Abstract
Thermographical investigation with a short inductive heating pulse can be well used for detection of surface cracks in metallic materials. Irregularities in the surface cause anomalies in the temperature distribution, therefore cracks become visible in the infrared images. But not only cracks, but also edges of the sample become warmer, which has to be distinguished. Separation of the geometry effects and real cracks is based on the evaluation of the time dependency of the temperature, calculating a phase value of the cosine transformation, similarly to the PPT technique. Equations are presented how the temperature changes at the sound surface and around a surface crack of a semi-infinite body and of a body with finite thickness. The calculation results are compared with experimental results giving very good agreement. Steps of image processing technique are presented, how the phase image can be evaluated to automatically decide whether the piece is sound or defective. To avoid the orientation dependency of the inspection, a new experimental setup with two generators is invented and results are shown. With this novel technique it is possible to detect cracks independently on their angle.
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