The honeycomb composites presented a challenging measurement situation because of the diversity of the different components. Several arrangements of electrode placement were devised in order to test as many of the component materials as possible. These electrode arrangements included placement on opposite sides (on the surfaces of the two graphite-epoxy sheets sandwiching the core material) and on the same side, as well as placing one electrode on the graphite-epoxy surface and making contact to the core material or to the adhesive backing with the other electrode lead.
The most straight-forward results were collected from an IM7/8552 composite with the Nomex core, locating the sensor electrodes at the graphite/epoxy surface and at the core. Figure 15 shows the low-frequency impedance values versus moisture content. The useful range of sensitivity in this case is roughly 0.15 % to 0.50%, in which the impedance steadily decreases as moisture content increases. It is believed that the Nomex core absorbed moisture more readily than the graphite-epoxy composite and controlled the impedance measurements.
For aluminum core honeycomb composite specimens, equivalent-circuit analysis was needed. The model of Figure 16 consists of two loops in series and a third imbedded loop. The R1/CPE1 parallel combination is a collapsed version of the monolithic composite circuit and is included to account for moisture absorption in the skin. The R2/C2 parallel combination with the imbedded R3/C3 parallel loop is based on a standard circuit model often used for the analysis of coated metals. In this case, R2/C2 represents moisture absorption in the adhesive joining the core and the skin. The R3/C3 portion represents activity at the adhesive/core interface. A final resistor is again placed in series with the circuit to represent any small "solution" resistance.
Figure 17 shows some moisture-sensitive parameters from equivalent circuit modeling of an AS4/3501-6/Al honeycomb. The situation of the honeycomb composite is complicated by the variety of materials present, and the differing rates at which they absorb moisture. Ideally, the model parameters will reveal not only how much moisture is in the system, but also in which materials it has accumulated. Continued evaluation of this complex combination of materials is necessary to develop the moisture sensor system to provide this information.
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.