Experimental determination of the fatigue life of modified threaded pipe couplings

To assemble tubular structures such as pipelines, risers, …, threaded couplings can be used. In service these couplings are often subjected to dynamic loads, and 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. In this study the fatigue life of the standard API Line Pipe coupling and two modified coupling configurations is determined experimentally. This way the influence of the coupling's global geometry on the connection's fatigue life is studied. Threaded pipe samples with a nominal size of 1″ are tested on a four-point bending fatigue test setup to obtain S-N curves. Both modified coupling configurations exhibit a fatigue life improvement. 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.