Fatigue Crack Propagation Evaluation Of The Typical Joint In Ultra Large Container Ship

Ultra large container ship has the characteristics of large hatch openings,small square coefficient,significantly changing section and high speed.Its cargo capacity has expanding dramaticly,high tensile thick steel plate is widely applied to the upper deck area of the container ship to ensure the structure longitudinal strength.Compared with the ordinary steel,high tensile steel has improved in the yield stress,but its ability to resist fatigue,corrosion does not increase accordingly.There exists high level alternating stress in the longitudinal joint and hatch corner position.Fatigue strength has become one of the most important aspects in structure performance of large container ship.It is meaningful to predict the fatigue life of large container ships accurately.The linear cumulative damage based S-N curve method is widely accepted and used in the project.However,these curves is suitable for the steel whose yield strength is less than 400 MPa and the yield stress of the steel used in large container ship is beyond the scope.The fatigue assessment method based on the crack propagation is more reasonable in theory and it can be checked by the measured results.It has gain more and more attention in the ship industry,but there still exists many problems needs to be solved.The main purpose of this paper is to implement the crack propagation method to the typical fatigue joint in the ultra large container ship.The main research contents are as follows:(1)Introducing the research background and research significance,discussing the ultra large container ship fatigue strength research,the stress intensity factor calculation research,wave load computation situation.Introducing the single curve model for fatigue crack propagation rate,and summarizing the empirical formulas of the stress intensity factor calculation widely accepted by the current ship building industry.Pointing out the significance of research on container ships based on crack propagation.(2)Proposing a stress intensity factor(SIF)calculation method for surface cracks in ship structures under wave loads so as to avoid using body element in the hot spot area in PATRAN finite element model for calculating the SIFs of a surface crack.The accuracy of the proposed method was validated by comparing the calculated results with that of widely accepted empirical formulas.(3)Calculating stress intensity factors of surface cracks at the weld toe of longitudinal stiffener end join with the finite element software ANSYS.Empirical formulas for calculating the stress intensity factors of surface cracks at the end of longitudinal stiffener join.By analyzing the characteristics of these joins and classifying them into four classes,the stress intensity factors of surface cracks in joins for all types of longitudinal stiffeners are given by means of modifying the base empirical formulas.(4)Forecasting hydrodynamic response and wave pressures of the container ship by using the finite element software WALCS.The wave pressures are transferred to the surface of the hull structure and the angular joint in the middle bottom of the ship is refined according to the guidelines for fatigue strength of ship structure of CCS,then the structural hot spot stress response is calculated.SIFs of surface cracks with different sizes in the joint under the wave induced pressure were calculated and dimensionless empirical formulas were summarized.Fatigue crack growths were calculated finally.(5)Using finite element software to simulate the crack propagation in the bracket under torsional stress field in the experiments and the results is compared with the experiment result.Then the method is applied to the hatch corner in front of the engine room of the container ship.SIFs of cracks in the circular corner and negative radius corner are calculated and dimensionless empirical formulas were summarized.Fatigue load spectrum is constructed based on spectrum analysis.Fatigue crack propagation is calculated using unique curve model and assessed using failure assessment diagram(FAD).