Co-authors​ :

 Ou YUNFU (CHINA), Wu LONGQIANG , Mao DONGSHENG  

Although fiber-reinforced polymer (FRP) composites are widely used in structural components because of their outstanding mechanical properties, delamination between fabric plies remains a well-known Achilles heel, weakening the laminated structures and limiting their more widespread applications. This study proposed to improve the interlaminar properties of FRP laminates via hierarchical reinforcement by combining CNT powders and fiber veils. Specifically, The CNT powders with an average length of less than 1 micrometer were adopted for toughening the epoxy matrix, which effectively avoids the "self-filtration effect" commonly observed in the Vacuum Assisted Resin Transfer Molding (VARTM) process, enabling the CNTs to penetrate the tiny gaps between carbon fibers and achieve uniform dispersion in the composites. Additionally, an ultrathin CNT fiber veil, approximately 100 nm thick, was interleaved as the toughening layer, offering the opportunity to strengthen the interlaminar connection with minimal weight penalty and effectively avoid reductions in the in-plane properties of the laminates. The results showed that the Mode I interlaminar fracture toughness (GIC) of laminate increased from 636 J/m2 to 840 J/m2 (increased by 32 %) when directly interleaving an ultrathin CNT veil (∼100 nm), while only incorporating 0.5 wt% CNT Powders in the matrix raised the GIC value to 1031 J/m2, enhanced by 62 %. Surprisingly, a significant improvement of GIC from 636 J/m2 to 1418 J/m2 (as much as 123 %) was achieved with the combination of both interleaving and matrix modification methods. This indicates a synergistic toughening effect in the composite whereby the multi-scale constituents induce a higher fracture resistance than the sum contribution of the individual constituents (123 % > 32 % + 62 %). The figure of merit for interlaminar reinforcement, considering changes in interlaminar properties normalized by interleaf thickness and ply thickness, reached an impressive value of 2460, surpassing the current state of the art in this field. Overall, this work suggests a highly effective strategy for enhancing the interlaminar properties of CFRP composites, opening up possibilities for broader applications of these materials in various structural components.