The Fatigue Life Prediction Of The Bow Structure Of A Steel Ship Based On Fracture Mechanics

There are many damage models of ship structure, fatigue destroy is one of main forms of ship structures destroyed. Some typical parts on the ship structure will have larger stress concentration phenomenon due to the cyclical wave loading in these parts, which will produce some serious fatigue problems. Therefore, it has become the focus of attention to assess the fatigue of ship structures.At present, there are two main types of methods to evaluate the fatigue life. One is the traditional S-N curve method based on the combination of S-N curves and Miner linear cumulative damage theory. The other is the fracture mechanics approach based on fatigue crack propagation theory. The S-N curve has a neglect of initial objection of material, which is it’s disadvantage. So applying fatigue crack propagation theory to assess the structural fatigue life has become the mainly development direction.In this paper, prediction of the fatigue life of bow structures based on the fatigue crack growth theory is done, mainly in the following several aspects.Firstly, based on the ship bow structures as the research object, using the finite element analysis software MSC.PATRAN, finite element analysis model of bow structure is established. Then, the loads are determined and statics analysis is conducted. Through comparison and analysis of the results, we obtain the stress concentration area and identifiy the parts to need to assessment fatigue life. In the end, competing the refined finite element modeling of hotspots identified, which provides results of the finite element analysis and the basic data for further assessment of fatigue life.Secondly, the definition of fatigue fracture parameters about model of bow structure is studied. Completing the determination of fracture pattern of the crack, the calculation of stress intensity factor and selection of fracture criterion. Meanwhile, the selection of the crack propagation rate formula is done and estimation process of structure fatigue life with initial crack is set out. A theoretical basis to analysis the structure fatigue life is provided.Finally, based on the fatigue crack propagation theory, using crack propagation analysis module of MSC.FATIGUE, evaluating the fatigue life of crack extension area. The evaluation of fatigue life is based on the stress distribution of the finite element model, load change information and material fatigue properties. The effects of different crack shape ratios and different initial crack sizes on fatigue life are also discussed.The research provides certain theoretical reference for the design of ship structure and assessment of the fatigue life.