| In the past decades, there were a lot of ship accidents causing serious damage, so ultimate strength of marine structures is realized more and more important. Ultimate strength is the most realistic and actual assessment method to evaluate the maximum carrying capacity of vessels. Over the ultimate strength, the vessels will present collapse damage. The analysis of ultimate strength is able to consider many factors, such as elastic-plastic of material, initial imperfection and welding residual stress and so on, and will give the real safety redundancy of structures.As the increase of deadweight cargo capacity of container ship, the application of high strength steel has been vastly used in marine structures and the stress level is much higher than before. Compared with the common steel, the high strength steel only improves its yielding strength, but its anti-fatigue and anti-corrosion abilities are not improved accordingly. So, it is necessary to analyze the residual ultimate strength of marine structures with crack and corrosion damage, which is also a topic research direction in ship mechanics.This thesis carries out comprehensively analysis of ultimate strength about container ships, and the following work has been completed:(1) The research methods of ultimate strength about marine structures have been summarized systematically, the research developments of residual ultimate strength with crack and corrosion have also been reviewed in detail, and the significance of ultimate strength analysis is illuminated.(2) Based on the thin-walled structure theory, the design relationships of similar model for bending ultimate strength experiment have been derived, a similar model of a 3100TEU container ship has been designed. The similarity of the model and the actual container ship is validated by calculating section properties, elastic strength, buckling strength and ultimate strength.(3) Then, the design relationships of similar model for torsion ultimate strength have been derived, another similar model of the 3100TEU container ship subjected to bending and torsion has been designed using optimization method, and the similarity of model and actual ship has been validated by NFEM.(4) A model experiment of bending ultimate strength has been carried out to validate the numerical method. The real collapse process of the model is recorded in detail and the ultimate strength of the model is measured. The collapse modes and stress distributions for plating and longitudinals are analyzed. Welding residual stresses and initial distortions are simulated approximately in ABAQUS and the results of ultimate strength obtained by the Arc-length method are compared with experimental results.(5) Numerical studies on the ultimate strength of cracked closed-section box girders subjected to torsional loading are carried out. The ultimate strength reduction of the box girders due to cracking damage are investigated by varying crack types, crack sizes, crack locations and girder thickness. Based on the numerical results obtained, a simple equation for predicting the residual ultimate strength of cracked closed-section box girders under torsional loads is developed.(6) The ultimate strength of open-section box girders with crack damage subjected to pure torque, compressive force, bending moment and combined loads is investigated. The ultimate strength reduction of the box girders due to cracking damage by varying crack types, crack sizes, crack locations and girder thickness are studied. Based on the numerical results, a simple equation for predicting the residual ultimate strength of open-section box girders with crack damage under single load and combined loads is proposed.(7) The effect of crack and corrosion on longitudinal ultimate strength of container ship is analyzed. Among the common crack types of container ships summarized by IACS Container Ships - Guidelines for Surveys, Assessment and Repair of Hull Structures, seven representative crack types are chosen to analyze the residual ultimate strength of container ships and then simple equations to predict residual ultimate strength are proposed. In order to analyze the effect of corrosion on ultimate strength, the corrosion addition recommended by ABS, CCS and DNV suggest that there is occurrence of substantial corrosion occurring in container ships and then the corrosion effects on the residual ultimate strength are analyzed.The main innovations of this thesis can be summarized as:(1) A bending-torsion ultimate strength similar model is designed by using optimization method. A model experiment of longitudinal bending ultimate strength has been carried out. The collapse modes and collapse sequences have been analyzed. The initial imperfections of test model have been approximately simulated by using thermal stress analysis and then the whole collapse process under vertical bending has been analyzed by the Arc-length method.(2) Simple equations for assess the effect of crack on ultimate strength of closed-section and open-section box girders have been proposed. The ultimate strength reduction characteristics of girders with crack damage under torsional load, axial compression force, bending moment or combined loads have been investigated. The effect of center crack, double-edge crack and edge crack on the ultimate strength has been analyzed.(3) The change of section modulus of container ship can be used to evaluate the effect of crack on ultimate strength. Simple equations to predict the residual ultimate strength of container ships have been proprosed. Purely from the view of corrosion, the hogging condition in aged vessels is the most dangerous. When the crack locates above neutral axis, the deck modulus can be used to predict the crack effect on ultimate strength. However when the crack locates below the neutral axis, the bottom modulus can be used. Uniform corrosion has a biggest reduction on hogging ultimate strength, then bottom section modulus, sagging ultimate strength and the smallest reduction on deck modulus.
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