MICROWAVE METHOD FOR LOCATING SURFACE SLOT CRACK TIPS IN METALS/

The detection of exposed (empty and filled) and dielectric covered fat igue/surface cracks on metal surfaces is an important practical issue. Recently, microwave techniques have shown the potential of detecting exposed and dielectric pled cracks and cracks covered with dielectric coatings (i.e. paint). An important issue associated with these invest igations is locating the tips of a crack. This is particularly importa nt from the repair point of view in steel bridges and other structures . ln this study open-ended rectangular waveguide probes are used to lo cate the tip of empty, dielectric filled, and covered cracks. In this paper the results of extensive measurements are discussed, including t he accuracy by which the tip location of a crack may be determined. Th e location of a crack tip can be determined using a two-dimensional cr ack characteristic signal (image of the crack), and more simply and ac curately by using the crack tip characteristic signal. For cracks/slot s used in this study, the results indicate that the tip location of ex posed (empty and filled) cracks may be identified to within 0.25 mm (0 .009 in.) of their actual position, while covered crack tips are locat ed within 2 mm (0.08 in.) of their actual locations. Using optimized m easurement parameters and/or a higher order mode defection scheme may result in improved tip location accuracy. Good agreement is obtained b etween the results of a theoretical model and the measurements, which strongly indicates the possibility of theoretically optimizing (increa sing) the accuracy by which a track tip location may be determined.