Experimental determination of the fatigue life of modified threaded pipe couplings

To assemble pipelines and tubular structures, threaded couplings can be used. In service these couplings are often subjected to dynamic loads. To maintain a secure connection, they are generally preloaded. The combination of preload and external dynamic loads results in a multiaxial stress distribution over the connection, where the coupling’s threads act as stress raisers, initiating fatigue cracks. The fatigue life of the connection depends on its global geometry, local thread geometry, interface and material properties. In this study the fatigue life of standard API Line Pipe couplings and two modified coupling configurations is determined experimentally. The influence of the coupling’s global geometry on the connection’s fatigue life was studied. Threaded pipe samples with a nominal size of 1” were tested on a four-point bending fatigue test setup. A sample was considered to be failed when a through thickness crack appeared. This was detected by pressurizing the tube samples. When a pressure drop was recorded, the test was stopped. The number of elapsed load cycles defines the fatigue life of the connection. Using this methodology, S-N curves were obtained for the standard API Line Pipe connection and two modified coupling configurations. Both configurations exhibited an improved fatigue life relative to the standard connection. In the final part of this paper, these experimental observations are explained by a combination of a damage evolution law and a finite element model.