Lock-in inductive thermography for surface crack detection

Inductive thermography is a non-destructive technique, which can be excellently used for detection of surface cracks in electrically conductive materials. In ferro-magnetic steel even a single short pulse with duration of 50ms up to is is enough to induce a Joule heating which makes shallow cracks well detectable in the infrared image sequences. In the case of non-magnetic materials with high electrical and thermal conductivity, as e.g. aluminum, the situation is much more difficult: on the one hand a short heating pulse duration is necessary, otherwise the thermal signal diminishes too quickly due to the thermal diffusion. On the other hand with a short heating pulse it is not possible to induce enough heat in the material; therefore the signal-to-noise ratio becomes too low for defect detection. A possibility to overcome this problem is to apply a sequence of short pulses, as it is also done in the lock-in thermography. It is investigated, how many pulses and which pulse duration is necessary to detect surface cracks with different crack depths in non-magnetic materials, as in aluminum. It is also studied, how the heating power, that means the temperature increase during one heating pulse, influences the detectability. Experimental results are presented, obtained for an aluminum sample with artificial cracks and they are compared also with numerical simulations.