Mechanical enhancement of woven composites with radially aligned carbon nanotubes (CNTs) : investigation of Mode I fracture toughness

Abstract

Composites have seen an increasing role in aerospace structures that demand lightweight, strong, and stiff materials. Composites are attractive structural materials with outstanding mechanical and physical properties, as well as directional fabrication control and tailorability, though these advantages come with increased complexity and challenging failure modes. Matrix-rich regions at ply interfaces especially are susceptible to damage and matrix cracking, leading to delamination and a reduction of mechanical properties. Several manufacturing solutions such as stitching, z-pinning, and braiding have been developed by the aerospace industry to provide through-thickness reinforcement and improve interlaminar properties, though these improvements come with concomitant reductions in important in-plane properties. This thesis describes the design, manufacturing, and testing of woven composites with aligned carbon nanotubes (CNTs) integrated into the bulk composite, focusing here particularly on interlaminar reinforcement at ply interfaces. Implementing aligned CNTs takes advantage of their scale and superior specific stiffness and strength, with in-plane properties maintained while interlaminar properties are enhanced by the CNTs bridging across matrix-rich interfaces. Significant improvement in Mode I fracture toughness is observed experimentally with over 60% increase in both initiation and steady-state Mode I fracture toughnesses (steady-state toughness improves from 2.1 to 3.7 kJ/m² ). This enhancement is attributed to CNT crackbridging and pullout, in agreement with a first-order model, confirming the viability of CNTs to improve composite interlaminar properties. Future work to follow this thesis will focus on development of a vacuum-assisted infusion manufacturing process implementation of the 'fuzzy'-fiber reinforced nano-engineered composite architecture with alternate fiber and polymer systems, and exploring multifunctional applications of these materials.