Load Sequence Effect On Fatigue Crack Propagation Of Marine Structures

Fatigue is a major cause for the failure of marine structures. With the wide application of high tensile steel, the weights of the ship structures are lower and the scandlings of the ship structures are smaller than ever before. Therefore, the stress level in ship structures is higher, and the fatigue damage problem is becoming more and more important. Fatigue strength is focused by shipbuilding industry, and is highly considered in design procedure.Nowadays, world famous classifation societies such as GL, ABS, and DNV have established their own fatigue life assessment methods. The methods share similarities and are also different to each other. The present thesis compares the fatigue life assessment rule codes of different classification societies. Moverover, the fatigue life assessment of structural details of a large Ro/Ro passenger ship is carried out to find out the limitation of such methods.Load sequence effect has a significant influnce on the fatigue life of marine structures. To predict the fatigue life precisely, lots of researches have been done on load sequence effect. In this thesis, the development and especially the state-of-the-art calculation models of load sequence effect in literature are reviewed and research trends are summarized. In the present thesis, a modified engineering model of fatigue crack growth under variable amplitude loading is introduced to extend the former model to cover fatigue crack propagation regionⅢ. To obtain satisfied results, a method to determineβshape parameter is given. And a comparison between the modified model and another two-parameter model has been made considering two different materials, 7075-T6 Al alloy and high tensile steel. In the end, the extended model of fatigue crack growth is validated by other 7075-T6 Al alloy and high tensile steel experimental data and a marine structure fatigue crack propagation real case calculation.