Figure 1: Capacitive sensors have been developed to inspect aircraft radomes such as the one being raised in this picture.
Figure 2: Fiberglass-honeycomb-fiberglass structure similar to many radome structural materials.
Figure 3: Coplanar concentric capacitive sensor electrodes (disc and ring) in surface contact with a multilayer dielectric structure.
Planar capacitive sensors have been developed to meet the practical need of detecting water ingression and imperfect repairs in modern aircraft radome structures (the raised part in Figure 1). The radome is a fiberglass-honeycomb-fiberglass sandwich structure, and has a similar configuration to the sample shown in Figure 2. Figure 3 shows the configuration of the sensor electrodes. The sensor consists of two electrodes: an inner disc and an outer annular ring. In the accompanying numerical model, the capacitance Cbetween the two electrodes is calculated using the relation C = Q/V, in which Q is the total charge on each electrode and V is the potential difference between the electrodes. By numerical modeling, aquantitative relationship is established between the sensor capacitance C and the thickness and permittivity of each layer in a multilayered structure [1]. This means that changes in measuredC can be related to permittivity changes in the test-piece materials.
Figure 4: Hand-held capacitive probe for materials evaluation of low-conductivity planar structures.
Figure 5: Fiberglass-honeycomb-fiberglass sandwich structure containing injected dielectric contrast agents: water and oil.
Figure 6: Capacitance measured as a hand-held capacitive probe scans over glassfiber-honeycomb-glassfiber sandwich panels containing injected high- and low-dielectric contrast agents water and oil.
To demonstrate the probe’s capability for defect detection in sandwich structures, different amounts (1, 3, and 5 cc) of water and oil have been injected to a fiberglass-honeycomb-fiberglass sandwich structure, Figure 5. Water has a high relative permittivity (≈ 80) and creates quite a high permittivity contrast with the surrounding material, whereas oil has a low relative permittivity (≈ 3). Figure 6 shows the measured capacitance as the hand-held probe scans a line directly over the cells containing the contrast agent. Figure 6 demonstrates the outstanding capability of the hand-held sensors in detecting relatively small contrast zones, e.g., 1 cc of low-contrast oil injected into the cells of a glassfiber sandwich panel was successfully detected.
