Low Frequency Eddy Current Testing of Insulators and Composites
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Eddy current testing (ECT), a non-destructive testing method widely used to evaluate defects within conductive materials, is explored in this study as it applies to insulators and non-uniformly conductive materials. Previous work has shown that at high frequencies, differences in electric permittivity can be detected with ECT. In this study, a new design of an ECT sensor that employs two resonance-tuned coils is evaluated. Results show that material inconsistencies in insulators are detectable due to spatial variations in permittivity and magnetic permeability, and that detection is possible at lower frequencies than previously demonstrated. In addition to determining signal dependence on individual electromagnetic parameters, sensitivity for defect detection in a carbon fiber-reinforced polymer (CFRP) composite is qualitatively determined. Although low signal-to-noise ratio is observed with a small-diameter coil, by increasing the coil diameter, the signal to noise ratio is increased while preserving adequate spatial resolution to detect defects in the sample. This study expands on previous studies of the application of ECT to insulators, and demonstrates that defect detection is possible in CFRPs.
KeywordsComposite materials Defects Eddy currents Material properties Magnetic materials
This research was supported by Northrop–Grumman Corporation. The sensors were designed and supplied by Exel Orbital Systems Inc. Solvay Inc. and Airtech International donated prepreg and consumable materials, respectively. Experimental assistance on the project was provided by Ellen Emerson. David B. Chang, PhD, provided assistance in the discussion of physical concepts.
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