Microwave Method for Locating Surface Slot/Crack Tips in Metals

The detection of exposed (empty and filled) and dielectric covered fatigue/surface cracks on metal surfaces is an important practical issue. Recently, microwave techniques have shown the potential of detecting exposed and dielectric filled cracks and cracks covered with dielectric coatings (i.e. paint). An important issue associated with these investigations is locating the tips of a crack. This is particularly important from the repair point of view in steel bridges and other structures. In this study openended rectangular waveguide probes are used to locate the tip of empty, dielectric filled, and covered cracks. In this paper the results of extensive measurements are discussed, including the accuracy by which the tip location of a crack may be determined. The location of a crack tip can be determined using a two-dimensional crack characteristic signal (image of the crack), and more simply and accurately by using the crack tip characteristic signal. For cracks/slots used in this study, the results indicate that the tip location of exposed (empty and filled) cracks may be identified to within 0.25 mm (0.009 in.) of their actual position, while covered crack tips are located within 2 mm (0.08 in.) of their actual locations. Using optimized measurement parameters and/or a higher order mode detection scheme may result in improved tip location accuracy. Good agreement is obtained between the results of a theoretical model and the measurements, which strongly indicates the possibility of theoretically optimizing (increasing) the accuracy by which a crack tip location may be determined