| The fatigue assessment of ship structures is particularly important with the ship designed large-scale and high strength steel widely used in ship structures. At present, the research works regarding ship structure fatigue majorly focus on how to prevent crack initiation without considering the damaged structures. Therefore, the fatigue strength check of classification society is mainly based on S-N curves by cumulative damage method. However, its limitations are obvious such as the significant diversions of the predicting results and incapability to analyze the crack growth and thus to estimate the remaining life of cracked structures. Therefore, prediction methods regarding fatigue crack growth (FCG) based on fracture mechanis are attracting extensive attentions in the fatigue problems.This dissertation has performed a series of studies on the fatigue life assessment, and the fatigue crack growth program FCG-System has been developed which is capable to conduct analyses on the fatigue crack propagation in cases with the combinations of unstiffened and stiffened panels with single crack or multiple cracks. In conjuction with the developed FCG-System, the theory of reliability is adopted to further sudy the probability of fatigue life, toward to establish a methodology to analysze and assess FCG of cracked hull structures.Firstly, based on ABAQUS using Python scripting language, FCG-System with four modules has been developed based on fracture mechanics. Automatic mesh method is proposed, and automatic modeling technology parameterized processing module of arbitrarily crack growth in complex structures is established. On this basis, combining with the Virtual Crack Closure Technique (VCCT), the fatigue crack growth analysis module of cracked plane has been developed. The fatigue fracture characteristic analysis of cracked plates has been carried out on simple single crack growth and simultaneous growth of complex multiple cracks, and the method and its application are validated by comparison to results from existing cases with experimental results or other established FEA predictions. In addition, systematic research is made on square plate with central non collinear cracks to investigate interfering crack propagations at varied initial positions and effects of interfering crack on the stress intensity factors of basic crack tips.Then, based on the nodal forces and nodal displacements of crack tip, a formula for shell elements to compute stress intensity factors is proposed in the view of VCCT. Under the framework of above-mentioned module, combining with VCCT, the FCG module was added into the FCG-System. In order to fit into large-scale marine structures, the substructure modeling technique is integrated in the system under the consideration of structural details and load shedding during crack growth. The accuracy and the robustness of FCG-System are demonstrated by several illustrative examples. No stability and convergence difficulties have been encountered in these cases and meanwhile, insensitivity to the mesh size is confirmed. In the following research, the fatigue crack growth in common longitudinal connection of oil tanker is simulated and the effect of soft toe, back bracket and tripping bracket on crack growth path and life is discussed subjected to the water pressure loading condition or the axial loading condition separately.Finally, reliability analysis of fatigue crack propagation is studied by further considering the effect of randomness such as geometric parameters, material parameters and load parameters. A Reliability analysis method based on the improved Kriging model is proposed combining the improved Kriging response surface method with MC method by adding new sample points to construct high precision response surface approximation method of limit state function step by step. The accuracy and the efficiency of the improved Kriging response surface method are demonstrated by three illustrative examples. The fatigue life reliability analysis method based on the improved Kriging response surface is proposed combining Kriging-based Monte Carlo simulation method with the FCG-System. Then the reliability analysis about a rectangular plate with a slant single-edge crack and a modified four-point bending SEN specimen has been made by the improved Kriging response surface method and compared with the results obtained by the classical response surface method. In addition, the method is applied to fatigue reliability analysis of other cracked structures. |
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