Limiting the permeability of composites for cryogenic applications

The correlation between thermal fatigue-generated networks of transverse micro-cracks and the permeability of carbon fiber polymer matrix composites (PMCs) was the focal point of this effort. Epoxy and bismaleimide matrix PMCs were cycled between liquid nitrogen temperature (−196 °C) and elevated temperature. The samples were then optically examined for micro-cracks and evaluated for their tendency for leakage in a fluid containment application. Composite coupons were tested for the rate of flow of low-pressure helium gas in the through-thickness direction at room temperature (RT) and liquid nitrogen (LN2) temperature as a function of the applied cycles. The choice of lay-up drastically influenced the permeability. Samples with [0/90]2S and [0/90/45/−45]S lay-ups resisted leakage at least 100 cycles more than the [0/45/−45/90]S samples. The temperature of the elevated portion of the thermal cycle also had a large impact on the initiation and propagation of transverse cracks and the eventual creation of complete through-thickness gas flow paths that were the source of permeability. For example, for the [0/45/−45/90]S lay-up of IM7/5250-4 a relatively moderate 57 °C reduction in the elevated hold temperature decreased permeability by up to 100 times.