Fatigue Life Prediction Of Critical Details In Ship Structure Based On Fracture Mechanics

The severe fatigue damages on the ship structure typical details occur due to the cyclical wave loading and the stress concentration in these details. The study on fatigue of these details based on fracture mechanics is rare at present. In this paper, the study on fatigue life prediction of ship critical details based on fracture mechanic has performed.At present, the most polular method to evaluate the fatigue life is the combination of S-N curves and Miner linear accumulative damage theory, which has a long history, easy to use and supported by rich experimental data. The S-N curves method can't explain the fatigue phenomena and mechanism. The fatigue life prediction method based on fracture mechanism has gradually solves these problems. But the fracture method still has a lot of problems to be studied, especially for its engineering application in ship details.The strength of cracked structure can be measured by stress intensity factor (SIF), J integration and crack opening displacement (COD). these factors have their own advantages and disadvantages, and they can be transformed to each other under some conditions. In fatigue life calculation of cracked components, the relationship between crack growth rate and stress intensity factor range, is commonly used. In this paper, the SIFR has been chosen as the parameter to calculate the fatigue life.In order to further improve the fatigue damage evaluation method of ship, a research has been done to study the fatigue life predicton method based on fracture mechanics. The research mainly includes SIF calculation at the crack tip, crack propagation paths and propagation rate, etc. In the calculation of SIF for ship typical detail, the ANSYS and the five-point method has been used to calculate SIFs. During this progress, the special circumstances about 3D element and complex stress field have been discussed, and a dada processing method has been proposed. For the crack growth path, a simplified method has been used to determine the crack propagation direction, and this method has been proved feasibility by comparison. As for the propagation rate, a unique crack growth rate method, which can consider the multiple influential factors, has been adopted and the fatigue life is calculated cycle by cycle iteration.In the end, the fatigue life of bracket with single crack and double cracks has been calculated and the feasibility of this method has been verified.In order to validate the proposed method and the effect of the loading sequence on crack growth,the fatigue life of conic-cylinder of submarine has been calculated. To show the load sequence effect of random load, a series of random load following Weibull distribution is generated through Matlab, the load sequence effect is demonstrated by different loading sequces.The preliminary research on fatigue life prediction of ship details provides the feasibility of the fatigue crack growth life prediction method on engineering ship typical detail.