Research On Fatigue Crack Propagation And The Establishment Of Fatigue Inspection Plan Based On Risk Analysis

Fatigue problem is one of the dominating reasons for structure failure in ship and ocean engineering. So how to predict properly the origin and propagation path of fatigue crack in structures, and how to understand the influence of the crack propagation in structures on the structural strength and reliability, how to establish the reasonable inspection and repairing plan about fatigue crack are significant problems in ship and ocean engineering. In this thesis, the problems mentioned above are discussed fully, the research results can provide basis and effective approaches for reduce the risk of structural failure induced by fatigue problem.The major works in this thesis are as follows:1) According the research results of the fatigue test data of a typical 3-dimension ship detail, the propagation rules of fatigue crack in stable propagation period are studied, and the dynamic model of fatigue crack propagation is established. It is pointed out that the log-number of crack length obey the normal distribution to the load circles, the mean value of crack length increases as the load circles increase, and the square variation is on the contrary, the mean value and square variation are inverse proportion.2) The acquired dynamic model of crack propagation is applied to analysis of the propagation life of crake in a platform. By the probability statistical theory and tolerant analysis method, the original crack length and criterion crack length are acquired. The residual strength interference model is adopted to establish the dynamic reliability model of the crack propagation in a platform. According the crack length acquired in inspection work, using Bayesian method, the crack propagation rules and the reliability of the structural fatigue crack propagation are modified. It is pointed out that the crack propagation rules obtained in test can not be applied in engineering practice directly due to the difference in structural style, structural size and . working circumstance between the specimen and real structures. When the rules gotten in the test are used in engineering practice, the rules must be modified by proper approach according to the inspection results.3) Two familiar methods (S-N curve method and local stress-strain method) are adopted to predict crack origin life of the specimen. The results are compared. The comparison result show that the local yield and mean stress affect the crack origin life significantly and the local stress-strain method can reflect the influence of the local yield and mean stress, so the result acquired by this method is more reasonable than S-N curve method.4) According linear elastic fracture mechanics theory, using universal FEM software-MSC. NASTRAN, crack propagation path and life are researched. The result show the fact that the path changed in beginning period of crack propagation under biaxial stress state. The path change of the crack may reduce the influence of the biaxial stress state. After some period, the crack path becomes straight line.5) The deterioration of the reliability of ship longitudinal strength with time due to the multi-crack propagation in ship section is studied. According the computed results of a ship in two state-cracks in components of deck and bottom propagate rapidly due to limit sea state in a voyage and the cracks in ship section origin and propagate in whole ship life, it is pointed out that cracks propagation in ship structure may brought the deterioration of the reliability of ship longitudinal strength, and it is necessary to arrange the inspection and repairing work about the crack propagation according the variation of the ship strength reliability. 6) The risk analysis concept is inducted. The consequence analysis in risk analysis is imposed onto the establishment of the inspection and repairing plan about fatigue crack. The research result show that the inspection and repairing plan established by risk analysis method can consider the safe and economic requirement comprehensively, so the inspection and repairing plan established by this method is more reasonable and acceptable.