Eddy-current lockin-thermography: Method and its potential

The basic idea is that an induction coil induces eddy currents in conducting materials where the involved resistive losses are used for heating. The remote modulated excitation generates thermal waves that interact with boundaries thereby revealing defects. As a thermography camera monitors the temperature modulation on top of the surface, the response to the coded excitation allows for fast imaging of defects in larger areas without the need of slow point-by-point mapping. This is an essential advantage as compared to conventional eddy current techniques. As compared to inductive heating with visual inspection of the thermographic sequence, the advantage is local phase information resulting in phase angle images that are independent of most artefacts. Due to the Fourier analysis performed at the lockin frequency on the temperature image sequence, the signal to noise ratio in the obtained images is significantly better than in single temperature shots. The inspection method provides images of defects within several seconds, but the applicability is confined to conductive materials. However, it can be applied not only to metals (crack detection etc) but also to carbon fibre reinforced laminates where even impact damage can be detected on the front surface.