The influence of the exciting frequency on crack detection by eddy current thermography

The influence of the exciting frequency on crack detection by eddy current thermography is investigated experimentally. The performance of a coil designated C16 with exciting frequency 32 kHz in the detection of eight cracks in circular aluminium plates, at various positions and orientations, is compared with the performance of two coils (designated C8 and C15) with exciting frequency 50 Hz. Crack detection is based not only on the thermograms but also on data processing techniques. The coil C16 proved to be the most effective as it detected seven cracks out of the eight, whereas the C8 coil detected five cracks and the C15 four. Furthermore, in most cases, it gave sharper images of the crack. Elevated heating rates are induced by higher exciting frequencies, resulting in elevated values of current density and heat flow. Therefore, wider detection regions and longer detection periods may be achieved.