Chapter 25 – Fatigue Capacity

In this chapter, factors affecting S–N curves will be discussed, and the determination of the stress range at the critical location (hot spot) of the joint is discussed as well. Methods for determining stress concentration factors will be presented. In part III, tubular joints and plated connections, termed “critical details” or simply “details,” are discussed. For the sake of consistency, discussions of fatigue criteria in this chapter will be mainly based on NORSOK. However, it is recommended that readers refer to the codes relevant to their projects such as IIW, Eurocode 3, IACS, ABS, and DNV, among others. In general, a thickness correction to the design equation for the S–N curve is required when plate thickness is thicker than reference thickness. In general, the fatigue life in seawater under free corrosion is 33% of the life in air at high-stress ranges. The classification societies define fatigue criteria in their rules and guidance/guidelines. The most efficient methods involve possible improvement of the design, such as reducing geometric stress concentration factors. The fatigue analysis procedure is based on ranges of cyclic principal stresses. To determine the stress range, two approaches have been developed, the “nominal stress” approach and the “hot-spot stress” approach. A stress concentration factor may be defined as the ratio of the hot-spot stress range over the nominal-stress range.