Simulation Of Structure Fatigue Crack Initiation And Growth

Dynamic stress variations experienced during the service period of a ship can initiate fatigue cracks in details which are inadequately designed, constructed or maintained. Subsequent crack propagation may cause failure of primary structure members leading to catastrophic consequences such as loss of the whole ship. Even if the fatigue cracking does not lead to complete failure, the cost of inspections and repaires can be high. Thus, fatigue cracking should, if possible, be avoided or kept at an acceptable level.S-N curve approach which is based on S-N curves and Miner linear cumulation damnification law and linear elastic fracture mechanics method which is based onParis crack growth law are studied in the paper separately to solve effectively the problem of computer simulation of structure fatigue crack initiation and growth. The following are what have been studied in this paper.Firstly, S-N curve approach for structure fatigue life analysis is introduced in the paper. The four styles of S-N curve approach which are applied in actual cases are reviewed in the paper: nominal stress method, hotspot stress method, notch stress method, notch strain method. Some methods to get and adjust approximately S-N curves are discussed in the paper. A cumulation damnification model is introduced. The formulae of structure fatigue lives prediction are deduced to form the basis of the following calculation and analysis.Secondly, the nominal stress method which is often used to calculate structure fatigue lives is discussed in the paper. The simplified calculation method and primary procedures to check ship structure fatigue lives is introduced. On the basis of the research and comparison of the guidelines for fatigue strength of ship structure of CCS and DNV, some parts of the guideline for fatigue strength of ship structure of CCS are modified in the paper. Accoding to the requires of the two guidelines, fatigue strength of some typical details of a 138,000m~3 LNG ship in the paper by the nominal stress method and the hotspot stress method separately. Not only the results are analyzed and compared, but the some useful suggestions are put forward in the paper.Thirdly, the basic principle and method of hotspot stress method which is veryuseful for calculating structure fatigue lives is discussed in the paper, and the hotspot stress method is compared with the nominal Stress method, some finite element models of longitudinals are created and the calculation is carried out. Fatigue strength of the forementioned details is calculated and analysed further by means of hotspot stress method combined with nominal stress method.Finally, three of the commonest methods for prediction of the onset of propagation and crack propagation angles have been outlined and discussed. The outlined models for estimation of crack extension angles rely purely on the stress intensity factors at the crack tip which can be determined by finite element procedures. The theory of the three models forms a solid basis for construction of a program for for simulation of mixed mode crack propagation .