Modeling the Complex Permittivity of Thermoplastic Composite Materials

Several models have been studied in an effort to predict the microwave frequency permittivities of unidirectional-fiberreinforced thermoplastic-matrix composite materials as a function offiber volume fraction, fiber orientation relative to the electric field, and moisture content. Specimens of the composite were tested at 9 GHz using a shorted wave guide technique. Simple “rule-of-mixtures” bounding models were found to approximate the composite perrnittivities for varied fiber volume fractions. The permittivities of oriented composites were modeled using a tensor rotation procedure. Moisture, especially free water, was found to have a strong effect on the loss factor of the composite.