Research On Fatigue Life Prediction Methods Of Welded Joints Under Complex Stress States

For large and complex mechanical structures, welded components are widely used to meet the requirement of lightweight in engineering. Owing to the extremely complex stress states and bad service conditions, the fatigue problems of welded structures are particularly prominent. Besides, there are some welded defects, such as cracks, porosities, inclusions, lack fusion, and lack penetration in welded joints, thus, welded joints are the weakest link of welded structures. Studying on fatigue life prediction of welded joints, making safety assessment and health management decisions and realizing the maximum use of welded structures can reduce economic losses and casualties greatly. So, it owns important theoretical meaning and engineering value to study on fatigue life prediction of welded joints.At present, two kinds of methods have been mainly used to predict fatigue life, one is based on S-N curve and fatigue damage accumulation theory, and the other is based on the fracture mechanics theory and crack propagation rate. In view of the limitations of these two kinds of methods and the complex loading conditions of welded structures, the methods to predict fatigue cumulative damage and probabilistic fatigue life for welded joints are carried out in this dissertation. The main research content includes the following aspects:(1) Development of a modified Manson-Halford model and a residual life prediction method considering load interaction effect.Usually, the fatigue life prediction model of welded joints ignores the load sequence effect and the influence of load interaction so that the accuracy can’t meet the demand of engineering. Thus, the dissertation proposes a modified Manson-Halford model, which introduces load interation effect by the form of index function. Because Manson-Halford model has a good consideration to the effect of external loading sequence for fatigue cumulative damage, the modified model can consider the influence of fatigue damage accumulation caused by different loading sequences and load interactions at the same time. Through the analysis of model predictions and experiment results of residual life, the modified model shows higher prediction accuracy and can be applied to fatigue life prediction of welded joints in engineering.(2) Development of a modified Corten-Dolan model and a residual life prediction method considering the damage and stress states.For some welded structures applied in engineering, such as welded frame, body, swing bolster and so on, mainly suffer from the action of small loads below the fatigue limit, which can also result in fatigue damage accumulation. Corten-Dolan model considers the effect of small loads on damage accumulation, but now, the determination method of the exponent d, a critical parameter that affecting fatigue life prediction accuracy in Corten-Dolan model, is still controversial. For this problem, considering that fatigue failure is often viewed as a result of continuous action of fatigue load under the condition that materials or structures have a certain damage. In this dissertation, a new method to determine the exponent d is investigated, and a modified Corten-Dolan model considering the damage and stress states is proposed, which meets the dual failure criterion, and characterizes the mechanism of fatigue failure better. It is verified that the modified Corten-Dolan can make the prediction error within 50% when it is used to predict fatigue life of welded materials, welded joints under two-level loading and multi-level loading conditions. Meanwhile, the calculation process is relatively simple and could be used to fatigue life prediction of welded joints under complex loading.(3) Development of a probabilistic fatigue life predication method and S-N curve uncertainty analysis based on generalized Polynomial Chaos theory.Since the specimens, conditions, operations and data reading exist uncertainty during experimental processes, as a result, the fatigue life of welded joints under constant loading owns dispersibility and brings about uncertainty for S-N curve. Therefore, in order to make the fatigue life predictions of welded joints approximate the actual situation well, this dissertation applies generalized Polynomial Chaos theory which has a good ability to deal with uncertainty, and combines with the modified Manson-Halford model and modified Corten-Dolan model, to present a probabilistic fatigue life prediction model considering the uncertainty of S-N curve. Results show that the fatigue life prediction method based on generalized Polynomial Chaos theory has approximate values compared with Monte Carlo with high computing efficiency, which verifies the rationality of application of Polynomial Chaos theory for fatigue life prediction, and the application range of Polynomial Chaos Theory is expanded, and fatigue life prediction methods based on S-N curve and cumulative damage theory are improved as well.(4) Development of a probabilistic fatigue life predication method based on initial crack size and equivalent crack size.At present, when the method based on fracture mechanics theory combining with crack growth rate curve are used to predict the fatigue life of welded joints, the initial crack size is difficult to determine, and the life prediction process under random loading is complicated. Aim at this, a method to determine the initial crack size which considers the influence of stress level is proposed based on equivalent initial flaw size, stress intensity factor model, and the boundary of crack initiation and propagation. It can reduce the dependence on human experience, save test cost and time as well. In addition, the concept of equivalent crack size and its calculation model are proposed to reduce the complexity of the calculation process of fatigue life prediction under random loading, and model uncertainty is melted into the prediction model of probabilistic fatigue life based on equivalent crack size. It is feasible, which has been verified, to take the influence of stress level into account when determining the initial crack size. Meanwhile, the proposal of equivalent crack size simplifies the calculation process of probabilistic fatigue life, and the consideration of model uncertainty is more conducive to assess the safety and reliability of the materials or structures.