Concentration-Dependent Interlaminar Fracture Toughening in Carbon, Glass, and Hybrid Fiber-Reinforced Polymer Laminates via MWCNT Interlayers

Fiber-reinforced polymer (FRP) laminates are prone to delamination because they lack continuous through-thickness reinforcement. Although multi-walled carbon nanotube (MWCNT) interlayers can improve interlaminar resistance, evidence has focused mainly on unidirectional carbon fiber-reinforced polymer (CFRP) and individual fracture modes. This study compared Mode I propagation and Mode II initiation toughness in CFRP, glass fiber-reinforced polymer (GFRP), and two carbon/glass hybrid laminates (HCRP and HGRP). Each system included unmodified (ORI), neat-epoxy interlayer (CON), and 0.1, 0.3, and 0.5 wt.% MWCNT conditions. The laminates were fabricated by N,N-dimethylformamide-assisted ultrasonication and hot pressing, and examined by ASTM D5528 double cantilever beam testing, ASTM D7905 end-notched flexure testing, and scanning electron microscopy. ORI and CON exhibited comparable toughness, indicating little contribution from the neat-epoxy interlayer alone. In all four systems, both toughness measures increased from 0.1 to 0.3 wt.% and decreased at 0.5 wt.%. At 0.3 wt.%, steady-state Mode I toughness increased by 31.2–51.2% and Mode II initiation toughness increased by 4.0–23.1% relative to CON. GFRP showed the highest absolute Mode I toughness, HCRP showed the largest Mode II improvement, and HGRP exhibited limited Mode II sensitivity. Matrix tearing, shear cusps, tortuous crack features, and distributed MWCNT pull-out at 0.3 wt.% contrasted with smoother regions, bundled or agglomerate-like structures, and cavities or pores at 0.5 wt.%. The effective MWCNT content therefore reflects a balance between nanotube-mediated energy dissipation and defect formation, modulated by fiber architecture and fracture mode.