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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1. Introduction Most metals suffer from degradation of mechanical properties in hydrogen environments, which was first described in the scientific literature 150 years ago (Johnson 1875). This degradation is often called hydrogen embrittlement (HE) and refers to the reduction of ductility or fracture toughness due to the influence of hydrogen (Lee 2016). It requires both the presence of hydrogen and of stress, causing localized cracking which can result in catastrophic failure (Lee 2016). It is a problem that remains relevant, especially as hydrogen has become interesting as a source of fossil-free energy to reduce carbon emissions (Ustolin et al. 2020). Applications where hydrogen embrittlement is relevant are found in aerospace, automotive, oil and gas (Briottet et al. 2019), power generation, and in production, transmission and storage of hydrogen gas. The conditions found in these applications vary, with temperatures ranging from cryogenic in hydrogen refuelling stations (Kobayashi et al. 2018) to incandescent in liquid propellant rocket engines (LPRE) (Jewett and Halchak 1991, Sutton 2003), and hydrogen gas pressures ranging from a hundred atmospheres in pipelines (Kanz et al. 2023) to a few hundred atmospheres in hydrogen refuelling stations and fuel cell vehicles (Kobayashi et al. 2018). Time is an important consideration too: a new hydrogen pipeline might have a service life of 50 years (Kanz et al. 2023), while a rocket engine is used for 10 minutes (Briottet et al. 2019). Material selection naturally depends on application conditions, involving for example stainless steel in hydrogen refuelling stations, fuel cell vehicles and pressurized water reactors in nuclear industry; nickel-base alloys in aerospace and sub-sea applications; ferritic-pearlitic steels in pipelines and high strength steels in automotive applications (Briottet et al. 2019). Hence, there is a wide range of materials that are subjected to conditions where they are susceptible to hydrogen embrittlement. Some of the earliest systematic testing campaigns to investigate the performance of metallic materials in gaseous hydrogen environments was done at the behest of National Aeronautics and Space Administration (NASA) in the USA in the 1960’s and 1970’s (Klima et al. 1962, Walter and Chandler 1969, Jewett et al. 1973), as part of the development of aircraft turbines and liquid propellant rocket engines using hydrogen (Klima et al. 1962), for example the RL10 and J-2 engines used in the Saturn V heavy-lift launch vehicle (Sutton 2003, Ha et al. 2023). These systematic testing campaigns focused on metals used in LPREs. Many of these were superalloys, owing to their ability to retain strength at high temperatures. A systematic testing survey performed by NASA under gaseous hydrogen conditions concluded that only a few stable austenitic stainless steels, aluminium alloys and copper alloys were resistant to hydrogen embrittlement while most superalloys were not (Jewett et al. 1973). For many alloys, susceptibility also depends on heat treatment or product form, e.g., cast or wrought (Jewett et al. 1973, Lee 2016) . Hydrogen has a high specific impulse, allowing for additional payload to be carried or higher orbits to be reached (Sutton 2003). It is still used in modern liquid rocket engines and many heavy-lift launch vehicles use hydrogen today (Ha et al. 2023), in the combination of liquid oxygen (LOX) and liquid hydrogen (LH 2 ) (Sutton 2003). Hydrogen is present both in liquid and gaseous form in LPREs at temperatures ranging from -253 to 760 ℃ and pressures up to 55 MPa (Jewett and Halchak 1991). Hydrogen environmental embrittlement (HEE) can occur under such conditions, as can internal hydrogen embrittlement (IHE) (Jewett and Halchak 1991) since the rocket engines are tested before launch, leading to possible charging of hydrogen into materials (Briottet et al. 2019). LPREs are made up of complex parts with many components consisting of assemblies of cast, forged and sheet parts that are joined together (Jewett and Halchak 1991). Components in LPREs must withstand harsh operating conditions while weight must be minimised (Jewett and Halchak 1991), resulting in small margins, making it important to understand material behaviour (Gradl et al. 2023). Additive manufacturing has been used to manufacture aerospace components as it allows for lead time reductions, cost savings, weight reductions and part consolidation by elimination of joints (Blakey-Milner et al. 2021), for example by multi-material builds (Gradl et al. 2022). It has also been used to develop novel alloys and to make conventionally manufactured alloys more affordable, for instance the hydrogen resistant NASA HR-1 alloy (Gradl et al. 2023). There is hence a need to understand material behaviour of additively manufactured components used in aerospace applications with gaseous hydrogen environments. Since most metals are susceptible to HE and such cracks can grow undetected, large safety margins are necessary in many applications (Yu et al. 2024). This requires large amounts of mechanical testing. There is thus a need for testing methods that can rapidly evaluate these materials in hydrogen environments under application conditions. Traditionally, in-situ mechanical testing in gaseous hydrogen has been done using pressure chambers filled with hydrogen gas with openings for rods that allow for the application of stress (Chandler et al. 1974). This time-tested method is unfortunately expensive (Vesely et al. 2002) as it requires technically complex machinery and safety