Mode II fracture toughness and delamination mechanism of vitrimeric CFRP laminates toughened with interlaminar carbon nanotubes

The vitrimeric CFRP (vCFRP) composites have gained widespread interest due to the healable ability of vitrimer matrix after damage. However, the inherent nature of laminated composite structures leaves them vulnerable to delamination. Carbon nanotubes (CNTs) have been widely used as reinforcing materials in both thermosetting and thermoplastic composites due to their excellent properties, whereas their application in emerging vCFRP has rarely been reported. In this study, carboxyl-functionalized CNTs were introduced into the interlaminar region of vCFRP laminates to improve their delamination resistance. The influence of CNT loading on the mode II fracture toughness was subsequently evaluated on the basis of load-displacement responses, R-curves, crack growth characteristics, and fracture surface morphologies. The experimental results indicate that the addition of CNTs alters the crack propagation path and enhances interlaminar performance. Within the investigated CNT content range, the specimens with relatively low CNT loadings exhibit a reduction in mode II fracture toughness. As the CNT content continues to increase, the initial fracture toughness is progressively improved, whereas the steady-state fracture toughness first rises and then declines, with a peak increment of 33.47%. The fracture morphology indicates that CNTs restrain crack propagation and improve the delamination resistance of composites through CNT fracture, pull-out, and enhanced fiber bridging effect. These findings confirm the applicability of CNT interlaminar toughening strategies in self-healable vCFRP composites.