Interlaminar fracture toughness of composite laminates with CNT-enhanced nonwoven carbon tissue interleave

A nonwoven carbon tissue (NWCT), composed of conventional carbon fibers, was coated with carbon nanotubes (CNT) to compose a multi-scale reinforcing interleave layer which can be directly incorporated into the composite laminate layup process. This CNT-enhanced NWCT layer was found to significantly improve the Modes I and II interlaminar fracture toughness of a carbon fiber reinforced polymer (CFRP) laminate, as measured by double-cantilever-beam (DCB) and end-notched-flexure (ENF) tests. GIC and GIIC of the CNT-enhanced NWCT specimens were compared with baseline CFRP specimens having no interleave, and also specimens having NWCT interleave with no CNT. The mean GIC of the CNT-enhanced NWCT specimens was measured to be 353% higher than the CFRP specimens, a great improvement over the NWCT specimens which showed a 5% reduction relative to CFRP. The mean GIIC of the CNT-enhanced NWCT specimens was 246% higher than the CFRP specimens, which is a significant improvement over the 194% increase measured for the NWCT specimens. Increased GIIC was mainly achieved via crack bridging provided by the NWCT carbon fiber morphology. Additional improvement in GIIC, and the increase in GIC (rather than decrease) of the CNT-enhanced NWCT specimens was due to local CNT reinforcement of the resin near the NWCT fiber surfaces. Scanning electron microscopy shows evidence of improved resin-to-NWCT interface adhesion, short fiber breakage, and CNT pullout at the fracture surfaces.