Millimeter-Wave Differential Probe for Nondestructive Detection of Corrosion Precursor Pitting

Critical aircraft structural components, such as wings and fuselages, are exposed to harsh environments that vary considerably in temperature and moisture content. In most cases, the corrosion is hidden under paint and primer and cannot be visually detected. The initiation of corrosion is preceded by the presence of corrosion precursor pitting. Near-field millimeter-wave nondestructive testing (NDT) methods have been successfully used for detecting corrosion precursor pitting in exposed as well as painted aluminum substrates. However, near-field millimeter-wave measurements are susceptible to clutter that may mask indications of small defects such as pitting. Standoff distance variation produces an unwanted intensity gradient on an image and may be considered the most undesired clutter-producing effect. This paper presents a differential millimeter-wave probe consisting of a pair of radiating apertures. It is shown that the differential nature of this probe tends to significantly reduce the undesired effect of standoff distance variation, thereby enhancing probe detection sensitivity. Furthermore, when this probe is used for the purpose of millimeter-wave imaging, it produces defect indications with unique features that help in distinguishing the defect from noise. This dual differential probe was used for detecting corrosion precursor pitting. The design of the probe and the results of detecting various pittings are presented in this paper.