Motivated by potential applications such as defect detection in aircraft radome sandwich structures, two prototype handheld concentric coplanar capacitive sensors with different target sensing penetration depths have been designed and built. The sensors have the same outer diameter (25.4 mm) but different gap width between the inner and outer concentric electrodes. The sensor penetration depth is defined by identifying the test-piece thickness for which the capacitance is 10 % smaller than its value when in contact with a similar but infinitely-thick test-piece. The sensors were designed with the aid of a numerical model in which Green's functions were used to derive the integral equation for the sensor surface charge density of a concentric capacitive sensor in surface contact with a five-layer dielectric. Using the method of moments, the integral equation was discretized to form matrix equations that are solved for the sensor surface charge density. The total charge Q on each sensor electrode can be obtained from the sensor surface charge density and the sensor output capacitance is calculated through C = Q/V, where V is the potential difference between the two electrodes. For a dielectric slab with relative permittivity µr = 4, the predicted penetration depths of the two sensors are 1.5 and 2.5 mm. Results of experiments investigating the penetration depth of the sensors on stepped Delrin (µr = 3.8 at 1 MHz) and HDPE (µr = 2.3 at 1 MHz) samples will be reported. Benchmark experiments comparing the measured capacitance of the handheld probes with numerically-predicted values, for the probes in surface contact with multi-layered dielectric test-pieces, also have been conducted. Measured and calculated capacitance agrees to within approximately 8 % for two-layer acrylic-glass laminates. The performance of the sensors for detection and characterization of water ingression in sandwich structures with glass fiber skins and honeycomb core will also be reported.


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    Title :

    A rotationally invariant capacitive probe for materials evaluation


    Additional title:

    Eine rotationsinvariante kapazitive Sonde zur Materialbewertung


    Contributors:

    Published in:

    ME - Materials Evaluation ; 70 , 2 ; 161-172


    Publication date :

    2012


    Size :

    12 Seiten, 8 Bilder, 4 Tabellen, 15 Quellen



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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