Multi-scale hierarchical carbon nanotube fiber reinforced composites towards enhancement of axial/transverse strength and fracture toughness

Achieving simultaneous enhancement of multiple mechanical properties in fiber reinforced polymer composites is a difficult challenge. In this work, unidirectional carbon nanotube (CNT) fiber reinforced polymer composites have been fabricated that exhibit improvement in not only axial and transverse strength, but also Mode I fracture toughness through multi-scale hierarchical structures. The multi-scale hierarchical microstructures in CNT composites are the result of pre-infiltrating polymer resin into the loosely connected CNT networks of the roving material to strengthen the bundle networks at the nano- and micro-scale. At the macro-scale, the polymer pre-infiltrated CNT fibers were laid up in a ‘brick-and-mortar’ pattern to produce an additional level of structural interlocking in the composite. The resulting multi-scale load transfer pathways combine to increase the deflection of crack paths, the distribution of damage ahead of the crack, and the dissipation of energy during deformation. Crack-bridging toughening mechanisms operating at multiple structural scales are responsible for the improved properties in the CNT composites. The measured specific axial tensile strength of the unidirectional CNT fiber composite was 0.85 GPa/(g/cm3), which was approximately 120% of the starting CNT material {0.71 N/tex [numerically equivalent to 0.71 GPa/(g/cm3)]}. The transverse tensile strength and Mode I fracture toughness, measured at a constant crack propagation, were 81.3 MPa/(g/cm3) and 0.670 kJ/m2, which are promising values for structural composite applications.