Critical review of the factors dominating the fracture toughness of CNT reinforced polymer composites

In this study, the main factors, which have substantial influence on fracture toughness of carbon nanotubes (CNTs)-based composites both at nano and macro scales, are discussed in detail. Based on the findings from the reviewed literature, the improvement obtained on fracture toughness of CNTs multi-scale composites are due to the effect of CNTs on crack bridging and pullout toughening mechanisms. It is highly recommended to reinforce raw composites with low content of CNTs in order to improve the dispersion and interfacial adhesion of CNTs with surrounding polymer. Long aligned MWCNTs with small diameters (high aspect ratio) are recommended to be used to reinforce FRP composite in order to obtain superior fracture resistance toward initiated cracks. We found out that the factors, which govern the fracture toughness in multi-scale composites, are strongly correlated to each other. The most critical factors that affect the fracture toughness include the alignment and structure (length of nanotube) followed by the type of functionalization conducted on the surface of CNTs. (topical review)