Thermal non-destructive testing (NDT) is commonly used for assessing aircraft structures. This research work evaluates the potential of pulsed -- transient thermography for locating fixtures beneath aircraft skins in order to facilitate accurate automated assembly operations. Representative aluminium and carbon fibre aircraft skin-fixture assemblies were modelled using thermal modelling software. The assemblies were also experimentally investigated with an integrated pulsed thermographic evaluation system, as well as using a custom built system incorporating a miniature un-cooled camera. Modelling showed that the presence of an air gap between skin and
fixture significantly reduced the thermal contrast developed, especially in aluminium. Experimental results show that fixtures can be located to accuracies of 0.5 mm.
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. Measurements of the piezoelectric induced strain constants (d33 and d31), relative dielectric constants (e33), longitudinal coupling factors (k33) and stiffness' (s33) of the varying volume fraction composites are compared to analytical expressions in order to extract the fibre properties. Results show 1-3 composite data accurately follows the analytical trends. The Viscous Plastic Process (VPP) fibres are found to exhibit optimum material properties, which approach bulk material values. Reduced piezoelectric activity in extruded fibres is thought to be associated with a small grain size and high porosity. A second study, an optimisation of interdigitated electrode design, was performed using the finite element software ANSYS. The effect of the IDE geometry (electrode width and spacing) and PZT substrate thickness on the strain output of bulk PZT substrates was modelled. Results show optimal actuation occurs at electrode widths equal to half the substrate thickness, and for thin substrates the electrode finger spacing can be reduced to enable lower driving voltages.
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.