Comparison between fatigue life values calculated using standardised and measured stress spectra of a naval high speed light craft

This paper presents a comparative study between assumed stress spectra and stress spectra derived from hull monitoring system data, with respect to the fatigue life at three different welded joints of a naval high speed light craft. The Palmgren-Miner rule, combined with Eurocode 9 construction details and fatigue resistance data, is applied to the stress spectra to calculate the fatigue life. This work has been motivated by the need to improve life-of-type evaluations of high speed naval vessels with semi-planing hullforms, which are required to perform demanding operations. It is shown that stress spectra derived from full-scale measurements are best modelled by Gaussian functions. Use of the largest stress cycle corresponding to design sagging and hogging load cases leads to conservative fatigue life estimates in comparison to analysis based on the derived spectra and fleet maintenance data, even when a 10^-8 probability of sustaining the design loads is considered. The use of standardised stress spectra in fatigue analysis of naval high speed light craft offers benefits though requires further consideration of the characteristic fatigue loads and wave environments.