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Carbon fiber reinforced polymer composites have the advantages of being lightweight, having high strength and designability, and having been extensively used. However, the interlaminar toughness and delamination resistance of these composites are relatively poor due to their laminated structure and intrinsic brittleness of resin matrix. In this paper, commercialized free-standing carbon nanotube (CNT) films, drawn from CNT forests, were used to toughen the interlaminar interfaces of the composites. The effects of resin infiltration state and thickness of CNT films on the interlaminar toughening effect were systematically investigated. The results show that the pre-infiltration treatment of CNT films with acetone diluted epoxy resin solution can effectively improve the degree of resin infiltration. Compared with the samples containing untreated CNT film, the Mode I and Mode II interlaminar fracture toughness of the treated samples were significantly improved. The GIC reached a maximum of 1412.42 J/m2 at a CNT film thickness of 5 µm, which was about 61.38% higher than that of the baseline. At a CNT film thickness of 15 µm, the GIIC reached a maximum value of 983.73 J/m2, approximately 67.58% higher than that of the baseline. The corresponding toughening mechanisms were also systematically analyzed. [ABSTRACT FROM AUTHOR]

"Interleaving" is widely used for interlaminar toughening of fiber-reinforced composites, and the structure of interleaving is one of the important factors affecting the toughening efficiency of laminates. Several experiments have demonstrated that compared to continuous and dense structures, toughening layers with structural heterogeneity can trigger multiple toughening mechanisms and have better toughening effects. On this basis, this work further investigates the application of heterogeneous toughening phases in interlaminar toughening of bidirectional GFRP. CNT was selected to construct toughening phases, which was introduced into the interlaminar of composites through efficient spraying methods. By controlling the amount of CNT, various structures of CNT toughening layers were obtained. The fracture toughness of modified laminates was tested, and their toughening mechanism was analyzed based on fracture surface observation. The results indicate that the optimal CNT usage (0.5 gsm) can increase the initial and extended values of interlayer fracture toughness by 136.0% and 82.0%, respectively. The solvent acetone sprayed with CNT can dissolve and re-precipitate a portion of the sizing agent on the surface of the fibers, which improves the bonding of the fibers to the resin. More importantly, larger discrete particles are formed between the layers, guiding the cracks to deflect in the orientation of the toughened layer. This generates additional energy dissipation and ultimately presents an optimal toughening effect. [ABSTRACT FROM AUTHOR]

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]