Research On Ultimate Strength Of Ships Structures With Low Cycle Fatigue Damage

There have been amount of studies about the effects of crash, aground and other serious damage on ultimate strength, and recently another damage influence, low cycle fatigue, has been brought to forefront by researchers. As the same with corrosion, fatigue is one of the most important time influences over strength. Traditional fatigue assessment method of ship structures is about high cycle fatigue, caused by repeated wave loadings, which damage has less effect on ultimate strength and thus be neglected. However, low cycle fatigue damage will decrease ultimate strength more than high cycle fatigue damages, so it is of crucial importance to estimate the cumulative low cycle fatigue damage effects on residual ultimate strength of aging ship's structures. Meanwhile, because low cycle fatigue stress amplitude is closed to material's yield stress and failure cycle number is lower than 104, itself also is an important assessment criterion of ship structures carrying capacity.As the development of modernized skills of design and construction of ships, more attention are paid to estimate the effect of low cycle fatigue damage on residual ultimate strength. For further study of ship structures'reliability and risk assessment, building its assessment models are also foundation to analyze the degradation rules of residual ultimate strength. The degradation models can be used to predict newer ship's residual ultimate strength, obtain the actual carrying capacity of aging ship structures, and also can be used as theoretical assistant of strength monitoring system in service.So this thesis aims at comprehensively reviewing the study process of residual ultimate strength assessment of low cycle fatigue damaged ship structures, discussing the analysis methods, investigating the effects of damages on the ultimate strength of typical ship structural elements, providing reasonable assessment methods and presenting residual ultimate strength assessment systems of complex ship structures and ship hull girders. In this thesis, to achieve this purpose, efforts and contributions have been made as the following:(1) Systematically summarizing current study methods and progress on residual ultimate strength of damaged ship structures; establishing reasonable assessment systems to analyze the effect of damages on residual ultimate strength;(2) Conducting reasons caused ship structures'low cycle fatigue damages, calculating low cycle fatigue stress amplitude, and establishing failure criterion and assessment procedure of low cycle fatigue; based on thermodynamic theory of continuum damage mechanics, defining damage variables with mechanical definition, building low cycle fatigue strength and damage models, and validating them with experiment results for next analysis of residual ultimate strength;(3) Taking damaged unstiffened plates as fictitious undamaged elements, and deriving general tensor expressions of ultimate strength of plate structures with low cycle fatigue damages; conducting finite elements analysis in order to assess low cycle fatigue damage's effect on material characters, structural extent and other factors which deteriorate ultimate strength, then obtaining empirical formula of ultimate strength of unstiffened damaged plates by simplifying tensor expressions;(4) On basis of conclusions about residual ultimate strength of unsitffend plate structures, taking damaged stiffened plates as fictitious undamaged elements, and expanding empirical formula of unstiffened plates to stiffened plates; conducting finite element analysis to verifying the empirical formula of ultimate strength of stiffened damaged plates;(5) Summarizing present ship hull girder residual ultimate strength methods, improving direct calculation method considering fatigue damages, and estimating local components' low cycle fatigue damage effects of on ship hull girder structures.From the foregoing, in this paper based on theories of continuum damage mechanics and plastic mechanics, it is studied the effects of low cycle fatigue damages on residual ultimate strength of aging ship structures. Combined with FE analysis results, assessment methods have been derived and it is indicated that low cycle fatigue damages will deteriorate ultimate strength seriously. Besides, since the damage positions and amounts are random and the deterministic models in this paper can be the foundation of random fatigue damage and risk assessment for next studies.