SAR imaging for inspection of metallic surfaces at millimeter wave frequencies

Millimeter wave near-field nondestructive testing (NDT) and imaging techniques are well-suited for the inspection of exposed (bare) and coated metallic surfaces for detecting surface-breaking discontinuities such as cracks and corrosion under coatings. However, one drawback of near-field imaging is the long scan time due to the dense grid requirements at these high frequencies. This paper presents a novel imaging technique for NDT of metallic surface for detecting surface-breaking cracks. This technique incorporates wideband measurements, synthetic aperture radar (SAR) based imaging algorithms, and surface wave propagation. The proposed technique vastly reduces the required scan time to produce an image. This paper introduces the imaging technique and studies its efficacy using full-wave numerical electromagnetic simulations. Results of measurements at K-band (26.5-40 GHz) and V-band (50-75 GHz) are provided illustrating the feasibility of the proposed technique for rapid detection of metal surface discontinuities.