Objective Transporting hydrogen-blended natural gas exposes high-grade steel pipelines to hydrogen-induced fracture. Girth welds serve as structural weak points due to their heterogeneous microstructure and inconsistent mechanical performance. To address existing model limitations in simultaneously capturing multiple hydrogen trap types, stress-assisted diffusion, and full damage evolution, there is an urgent need to conduct fracture and damage assessments of high-grade steel pipeline circumferential welds in a hydrogen environment.

Methods Based on Oriani’s equilibrium theory, hydrogen diffusion governing equations and mass conservation, this study proposes a calculation approach to quantify the concentrations of lattice hydrogen, alongside hydrogen trapped by dislocations, carbides and martensitic grain boundaries. The evolution of dislocation trap density induced by concentration gradients, hydrostatic stress gradients and plastic strains is incorporated into the diffusion governing equations. On the basis of the complete Gurson model, this work introduces the acceleration effect of lattice hydrogen and trapped hydrogen on void nucleation and growth. The Thomason void coalescence criterion is modified with the hydrogen-softened matrix flow stress, thereby establishing the constitutive model for fracture damage of girth welds in high-grade pipeline steels under hydrogen environments. Coupled computations of hydrogen diffusion, stress, plastic deformation, and damage evolution are realized via the User Material Heat Transfer Subroutine (UMATHT), User Material Subroutine (UMAT), and User External Database Subroutine (UEXTERNALDB). The proposed model is verified using a two-dimensional Compact Tension (CT) finite-element case and Resistance Curve (R-curve) experiments of girth welds.

Results For the CT specimen case with a stress intensity factor of 30 MPa·m^0.5 and a boundary hydrogen concentration of 2.659×10²² m⁻³, the proposed model reproduces the enrichment and spatial distribution of total hydrogen and hydrogen trapped by three categories of traps at the crack tip. The variation trends are consistent with those from existing research. At room temperature, a total pressure of 10 MPa (partial pressures of H₂ and N₂ are 2 MPa and 8 MPa, respectively) and a loading rate of 0.01 mm/min, the girth weld R-curve under the environment with a 20％ hydrogen blending ratio is obtained after 33 loading-unloading cycles. The fracture toughness determined by the 0.2 mm crack growth offset line is 0.2457 mm. The average relative error between the prediction curve of the constitutive model for fracture damage of girth welds in high-grade pipeline steels under hydrogen environments and experimental data is approximately 8％.

Conclusion Hydrogen enrichment and fracture damage at the crack tip are jointly governed by hydrostatic stress, plastic deformation and hydrogen trapping effects. The newly developed constitutive model can quantify how hydrogen in different existing states affects void nucleation, growth and coalescence for high-grade pipeline girth welds under hydrogen exposure. This model provides references for crack propagation prediction and structural integrity evaluation of girth welds of hydrogen-blended pipelines.