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To evaluate fatigue damage in ship hulls and offshore structures over their design service life, consideration of the long-term frequency distribution of wave-induced stresses is essential. For ships ...equipped with Type-B tanks, it is particularly important to verify that the Leak Before Failure (LBF) condition is satisfied under an appropriate stress history derived from stress frequency distributions. However, despite the availability of various methods for generating stress histories, no consensus has been established on the most appropriate approach. In this study, multiple representative stress histories were constructed from a common stress frequency distribution, including those recommended by classification societies. Fatigue crack propagation analyses were performed under those conditions using two approaches: the conventional Paris’ law and an advanced fracture mechanics model incorporating the Re-tensile Plastic zone Generating (RPG) load criterion, as well as the method proposed by Toyosada et al., which accounts for crack opening and closing behavior. The results revealed that Paris’ law significantly overestimated crack growth under variable amplitude loading, as it failed to capture overload-induced crack growth retardation. In contrast, the RPG-based model predicted slower crack growth due to the retardation effect following high tensile stress peaks, which are not considered in the Paris’ law. Early stress peaks in the loading history triggered earlier retardation, thereby decelerating crack propagation. These findings are consistent with the known behavior of actual materials, where overload-induced retardation is commonly observed, suggesting that fatigue life predictions based solely on Paris’ law may be overly conservative. Furthermore, the influence of different stress history modeling methods on fatigue life evaluation was examined. The numerical results indicate that certain modeling approaches provide more practical and realistic assessments, particularly when crack arrest and retardation effects are considered.続きを見る
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