The estimation of fatigue crack growth for a ship structure is a complicated subject, especially when loads vary. Several researchers studied fatigue crack growth under varying loads with simple block loads, and many models have been proposed. More research is required to properly evaluate the storm loads, representing complex actual loads a vessel experience on the sea.
This study proposes a new simple fatigue crack growth model that incorporates only two parameters of the stress ratio factor and the load history factor taking account of the underload effects using empirical data in the literature. The procedures for formula modification and optimum model selection are introduced. The new model is optimized and validated by comparing it with several simple block load tests and existing fatigue crack growth models.
Finally, the proposed crack propagation model is applied on a spherical IMO Type B LNG tank deployed on a 150K class LNG carrier. IMO Type B tanks require a high level of safety. Therefore, analyses such as strength evaluation, fatigue analysis, fatigue crack propagation analysis, and LNG leak rate should be conducted to ensure the safety.
Various fatigue crack growth models were proposed for fatigue crack propagation analysis. The Forman model and the Walker model consider the stress ratio. Furthermore, the Huang model, and the proposed model consider the stress ratio and the effects of overload and underload present in the past load history on crack propagation. To examine the effects of stress ratio and load history for the fatigue crack propagation models, the crack growth of the LNG tank is evaluated by considering the stresses under full load and ballast load operating conditions, and three stress amplitude sequence cases.
Generally, the new model shows competitiveness and good agreement with the experiment results. The proposed model is expected to be a potential solution, with a simple formula, to estimate fatigue crack growth in ship structures under variable loads including storm loads.