[Paper Review] Coupled thermo-chemo-mechanical phase field-based modelling of hydrogen-assisted cracking in girth welds

L. Castro, Y. Navidtehrani. C. Betegón+1 more|arXiv (Cornell University)|Jan 1, 2026

Hydrogen embrittlement and corrosion behaviors in metals0 citations

TL;DR

The paper presents a new computational framework that couples thermo-mechanical welding simulations with a multi-trap hydrogen diffusion and hydrogen-sensitive phase-field fracture model to predict weld integrity in X80 pipelines under hydrogen exposure.

ABSTRACT

A new computational framework is presented to predict the structural integrity of welds in hydrogen transmission pipelines. The framework combines: (i) a thermo-mechanical weld process model, and (ii) a coupled deformation-diffusion-fracture phase field-based model that accounts for plasticity and hydrogen trapping, considering multiple trap types, with stationary and evolving trap densities. This enables capturing, for the first time, the interplay between residual stresses, trap creation, hydrogen transport, and fracture. The computational framework is particularised and applied to the study of weld integrity in X80 pipeline steel. The focus is on girth welds, as they are more complex due to their multi-pass nature. The weld process model enables identifying the dimensions and characteristics of the three weld regions: base metal, heat-affected zone, and weld metal, and these are treated distinctively. This is followed by virtual fracture experiments, which reveal a very good agreement with laboratory studies. Then, weld pipeline integrity is assessed, estimating critical failure pressures for a wide range of scenarios. Of particular interest is to assess the structural integrity implications of welding defects present in existing natural gas pipelines under consideration for hydrogen transport: pores, lack of penetration, imperfections, lack of fusion, root contraction, and undercutting. The results obtained in hydrogen-containing environments reveal an important role of the weld microstructure and the detrimental effect of weld defects that are likely to be present in existing natural gas pipelines, as they are considered safe in gas pipeline standards.

Motivation & Objective

• Characterize residual stresses and heat-affected zone (HAZ) properties from multi-pass girth welding of X80 steel.
• Develop a coupled deformation-diffusion-fracture model that includes plasticity, hydrogen trapping, and phase-field fracture.
• Assess weld integrity under hydrogen environments and identify effects of common weld defects.
• Evaluate how weld microstructure and defects influence hydrogen-assisted cracking in girth welds.

Proposed method

• Model the welding process as a sequential thermo-mechanical problem with temperature-dependent material properties.
• Use Abaqus with a UMAT to simulate elastic-plastic thermo-mechanical response during multi-pass SMAW welding.
• Define three weld regions (BM, WM, HAZ) with distinct properties and couple the welding results to subsequent fracture analysis.
• Implement a two-level (lattice-trap) hydrogen transport model with multiple trap types and trap creation driven by plastic deformation.
• Apply a hydrogen-sensitive elastic-plastic phase-field fracture model where crack growth is driven by a history field and material toughness depends on lattice hydrogen.
• Utilize a phase-field AT2 formulation with a length scale parameter linked to material strength to capture crack initiation and propagation.

Experimental results

Research questions

• RQ1 How do residual stresses from multi-pass girth welding influence hydrogen diffusion and trapping in different weld regions (BM, WM, HAZ) at service temperatures?
• RQ2 Can a coupled thermo-mechanical welding simulation combined with multi-trap hydrogen diffusion and phase-field fracture accurately predict hydrogen-assisted cracking in X80 girth welds?
• RQ3 What is the impact of typical weld defects (porosity, lack of fusion, undercutting, etc.) on hydrogen embrittlement and fracture risk in welded pipelines?
• RQ4 How does the evolving dislocation trap density during plastic deformation affect hydrogen transport and fracture resistance?
• RQ5 How well does the model reproduce experimental fracture behavior in BM, WM, and HAZ specimens under hydrogen-rich environments?

Key findings

• Thermo-mechanical welding simulations identify the BM, HAZ, and WM regions and reveal residual tensile stresses up to around 730 MPa near the weld root.
• The HAZ width is found to be about 3 mm, consistent with experimental observations, and the residual stress field shows thickness-dependent variation.
• The coupled deformation-diffusion-fracture framework captures hydrogen trapping and diffusion effects, enabling prediction of crack initiation and growth under hydrogen exposure.
• Weld defects commonly present in natural gas pipelines (porosity, lack of penetration, lack of fusion, root contraction, undercutting) have significant detrimental effects on structural integrity in hydrogen environments.
• The model demonstrates good agreement with laboratory fracture experiments across BM, WM, and HAZ, and can estimate critical failure pressures for pipelines with defects.