Eddy current thermography in circular aluminium plates for the experimental verification of an electromagnetic–thermal method for NDT

The performance of an electromagnetic–thermal method for nondestructive testing is investigated experimentally by eddy current thermography. The experiments concerned the detection of the crack in six circular aluminium plates at various positions and orientations. Two circular coils are employed for the excitation, C8 and C15, with outer diameter smaller (C8) and greater (C15) than the plate diameter. The coil C8 is more effective in detecting cracks perpendicular to the current flow, while C15 excels in the detection of cracks perpendicular to the heat flow. Crack detection is based not only on the thermograms but on data processing techniques as well, such as the depiction of the norm of spatial derivative of temperature and Fourier transform. The detection of cracks perpendicular to the current flow is enhanced considerably by Fourier transform (phase images). Both the norm of the spatial derivative of temperature and Fourier transform (amplitude images) give the adequate results for the cracks perpendicular to the heat flow. Notwithstanding the fact that the performance of the infrared camera is relatively low, the use of the two coils, combined to techniques of data processing, permitted for the detection of all six cracks in the plates.