Many critical mechanical devices and structures like aircraft’s wings and fuselages are exposed to harsh environmental conditions that lead to corrosion of these parts. The beginning of corrosion is preceded by the presence of corrosion precursor pitting. Early detection of corrosion precursor pitting in these devices is an important practical issue when considering maintenance and repair. The size (area and depth) of a precursor pitting is very small (micron or fractions of a millimeter).These micro-cracks at the surface of metals can be detected and imaged by near-field microwave techniques from the crack-induced variations of the resonance frequency and of the resonant circuit quality factor. Corrosion that is hidden under paint may not always be visually detected. However, microwave and millimeter wave signals are capable of penetrating paint and interacting with surface anomalies and measurements can be conducted in a rapid, one-sided and non-contact fashion.
Microwave and millimeter wave nondestructive testing (NDT) techniques is used for detecting corrosion and precursor pitting under paint and other thin and thick dielectric laminates. Near-field approaches are applied to estimate the dimensions of a surface defect.
Measurements are conducted using Near-field microwave NDT techniques. An open-ended rectangular waveguides to inspected complex composite structures with 1 mm distance between the probe and the composite under test, referred to as the standoff distance is optimized to increase measurement sensitivity.
When operating in the near-field and for the purpose of microwave and millimeter-wave imaging, the electromagnetic properties such as electric field distribution and the aperture size of the probe significantly influence the resulting system spatial resolution and radiation efficiency. Standard open-ended rectangular waveguide probes filled with dielectric at operating at 35GHZ and 75 GHz detected 200 µm wide pits.