Investigation of permeability and leakage in composite materials for application in cryogenic storage systems

Liquid hydrogen storage systems made from composite materials offer promising advantages for future lightweight aircraft. However, to ensure functionality and safety, these tanks must exhibit both low hydrogen permeability and high resistance to microcrack formation - especially under cryogenic conditions. Testing either of these parameters is not straightforward. In the present work, different test setups are employed to quantify permeability and leakage, and to differentiate between these two phenomena. Pragmatic approaches for elevated, room and low temperature permeability testing are presented, including a direct comparison of helium and hydrogen permeability, and permeability values down to cryogenic temperatures are provided. Permeability at 77 K and 20 K is estimated through regression-based extrapolation of experimental temperature-dependent permeability data. In addition, the effectiveness of clay-based barrier materials in further reducing gas permeation is quantified. Regarding leakage, the limitations of unidirectional thermo-mechanical loading with respect to crack network formation are demonstrated. By applying biaxial loading, crack networks are introduced and the temperature- and pressure-dependence of gas flow through these networks is analyzed. The presented methodology and results offer practical tools for material evaluation and support the design of composite structures for cryogenic hydrogen storage. • Permeability and leakage in CFRP are experimentally distinguished and quantified. • Helium and hydrogen permeability are directly compared. • Cryogenic permeability is estimated via regression-based extrapolation approach. • Biaxial loading induces through-thickness crack networks and leakage in CFRP. • Clay-based film barrier reduces helium flux by over 80x under steady-state conditions.