Influence of hollow specimen geometry and high-pressure hydrogen on tensile properties for AISI 304 stainless steel

Tensile tests using hollow specimens filled with high-pressure hydrogen enable measurement of tensile properties over a wide range of pressures and temperatures, but possible geometry effects are not fully understood. This study investigates hydrogen embrittlement in hollow tensile specimens of stainless steel AISI 304 with constant gauge length and varying inner and outer diameters. Slow strain rate tests were performed in hydrogen and, for reference, in nitrogen, with three tests per geometry. Specimens were mainly manufactured by deep hole drilling, with one batch by conventional HSS drilling to assess machining effects. Electron backscatter diffraction and surface roughness measurements were used to characterize the inner surface. The tensile properties show only a slight dependence on the ratio of inner to outer diameter, within the range of experimental scatter, for normal specimen sized specimens, meaning neither using sub-sized nor oversized geometries. This dependence could only be resolved through repetitive testing. A mechanism based on crack initiation is proposed. Finally, recommendations for preferred geometry for reliable testing are given. The global warming caused by fossil fuel use threatens the long-term habitability of the Earth. Transitioning to low-carbon energy systems can reduce greenhouse gas emissions while securing global energy supply. Hydrogen is a key option for storing and transporting energy from fluctuating wind and solar power. Safe and efficient use of high-pressure hydrogen in energy and transport systems requires reliable mechanical property data under representative gas conditions. The hollow specimen technique is a promising alternative to autoclave testing, but the effect of specimen geometry on tensile test results has not been systematically studied. This work provides a systematic experimental investigation of how hollow specimen geometry and high-pressure hydrogen jointly affect the tensile properties of AISI 304 stainless steel, using slow strain rate tests on specimens with constant gauge length and varying inner and outer diameters. It advances understanding of hydrogen embrittlement mechanisms, quantifies geometry effects on tensile properties, and provides technical input for the ongoing revision of ISO 7039. By improving the reliability and comparability of mechanical data in high-pressure hydrogen, this study supports safer component design and the broader deployment of hydrogen technologies.