PSI - Issue 77

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Behzad V. Farahani et al./ Structural Integrity Procedia 00 (2026) 000–000

Behzad Vasheghani Farahani et al. / Procedia Structural Integrity 77 (2026) 424–431 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSI organizers

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Keywords: Hydrogen embrittlement; Hydrogen-Induced Damage; Heterogeneity;FEM

1. Introduction The transition towards sustainable energy systems has renewed interest in hydrogen as a clean energy carrier. When produced via renewable methods such as wind- or solar-powered electrolysis, green hydrogen offers a promising route to decarbonize transportation, industry, and power generation. However, adoption is limited by material degradation, notably hydrogen embrittlement (HE) (Campari et al., 2023). It compromises the structural integrity of metals and alloys in hydrogen-rich environments. HE is a complex process in which absorbed hydrogen atoms reduce ductility and toughness, potentially leading to early failure under stress. This phenomenon is particularly critical for high strength steels/alloys used in pipelines (Li et al., 2020). The occurrence of HE can be related to hydrogen-induced cracking (Pourazizi et al., 2020), void formation (Morrissey et al., 2019), and grain-boundary decohesion (Yamaguchi et al., 2025), all influenced by hydrogen concentration, microstructure, and applied stress. To mitigate these risks, predictive tools that accurately simulate material behavior in hydrogen environments are essential. The finite element method (FEM) has therefore become a key method to study the coupled phenomena of hydrogen diffusion, mechanical deformation, and fracture (Negi et al., 2024). Building on this discussion, pipeline steel remains a central focus due to the increasing use of hydrogen as an energy carrier. Welded pipelines introduce additional challenges: microstructural variations in the weld metal (WM) and heat-affected zones (HAZ) can expand susceptibility to localized damage. Evaluating their performance under hydrogen is thus critical for ensuring structural integrity and safe operation. As highlighted by (Zerbst et al., 2014), accounting for weld microstructure variations is essential for accurate assessment of weld defects. Fig. 1-a) presents a macrograph of a girth-welded (GW) joint showing distinct zones. The base metals (BM) (here, API 5L-X70 pipeline steel) are considered homogeneous, while the WM and HAZ are heterogeneous due to temperature-induced property variations during welding (S. Zhang et al., 2019). This heterogeneity significantly affects the material’s mechanical response and its susceptibility to hydrogen-assisted degradation. Understanding the heterogeneity of girth-welded joints is essential for studying HE, as BM, WM and HAZ may exhibit differences in mechanical behavior, hydrogen diffusion, and susceptibility to cracking. Numerical damage models must account for this heterogeneity to accurately predict the structural integrity of welded pipelines used for hydrogen transport (Tang et al., 2025). Accurate characterization of the material’s mechanical behavior is crucial for reliable numerical damage modelling, particularly in hydrogen simulations (Merheb et al., 2025). The flow stress–strain response serves as the fundamental input for these simulations and can be obtained using several approaches. Experimental tensile testing is commonly employed to characterize a material’s elastoplastic properties, including yield strength, ductility, and strain hardening. Alternatively, hardness measurements provide indirect insight into the material’s flow behaviour (Hertelé et al., 2014; Kollenberg, 1991). Among various methods, Vickers microhardness is widely used due to its applicability in small or localized regions such as WM and HAZ (Moore & Booth, 2015). By applying a controlled load with a diamond indenter, the indentation size correlates with material hardness, allowing evaluation of local heterogeneous mechanical properties in welded joints that are difficult to measure directly with conventional macroscopic tensile tests. Most numerical models for hydrogen embrittlement only consider homogeneous material response. In this work, a previously established Gurson-type damage model for hydrogen embrittlement (Depraetere et al., 2021) is extended to consider the heterogeneity in the girth-welded regions. Numerical damage simulations were performed on notched round bar (NRB) samples, with the notch positioned in the BM and the WM of girth-welded joints of X70 pipeline steel. The BM was characterized directly through tensile testing, while the WM and HAZ were characterized using Vickers hardness (HV) measurements, which were converted into flow stress/strain data. Simulations were conducted

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