Analysis of the impedance spectra of short conductive fiber- reinforced composites

The presence of small amounts of short conductive fibers in a composite of finite matrix conductivity results in the subdivision of the one matrix impedance arc into two separate low and high frequency arcs in the complex impedance plane. These features are attributable to a " frequency-switchable " interfacial impedance on the fiber surfaces, rendering them insulating at DC and low AC frequencies, but conducting at intermediate frequencies. A combination of physical simulations (single wires in tap water) and pixel-based computer modeling was employed to investigate the roles of fiber pull-out, debonding, and orientation on the impedance response of fiber-reinforced composites. The ratio of the low frequency arc size to the overall DC resistance (γ-parameter) is sensitive to pull-out and/or debonding, especially when a fiber just barely makes contact with the matrix. The γ-parameter is also quite sensitive to fiber orientation with respect to the direction of the applied field. Ramifications for the characterization of cement, ceramic, and polymer matrix composites are discussed. C 2001 Kluwer Academic Publishers 1. Introduction The addition of fibers to an otherwise brittle matrix has a profound influence on the mechanical properties of the composite. By " fiber " is meant an object that is approximately axisymmetric, and much longer than it is wide. The aspect ratio, the ratio of length to width, is usually on the order of 100 or more for typical fibers. The addition of fibers can also have a strong effect on the electrical properties (DC conductivity and AC impedance) of the composite, but only when the added fibers are highly conductive compared to the matrix [1]. Long, thin inclusions like fibers, which are insulating with respect to the matrix, have little effect on overall electrical properties [1], although of course they can still affect mechanical properties. But if the fibers, which are usually added for mechanical purposes, are highly conducting with respect to the matrix, electrical measurements, which are indirect and non-destructive, are sensitive to the presence of fibers. If the properties of the fibers with respect to the matrix varies with frequency , then effects of the fibers on the overall electrical properties will be seen in the complex impedance plane. The present study addresses the microstructure-electrical property relationships of such a composite