Fatigue Reliability Analysis Of Ship Stiffened Plates Under Combined Loads

Fatigue fracture is one of the main factors which affect the safety of ship and offshore structures. As the important part of hull, stiffened panels may probably be broken by crack growth under all kinds of the alternate loads such as tensile load, bending load if there are initial cracks in them. Considering the importance of the safety of stiffened panels, in this paper the fatigue reliability of a stiffened panel which has initial cracks subjected to tensile load and bending load at the same time is analyzed based on fracture mechanics and theory of reliability.In order to consider the effect of combined loads, the stress intensity factor of Paris formula is considered as a superposition which is related to the phase of tensile and bending load. In the calculation, according to the superposition of the stress intensity factor, the Paris formula is expanded to the tensile load term, the bending load term and the coupling term.The stress intensity factor for the different combinations of the crack sizes in the plate and the stiffener are calculated by finite element method. Singular elements are used to establish the stiffened panel model. Then the geometry correction functions of the stiffened panel are obtained. The wave loads applied on the stiffened panel are predicted based on the 3D potential flow theory. Then the probability distribution of the local loads is achieved.According to the characteristic of each item, the Monte Carlo method is used to calculate the statistical descriptors of the crack growth, at last the least square method is used to fit the date and then get the probability model of the crack growth of every term. The probability model of the crack growth of the stiffened panel can be got by combining these data in accordance with the Paris formula.The fatigue assessment diagram is used for estimating the critical crack length. The fatigue reliability index and the failure probability are analyzed finally based on the second level method and the probabilistic model of the stiffened panel subjected to tensile load and bending load of different phases under different correlation factors.