Fatigue design rules for offshore structures are based on research work of relatively small welded joints for financial reasons and for ease of experimentation. However, extrapolation of the results to bigger and thicker welded joints may not be safe, especially for extra thick joints that are even up to 100 mm thick and more. In this paper, the hot-spot stress analysis procedure as well as the thickness effect correction method, recommended in the current fatigue design rules, was examined for extra thick welded joints by using Finite Element (FE) calculation. Both solid and shell elements were used in the hot-spot stress analysis to investigate the influence zone of notch stress concentration due to the weld toe, appropriate element size and positions of the reference points for the stress extrapolation. The thickness effect was studied based on fracture mechanics theory. The stress intensity factor (SIF) of a surface crack at the weld toe, obtained from FE analysis, was used to calculate the fatigue life of welded joints with different plate thicknesses. The results show that the proposed hot-spot stress analysis method is applicable to the fatigue assessment of extra thick welded joints.

INTRODUCTION

The hot-spot stress approach for the fatigue strength assessment of welded structures has been widely used in engineering. The approach was firstly applied in the 1970's to fatigue analysis of tubular joints in offshore structures in a combined effort by classification societies and operators of offshore installations together with research institutes (Fricke, 2003). It is generally accepted that the stress distribution approaching the weld toe depends on the plate thickness. Consequently, the hot-spot stress can be obtained by extrapolation from stresses located at distances, which are some proportions of the plate thickness, away from the weld toe. Although the hot-spot stress concept is considered to be an efficient methodology for engineering fatigue analysis of structural details, the appropriate selection of reference points for the stress evaluation and the definition of the hot-spot S-N curve are still under debate.

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