Capacitance mapping of composites

Non-contact capacitance mapping is proposed as a complimentary technique for structural health monitoring of dielectric media in general and composites in particular. An XY scanner has been fabricated with parallel electrode as probes with a provision for maintaining a constant air gap, both at the top and bottom surface of flat samples to enable spatial mapping of capacitance. The probes connected to an impedance analyser provides the real and imaginary parts of capacitance/ impedance respectively over a frequency range of 100 Hz to 1 MHz . Spatial maps of the complex capacitance are obtained for 50 mm x 200 mm Glass fibre reinforced composite and 30 mm x 100 mm Plexiglas with and without defects. A discussion on the role of electrode dimensions, probe lift-off and scan step would be presented. Results on the contrast between defect-free and defective regions and the effect of sample thickness variations on the measured capacitance would be described. Finally, a scheme involving model-assisted extraction of dielectric constant from measured capacitance will be presented.