Research On The Multiaxial Fatigue Life Prediction Of Notched Members

Most of engineering structures and mechanical parts are served under complex fatigue loads. What's more, there exists stress concentration phenomenon in the practical engineering inevitably caused by the existence of notches, or other kinds of discontinuity features in these components. At present, many domestic and abroad scholars have carried out extensive research on the mechanism of multiaxial fatigue failure and the methods of notched multiaxial fatigue life prediction, but a complete and systematic multiaxial fatigue failure analysis model including notched multiaxial fatigue problems has not been established yet up to now. Hence, further study of multiaxial fatigue failure and accumulative damage mechanism and multiaxial fatigue life prediction methods of notched components has an important significance in practical engineering.Firstly, several widely used multiaxial fatigue damage parameters under nonproportional loading were systemically reviewed and analysed. According to the way to deal with the nonproportional additional hardening effect the damage parameters were classified into two groups. Four typical damage parameters were evaluated and discussed with the collected multiaxial fatigue test data. Analyses of the evaluation result led to conclusions as follows: Itoh parameter is suitable for the low cycle fatigue of most cyclic hardening materials; Susmel parameter is suitable for medium/ high cycle fatigue of most of cyclic softening materials; Kanazawa parameter and Spangnoli parameter are respectively suitable for low and high cycle fatigue problems of all materials.Secondly, a number of multiaxial fatigue accumulative damage theories are summarized to four categories according to their different theoretical basis. In this paper, some multiaxial variable loading tests' results were collected and analyzed. The percentage of the test data points within a factor of two scatter bands is counted. The conclusions obtained through the collected tests' results were summarized as follows: Miner accumulative damage theory and the improvement models based on Miner's model widely used to deal with various load spectrum could have a good effect; the nonlinear models based on the damage curve model and continuum damage mechanics have a high accuracy for two steps load spectrum and block load spectrum; now the application of the accumulative damage models based on critical plane theory, especially the nonlinear models, is rare, but the accuracy of these models are satisfactory.Lastly, two kinds of notched multiaxial fatigue life estimation methods which have a broad prospect of engineering application are expounded in detail. They are stress field intensity method and the critical distance method. Combining with the evaluation results of the multiaxial fatigue failure criteria and the multiaxial fatigue accumulative damage theories, the effectiveness of the above two methods are demonstrated with the collected test data. For comparison, direct critical plane method was also used to predict the multiaxial fatigue lives of two different notched members. The results show that the stress field intensity method and the critical distance method both could have a high prediction accuracy for notched components subjected to complex multiaxial loading paths, but the critical distance method has a smaller calculating quantity and is more suitable for engineering application.