Influence of sprayed multi-walled carbon nanotubes on mode I and mode II interlaminar fracture toughness of carbon fiber/epoxy composites

Unidirectional carbon fiber/epoxy prepregs were sprayed over their surfaces with different multi-walled carbon nanotubes (MWCNTs) concentrations in order to evaluate their effect on interlaminar fracture toughness of composite laminates under modes I and II loading and the influence in fracture mechanisms. To this aim, determination of the mode I and mode II interlaminar fracture toughness of composite laminates reinforced with sprayed MWCNTs were experimentally investigated using double cantilever beam and end notched flexure tests. Composite laminates without MWCNTs were also fabricated as a reference material. MWCNT characterization using Fourier Transform Infrared and Raman spectroscopy was also carried out in order to evaluate their functional groups and structural defects. The results show that the mode I and mode II interlaminar fracture toughness of the composite laminates can be moderately increased as MWCNT concentrations varying from 0.05 to 0.2 wt.%. Scanning electron microscope images of fracture surfaces provide evidence that the MWCNTs bridged and pulled out at the interface of composite laminates. It was also demonstrated that the optimum weight percentage of MWCNTs needed to improve the mode I and mode II interlaminar fracture toughness was 0.05%, due to the fact that higher concentrations promote the agglomeration of MWCNTs weakening the laminate interfaces.