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Ultrathin CNT fiber veils (100–300 nm thick) were used as interleaves to improve the interlaminar fracture toughness of unidirectional carbon fiber reinforced polymer composites. These veils, drawn from vertically aligned CNT forests, were directly deposited at the mid-plane of CFRPs either longitudinally or transversely to the carbon fiber. The results revealed that triggering multiple toughening mechanisms is the key to achieve high toughness FRP composites. When the veils were oriented parallelly to the carbon fiber, the crack propagates in a "zig-zag" manner, not only interacting with more CNTs, but also triggering more carbon fiber bridgings, leading to a steeply "rising" R-curve. The superiority and effectiveness of this technique is exemplified by the extremely thin toughening layers (about 300 nm thick) needed to remarkably improve the propagation value of Mode I interlaminar fracture toughness of the CFRP by approximately 107%. The Mode I toughening factor (η I), defined as the change in Mode I interlaminar fracture toughness per interleaf-to-ply thickness ratio, comes out as high as 713, which is far outweigh the state-of-the-art. However, transversely-placed CNT veils will seriously block the formation of carbon fiber bridgings, remarkably degrading the Mode I interlaminar toughness of the laminates. [Display omitted] [ABSTRACT FROM AUTHOR]

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 fiber/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%). A toughening factor, defined as the relative increase in toughness per interleaf-to-ply thickness ratio, give a record-high value of 15.8 for Mode II IFT. The results help establish the role of interlayer infiltration, interlaminar crossings and formation of a carbon fiber bridgings, for interlaminar reinforcement with interleaves. [ABSTRACT FROM AUTHOR]