Study on the fatigue strength of Ships and Offshore Structures under Variable Amplitude Loading Conditions

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The purpose of this research is proposing the practical fatigue-strength diagram for designing of ships and offshore structures.For that purpose, the fatigue-strength characteristic under the variable amplitude loading conditions of structure material, corrosion environmental conditions, and the variation on works need to be taken into consideration.We inquired in the order of development of examination of a crack propagation behavior, measurement of a crack opening-and-closing action, the fatigue test that used the test pieces of a welded joint, and a development of crack propagation analysis program. The fatigue-strength diagrams ware created using the developed program.The obtained results are as follows.・Proposed of Kop presumption formula which can be used for crack propagation analysis.・Two or more formulization which can be used for crack propagation analysis was proposed about the interference influence of a surface crack.・The crack propagation analysis program which can take into consideration of variable amplitude loading, multiple surface crack, and corrosion environmental conditions was developed.・By comparison with the variable amplitude loading test result by this research and other researchers, it checked that the analysis result by the developed program was appropriate.・The initial crack setting-up method for crack propagation analysis was proposed.・On the basis of crack propagation analysis, the fatigue-strength diagram which can be used for an initial design was proposed.・Variation in the fatigue strength by change of initial cracks was clarified.