Fatigue Life Prediction Of Metal Structure

In this thesis, the methods for fatigue life prediction of notch component and welded joints are investigated. The main research contents and results are given as follows.Prediction method of the fatigue life for notched components is constructed with the combination of Taylor's critical distance theory and stress field intensity approach. The stress distribution of specimens with notch which are made from alloy LY12-CZ, 2024-T3 and 7075-T6 plates is determined by Finite Element Method, and then the fatigue life is evaluated. It is found that the predicted life and experimental results are close in middle and high cycle range, and that the Critical Distance Approach which is originally for the estimation of fatigue strength can be extended to predict the fatigue life of notched components.The fatigue notch factor (FNF) of welded joints is introduced based on the consideration of hot spot stress as controlling stress for fatigue assessment. The critical distance theory in combination with the implementation of finite element method gives fatigue notch factors of welded joints. The influence factors of fatigue assessment by hot spot stress method are revealed with the analysis of the effect of joint type, weld size, plate thickness and the determination approach to obtain the hot spot stress on the scattering of FNF. The results from T shaped joints, cruciform joints and butt joints show that the influence of weld size and the web plate thickness on FNF is small in contrary to the obvious effect of the main plate thickness on it. and the latter is predicted for welded joints with fillet welds and butt welds by the analysis of FNF and is found to be in good agreement with existing empirical relation. Also the results indicate that the two types of extrapolation method to obtain hot spot stress are valid in reducing the diversity of fatigue data and the dependence of fatigue data on the types of joints and loading conditions in view of the low scattering of corresponding fatigue notch factors. Based on the theory of critic distance (TCD) proposed by Taylor, fatigue notch factors for welded joints are estimated and they are used to predict effectively the variance of fatigue strength corresponding to the tension-compression loading and bending loading in main plates respectively. The gradient effect of hot-spot stress on fatigue behavior is considered by introducing effective hot-spot stress, which is determined with the reduction of the component of bending stress involved. Cruciform joints and right-angle shaped joints of specified geometrical size are examined. It is shown that fatigue notch factors for these joints are irrespective to both joint type and loading mode in main plate if the control stress is effective hot-spot stress, also the thickness effect of main plate can be estimated by existing power law type relation. In terms of the concept of effective hot-spot stress, the re-analysis of S-N curves for several types of welded joints subjected to variant loading is carried out. It is found that a identical S-N curve can be drawn to deal relevant fatigue test data with the scattering band becoming narrow and standard deviation getting smaller.The characteristics of local approach used in fatigue evaluation of welded joints are examined. The fatigue strength for non-load-carrying cruciform joints and load-carrying cruciform joints made from steel and aluminum is expressed in terms of two types of equivalent stresses respectively. Following conclusions are summarized, the fatigue strength of cruciform joints for fatigue cracks to initiate at weld toes can be represented by an average stress with rather narrow scattering band, and the magnitude of equivalent stress can be applied to predict the location of fatigue failure from weld toe or weld root, also, the fatigue strength of steel welded joints with residual stress relieving can be evaluated by multiplying fatigue strength of corresponding as welded joints with a beneficial factor that is about 1.28 for the case under investigation.