Modelling the Electromagnetic Properties of Conductive Nonwoven Fabrics

This thesis presents micro-structure models of wet-laid conductive nonwoven fabrics allowing the sheet conductance and shielding effectiveness to be simulated and compared to experimental measurement. Conductive nonwoven fabrics are used within the aerospace and defence industries to provide lightweight, functional electromagnetic enhancement to composite structures. They are materials borne from stochastic processes with anisotropic distributions of fibre and parameters that vary from point to point on the local scale. Monte Carlo models of the material’s micro-structure have been constructed by writing a series of algorithms which pseudo-randomly generate the material’s structure by incorporating key physical parameters such as the density, areal concentration and fibre angle distribution. To define the last of these parameters, a completely new optical method has been developed making use of the Hough Transform. These models have predicted the anisotropic sheet conductance to within 1-2% of experimental values, with an estimated inter-fibre contact resistance of Rj = 8.6kΩ, and a measured geometry factor of Φx = 0.727, Φy = 0.273. Analytic models of the material are derived from first principles enabling the rapid calculation of the sheet conductance, whilst also providing an understanding between the key parametric relationships. The analytic model, Monte Carlo model and experimental measurements are compared and give good correspondence. The micro-structure models are finally applied to a full wave electromagnetic simulation technique and shown to produce close correlation to polarisation specific measurements of the shielding effectiveness. High frequency (up to 200GHz) simulations of lightweight nonwoven structures suggest an eventual fall in the shielding effectiveness, attributed to the material’s sub-wavelength apertures.