Evaluation of the Effect of Different Mean Stress Levels on the Fatigue Resistance of OCTG Premium Threaded Connections

Identification of the effect of mean stress for fatigue performance of the premium threaded connection for the OCTG pipes, was conducted via full-scale test. API standard material [grade L80-1] was used for the test. The nominal pipe outside diameter and wall thickness were 244.48 mm (9-5/8 inch) and 11.99 mm (0.472 inch), respectively. The fatigue life of the specimens tested with no mean stress was longer than that of the specimens tested with a tensile mean stress. Through-wall cracks were found at the imperfect thread area of the male embodiment, but the crack initiation site depended on the mean stress. However, the primary root cause of the failure on both mean stress levels can be regarded as the fretting fatigue. Fatigue life was also able to be estimated using modified Goodman relation.