Gas permeability of three-dimensional stitched carbon/epoxy composites for cryogenic applications

3D through-the-thickness stitching can be used to improve interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, the effects of cryogenic cycling on the gas permeation of three-dimensional stitched PMCs are investigated. Stitched and unstitched carbon/epoxy composites of standard modulus (237 GPa) and intermediate modulus (290 GPa) carbon fibers were thermally cycled and the helium gas permeability was measured at selected cryogenic cycles. Results show that stitching increases gas permeability in PMCs, with the standard modulus stitched composites having approximately twice the permeability of the intermediate modulus stitched specimens. Optical microscopy images show that cracks develop in the resin rich areas around the stitching seams. These internal cracks produce the increase in the midspan microcrack density that leads to the increase in permeability of stitched composites.