Quantifying the Deleterious Effect of Sea Water on the Fatigue Life of Welded Steel Joints Using a Fracture-Mechanics Approach

Abstract

Numerous studies have been conducted to investigate the deleterious effect of sea water on the fatigue life of welded joints for offshore structures subjected to wave loading (0.15 Hz to 0.5 Hz). In contrast, little research has been conducted to quantify this effect on the fatigue life of marine structures subjected to lower cycle frequencies. In the present study, a fracture-mechanics based approach in conjunction with the finite-element method has been used to model fatigue tests carried out on tee-butt welded joints in aqueous 3.5 wt.% NaCl (an analogue for sea water) under freely corroding conditions and various cycle frequencies (from 1 Hz to < 0.001 Hz) subjected to a constant stress range. This type of analysis can now be carried out because a more comprehensive corrosion-fatigue crack-growth-rate dataset for these low cycle frequencies has recently been published. Fatigue cracking in tee-butt welded joints was successfully modelled, and the results of this modelling, which are consistent with corrosion-fatigue crack-growth rates, indicate that the fatigue life of these tee-butt welded joints can be reduced by up to about 6.7 times in a salt-water environment for very low cycle frequencies (< 0.001 Hz) compared with the fatigue life in air.