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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. NomenclatureA damage parameter S m mean stress [MPa] D damage parameter [-] S r stress range [MPa] E Young's modulus [MPa] STD standard deviation F force [N] damage parameter [-] N number of cycles [-] eq equivalent von Mises stress [MPa] N f number of cycles to failure H hydrostatic pressure [MPa] R load ratio [-] y yield stress [MPa] 2 J. V. Wittenberghe et al./ Procedia Engineering 00 (2010) 000-000 R V triaxiality function [-] UTS ultimate tensile stress [MPa] S a stress amplitude [MPa] Poisson's ratio [-] S f fatigue stress [MPa]

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To ensure a reliable connection between two pipe sections, an initial make-up is applied to the threaded connections to induce a favorable stress state. Using finite element analysis techniques, it is possible to predict the internal strains and stresses of the connection when torque is applied. This article presents the outline of an experimental setup, which allows to directly validate the occurring strains together with the torque versus turn diagram and indirectly the contact pressures. The strains are measured by means of digital image correlation and strain gages. Both methods provide similar results and comply with the predicted finite element analysis strains when taper mismatch is taken into account. In an effort to qualitatively validate the simulated contact pressures, the temperature of the box is measured during make-up by means of infrared monitoring. The maximum temperature increase occurs near the vanishing threads where contact pressures are larger. Despite promising results, no decisive validation for the contact pressures could be obtained