Short conductive fiber composites, below their percolation threshold, can exhibit dual arc impedance behavior. The subdivision of the bulk arc is attributable to a frequency-switchable surface impedance on the fibers. This impedance renders the fibers insulating at DC and low AC frequencies. At intermediate frequencies, however, the surface impedance disappears and the fibers act as microstructural short-circuits. The residual resistance is a combination of inter-fiber regions and spreading resistance zones near the fiber tips.
Physical simulations (wires in tap water cells) combined with pixel-based numerical computations have demonstrated how several factors−fiber pull-out, fiber/matrix debonding, and fiber orientation−can influence the fiber-induced bulk arc subdivision. This can be quantified by the parameter γ, defined as the ratio of the high frequency arc width to the overall DC resistance. The γ parameter can be quite large for a fully submerged (intact), bare (fully bonded) fiber aligned in the direction of the applied field, meaning that the fiber greatly reduces the composite resistance at intermediate frequencies. However, the γ parameter is relatively unaffected by pull-out or debonding of the fiber until the final stages of either process. In contrast, the bulk arc subdivision is significantly influenced by the fiber orientation with respect to the electric field direction. The γ parameter is largest when measured in the fiber direction, and least (or zero) when measured perpendicular to it. These results explain, at least qualitatively, the orientation dependence of impedance measurements on highly oriented short conductive fiber composites.
It is clear that in the case of highly conducting fibers in a poorly conducting matrix, where the fibers have a frequency-switchable coating impedance, many of the fiber aspects that are important in mechanical behavior, like fiber pull-out, fiber debonding, and fiber orientation, can be quantitatively observed with impedance spectroscopy. Since impedance spectroscopy is a non-destructive technique, these results open the door for non-invasive monitoring of the mechanical state of composites, which are mechanically dominated by the presence of fibers.