Effect of weld reinforcement on the fatigue strength of closely spaced S355 girth welds under cyclic tensile loading

The fatigue strength of one-sided butt-welded tubular structures is critical in offshore and structural applications subjected to cyclic loading. The butt-welded joints need to be designed to withstand the cyclic loads where the fatigue performance of the designed weld has to be checked accordingly to the corresponding S-N curve defined in the standard. Although the fatigue strength of one-sided butt welds is well established for single welds, resent research have highlighted concerns and risks regarding fatigue performance of closely spaced welds. Therefore, this study investigates proximity welds and how as-welded geometry, and flush-ground welds affect fatigue strength. In the study 72 small scale specimens extracted from S355 tubular sections was tested, each containing two girth welds at spacings of 5, 10, and 15 mm; 36 specimens were as welded with a stress concentration factor (SCF) of approximately 1.5 and 36 specimens were flush ground. Constant-amplitude axial fatigue tests under cyclic tensile loading were evaluated using the hot-spot stress method and regression analysis of S–N data. The study has developed S–N curves for each spacing and condition, compare slopes and intercepts, and relate fracture initiation/propagation to local geometry. The results show how welds located at close proximity affects the fatigue life, and how as welded but-welds reduce fatigue strength while removing the weld notches and lowering stress concentration with flush grinding extends the fatigue life. The findings provide experimental validation for the relevant fatigue Class used in assessing the fatigue performance of welds, offering practical guidance for integrity assessment of welds located in close proximity in tubular joints