Influence of Dual‐Crack Evolution on Fatigue Life of Hydrogen‐Transmission Welded Steel Pipelines Under Multifield Coupling

Repurposing pipelines for hydrogen‐blended natural gas raises concerns that welding residual stress, cyclic pressure, and hydrogen diffusion affect coexisting cracks. This study examines coplanar dual semielliptical cracks in X80 pipeline steel (22‐mm wall) under multiphysics coupling using ABAQUS‐FRANC3D. Peak hydrogen concentration follows a four‐stage evolution with under 1% variation across 1.5%–15% blending. Post‐weld heat treatment cuts residual stress from 69.3% to 172 MPa, yet fatigue life gain drops from 2.8% to 1.5% as hydrogen rises from 0% to 15%. During coalescence, a 4.4‐mm crack yields 91.5% higher stress intensity factor than a 1.1‐mm crack. Under strong through‐thickness constraint, deep cracks keep elevated mid‐region SIF at failure, with the 4.4‐mm crack reaching about 72 MPa·m 0.5 at the mid‐wall, roughly 14% above the 1.1‐mm value of about 63 MPa·m 0.5 . Sensitivity ranks crack depth above blending ratio and spacing.