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This paper investigates whether the toughening mechanisms of a biological crossed-lamellar microstructure can be reproduced in a synthetic high-performance carbon fibre/epoxy matrix composite. The mechanics of the failure process in synthetic crossed-lamellar microstructures was investigated using the Finite Element Method. This enabled the design of a high-performance carbon-fibre reinforced polymer (CFRP) with such microstructure. Two different procedures were then developed to synthesise the first crossed-lamellar microstructures in CFRP in the literature. Test specimens were subsequently manufactured. Three-point bend tests were carried out in an SEM environment, showcasing the damage diffusion capability of the microstructure under stable conditions. The results show that the crossed-lamellar microstructure can be synthesised in CFRP with good accuracy, and that the mechanical toughening mechanisms associated with the natural crossed-lamellar microstructures can be reproduced in this synthetic material.

In this paper, a carbon-fibre/epoxy (CF) composite with nacre-inspired tiled micro-structure is designed and synthesised. The aim is to investigate the interaction between the CF discontinuous micro-structure and continuous glass-fibre/epoxy (GF) layers, which are intended to act as crack stoppers, similarly to the organic interlayers that separate layers of ceramic tiles in natural nacre. Firstly, we use a GF skin to trigger unstable failure in nacre-like mesolayers, and show how the damage mode in the latter changes from pull-out to brittle fibre fracture due to the interaction with the GF skin. Secondly, we demonstrate how continuous GF interlayers can succeed in arresting unstable crack propagation in the nacre mesolayers. Furthermore, we show that they can also change the morphology of damage in the nacre, promoting a transition from brittle tile fracture to more damage-tolerant tile pull-out.

A promising strategy to increase the tensile failure strain of carbon fibre-reinforced composites is to hybridise carbon fibres with other, higher-elongation fibres. The resulting increase in failure strain is known as the hybrid effect. In the present article, a global load-sharing model for hybrid composites is developed and used to carry out a parametric study for carbon/glass hybrids. Hybrid effects of up to 15% increase in failure strain are predicted, corresponding reasonably well to literature data. Scatter in the carbon fibre strength is shown to be crucial for the hybrid effect, while the scatter in glass fibre strength is much less important. In contrast to reports in earlier literature, the ratio of failure strains of the two fibres has only a small influence on the hybrid effect. The results provide guidelines for designing optimal hybrid composites. publisher: Elsevier articletitle: Global load-sharing model for unidirectional hybrid fibre-reinforced composites journaltitle: Journal of the Mechanics and Physics of Solids articlelink: http://dx.doi.org/10.1016/j.jmps.2015.08.009 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Journal of the Mechanics and Physics of Solids vol:84 pages:380-394 status: published

The translaminar fracture toughness of fibre-reinforced composites is a size-dependent property which governs the damage tolerance and failure of these materials. This paper presents the development, implementation and validation of an original analytical model to predict the tensile translaminar (fibre-dominated) toughness of composite plies and bundles, as well as the associated size effect. The model considers, as energy dissipation mechanisms, debonding and pull-out of bundles from quasi-fractal fracture surfaces; the corresponding lengths are stochastic variables predicted by the model, based on the respective bundle strength distributions and fracture mechanics. Parametric studies show that composites are toughened by stronger fibres with large strength variability, and intermediate values of interfacial toughness and friction. Predictions are validated against four different composite ply systems tested in the literature, proving the model’s ability to capture not only size effects, but also the influence of different fibres and resins.

The homogenization problem of linearly piezoelectric fibrous composites with a periodic microstructure is formulated by adopting the free- and complementary-energy minimum principles, as well as the Hashin Shtrikman principles. The auxiliary electroelastic problems involved in the Hashin-Shtrikman principles are solved by a transformation from the space domain to the Fourier domain. Approximations of those variational principles, suitable for numerical computations, are built up by adopting the Fourier-series method and the finite-element method. In this way, rigorous upper and lower bounds on the overall material moduli are obtained. An example is presented, in order to show the effectiveness of the proposed approximation techniques and to compare them.