Anisotropic material properties can be induced in ceramic-polymer composites by applying an alternating electric field of moderate strength during processing. Under suitable conditions, particles of a ceramic filler material that are randomly dispersed in a liquid polymer or pre-polymer can be polarized and they then exhibit a collective response to localised gradients in the electric field. Typically the particles experience a mutually attractive force which causes them to form 'pearl-chains' or columnar structures spanning the gap between electrodes. If the fluid is solidified, for example by curing the polymer resin, then the newly formed structures can be fixed in place to produce a composite with directional electrical and mechanical properties. Direct visual observations were made for low volume fraction dispersions of pure lead titanate in an epoxy pre-polymer under the influence of an electric field. The observed interaction was correlated with low-field dielectric measurements and existing theory to identify optimum assembly conditions. The dielectric properties of the fluid are predominant and the formation of chain-like structures is found to be both field strength and field frequency dependent. The dielectric permittivities of a range of structurally modified composites were measured and compared with existing theoretical models of di-phasic materials.

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The paper reports results of preliminary experiments on a continuous curing carbon fibre reinforced epoxy composite, designed to achieve simultaneous characterizations of the state of cure of the resin and the development of strain in the reinforcing fibres. The measurements were carried out using embedded dielectric microsensors and in-fibre Bragg gratings (IFBGs) to follow changes in the conductivity of the matrix resin and in the internal strain in the embedded optical fibre respectively. This was performed using an automated data acquisition system, part of which uses real-time dielectric measurements to follow the industrially relevant parameters of curing. The system can multiplex several sensors and could be used in a quasi-distributed system. The IFBG sensors were demodulated using a scanning fibre Fabry-Pérot interferometer. The results obtained with this combined cure monitoring system show that it is possible to monitor the strain levels in the optical fibre resulting from the onsets of liquification, gelation and vitrification within the surrounding resin matrix. The combined measurements constitute a first step towards a scientific study of the development of internal residual stresses in a cured composite, which will be crucial to the ultimate elimination of warpage and `spring-back' in large composite structures.

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