Study On Safety Life Assessment Method For Marine Structures Based On FCP Theory

Fatigue is one of the main failure modes in navel architecture and ocean engineering structures. Nowadays, fatigue strength assessment method used by each classification society and shipyard is mainly based on CFD theory. However, since CFD theory does not include some important factors such as initial crack defect, loading sequence interaction, etc., the predicted results of CFD theory are scattered. And also the fatigue assessment results cannot be verified by nondestructive method. At the same time, the yield strength of high strength steel plates applied to some marine structures have exceeded 400MPa which is the maximum yield strength permitted by the fatigue rules of classification societies. In essence, the fatigue assessment results based on the current rules of marine structures are relatively reference value. For aged marine structures, the assessment results by CFD method are more unreliable.More and more researchers have realized the importance of FCP theory. FCP theory can explain the effect of loading sequence interaction easily, and it can define the initial and final states of fatigue failures accurately. And fatigue assessment results can be detected by nondestructive testing method. In the paper, the safety life assessment methods of marine structures based on FCP theory under complex loading conditions are discussed, and the main works are as follows:(1) The basic equations of unified fatigue crack growth model and unique curve model proposed by our group and SIF calculation formulas of typical welded structures are introduced respectively.(2) The unified fatigue crack growth model is combined with SIF calculation equations for surface crack in the weld toe, and the procedure to predict the fatigue crack growth behavior of a submarine structure under complex loading conditions is discussed. Fatigue crack growths of a submarine structural welded joint under three typical spectrum loadings are predicted.(3) Storm wave loading model is combined with a unique crack growth rate curve model and SIF calculation equations for surface crack in the weld toe, and the procedure to predict the fatigue crack growth behavior of ship structures under complex loading conditions is discussed. Fatigue crack growth behavior of surface crack at weld toe of butt welded joint in ship hull under different storm wave loading conditions are predicted.(4) The fatigue loading is obtained based on Weibull long-term checking stress range distribution. A procedure which consists of FAD and crack growth life prediction method is used to assess the security life of high tensile strength thick steel plates under complex loading conditions. Fatigue life of an YP47 steel plate under complex loading condition is predicted. Equivalent stress method is used to transform random loading spectrum into constant loading spectrum, and fatigue lives of a surface crack and an embedded crack located at butt welded joint in the upper deck of 8530TEU container ship under complex loading conditions are assessed respectively.