Loading-rate and waveform effects in hydrogen-assisted fracture and fatigue are commonly interpreted in terms of hydrogen transport to the crack tip, but direct measurements of time-dependent crack opening under sustained loading in gaseous hydrogen remain scarce. X65 single-edge-notched tension specimens were tested in air and 40 MPa hydrogen under monotonic loading with and without a 168 h peak-load hold and under cyclic loading with triangular and trapezoidal waveforms. Under monotonic loading at an initial Kmax of 44.2 MPa·m1/2, crack-mouth opening displacement (CMOD) changed little during the hold in air, whereas in hydrogen the CMOD response diverged from an independently measured no-load drift trend. Post-test residual crack-opening displacement (residual COD) was also greater under the hold condition than under the no-hold condition in hydrogen. In the hydrogen hold specimen, no side-surface crack advance was detected optically, while scanning electron microscopy revealed pronounced blunting and a squared-off crack-tip profile. Under cyclic loading at an initial Kmax of 42.5 MPa·m1/2, differences in CMOD and residual COD between the triangular and trapezoidal waveforms were small in air. In hydrogen, however, trapezoidal loading with a 300 s peak-load hold in each cycle showed markedly greater CMOD accumulation and residual COD than did triangular loading. These results indicate a hydrogen-enhanced, hold-time-dependent crack-tip deformation response, referred to here as creep-like deformation. The time spent at peak load should therefore be considered when interpreting fracture and fatigue tests in gaseous hydrogen.

The influence of hydrogen on fatigue crack growth (FCG) was investigated in 90 MPa hydrogen gas using a 3Mn0.2C martensitic steel with a tensile strength of 1500 MPa. Surprisingly, this steel exhibited significant crack closure in hydrogen gas under specific loading conditions, resulting in considerably lower FCG rates compared to those in air. The roughness- and plasticity-induced crack closure markedly retarded the FCG in hydrogen in a relatively low stress intensity factor regime. However, intrinsic FCG resistance, excluding the effect of crack closure, was indeed degraded by hydrogen. Further analysis revealed that the reduction in cohesive strength of block boundaries depended on the plastic zone size and the inclination angle between the longitudinal axis of the blocks and the macroscopic FCG direction. Consequently, hydrogen-enhanced plasticity-mediated decohesion led to cycle-dependent acceleration of FCG, even in such high-strength steel under high-pressure hydrogen environments.

Understanding and predicting mechanical properties such as proof stress (yield strength), ultimate tensile strength, elongation to failure, and reduction in area are essential for screening the application of austenitic stainless steels in adverse chemomechanical environments. However, experimental determination of these properties is time-consuming, labor-intensive, and costly—especially under extreme conditions, which requires advanced experimental capabilities. In this study, we leverage a large-scale, curated dataset comprising 2180 experimental entries of austenitic alloys to develop machine learning (ML) models capable of predicting these key mechanical properties as functions of composition, solution treatment condition, and testing temperature. We systematically evaluate a range of ML algorithms, namely linear regression, kernel ridge regression, extreme gradient boosting, and artificial neural network. Among these, the extreme gradient boosting achieves the highest predictive accuracy, with R2 scores of 0.946 and 0.985 for proof stress and ultimate tensile strength, respectively. To further enhance model performance, we explore ensemble learning and transfer learning strategies. The transfer learning approach that leverages interdependencies between mechanical properties reduces the mean percentage error by 21.0% and increases R2 score by 3.1% in predicting reduction in area, compared to the original artificial neural network model. Our results show that ML models trained on well-structured experimental data can serve as an efficient and reliable tool for exploring the effect of composition on screening metrics. This work highlights the potential of data-driven approaches to accelerate the design and optimization of high-performance austenitic alloys. Furthermore, obtaining feature importance and insights from extreme gradient boosting using Shapley Additive Explanation tool provides valuable understanding of how features contribute to the prediction of mechanical properties.

Environment-assisted, time-dependent subcritical cracking is a significant concern in high-pressure storage systems and pipelines, as minor flaws can evolve into critical failures under sustained stress over time. Moreover, hydrogen gas storage facilities are known to experience pressure and temperature fluctuations during pressurization and depressurization. In this study, the impact of such fluctuating conditions on hydrogen-induced delayed fracture was investigated. Constant-displacement bolt-load tests were conducted using low-alloy steel JIS-SCM435H. Threshold stress intensity factors for crack arrest, KTHa, were measured in high-pressure hydrogen gas under both constant and fluctuating pressure and temperature conditions. For tests conducted in constant environments, KTHa values increased at high temperatures compared with room temperature, which may be associated with lower hydrogen trap-site occupancy at elevated temperatures. Furthermore, under temperature and pressure fluctuations, KTHa values appeared to correlate with the hydrogen trap-site occupancy, which in turn was likely affected by the instantaneous combination of temperature and pressure. Consequently, it was revealed that the mechanical state at the crack tip, potentially influenced by yield strength and hydrogen occupancy, could govern the subcritical crack propagation behavior in the material. This study provides valuable insights for mitigating hydrogen embrittlement risks in practical applications and offers a comprehensive understanding of how environmental fluctuations can influence material integrity in hydrogen storage systems.

