  The introduction of nanostructured interlayers is one of the most promising
strategies for interlaminar reinforcement in structural composites. In this
work, we study the failure mechanism and interlayer microstructure of
aerospace-grade structural composites reinforced with thin veils of carbon
nanotube produced using an industrialised spinning process. Samples of
unidirectional carbon fibre/epoxy matrix composites interleaved with different
composition CNT veils were prepared using hot press method and tested for
interlaminar fracture toughness (IFT), measured in Mode-I (opening) and Mode-II
(in-plane shear), and for interlaminar shear strength (ILSS), evaluated by the
short beam shear (SBS) test. The crack propagation mode could be directly
determined through fractography analysis by electron microscopy and resin/CNT
spatial discrimination by Raman spectroscopy, showing a clear correlation
between interlaminar reinforcement and the balance between cohesive/adhesive
failure mode at the interlayer region. Composites with full resin infiltration
of the CNT veils give a large increase of Mode II IFT (88%) to 1500 J/m2 and a
slight enhancement of apparent interlaminar shear strength (6.5%), but a
decrease of Mode I IFT (-21%). The results help establish the role of
interlayer infiltration, interlaminar crossings and formation of a carbon fibre
bridgings, for interlaminar reinforcement with interleaves.
