Fracture Toughness and Impact Damage Resistance of Nanoreinforced Carbon/Epoxy Composites

In this study, the objective was to develop, manufacture, and test hybrid nano/microcomposites with a nanoparticle reinforced matrix and demonstrate improvements to damage tolerance properties in the form of Mode-II fracture toughness and related impact damage absorption. The material employed was a woven carbon fiber/epoxy composite, with multi-wall carbon nanotubes (CNT) as a nano-scale reinforcement to the epoxy matrix. A direct-mixing process, aided by a block copolymer dispersant and sonication, was employed to produce the nanoparticle-filled epoxy matrix used in composite fabrication. Composite samples were tested as End Notched Flexure (ENF) specimens in three point bending to determine the static Mode-II fracture toughness, showing improvement of approx. 30 % for nano-reinforced composite over reference material. Certain testing and material difficulties were noted with useful implications for both the testing technique as applied to woven composite materials and the material properties of the nano-reinforced composite. Impact tests were then performed in a falling-weight drop tower to generate delamination damage in samples of hybrid and reference composite. Impact damaged specimens were imaged by ultrasonic c-scans to assess the size and internal geometry of the damage zone, showing a consistently smaller mean damage zone diameter (approx 15 %) for hybrid composite over reference material. This translated to a nominally higher Mode-II fracture toughness in the hybrid composite (approx 30 %) regardless of specific impact energy, agreeing with static Mode-II fracture toughness tests.