The decarbonization of aviation is legally required worldwide by the Paris Agreement, in the EU by the Green Deal and in Germany by the Climate Change Act. Hydrogen (H₂) thereby takes a central position as a potential future energy storage medium. Due to the lower volumetric energy density compared to kerosene, the aim is to store H₂ in liquid form (LH₂) for long-haul aircrafts. Metallic and carbon fiber reinforced plastic (CFRP) composite structures are currently competing as possible materials storing LH₂, with the latter being predicted to have a significant weight advantage. However, CFRP has disadvantageous properties with regard to its impermeability.

In order to prove the impermeability of a CFRP structure to H₂ under load, uniaxial or, in rarer cases, biaxial tests are currently carried out on simple reference specimens. These tests are performed under cryogenic conditions and, in a subsequent step, microcracks are detected, e.g. by micrographs or CT images, or leak tests are carried out. As the thermal and mechanical loading is performed separately from the subsequent analysis, an unrepresentative material behavior can occur, such as the closure of microcracks. This can lead to inaccurate or non-conservative results.

A bulge test rig is introduced to close this research gap by applying a multi-axial stress state, through pressure and temperature, to a clamped, circular and spherically curved test specimen while simultaneously monitoring the tightness via a leak detector. The test stand is operated with helium as a tracer gas for leak detection and liquid nitrogen for cooling. The practical tests are backed up by supporting FE simulations of the test specimen. These are necessary, among other things, to determine the representative area of the test specimen with regard to a uniform stress distribution, as the clamping distorts the real conditions on the tank.

The results are intended to help with material screening, to validate simulation methods and to contribute to a better understanding at material level of the leakage behavior of CFRP under realistic conditions. Additionally, the findings should minimize costs and risks of more resource-intensive full-scale test procedures. In later expansion stages, more complex test specimens with feed-throughs and fatigue tests are also possible.