Mechanical behavior of advanced nano-laminates embedded with carbon nanotubes – a review

Abstract

Embedding carbon nanotubes (CNTs) in load-bearing composite laminate hosts to turn them into nano-laminates is a rapidly emerging field and has tremendous potential in enhancing the mechanical performance of the host laminates. This state-of-the-art review intends to provide a physical insight into the understanding of the enhancing mechanisms of the processed and controlled CNTs in the nano-laminates. It focuses on four aspects: (1) physical characteristics of CNTs, including CNT length, diameter, weight percentage and surface functionalization; (2) processing and control techniques of CNTs in the fabrication of nano-laminates, including distribution, dispersion and orientation controls of CNTs; (3) mechanical properties along with their testing methods, including tension, in-plane compression, in-plane and interlaminar shear (ILS), flexure, mode I and mode II fracture toughness as well as compression-after-impact (CAI), ballistic protection and fatigue; and (4) CNT–matrix load transfer and enhancing mechanisms along with a few major governing factors. The selective and uniform production of CNTs with specific dimensions and physical properties has yet to be achieved on a consistent basis. Moreover, the processing details of CNTs vary very significantly among different researchers so that the processed CNTs share little common characteristics. There is little control over the CNT orientations in most fabrication processes of the nano-laminates except for some cases associated with chemical vapor deposition (CVD). There are only two reports on in-plane compression and there is only one on in-plane shear. For reinforcement-dominated mechanical properties such as longitudinal tension and flexure, there was little enhancement reported. However, the substantial enhancement in in-plane compression strength was also reported. For matrix-dominated mechanical properties, such as transverse tension, in-plane shear, ILS strength and mode I and mode II fracture toughness, a significant enhancement, albeit with substantially varying degrees, was reported for ILS strength and mode I and mode II fracture toughness values. Meanwhile, the lack of consistent characterization of those properties was also noticeable. There is little established understanding of the enhancing mechanisms in nano-laminates.