A crosslinked epoxy polymer has been modified by the addition of nano-silica particles. The particles were introduced via a sol-gel technique which gave a very well dispersed phase of nano-silica particles which were about 20 nm in diameter. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The fracture energy, GIc, increased from 100 J/m 2 for the unmodified epoxy to 460 J/m2 for the epoxy with 20 wt. % of nano-silica. The microscopy studies showed evidence of debonding of the nanoparticles and subsequent plastic void growth of the epoxy polymer. A theoretical model of plastic void growth was used to confirm this mechanism. The cyclic-fatigue behaviour of the epoxy polymers has also been studied and the fatigue properties were clearly enhanced by the presence of the nano-silica particles. Indeed, it was found that the values of the strain-energy release rate at threshold, Gth, from the cyclic-fatigue tests increased steadily as the toughness, GIc, also increased, i.e. as the concentration of nano-silica particles was increased