This study investigates the safety, hydrogen compatibility, and fatigue performance of thick-walled Type 2 pressure vessels for high-pressure hydrogen storage up to 93 MPa. The vessels employ Cr-Mo and Ni-Cr-Mo steel liners (JIS-SCM435 and JIS-SNCM439) that are hoop-wrapped with carbon-fiber-reinforced plastic (CFRP). Seamless steel pipes for liners exceeding 50 mm in thickness were manufactured using two distinct quenching processes, and their mechanical properties were evaluated through hardness measurements, microstructural observations, and Charpy impact testing. To characterize hydrogen-assisted degradation, slow strain rate tensile (SSRT) tests and fatigue crack growth (FCG) tests were carried out under high-pressure hydrogen gas, demonstrating reduced ductility and up to a 30-fold increase in crack growth rates relative to air. Small-scale vessels containing artificial inner-surface defects were subjected to pressure-cycling tests between 35 and 93 MPa in both hydraulic fluid and hydrogen environments. Stress analysis considering liner–composite interaction and FCG-based life prediction in accordance with ASME Section VIII, Division 3 were conducted. The analytical predictions successfully reproduced the experimentally observed Leak-Before-Break (LBB) behavior and the number of cycles to leakage when hydrogen-specific FCG data were applied. Overall, the combined experimental and analytical investigations clarify the conditions necessary to achieve hydrogen-compatible microstructures in thick-walled liners, highlight the influence of quenching route and alloy composition on embrittlement resistance, and demonstrate that LBB-based assessment is applicable to Type 2 steel-liner pressure vessels in a high-pressure hydrogen environment. These findings support material selection, heat-treatment optimization, and fracture-mechanics-based structural integrity design for ultra-high-pressure hydrogen storage systems.

Interstitial hydrogen at grain boundaries (GBs) can significantly compromise material strength, leading to catastrophic intergranular fracture. However, the intricate interaction between hydrogen and GBs remains inadequately understood, particularly under complex external loading conditions. In this study, we use atomistic simulations and a geometrical algorithm to elucidate the hydrogen segregation energy spectrum at the GBs of polycrystalline nickel under various loading strategies. Three distinct peaks are identified in all spectra, with segregation energy decreasing under increasing tensile loading. Four types of loading-triaxial compression, uniaxial tension, uniaxial straining, and triaxial tension-are applied, with triaxial tension causing the most dramatic spectrum shift. Notably, a linear relationship between hydrogen segregation energy and local volume change is established for the first time. This relationship reveals that hydrogen solution is almost exclusively determined by local volume change, irrespective of the loading conditions. Uniquely in the spectrum of the uniaxial tension case, a fourth peak emerges, signifying a group of super-trapping sites formed through earlystage dislocation-GB interactions. These findings underscore the distinguishable impact of both elastic and plastic deformation on hydrogen distribution in polycrystals. Furthermore, hydrogen diffusion coefficients are derived through mean square displacement analysis, revealing the hydrogen diffusivity in the lattice and GBs under various loading conditions. This study provides critical insights into hydrogen embrittlement in polycrystalline materials, essential for developing more resilient hydrogen storage and transport systems.

Given the potential of hydrogen as an energy carrier in achieving carbon neutrality, assessing the fatigue crack growth rate behaviour of new and vintage pipeline materials exposed to hydrogen for new or repurposing of existing infrastructure is vital. Fatigue crack growth rate (FCGR) curves were established under in-situ electrochemical hydrogen charging at 1 Hz, observing up to 10 times acceleration compared to air. Constant Delta K testing at 11 MPa'm0.5and 18 MPa'm0.5 under varying frequencies (1 Hz, 0.1 Hz, and 0.01 Hz) was performed to investigate the frequency dependence of hydrogen accelerated FCGR. The acceleration factors (AF) were observed to vary significantly, with the modern steel exhibiting an AF of 6.3 at 0.01 Hz and Delta K of 11 MPa'm0.5 and 22.9 at Delta K of 18 MPa'm0.5. The vintage material showed AFs of 1.5 and 30 under the same conditions, respectively. The increase in AF was associated with a larger fraction of quasi-cleavage fracture. Electron channelling contrast imaging revealed evidence of plastic deformation even in the accelerated regime. The findings indicate notable differences in fatigue behaviour influenced by the microstructure and hydrogen environment, consistent with previous research and providing insights into the feasibility of repurposing existing pipelines for hydrogen transport considering existing FCGR design curves.

