A Phase-Field Assessment of Hydrogen-Assisted Fatigue Crack Growth in X52 Pipeline Steel: Three-Stage Crack Evolution and Hydrogen Concentration Trends

Abstract Hydrogen embrittlement significantly accelerates fatigue crack growth in pipeline steels, threatening the safe operation of hydrogen transmission pipelines. A coupled phase-field model integrating hydrogen diffusion, fatigue degradation, and brittle fracture is used to investigate hydrogen-assisted fatigue crack propagation in X52 pipeline steel under internal pressure fluctuations. The process comprises three stages—energy accumulation, accelerated growth, and rapid fracture—accounting for approximately 20%, 72%–74%, and 3%–6% of the simulated fatigue life under the adopted stage definitions. The relative hydrogen concentration in the crack-tip region peaks at crack initiation (point N2) and then decreases. In the present model, the maximum hydrogen concentration occurs approximately 0.1–0.2 mm ahead of the crack tip, and this distance increases with crack growth and hydrogen pressure. The relative critical energy release rate decreases with crack extension; within the linear-elastic pressure range considered, degradation peaks in Stage II. Within the linear-elastic pressure range, the model indicates that higher hydrogen pressure promotes earlier degradation, whereas fatigue effects dominate during the prolonged propagation stage. These model-based findings illustrate trends under the stated assumptions and are not validated quantitative criteria for predicting the fatigue life of hydrogen pipelines.