The mechanics and mechanisms of fracture of nano-modified polymers

Abstract

The present paper considers the general mechanical, fracture and cyclic-fatigue properties of four different epoxy polymers containing various concentrations of well-dispersed silica nanoparticles. Firstly, it was found that, for any given epoxy polymer, their Youngs modulus steadily increased as the volume fraction, vf, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy-polymers lay between upper-bound values set by the Halpin-Tsai and the Nielsen no-slip models, and lower-bound values set by the Nielsen slip model; with the last model being the more accurate at relatively high values of vf. Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature, Tg, and molecular weight, M c, between cross-links of the epoxy polymer, and (b) the adhesion acting at the silica-nanoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear-bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void-growth of the epoxy polymer. Fourthly, for one formulation the cyclic-fatigue properties have been studied and a significant improvement was found to arise from the addition of the silica nanoparticles. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally measured values and the predicted values of the fracture energy, Gc, for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silicananoparticle/ epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy, Gc, of the nanoparticle-filled polymers.