PSI - Issue 68

Robert Sundström et al. / Procedia Structural Integrity 68 (2025) 1081–1090 Robert Sundström / Structural Integrity Procedia 00 (2025) 000–000

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The use of a hollow specimen for evaluation of hydrogen embrittlement appears to date as far back as the 1950’s, where it was used at the Chemical Engineering Department of Yale University in New Haven, Connecticut, USA (Dodge 1953). Specimens were drilled to have a 3.175 mm hole, and the hollow was pressurized with hydrogen gas while being held in a furnace at elevated temperature from 300 to 500 °C, at pressures from 1013.5 to 2027 bar and exposure durations of 4 to 10 weeks. The specimen was removed from the furnace and tested in a standard tensile testing machine under ambient conditions to determine ultimate tensile strength (UTS) and elongation. As such, this was a type of ex-situ testing using gaseous thermal charging rather than an in-situ test with hydrogen gas present during the mechanical test (Freitas et al. 2024). The hollow specimen was also used by subcontractors of NASA for low cycle fatigue (LCF) in-situ testing in gaseous hydrogen (Chandler et al. 1974). More recently, a hollow specimen has been used by some researchers to assess the mechanical properties in gaseous hydrogen mainly by slow strain rate tensile testing (Ogata 2008a, b, 2010, 2012, 2015, 2018, Ogata and Ono 2019, Ueno and Benjamin 2019, Boot et al. 2021b, Boot et al. 2021a, Michler et al. 2021, Michler et al. 2022, Faucon et al. 2023, Michler et al. 2023, Konert et al. 2024, Michler et al. 2024) and fatigue testing (Ogata 2018, Ebling et al. 2022) but has also been used in other environments to investigate influences from reactor water (Twite et al. 2016, Asada et al. 2017, Xiong et al. 2020) and liquid metal embrittlement (Ding et al. 2023). There was no standardised testing method for the hollow specimen until a standard (ISO 7039:2024) was published for slow strain rate testing using a gas-filled hollow specimen in 2024 (ISO 2024). The standard established some limits and recommendations on testing parameters, for example gas purity, strain rate, specimen dimensions and purging procedures. Previous work with testing in autoclaves has indicated higher variability for tests in gaseous hydrogen than air (Vesely et al. 2002). No records can be found of round-robin programs using hollow specimens with hydrogen gas in the published literature. Lacking that, judgments about reproducibility are yet to be made. To address this, a project in Germany is planning a round-robin program for the hollow specimen (Freitas et al. 2024). 2.2. Surface finish effects for solid specimens As the hollow specimen contains an inner surface that is difficult to polish, hole manufacturing and post-processing methods have been the focus of some studies to determine its influence on mechanical testing results. Comparisons between solid and hollow geometries have also been made. To begin this section, the results of surface conditions on solid specimens is briefly reviewed. Hole manufacturing methods for the hollow specimen and inner surface effects are then discussed, followed by a review of papers that have compared solid and hollow specimens. Gaseous hydrogen embrittlement depends on crack initiation and crack growth at the surface, so surface finish may influence embrittlement testing results (Lee 2012). A small influence of different ground surface conditions on unnotched bar strength and ductility of directionally solidified superalloy MAR-M246 in gaseous hydrogen tested in an autoclave using solid specimens has been reported (Fritzemeier and Chandler 1989), but the scatter of the data might also have been within normal variability. The presence of a surface oxide layer had a much stronger influence on embrittlement than the surface finish. For Inconel 718, one author (Lee 2012) compared turning, grinding and electric discharge machining (EDM) of solid specimens, and the turning process was varied using different turning rates and tools on notched specimens. The different surface conditions had a much larger influence on notch tensile strength (NTS) in gaseous hydrogen than helium in an autoclave, and NTS ratios ranged from 0.76 to 0.91. The ground surface had a ratio of 0.82 and EDM surface a ratio of 0.91, indicating less embrittlement when EDM was used. Tensile tests on metastable austenitic stainless steels permeated with hydrogen after cathodic charging have been done to investigate the influence of surface roughness and martensite transformation (Queiroga et al. 2019). The solid specimens were manufactured to have different surface roughness. Metastable austenitic stainless steels exhibit strain induced martensite formation which can take place during machining of specimens. They found that martensite formed in the surface layer of a metastable stainless steel for the fine surface condition (Ra = 0.18 µm), but not for the rough one (Ra = 16 µm) which had a faster rate of material removal during turning. Higher levels of lattice strain were found in the rough condition than the fine. Martensite and lattice strain in the surface layer both influenced hydrogen embrittlement susceptibility, but martensite had a stronger influence. There are effects on high cycle fatigue lives of low-alloy steel when testing solid specimens in gaseous hydrogen, and these effects depend on machining method: grinding, grinding followed by annealing, or polishing (Wada et al. 2007). These effects depend on both the roughness and the residual stresses induced by the different treatments.