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Selected EIS spectra for a Navy graphite-epoxy material are shown in Figure 12. Same-side and opposite-side measurements gave similar results as did measurements with the electrodes oriented along the outer fiber direction and at 45° to this direction. The overall impedance values for these composites are generally lower then the impedance values for the glass/polyimide composites just discussed. Also, the change in impedance as a function of moisture content is less pronounced in the graphite-epoxy composites as compared with the glass-polyimide specimens. We attribute both of these effects to the presence of the conductive graphite fibers in the composite structure, which serve to both lower the overall impedance and to alter the effects of moisture absorption. In most cases, the impedance behavior remained capacitive over the entire frequency range and did not have a low-frequency resistive component. Nonetheless, spectra information could be correlated with moisture levels. To better understand and track the changes these composites underwent as a result of absorbing moisture, equivalent circuit modeling was used. This approach uses the data collected at all frequencies but is dependent on the choice of circuit model.
Equivalent circuit analysis focused on identifying circuits that were physically significant with the intention of identifying circuit elements particularly sensitive to changes in the quantity of moisture absorbed. The method was based on tracking the changing values of several moisture-sensitive circuit elements as a function of moisture content. Identifying and monitoring the trends of more than one circuit element increases confidence in this method of moisture content prediction.
The model selected for the monolithic samples is shown in Figure 13. The model consists of three parallel R/CPE combinations in series with a "solution" resistance. This circuit was de-signed to describe the expected behavior of same-side contact measurements. The first and third R/CPE loops were constrained to be equal. Each was intended to describe the signal response of the resin-rich layer at the composite surface. The middle loop, R2/CPE2, was intended to model the behavior of the composite interior, where the signal is governed with the graphite fiber system. Although exact material values such as resistivity and dielectric constant were not known for the composite material, reasonable estimates were applied during model fitting in order to further constrain the parameters in favor of a physically realistic result.
Equivalent circuit modeling of the data shown in Figure 12 revealed several parameters with sensitivity to moisture content. Examples of correlations between moisture content and particular circuit elements modeled for the 7-ply IM7/8552 sample spectra are shown in Figure 14. The parameters vary with moisture in a regular way. By fitting models to data in multiple formats and by tracking several moisture-sensitive parameters, it becomes possible to predict values of moisture content for these types of materials more reliably than by tracking only one parameter.
In-Situ Sensor to Detect Moisture Intrusion and Degradation of Coatings, Composites, and Adhesive Bonds, G.D. Davis, C.M. Dacres, and L.A. Krebs, DACCO SCI Inc.