Comparison of Fatigue of Girth-Welds in Full-Scale Pipes and Small-Scale Strip Specimens

As part of a study of fatigue in girth-welded steel pipes, tests were performed under constant amplitude loading on both full-scale pipes and strip specimens cut from such pipes. Significant differences were found in their high-cycle fatigue lives, which extended to around 10⁸ cycles, and apparent fatigue endurance limits, the small-scale strips displaying superior fatigue properties. The reasons for this were investigated considering the fatigue crack initiation site, weld geometry, type of pipe, loading conditions, residual stresses, the re-testing of unfailed specimens and size effects. Fracture mechanics fatigue crack growth calculations were also performed using a K solution specially calculated by FEA for the girth weld. Conclusions are drawn about the suitability of strip fatigue test specimens for representing the fatigue behaviour of full-scale girth welded pipes and the scope for re-testing unfailed full-scale pipes.