Mode II interlaminar fracture toughness of carbon nanotubes/epoxy film-interleaved carbon fiber composites

In recent years, the demand for composite materials in various industries such as aerospace, defense, and automotive industries has increased significantly. Furthermore, the mechanical properties of carbon fiber-reinforced plastic (CFRP) composites have been improved significantly. However, the out-of-plane (thickness direction) mechanical properties of CFRP laminates are considerably inferior to their in-plane mechanical properties. Owing to their unique properties, carbon nanotubes (CNTs) have been used to improve the mechanical properties of CFRP composites. However, it is challenging to disperse high concentrations of CNTs in the polymer resin matrices of such composites. In this study, a high concentration of CNTs was incorporated in CFRP laminates in the form of CNT/epoxy films by using ultrasonication and three-roll milling. Furthermore, unidirectional and plain woven prepregs were used for reinforcement. The mode II interlaminar fracture toughnesses of the CNT/epoxy film-interleaved CFRP composites with different CNT concentrations were measured by carrying out their end-notched flexure (ENF) tests. Optical micrographs were obtained to observe the microstructures of the specimens with different CNT loadings. The fracture surfaces obtained after the ENF tests were examined using a scanning electron microscope to investigate the toughening mechanism of the composites.