Microwave Imaging of Subsurface Flaws in Coated Metallic Structures Using Complementary Split-Ring Resonators

Microwave testing can detect surface and sub-surface flaws and anomalies under coatings, paint, or dirt in metallic structures. A major application of microwave sensors with an increasing interest is microwave imaging of coated metallic surfaces. Microwave imaging has the capabilities to determine the shape, size, and location of buried flaws using scattered field measurements, because microwave signals can penetrate dielectric coatings. Electrically small complementary split-ring resonators etched on printed circuit boards are strong candidates for microwave imaging as they can provide the benefit of one-sided small footprint sensors while resonating at wavelengths much lower than their dimension to provide high lateral resolution. This paper introduces and investigates the use of complementary split-ring resonators in the microwave regime for imaging subsurface flaws in coated metallic structures. Both transmission and reflection coefficients were studied to build images using magnitude and phase information. Aluminum plates with different corrosion-coated regions were studied numerically and experimentally. The results of a raster scan mechanism demonstrate the viability of the proposed imaging system.