Study on Microstructure, SCC Crack Propagation Paths, and Mechanical Field at the Tip of the Stationary-Propagating Crack of Pipeline Steel Girth Welded Joint

Abstract During the service life of oil and gas pipelines, welded joints are critical areas that are susceptible to fracture. This study aims to investigate the influence of mechanical heterogeneity on the mechanical fields at the crack tip and the crack propagation paths in welded joints. Firstly, the microstructure of the welded joint was analyzed, and the distribution of mechanical properties in specific local areas was determined through hardness tests. Subsequently, the mechanical properties of the dissimilar materials within the welded joint were characterized using the “Field” subroutine, and the influence of mechanical heterogeneity on the mechanical fields at the crack tip was examined. Additionally, the crack propagation paths at various locations of welded joints, influenced by mechanical heterogeneity, were analyzed from a mechanical perspective. Finally, based on the identified crack propagation paths, the variation of mechanical fields at the crack tip during the propagation process was captured using de-bonding techniques. The results reveal significant differences in the microstructure across different regions, affecting the distribution of mechanical properties. Mechanical heterogeneity influences the distribution of mechanical fields at the crack tip, thereby affecting the trajectory of crack propagation. Stress corrosion cracking tends to propagate toward regions with higher yield strength, as increased yield strength facilitates crack propagation. Crack propagation redistributes the stress-strain field at the crack tip, resulting in an unloading process that relieves stress at the crack tip. Consequently, the stress-strain at the tip of the propagating crack is lower than that at the stationary crack tip.