L J Nelson

Manufacture and characterization of high activity piezoelectric fibres

Piezoelectric fibres are finding increasing application in a variety of piezoelectric composites, including active fibre composites (AFCs). This paper describes the manufacture and characterization of lead zirconate titanate (PZT) fibres manufactured by viscous plastic processing (VPP). The manufacturing method will be described along with a systematic characterization of the macrostructure, microstructure, phase composition and low and high field piezoelectric properties. A comparison with other available PZT fibres will be made, which demonstrates that the VPP PZT fibres display high piezoelectric coefficients.

A novel and robust thermal wave signal reconstruction technique for defect detection in lock-in thermography

In lock-in thermography, given sufficient time for periodic heating, the surface temperature will evolve periodically in a sinusoidal pattern from the transient state to the steady state. A phase image at the frequency of periodic heating can be calculated using a Fourier transform of the steady-state temperature sequence for defect detection. It has been found that the transient state surface temperature possesses superior properties, which can be utilized for defect detection. As compared to the steady state, the contrast in the transient state is 60% higher. The surface temperature can be best approximated by a hybrid polynomial model, which consists of sinusoidal and ordinary polynomial functions. A novel and robust thermal wave signal reconstruction (TWSR) technique has been derived from these properties. With this technique, the hybrid polynomial function is fitted to each pixel in the thermographic sequence and the fitted coefficients are used to reconstruct phase and background levelled images. Phase images generated in this way are less prone to noise problems and the need for using Fourier transformation is eliminated. However, better defect detection has been achieved with levelled images. Results obtained using a 3 mm thick CFRP sample show that the technique is highly repeatable and probes 43% deeper than the conventional lock-in phase image technique. The high signal-to-noise ratio in the transient state also implies the possibility of earlier defect detection. Levelled images have been found to be best at exploiting this property. It is shown that the duration of periodic heating can be reduced substantially from the times necessary for conventional steady-state lock-in imaging.

High-field behavior of piezoelectric fiber composites

This paper analyses strain and polarisation responses of 1-3 composites, which are related to the fibre and matrix properties. The validity of equations that predict the strain and polarisation of fibres from composite responses, and associated errors at high electric driving fields, are discussed. Surface profile measurements of single PZT rods in a polymer matrix, subjected to a static voltage, were made to investigate the effect of fibre aspect (diameter to length) ratio. Surface profiles, which show the active PZT rod extending from the passive polymer matrix, agree well with predictions made using finite element analysis. The results show that for a 1-3 composite to be treated as a homogeneous medium the fibre aspect ratio needs to be low. Commercially available PZT-5A composition fibres fabricated using four production methods were incorporated into 1-3 composites with fibre volume fractions ranging from 0.02 to 0.72, and with various aspect ratios, were evaluated. Strain-field and polarisation-field curves for the composites were obtained by testing the composites under electrical field cycles of ±2 kVmm-1. From these curves the strain and polarisation response of the fibres have been extracted using appropriate analytical equations. The saturation strain, saturation polarisation and coercive field values are reported for the four fibre types. The Viscous Plastic Process (VPP) and Viscous Suspension Spun (VSSP) fibres develop strains of approximately 4000 ppm. Reduced piezoelectric activity is seen in extruded fibres, which develop strains of 3000 ppm.

Evaluation of High Field Responses of Fine Scale Piezoelectric Fibres

A methodology for the measurement of high field strain and polarisation responses of fine scale piezoelectric fibres is presented. Using analytical models, the response of the fibres is extracted from measurements made on 1–3 composites. Analytical modelling shows that composites with a high fibre volume fraction are desirable for accurate prediction of the fibres response. Poor model response at low fibre volume fractions is attributed to non-linear material properties, and deviation from the model assumptions.