Preparation and Investigation of PA66 Veil/CNTs Multiscale Interlayer Toughened Carbon Fiber/Epoxy Composites with Improved Interlaminar and Mechanical Properties

Carbon fiber/epoxy (CF/EP) composites have poor interlaminar toughness due to the influence of layered structure characteristics and intrinsic brittleness of epoxy resin.To address this issue, carbon nanotubes (CNTs) with varying areal densities were sprayed onto carbon fiber fabrics, which were then combined with wet-laid polyamide 66 veils (PA66V) to fabricate CF/EP composites via the vacuum-assisted resin infusion (VARI) process. The Mode I ( G IC ) and Mode II ( G IIC ) interlaminar fracture toughness of both reference and toughened CF/EP composites were evaluated, while the corresponding toughening mechanisms and the influence on other mechanical properties were systematically investigated. A significant improvement in interlaminar fracture toughness was achieved. For the samples denoted as PV4CNTx, where the two numbers represent the areal densities of PA66V and CNTs (in g/m²), respectively, the variant with 4 g/m² PA66V and 2 g/m² CNTs (PV4CNT2) exhibited increases of 26.9% in G IC-initiation and 190.1% in G IIC . Its toughening mechanisms mainly involve bridging and pull-out of the PA66 veil, as well as plastic deformation and crack deflection induced by the CNTs. Among the three toughened samples, with PA66V fixed at 4 g/m² and CNT areal densities of 1, 2, and 3 g/m² (denoted as PV4CNT1, PV4CNT2, and PV4CNT3, respectively), PV4CNT3 exhibited the highest tensile strength and flexural modulus. Notably, its tensile strength was 9.2% lower than that of the untoughened sample, while its flexural modulus was 3.3% higher. PV4CNT2 showed the highest tensile modulus and interlaminar shear strength, with increases of 2.1% and 3.6%, respectively, compared to the untoughened sample. PV4CNT1 achieved the highest flexural strength, which was 2.3% higher than that of the untoughened sample.