  The susceptibility to delamination is one of the main concerns in fiber
reinforced polymer composites (FRPs). This work demonstrates improvements of
60% in Mode-I fracture toughness after integration of thin (~30 micron),
continuous veils of carbon nanotubes (CNTs) directly deposited onto carbon
fiber fabric as the CNT are drawn from the gas-phase using a semi-industrial
process. A combination of optical imaging, scanning electron microscopy and a
Raman spectroscopy provide a new rapid tool to unambiguously determine the
crack propagation path by simple visual inspection of fracture surface. The
results show that interlaminar crossing between CNT veil/CF interfaces is of
paramount importance. The crack front alternatingly propagates above and below
the CNT-toughened interlayer, significantly improving the fracture toughness of
resultant laminates. This mechanism is strongly influenced by the method used
to integrate the veils onto the CF. CNT veils directly deposited onto the
fabrics as a low-density layer lead to large improvements in interlaminar
properties, whereas compact CNT veils densified by solvent exposure prior to
their integration in the lay-up act as defects.
