A Phenomenological Model for Fatigue Crack Growth Rate of X70 Pipeline Steel in H₂S Corrosive Environment

Long distance pipeline transmission is considered to be the most economic and safest method to transport natural gas in recent days. The pipeline safety problem is a focus of concern in the academic and industrial circles. Wildly used material X70 pipeline steel is tested in this paper. Based on actual conditions of long distance pipeline, experiments were designed by using orthogonal method. Corrosion fatigue crack growth tests were carried out under different H₂S concentrations (2000 ppm, 1000 ppm, 500 ppm), stress ratios (0.9, 0.8, 0.7), and load frequencies (0.0067 Hz, 0.01 Hz, 0.013 Hz). A phenomenological model was built based on test data and Paris formula, which corrects parameters C and n of the Paris formula at the same time. The boundary element method (BEM) was used to simulate the process of corrosion fatigue crack propagation. Fatigue crack growth rates (FCGRs) under different loading conditions were obtained and all the parameters of the phenomenological model were calculated from test data. Compared with traditional Paris formula, this new model shows coupling interactions of stress ratio, load frequency, and H₂S concentration to both slope and intercept of FCGR curve in log-log space. The simulation results show that the largest difference between actual test data and BEM simulation is less than 10%. According to the analysis of coupling interactions of each factor, the slope of fatigue crack growth rate (FCGR) curve in log-log space is mainly influenced by frequency; the influence of H₂S concentration is nonlinear, and FCGR increases significantly with the increase of stress ratio and decrease of frequency. The BEM is suitable to be used in the engineering field to predict the residual life of pipelines.