Hollow specimen method for mechanical testing in gaseous hydrogen

Hydrogen embrittlement remains a problem in many applications. Rocket engines that use hydrogen as a propellant have small margins, operating in harsh conditions with weight-minimizing design requirements, and additive manufacturing is increasingly used to meet these goals. Rapid, inexpensive testing methods for assessing mechanical properties in gaseous hydrogen are needed to characterize additively manufactured alloys. A hollow specimen geometry has been used for evaluating these properties, and a standard was published in 2024. Research using autoclaves indicates that surface condition can influence results. The hollow specimen has an inner surface that is difficult to polish. Studies show that this geometry gives higher relative reductions of area in slow strain rate testing compared to solid specimens in hydrogen. This paper provides an overview of the benefits, drawbacks, status, and future developments of this method. Based on a literature review, it discusses the history, its use in other fields, manufacturing methods, surface condition, gas purity, and comparisons with solid specimens. The method requires attention to inner surface condition as it can influence results when ranking materials based on embrittlement indicators. Current research indicates it is potentially suitable for quasi-static tensile testing, providing a hydrogen embrittlement index based on ductility. Further work is needed to refine testing parameters, investigate reproducibility, and assess its suitability for fatigue testing in gaseous hydrogen.