Ultra-Resonance Microwave Defectoscopy of Metal Surfaces

Abstract

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 the case of metal structures due to intrinsically high reflection magnitude from the metal surfaces masking defect a 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 a 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, a phase-modulated near field imaging technique is proposed and experimentally evaluated in the case of a defected metal plate. This technique is based on fast variation of the reflection phase in the narrow frequency band around the resonance, which essentially enables elimination of background a microwave signature from the reflected signal. The proposed imaging technique should find applications in non-destructive surface testing and evaluation of metal and alloy structures.