Fatigue crack propagation prediction for marine structures based on a spectral method

A method based on spectrum analysis is proposed for predicting fatigue crack propagation (FCP) of a crack in marine structures. The stress intensity factors (SIF) for a crack can be different under different load cases, such as loading conditions, heading angles and wave frequencies, even at the same level of nominal or hotspot stress. Therefore, the load spectrum obtained from the stress transfer function and wave spectrum may be failing to calculate the FCP accurately. In current study, SIF transfer functions were evaluated through detailed structural analysis for different crack sizes, and the short term SIF distribution obtained from the SIF transfer function. FCP life could then be calculated from numerous short term SIF distribution and the improved Paris formula. A sub-model technique was integrated, facilitating SIF re-analysis due to crack growth, and an Improved Euler method was adopted to reduce the computational steps. FCP of a semi elliptical surface crack at the weld toe between hatch coaming and forward bulkhead of a container ship superstructure was used to demonstrate the application of the proposed spectral method. This paper offers a way of combining the spectral analysis and fracture mechanics in fatigue crack growth calculation for marine structures.