Research On Uncertain Methods For Fatigue Life Prediction

Fatigue failure is the main failure mode of mechanical components. As the modern machine develops to high temperature, high-speed and large-scale direction, a large number of components work in the complex and poor working environment. Uncertainties significantly increase, and the accidents caused by fatigue failure emerge in endlessly. Therefore, it is very important in theory and engineering to further study the mechanism of fatigue failure and the fatigue life prediction methods of mechanical components for our country's modernization. Based on the deterministic prediction method for fatigue life and general reliability design, this paper makes a system study on the application of three uncertain methods including probability theory, fuzzy mathematics and grey system theory for fatigue life prediction and fatigue reliability design. The researching works are just as follows:A lot of fuzzy phenomena during fatigue life prediction are analyzed in the paper, and the opposition and unification between"accuracy"and"fuzziness", the difference and relationship of"randomness"and"fuzziness"are also discusses. According to the different circumstances for fatigue life prediction of components in engineering, the concept of"fuzzy zone of stress"and its two kinds of extending modes are put forward for the first time in the paper. The paper exposits the method of using membership function to describe quantitatively the fuzziness in fatigue life prediction. The examples show that the new method is better and more reasonable than the traditional method of ignoring fuzziness.For the traditional Miner rule can not reflect the influence of load sequence to components'fatigue life, the paper researches the fuzziness of fatigue damage under different load sequence. The concept of"threshold value of fatigue damage"and"expanding value of fatigue limit"are put forward, and the fuzzy mathematics is firstly used to describe quantitatively the"fuzzy zone"of fatigue damage under different load sequence. In order to analysis the influence of load sequence to fatigue life further, the fatigue experiment is carried out in which the material of the test article is 6005AT6 aluminum alloy. The examples in which the fatigue experiment data are used show that the new method can reflect the influence of load sequence better, and the prediction results by the method are more accurate and reasonable. In addition, the paper analyzes the influence of choosing membership function and its parameters to the prediction error of fatigue life, and summarizes the law of choosing membership function and its parameters. In this way, some significant conclusions are achieved, by which we can help to guide engineering practice. The infinite fatigue life reliability design and finite fatigue life reliability design of constructional elements are expatiated in the paper. For the main problem of finite fatigue life reliability design under multilevel load, considering about the objective law and actual process of fatigue failure, a new interference model of"cumulative damage - critical life"is put forward based on the Miner rule and the theory of general reliability design. The model is proved to be correct and reasonable by theoretical analysis and a large number of experiment data. The formula for calculating reliability is also deduced by this model, and it establishes the foundation for fuzzy fatigue reliability design.The fuzziness of stress and strength is analyzed in the paper, and the infinite fuzzy fatigue life reliability design and finite fuzzy fatigue life reliability design of constructional elements are expatiated. For finite fuzzy fatigue life reliability design, the fatigue failure is considered as a fuzzy event based on the interference model of"cumulative damage - critical life". In the paper, it is considered that the transformation from serviceable state to failure state is not accomplished in an action. There is a gradual transition stage from serviceable state to failure state. Fuzzy theory is firstly used to describe the model uncertainty quantitatively, which can not be disposed by general fatigue reliability design. The formula for calculating reliability is also deduced in the paper. The examples show that prediction result by the new method is more proper to the data of fatigue experiment than the other methods, and this method works with a wider range of adaptability and flexibility in engineering.For the complexity of fatigue phenomenon, the paper researches the application of grey system theory in fatigue life prediction. The fatigue phenomenon is regarded as being in a grey system. By using grey system theory, a new method for predicting the fatigue life of components based on Grey model GM(1,1) is proposed. The method is proved to be accurate and practical in engineering by the result of example analysis.