The effects and mechanisms of various alloying elements on the characteristics of Ni-based alloys have not yet been systematically investigated, despite the widespread application of such alloys in diverse domains. To address this gap, in this study, we investigated the effects of the substitutional alloying elements, specifically Fe, Cr, Mo, and Mn, on the lattice expansion, mechanical properties, hydrogen diffusivities, and solubilities of Ni. These elements led to both austenite lattice expansion and strengthening. We measured hydrogen diffusivities under high-pressure hydrogen environments (100 MPa) and desorption at constant temperatures. Notably, all the examined alloying elements reduced the hydrogen diffusivity of Ni in the order: Mn < Mo approximate to Fe < Cr. The effects of alloying cannot be simply explained by lattice expansion or strengthening but are attributed to increased activation energy for hydrogen diffusivity. We also assessed the hydrogen solubility through thermal desorption analysis (TDA) after exposure to highpressure hydrogen (100 MPa). Except for Fe, alloying elements increased hydrogen solubility in the order: Cr < Mo approximate to Mn. These effects are attributed to changes in the activation energy of hydrogen solubility. Additionally, TDA spectra for almost all the alloys, simulated based on the temperature dependence of hydrogen diffusivity, indicated that hydrogen diffusion through the face- centered cubic lattice remained unaffected by hydrogen trapping.

Transporting high-pressure gaseous hydrogen through the existing pipeline network - both subsea and onshore - is a promising strategy for cost-effectively expanding the hydrogen infrastructure, supporting the transition to sustainable fuels. However, repurposing pipelines to transport high-purity hydrogen poses challenges to material integrity, as hydrogen can accelerate fatigue crack growth in pipeline steels, especially under their typical operating conditions characterized by cyclic loading due to daily pressure fluctuations. Additionally, loading peaks, such as those from pressure testing, may further influence crack propagation. Despite these concerns, the effect of hydrogen on overload-affected fatigue behavior remains underexplored. This study examines the impact of hydrogen on fatigue crack acceleration in a vintage Norwegian X65 pipeline steel (originally installed in 1982), focusing on how overload (OL) conditions modify crack growth in three scenarios: (a) overload in air followed by FCG in air, (b) overload in highpressure hydrogen followed by FCG in the same environment, and ( c) overload in air at atmospheric pressure followed by FCG in high-pressure hydrogen. Preliminary results show that hydrogen significantly impacts the sudden acceleration in the FCG typically induced by the overload, but it does not seem to affect the overload-induced delayed retardation in the FCG. In any case, hydrogen leads to accelerated crack propagation and can potentially jeopardize material integrity. In cases where overloading occurs in air and is followed by FCG in hydrogen a scenario potentially similar to pressure testing before hydrogen injection in pipelines - a specific behavior in the hydrogeninduced crack acceleration was observed, underscoring the need for thorough safety considerations in pipeline repurposing.

The hydrogen compatibility of two X65 pipeline steels for transport of hydrogen gas is investigated through microstructural characterization, hydrogen permeation measurements and fracture mechanical testing. The investigated materials are a quenched and tempered pipeline steel with a fine-grained homogeneously distributed ferrite-bainite microstructure, and hot rolled pipeline steel with a ferrite-pearlite banded microstructure. All tests are performed both under electrochemical and gaseous hydrogen charging conditions. A correlation between electrochemical hydrogen charging and gaseous charging is determined. The results point to inherent differences in the interaction between hydrogen and the two material microstructures. Further research is needed to unveil the influence of material microstructure on hydrogen embrittlement.

To advance carbon neutrality, structural materials for high-pressure hydrogen environments must be designed based on fundamental principles. However, the atomic-scale complexity of random alloys hinders the development of interatomic potentials that can accurately reproduce hydrogen behavior influenced by alloying elements. This study develops a machine-learning interatomic potential (MLIP) for the Ni–Mn–H ternary system by efficiently sampling training data through an active learning strategy that combines atomic-force uncertainty and structural descriptors of diverse atomic environments. Molecular dynamics simulations employing the constructed MLIP quantitatively reproduce the experimentally observed non-monotonic dependence of the hydrogen diffusion coefficient on the Mn content. Two competing Mn-addition effects are found: increased and decreased activation energies from repulsive Mn–H interactions and lattice expansion, respectively, the balance of which shifts with the Mn content and governs the diffusion behavior. This approach enables accurate prediction of hydrogen diffusion in random alloys and provides atomic-level insights into alloying effects. Hydrogen-rich environments, such as in energy applications, can cause embrittlement of high-performance alloys, making it important to predict hydrogen behavior. Here, a machine-learning interatomic potential is reported for the Ni-X-H ternary system, and used to predict hydrogen diffusion in random alloys.

