Hollow specimen method for tensile tests in high-pressure hydrogen gas: The role of strain rate and surface quality

Abstract The hollow specimen method enables materials evaluation in high-pressure hydrogen gas, however, open questions concerning specimen geometry, manufacturing, and gas quality have prevented its standardization for hydrogen applications. This work assesses how inner-hole surface quality and strain rate govern the hydrogen-induced degradation of metastable austenitic stainless steel AISI 304. Inner holes were produced by conventional drilling and by electrical discharge machining followed by honing, and characterized by roughness, microstructure, and residual stress. Slow strain rate tensile tests were performed in 15 MPa argon and hydrogen at strain rates from 1E−2 to 1E−6 s −1 . Both production methods showed the same trends in elongation at leakage and reduction in area, with drilled specimens considerably more degraded. Degradation increased with decreasing strain rate up to a plateau at 1E−4 s −1 . Diffusion-length estimates and fractography indicate that the degradation is governed by hydrogen-assisted crack initiation at the inner surface rather than bulk hydrogen transport. Impact statement Replacing fossil fuels with hydrogen requires that the metals used in pipelines, storage vessels, and refueling systems be qualified in the gas they will carry. Such testing is currently performed in autoclaves that consume large volumes of high-pressure hydrogen, demand extensive safety infrastructure, and are available to few laboratories worldwide. The hollow specimen method removes that barrier by confining the gas inside the specimen itself, therefore reducing significantly the amount of hydrogen used. The method was standardized, resulting in the ISO 7039:2024, however three questions raised during the standardization process, concerning specimen geometry, inner-hole manufacturing, and gas quality, have kept it from being a standard tailored to hydrogen testing. This work addresses the influence of the manufacturing process on the inner-hole surface of hollow specimens and its response to hydrogen effects. Damage begins at the machined inner surface rather than building up as hydrogen works its way into the bulk. As a result, the same steel can appear highly susceptible or barely affected depending only on how that surface was produced. Inner-surface quality must therefore be specified and reported alongside hollow specimen results. This is directly relevant to the ongoing revision of the ISO 7039:2024 for hydrogen testing.