Gas permeability mitigation of cryogenically cycled stitched composites using thin plies

Abstract

3D through-thickness stitched composites can be used to produce lightweight, unitized structures for cryogenic applications. However, to fully utilize stitched composites for these applications, cryogenic fuel leakage through transverse cracks in these structures must be prevented. In this study, thin plies (cured thickness ≈ 0.032 mm) are incorporated directly into three-dimensional stitched polymer matrix composites (PMCs) to reduce gas permeability and prevent through-thickness cracking. Stitched carbon/epoxy composites ([+45/−45]4s) with standard thickness plies were used as baseline laminates. Thin plies were added to the baseline design to fabricate three different laminates with one thin ply placed on (a) both laminate surfaces, (b) two plies from each laminate surface, and (c) at the laminate midplane. Test coupons were thermally cycled from ambient (23 °C) to cryogenic (−196 °C) temperatures. Gas permeability and ply-level microcrack densities were measured as a function of cryogenic cycles. Results show that both the presence and location of the thin plies impact the gas permeation characteristics of stitched composites. Thin plies were effective in resisting microcrack growth and acted as integral liners to prevent excessive gas flow through the stitched specimens.