Characterisation and modelling of active fibre composites

• Main Author: Nelson, Luke J
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2005
• Subjects: Materials science
• ISBN: 9781073281053; 1073281051

The objective of this study is to examine the major components of a newly developed active composite configuration, and to use the understanding gained to optimise its performance. The composite comprises active piezoelectric fibres in a passive polymer matrix, this it falls into the research field termed 'smart structures and materials'. The composite, typically known as the Active Fibre Composite (AFC), is made 'active' by applying a voltage to the driving electrodes, which are positioned on the top and bottom faces of the composite ply. By developing techniques to measure the properties of piezoelectric fibres, a fundamental understanding into their characteristics will be obtained. It is expected that by combining this understanding with research into the influence of the interdigitated driving electrodes on the composite performance, the composite configuration can be optimised for inducing maximum strain. The model was used to extract high field strain and polarisation responses from the same set of fibres that had had their low field properties characterised. As with the low field properties, the high field responses were found to differ markedly between the fibre types. Fibres manufactured using the VPP and VSSP techniques exhibited the greatest piezoelectric activity, generating strains in the region of 4000 ppm for an electric field cycle of ±2.0 kV mm -1. The large variation in the fibre properties has been linked to the physical and chemical properties of the fibres including porosity, grain size and phase composition. In general, large grain sizes, high densities and compositions closer to the tetragonal side of the morphotropic phase boundary (MPB) contributed to a large piezoelectric activity. Finite element analysis was used to gain an understanding into the effect of the interdigitated electrode (IDE) design on the performance of the AFC device. The electrode width, substrate thickness, and electrode finger separation were varied, and the resultant strain measured. It was found that the optimum electrode finger width was equal to half the substrate thickness. It was also shown that to maintain at least 80% of the ideal d33 response the electrode separation must be at least four times greater than the substrate thickness. The underlying reasons for these results were investigated by examining the electric field strength and direction within the substrate.