Fatigue Damage And Crack Propagation Calculation Method Based On The Spectral Analysis Method

Fatigue is one of the failure modes of ships and marine structures.Considering the liquid cargo carrying ships,the liquid tanks in those ships will suffer dynamic cargo pressure induced by ship motion acceleration,and the wave load transferred by the ship.As a result,fatigue damage will be caused and the fatigue life of ship tanks should be calculated during the design phase.The spectral analysis method is a direct calculation method for assessing fatigue damage in ships and marine structures,and it should be used in the fatigue assessment of ship tanks.However,due to the nonlinear relationship between the dynamic cargo pressure and the wave load,it is not proper to calculate the fatigue damage using the traditional spectral method based on the linear system method.How to calculate the fatigue damage of tanks under nonlinear fatigue load using spectral method is a problem which worth researching.The crack propagation method based on the fracture mechanics is a better way to calculate the fatigue damage.It is necessary to research the crack propagation calculating method based on the spectral analysis.Based on that background,the fatigue damage calculation and the crack propagation calculation for ship tanks based on the spectral method are researched in this paper.The researches are as below:(1)The background of this study is introduced.The review of the research status of the time domain method and the frequency domain method for fatigue damage calculation and the crack propagation method is given.(2)The spectral method based on the linear system method is introduced in detail.The nonlinear relationship between the dynamic cargo pressure and the wave load in ship liquid cargo tanks is analyzed,and the deficiency of the traditional spectral method based on the linear system method in calculating fatigue damage of ship tanks is researched.An improved spectral method which can take the nonlinear relationship into consideration is proposed in this paper.(3)The time domain method for calculating the fatigue damage of ship tanks is proposed.Take an independent tank in a LNG carrier for example,the accuracy of the improved spectral method and the traditional spectral method are assessed by comparing the fatigue damage results calculated by the time domain method.The assessment results show the traditional spectral method will cause relatively big errors,and the improved spectral method is more accurate.(4)A method for calculating stress intensity factor(SIF)based on submodel technology of MSC.Patran and ANSYS is proposed.The feasibility and accuracy of the proposed method are tested.(5)Based on the spectral method and the calculation method for the SIF proposed in this paper,A solution for fatigue load spectral generation and SIF calculation of a crack in details of LNG tank is given.The crack propagation life of a detail in the tank of a LNG ship is calculated.