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In the current work, the microstructure and fracture performance of carbon fibre-reinforced polymer (CFRP) composites based upon matrices of an anhydride-cured epoxy resin (formulated with a reactive diluent), and containing silica nanoparticles and/or polysiloxane core-shell rubber (CSR) nanoparticles, were investigated. Double cantilever beam tests were performed in order to determine the interlaminar fracture energy of the CFRP composites, while the single-edge-notched bend specimen was employed to evaluate the fracture energy of the bulk polymers. The fracture energy of the bulk epoxy polymers increased from 173 J/m 2 for the unmodified polymer to a maximum of 1237 J/ m 2 with the addition of 16 wt% of CSR nanoparticles. The toughening mechanisms were identified as (a) localised plastic shear yielding and (b) cavitation of the CSR particles followed by plastic void growth of the matrix. The steady-state propagation value of the interlaminar fracture energy of the CFRP composites increased with increasing nanoparticle concentration, from 1246 J/m 2 for the unmodified epoxy matrix to a maximum of 1851 J/m 2 with 4 wt% of silica nanoparticles and 8 wt% of CSR nanoparticles. Crack growth in the CFRP composites was dominated by fibre-bridging toughening mechanisms. The efficiency of the transfer of toughness from the bulk polymers to the carbon fibre composites was considered. The measured fracture energy of both bulk and composite materials decreased at a test temperature of -80°C, compared with room temperature, i.e. 20°C. Nevertheless, the toughening effects of both the silica and CSR nanoparticles on the bulk epoxy polymers and the CFRP composites, compared with the unmodified epoxy polymers, were still evident even at the lower temperature. Indeed, the toughening effect of the silica nanoparticles was greater at -80°C than at room temperature.

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Publication informationEngineering Fracture Mechanics, 78 (17) AbstractThe fracture toughness of five grades of polycrystalline cubic boron nitride (PCBN) has been determined using Single Edge V-Notched Beam specimens.Both coarse and fine grade materials were considered, containing CBN grain sizes of between 1 µm and 22 µm. The influence of notch root radius on the measured fracture toughness was examined. The notch root radius was found to have a major effect for materials with smaller CBN grain sizes while only a small effect was noted for the material with large CBN grain sizes. A simple analytical model was developed to explain the effect of the notch root radius on the fracture toughness and was found to agree well with experiment for all the materials tested. It was shown that the effect of notch root radius is directly linked to the size of the CBN grain. It is proposed that this effect results from the interaction between the microstructure and the stress field around the notch tip.

An arbitrary crack propagation model using cell-centre finite volume based method is presented. Crack growth in an elastic solid, across an interface perpendicular to the initial crack path and into a second elastic solid is analysed. Crack initiation and the subsequent path of propagation are shown to arise naturally out of the selection of appropriate cohesive parameters.It is shown that the allowable crack propagation path is restricted by the underlying mesh. Results are presented for a number of values of interfacial strength and ratios of elastic properties between the two elastic solids. For higher values of interfacial strength, the crack is shown to propagate straight through the interface, while for lower values of interfacial strength, the crack is shown to change direction and propagate along the interface. It is shown that with careful selection of material and interface parameters it is possible to arrest a propagating crack at the interface. The method represents a useful step towards the prediction of crack propagation in complex structures.

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