The Multiaxial Fatigue Life Estimation Of Metal Structures Under Random Loading

In many areas of engineering, fatigue is a common cause of failure in metallic structures and components subjected to cyclic loading. Thus, fatigue life prediction is an important field in engineering and academic research. The research has focused on constant amplitude loading, and resulted in a considerable wealth of fatigue life prediction models. However, the majority of engineering structures and components in service are generally subjected to random loading, and often multiaxial cyclic loading. In this case, the values and directions of the principle stresses and strains are varying during a loading cycle, and the cumulative damage becomes difficult to assess. Compared with constant amplitude loading fatigue analysis, the multiaxial fatigue life prediction under random loading has not ever reached a satisfactory level. Special attention should be paid to this aspect of research. In this thesis, the hybrid frequency-time domain method and equivalent Lemaitre stress method are proposed to estimate fatigue life of metal structures under multiaxial random loading, and the influence of mean stress on multiaxial fatigue life is analyzed. The main contents are included as follows:First of all, the random vibration fatigue tests are conducted on notch specimens of 7075-T6 aluminum alloy. The vibration response and fatigue damage characteristics of specimen under narrow-band and wide-band random loading are studied, and the effect of bandwidth on dispersions of fatigue life is analyzed.Secondly, a hybrid frequency-time domain method for predicting multiaxial fatigue life under random loading is developed on the basis of combination of the frequency domain and time domain analysis, which is combining qualitative analysis and quantitative analysis. The fatigue life prediction by the proposed method and the frequency domain equivalent von Mises stress method are compared with the random vibration fatigue tests of 7075-T6 aluminium alloy. The result shows that the hybrid model is more adequate for complex loading fatigue analysis, and has a computational advantage. The effects of multiaxial stress and bandwidth on fatigue life under random loading are analyzed.Besides, in order to facilitate the optimization design when the fatigue life is included in the objective function, the frequency domain equivalent Lemaitre stress method taking into account the effect of hydrostatic stress is presented to estimate fatigue life under multiaxial random loading. The results obtained by applying the equivalent Lemaitre stress method to the random bending-torsion fatigue tests from the literature are compared to the results obtained by using the equivalent von Mises stress method and PbP method. It has been shown that the results of fatigue life calculated by the equivalent Lemaitre stress method are well correlated with the experiment, and the equivalent Lemaitre stress method is conservative and convenient for application. On this basis, numerical simulations for a number of bending-torsion random loadings with different variance ratios and S-N curves have been carried out. The result shows that the equivalent Lemaitre stress method could provide accurate results for combined bending-torsion loading for different types of S-N curve, especially for pure torsion. The equivalent Lemaitre stress method is applied to predict the life of random vibration fatigue tests of 7075-T6 aluminium alloy. The result indicates that the equivalent Lemaitre stress method gives much better results than the equivalent von Mises stress approach, and is less accurate compared to the hybrid frequency-time domain method.Finally, the fatigue life of metallic structures under combined thermal-acoustic loadings is predicted based on critical plane model. In order to take into account the effect of mean stress induced by temperature loading, a new critical plane model based on shear strain is proposed. The model employs the maximum shear strain range △γmax as based damage parameter, which also incorporates the normal strain rang △εn, the maximum normal stress σn,max and mean normal stress σn,m/E on the maximum shear strain plane. The stress terms σn,max and σn,m/E are seen to be capable of accounting for the effect of mean stress on multiaxial fatigue life. The proposed model is validated with experimental data from literature through testing four metal materials under various stress/strain paths with zero/non-zero mean stress. The performed validation seems to support the idea that the proposed approach is reliable for estimating fatigue life under multiaxial loading conditions with and without mean stress. On this basis, the snap-through motion of metallic structures under combined thermal-acoustic loadings is analyzed, and the proposed critical plane model is used to predict the fatigue life of metallic structures subjected to combined thermal-acoustic loadings. It is shown that the fatigue life is decreasing with the increase of temperature under the same sound pressure level. Because of thermal stress caused by the temperature loading, a higher stress response and larger mean stresses arise in the process of structural vibration, thus reducing the fatigue life of structure.