Fatigue Reliability Analysis Of Ship Stiffened Panel Structure Subjected To Multiple Cracks

Fatigue failure is one of the main factors which endanger the safety of naval architecture and ocean engineering structure.Fatigue life assessment is an essential process in ship structure design.Stiffened panels have a wide range of application in main ship structure,the reliability of which affect the reliability of the whole ship.Compared to the traditional S-N curve fatigue calculation,fatigue cracks growth method based on fracture mechanics can consider initial cracks in the structure and the process of crack growth,which is closer to the practical failure in the structure.In the service life of ship,stiffened panels generally have a crack even multiple cracks under the alternate loads induced by waves.The initial cracks can increasingly extend to the critical size and then lead to fatigue failure of structure.To calculate the reliability of stiffened panels with multiple cracks,fatigue assessment based on fracture mechanics and reliability theory are applied to research the fatigue reliability of stiffened panels subjected to multiple cracks in this paper.Through finite element analysis,the stress intensity factors are calculated with cracks simulation by the singular elements.To study the effect of stress filed of parallel cracks,empirical formulas of the geometry correction functions varying with distance are obtained.The stress intensity factor of one crack is affected by other cracks in the stiffened panels with multiple cracks.Chosen a distance within the stress field,a series of models with different combinations of plate and stiffener crack sizes are established.Then the formulation of the geometry correction function which controls crack growth can be achieved.A program that predicts the crack propagation is coded using the Paris-Erdogan law.Input the variables such as initial crack size,material parameters,stress range and the geometry correction function,Monte-Carlo simulation is employed to simulate the process of cracks growth.The crack sizes as output are used to get mean crack sizes of different years.The least square method is applied to make the regression of the results of crack growth from Monte Carlo simulation.The statistical descriptors of parameters of geometry correction functions can be calculated through curve fitting.Then the probabilistic model of the stiffened panel subjected to correlated crack growth of multiple cracks is achieved.The probabilistic model of the stiffened panel subjected to correlated crack growth of multiple cracks is used to calculate the mean values and standard deviation of crack sizes in different years.Combined the critical crack length and limit state equation for fatigue failure,the fatigue reliability index and the reliability of structure are analyzed.Considered the effect of the failure of every crack on the stiffened panel subjected to multiple cracks,the joint failure mode of multiple cracks is investigated.And the time-varying fatigue reliability index and reliability of overall structure are achieved.Based on the above,planning for the inspection and maintenance of ship structure are proposed in order to guarantee safe navigation.