This study aimed to identify the microstructural factors governing the torsional fatigue strength of bainitic steels. Torsional fatigue tests were performed on two bainitic steels with banded microstructures comprised of soft and hard layers. The soft layers were coarse-grained with low Vickers hardness (HV), while the hard layers were finegrained with high HV. Both materials possessed similar average HV values but differing band morphologies: a coarse band (CB) with HV = 329 and a fine band (FB) with HV = 314. Interestingly, the FB exhibited a 30 % higher fatigue strength than the CB. Through microscopic observations and finite element analysis, it was established that different fatigue strengths could be attributed to the particular width and array of the bands. The reticular band array in the FB steel raises crack initiation resistance due to the constraint of cyclic plastic deformation. In addition, the narrower spacing of hard layers can impede crack propagation when the extension mode transitions from shear mode to Mode I. In contrast, the columnar array and wider spacing of the bands in the CB steel are likely to provide weaker resistance to crack initiation and propagation, resulting in an inferior fatigue strength.

This article presents a novel model for assessing the fracture toughness in hydrogen pipelines. By combining experimental fracture toughness data with recognized hydrogen uptake and diffusion models, the local hydrogen-affected fracture toughness across the wall thickness of a hydrogen pipeline can be determined. The model specifically accounts for hydrogen concentration gradients across the pipe wall thickness and the existence of surface barriers. Presented results demonstrate that embedded defects may possess higher fracture toughness values compared to inner surface flaws for the same operating pressure, reducing their criticality compared to that of inner surface defects. The model has thus the potential to reduce conservative selection of hydrogen-affected fracture toughness, yielding less restrictive flaw-acceptance criteria in pipeline integrity assessments. The approach can efficiently be applied in combination with various integrity assessment methods as a readily accessible and easy to use engineering tool for fracture analysis of hydrogen pipelines and pressure vessels.

Ausformed specimens of the chromium-molybdenum steel JIS-SCM440 were subjected to fatigue tests in both air and 90 MPa hydrogen gas. The results were compared with those of non-ausformed specimens of the same material with similar tensile strengths (approximate to 950 MPa and approximate to 1050 MPa). The ausformed materials demonstrated excellent resistance to hydrogen-induced acceleration of fatigue crack-growth (FCG), effectively reducing the crack propagation rate under cyclic loading in hydrogen environments compared to their non-ausformed counterparts. They maintained an acceleration ratio (i.e., relative FCG rate in hydrogen with respect to that in air) within 10 to 40 times, an order of magnitude lower than that of the non-ausformed counterparts. Despite their high strength levels (i.e., tensile strengths greater than 900 MPa), the FCG rate in the ausformed materials was almost independent of loading frequency at a stress intensity factor range of 20 and 30 MPa & sdot;m1/2. Fractographic observations revealed that no intergranular fracture occurred in the ausformed materials, unlike in the non-ausformed ones. These findings suggest that two factors possibly caused the mitigation of FCG rate in hydrogen: (i) modification of the microstructure morphology, i.e., refinement and elongation, and (ii) an increase in the cohesive strength of interfaces under the influence of hydrogen.

The global promotion of hydrogen refueling stations for fuel cell vehicles (FCV) is driven by the vision of achieving a Hydrogen Society. In Japan, a design pressure of 70 MPa has become the standard to enhance the cruise distance of FCV. However, this higher gas pressure increases the risk of hydrogen leakage from mechanical joints. Consequently, the infrastructure industry emphasizes the need to understand the hydrogen compatibility of welding joints to ensure the safer and more cost-effective operation of these facilities. This paper presents the results of slow strain rate tensile (SSRT) tests conducted in a high-pressure gaseous hydrogen environment to assess hydrogen embrittlement in austenitic stainless joints. During specimen preparation, U-grooves were machined at the edges of the plates, which were then butted together to form filler welded joints using Gas Tungsten Arc Welding (GTAW) with commercially available AWS ER 316L filler wire (high Ni-equivalent) under two different weld heat inputs (9.9 and 36.0 kJ/cm). The welded joints exhibited a low susceptibility to hydrogen embrittlement, despite the increase in the amount of crystallized delta ferrite in the weld metal with a decrease in the weld input. Based on the results, we conclude that the Type 316L welded joint with AWS ER 316L filler is suitable for various components in hydrogen refueling stations with the applied welding conditions. This valuable insight contributes to realizing safer and more reliable hydrogen refueling facilities while also reducing maintenance costs.