewed. On the base of the existing research result of the multiaxial fatigue, by the ways of combination between the time-domain and frequency-domain method, the fatigue life prediction method under the multiaxial random loading was studied deeply.First, establish a reference coordinate system according to the variance of the load amplitude. And the load path of stress deviator in space is projected by the reference coordinate system. The original load is equivalent to a multistage uniaxial load history. The amplitude of equivalent shear stress is defined and evaluated by vector norm. Because fatigue damage is closely related to the variance of the load amplitude, the amplitude of equivalent shear stress obtained by this method reflects the influence of material damage.Then, fatigue damage caused by equivalent uniaxial load-history is calculated considering the influence of the mean stress. The two load-time histories are counted synchronously in the process. Based on this, the compression method of the multiaxial random load and the multiaxial cycle count method are given. Not only can this method effectively simplify the random load history, but also realize the synchronization of the choice of the load. And it is possible to cycle count for the couple of loadings at the same time. Next, multiaxial fatigue life estimation model in the time domain is established according to modified S-N curve. And the model can be applied to uniaxial, multiaxial proportion, multiaxial non-proportion, multiaxial amplitude loading and so on. The results were verified by three materials test data. The error between predicted results and test results is small, and it tends to safety.Finally, multiaxial fatigue life estimation model in frequency domain is established based on the multiaxial fatigue life estimation model in time domain and the research methods in frequency domain. And the model is also applied to uniaxial, multiaxial stationary Gaussian stress process. Through the verification by the experimental data in the literature cited, the predicted results are within two factors. And the predictive ability is better.