| Fatigue life prediction is of great importance to the fatigue design and evaluation of steel bridges.From the point of view of fracture mechanics,the fatigue life prediction and fatigue reliability evaluation methods for steel bridges are systematically studied in this dissertation by combined applications of extended finite element method,crack growth theory,cumulative fatigue damage theory and structural reliability theory.Both the variation characteristics of fatigue life and the fatigue life reliability index within the design period of the structure are obtained by the present approach,indicating the applicability and accuracy of the proposed method in solving the fatigue and fracture problems of engineering structures.The major work in this dissertation is described as follows:(1)The research history of fatigue problems is presented first,followed by an introduction to the two major types of approaches for fatigue life prediction based respectively on S-N curves and fracture mechanics.The numerical methods of fracture mechanics are summarized,and basic concepts of fatigue reliability theory are also introduced.(2)The linear elastic fracture mechanics and extended finite element method theory are introduced.The numerical equations of the extended finite element method are derived and three types of approaches for stress intensity factor are summarized.Finally,two numerical example is given to test the accuracy of extended finite element method for fracture mechanics problem.(3)A fatigue life prediction method based on the extended finite element method is presented in which the Paris law and fracture mechanics theory are is used.In order to improve the efficiency,the fatigue life prediction method is improved and Fast algorithm for fatigue crack growth life is proposed.In addition,in order to consider the variation of load,a fatigue life prediction approach based on XFEM is proposed for the case of variable-amplitude(CA)loading,in which the fast algorithm for fatigue crack growth life is used in conjunction with Miner rule for linear cumulative damage.Finally,the fatigue life of I type crack and mixed mode crack problem is calculated by using the method in this chapter.(4)To account for the inherent uncertainties in fatigue problems,the reliability analysis method for fatigue crack growth life based on XFEM is proposed by combining with response surface-Monte Carlo method.Finally,a numerical example is given to test the feasibility and effectiveness of the proposed method.(5)Using the reliability analysis method for fatigue crack growth life based on XFEM,the statistical characteristics of the fatigue life and the fatigue life reliability index within the design period are obtained for some fatigue details of a railway bridge.It has been shown that wide applicability and high accuracy can be obtained by extended finite element method for fracture mechanics problem.High accuracy and efficiency can also be observed by using reliability analysis method for fatigue crack growth life based on XFEM,since efficiency is improved by fast algorithm for fatigue crack growth life with almost no loss of the accuracy.The feasibility and effectiveness of the proposed approach is well validated by the successful application of the method to the fatigue reliability analysis of a railway bridge. |
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