Experimental study on fatigue performance of girth welds in X80 natural gas pipelines

Butt girth welds are recognized as vulnerable points within the natural gas transmission pipeline system, as their internal defects are prone to fatigue failure under the influence of alternating loads. Focusing on girth welds in X80 natural gas pipelines as the research object, high-frequency fatigue performance tests were conducted under various loading stresses. Subsequent tests examined the crack propagation rate, considering stress ratios of 0.1, 0.4, and 0.7, along with a morphological analysis of the fatigue fracture surfaces. An S-N curve was plotted from the results of these tests, followed by the establishment of an S-N curve equation by fitting. This equation was utilized to determine the fatigue limit of girth welds in these pipelines, which was found to be 191.96 MPa. Furthermore, after plotting the crack propagation rate curves, a calculation equation reflecting correlations between crack propagation rates and stress intensity factors was established through a fitting process. This equation was then used to establish the fatigue crack threshold. Based on this threshold, the range of internal pressure fluctuations contributing to crack propagation in girth welds was identified as 4.62 MPa through conversion. Additionally, the primary factors influencing the fatigue fracture of girth welds were identified based on the morphological features observed on the fracture surfaces of the girth weld specimens. These research findings provide a reference for predicting the fatigue life as well as for troubleshooting and repairing girth welds in X80 pipelines.