Bridging S-N Method and Fracture Mechanics in Fatigue Evaluation of Fillet Welded T joints of an Oil Tanker

Fatigue failure in fillet welded T joints is a critical concern for marine vessels, particularly for oil tankers which are constantly exposed to fluctuating loads in harsh sea environments. This paper explores the fatigue performance of those joints through a combined approach involving the hotspot stress method and fracture mechanics. The study followed the design provisions of Eurocode 3 for application of loads. Finite element analysis was used to calculate the hotspot stresses which were then applied in the S-N fatigue assessment. In addition, crack propagation of the joints was modeled using FRANC 2D with application of Paris Law. The objective is to predict fatigue life at critical locations susceptible to cyclic loading and verify whether the joints meet the code-specified design resistance. Under ship’s actual design load, the joints are expected to sustain more than 2.5 million cycles which is significantly exceeding Eurocode based estimation (as low as 0.5 million cycles) reflecting the conservative nature of Eurocode. The fatigue life under the simulation of crack closely aligns with the S-N prediction and previously published experimental findings, lending confidence to robustness and validating the accuracy of the adopted methods. Ultimately, the study identifies the fatigue critical zones with clarity and offers a practical framework for assessing the durability and safety of the welded joints in marine structures, contributing to the development of more reliable fatigue resistant designs.