Scale matters: A perspective on structural hierarchical carbon fibre composites incorporating carbon nanotubes

Composites have long played a vital role in material science due to their lightweight, stiff, strong, and durable construction. Composites consist of at least two complementary materials, typically comprising reinforcing elements, prominently carbon or glass fibres, held in place by a surrounding polymer matrix. Conventional fibre composites already display a structural hierarchy from fibres within tows, to plies, to laminates forming large-scale structures. The term “hierarchical composites” specifically refers to materials that integrate reinforcements spanning additional length scales, down to the molecular range, most notably nanoscale reinforcements that complement microscale fibres. Natural structural materials rely extensively on hierarchical motifs to maximise performance, though using constituents limited by abundance and ambient aqueous processing. Technical hierarchical composites are broadly inspired by natural multiscale systems, sometimes implementing specific mechanisms from nature in new material classes. In hierarchical composites, the largest reinforcement, fibres, dominate in-plane mechanical properties. In contrast, nanoscale reinforcements may address matrix-dominated responses by, for example, improving shear properties that control stress transfer and kink band initiation, introducing additional toughening mechanisms to limit debonding or delamination, and providing direct reinforcement, particularly through-thickness. Nanomaterials can provide other benefits, such as improved fatigue life, acoustic damping, and solvent/fire resistance. The addition of nanomaterials may also imbue composites with multifunctionality, obviating other constituents or components and reducing system weight. We critically discuss the progress in developing hierarchical fibre reinforced carbon nanotube composites over the past decade and provide insight into manufacturing and their structural and functional performance.