Scanning pulse phase thermography with line heating

Thermographic investigations with a line heating source have been carried out, to localise small subsurface defects. The object is moved along the heating source, i.e. an induction coil in our experiments. The infrared camera is recording the temperature distribution either in reflection or in transmission mode. From the recorded images, the pixel columns are extracted and a new image sequence is created. After adjusting the images according to the shift between two consecutive recorded images, the sequence represents the temporal change of the temperature after a short homogeneous heating pulse. With Fourier transformation a phase image is created, therefore the technique is called scanning pulse phase thermography. In order to find the optimal parameters, as inspection time and motion speed, finite element simulations and analytical calculations are carried out. Experimental results on ferromagnetic steel samples are presented, showing that in reflection mode a 6 mm defect in a 9 mm thick slab in a depth of 7 mm can be detected (aspect ratio = 6/7). In contrast, in transmission mode, even a 4 mm defect in 8 mm depth could be made visible (aspect ratio = 0.5).