LEAKLAB · TECHNICAL INSIGHTS
Helium tests must distinguish diffusion through intact polymer from leakage through a connected crack network.
An investigation by DLR, JAXA, AIST and TU Braunschweig examines gas transport through composites intended for cryogenic tanks. A mechanically sound carbon-fibre laminate can still transmit gas through its polymer matrix. In a double-wall hydrogen tank, gas entering the vacuum insulation can increase heat input.
Two transport mechanisms
Intact material transfers gas by dissolution and diffusion. Isolated microcracks do not necessarily create a through-path; a connected crack network can produce leakage governed by crack opening, pressure and temperature. A material permeability coefficient should not be used to describe all of these mechanisms interchangeably.
Measurement arrangement
A specimen separates a pressurised tracer-gas side from a vacuum side connected to a mass-spectrometer detector. Edge-seal leakage must be excluded. DLR’s room-temperature helium rig uses six cells: one is monitored continuously during the transient, then the remaining cells are sampled for steady flow and scatter.
For steady throughput QL, thickness d, area A and pressure difference Δp, the study uses P = QLd/(AΔpRT), with P in mol/(s·m·Pa). Convert units consistently: 1 mbar·L/s = 0.1 Pa·m³/s.
Temperature and waiting time
For the tested 1 mm cross-ply CFRP at 5 bar, the reported approach to a plateau ranged from about 8.5 hours at +81°C to 236 hours near −54°C. At −97.4°C, no change was detected during 228 hours; the fitted model indicated a much longer equilibration time. A short test with no signal does not prove zero permeability.
The reported helium-to-hydrogen permeability ratio of about 3.1 was measured on HDPE, not CFRP. It is not a universal conversion factor. Values extrapolated to 77 K or 20 K are model estimates, not direct measurements of a full tank.
Barrier layers and qualification
A 60 µm clay barrier layer reduced steady flow by a factor of 83 in the authors’ comparison, but delamination concerns mean that barrier performance and mechanical compatibility must be assessed together. Coupon measurements, thermal cycling, mechanical loading and full-component tests answer different questions. Hydrogen work requires a separately engineered gas-safety arrangement.
For a practical programme, define specimen geometry, gas, pressures, temperatures, conditioning time, edge-seal checks, background, reference response and the distinction between steady permeation and defect leakage.
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Research discussed in the Russian article: J. Koord and colleagues, 2026 (CC BY 4.0).