Fracture Mechanics Assessment Of Fatigue Life Of Hatch Corners Of An Ultra Large Container Ship

Due to large opening characteristics of the ultra large container ship,shear stress and warp stress occupy a large proportion of the whole stress the ship suffered,which is very unfavorable to the flexural strength,transverse strength and torsional strength of the ship.As a result,more and more high-strength thick steel plates are used in ultra large container ships which brings more severe fatigue problems even though increases direct strengths.At present,the CFD method and FCP method are mainly used for the fatigue assessment of steel structures.The former is widely used in engineering and the latter is in the ascendant due to the ability of taking load ratio and load sequence into consideration while predicts fatigue life.However,whether the S-N curves can be extrapolated(the rules require that yield strength of material should be less than 400 MPa)to high-strength steels remains to be further studied and CFD method can't evaluate the residual life of structural details that have cracked.Therefore,this thesis takes FCP method to evaluate the fatigue life of typical structural details of container ships.As to structural details in container ship with simple loads and empirical formula of SIF,the fatigue load spectrums are calculated by the simplified approach recommended by guideline of CCS.Hatch corners suffered complicated loads and have no empirical formula of SIF,so empirical formula of SIF are regressed by direct finite element analysis and the fatigue load spectrums are calculated by spectral analysis.The main research contents are as follows:(1)Introduce the research background and significance of the thesis,summarize the research status on the fatigue assessment of container ships,and discuss the construction methods for fatigue load spectrums.(2)This thesis introduces the simplified approach to calculate fatigue load spectrums recommended by the Guidelines for Fatigue Strength of Hull Structures(CCS)and the spectral analysis method recommended by the ABS fatigue guideline.The calculation method and process of stress intensity factor under complex loads,Huang Model and FAD technique are introduced.(3)Based on the simplified approach,the fatigue load spectrum sequence under multiple working conditions is studied and applies it to the fatigue assessment for typical longitudinal structural details with known empirical formula of SIF.For the convenience of calculation and engineering application,compile a powerful,user-friendly GUI program.(4)Direct finite element analysis method is used to study the crack propagation at several hatch corners of an ultra large container ship under different spreading angles and frequencies.On the basis of large amount of calculation,the rules of crack propagation are analyzed and a unified empirical formula for SIF calculation is regressed.And verify the reliability of the regression formula with the result of the direct finite element calculation of another container ship.(5)Analysis factors that affect fatigue crack growth life,such as wave spectrum,speed and spreading angle,initial crack size and load sequence and so on.