Ultra-resonance microwave defectoscopy of metal surfaces

A novel microwave high-resolution near-field non-destructive testing technique is proposed and experimentally evaluated in reflectometry imaging scenarios involving planar metal surfaces. Traditionally, microwave reflectometry does not provide high dynamic contrast between the defect and background material in case of metal structures due to intrinsically high reflection magnitude from the metal surfaces masking defect microwave signature. A high-Q resonant sensor based on the loaded aperture is designed to interact very strongly even with small defects on the metal surface providing very high two-dimensional spatial resolution of approximately one tenth of a wavelength, λ, at λ/20-λ/10 standoff distance. Experimental results demonstrate defect-to-background contrast greater than 5 dB amplitude and 50° phase in raw microwave data. To further enhance the spatial resolution and defect contrast, two techniques are proposed and experimentally evaluated: 1) phase-modulated near field imaging is based on sharp variation of the reflection phase in the narrow frequency band, which essentially enables elimination of background microwave signature from the reflected signal; 2) numerical cross-correlation signal processing that allows for defect signature enhancement by numerical elimination of the background response. The proposed imaging technique should find applications in non-destructive surface testing and evaluation of metal and alloy structures and elements of construction.