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.