Synergistic toughening on CFRP via in-depth stitched CNTs

Efficient toughening of the interlaminar fracture toughness for CFRP composites without sacrificing in-plane mechanical performance remained an unresolved challenge. Here, we present a synergistic toughening strategy by construction of hierarchical architecture, within which carbon nanotubes (CNTs) in-depth stitched into the nano-channel between neighboring carbon fibers at the interlaminar region. Mode I fracture test revealed that, even at low concentration of CNTs (0.3 wt%), a considerable improvement (50%) on mode I fracture energy G_Ic of composites can be realized, from 1098.6 to 1647.8 J/m² which is nearly two times greater than that of most aerospace CFRP laminates (∼500 J/m²). The excellent fracture toughness is predominantly attributed to the desired hierarchical architecture, which simultaneously triggered the intrinsic toughening by CNTs nano-bridging and extrinsic toughening mechanisms due to carbon fiber bridging, as was demonstrated by SEM images of fracture surfaces and further verified by finite element simulations. These findings offer significant guidelines for designing CFRP composites with high fracture toughness by application of low content CNTs using cost-effective resin mixing process.