Session: 03-02-01 Fatigue Performance and Testing

Submission Number: 180994

Numerical Study on the Stress History Model for Fatigue Strength Verification of Ship Hull Structures 

To evaluate fatigue damage in ship hulls and offshore structures over their design service life, it is essential to consider the long-term frequency distribution of wave-induced stresses. For ships equipped with Type-B tanks, it is particularly important to verify that the Leak Before Break (LBB) condition is satisfied under an appropriate stress history derived from data extracted from the stress frequency distributions. However, although various methods exist for generating stress histories from frequency distributions, there is no consensus 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 these loading 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 crack growth retardation phenomena induced by overloads. 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 study examined the influence of different stress history setting methods on fatigue life evaluation. Numerical results indicated that certain history-setting approaches yield more practical and realistic assessments, particularly when crack arrest and retardation effects are considered.

Presenting Author: Yuji Miyoshi Oshima Shipbuilding co., Ltd.

Presenting Author Biography: Mr. Yuji Miyoshi is the Deputy Manager of the Design Department at Oshima Shipbuilding Co., Ltd.
He was awarded a Master's degree in Naval Architecture and Ocean Engineering from Kyushu University in 2003.
He is also currently enrolled in the Doctoral Program at Kyushu University Graduate School.
In his current research and development work, he is engaged in developing structural integrity evaluation for ships and offshore structures, fatigue strength evaluation, and specifically fatigue crack propagation analysis.

Authors:

Yuji Miyoshi Oshima Shipbuilding co., Ltd.
Kazuki Matsuda Kyushu University
Koji Gotoh Kyushu University