Scanning pulse phase thermography with line heating

Abstract

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).

Comparison of pulse phase and thermographic signal reconstruction processing methods

“…The width of the induction coil is 5 mm, with scan speeds in the range of 0.02-0.2 m/s, and heating duration 0.025-0.25 s. The sample passes under the induction coil, and then into the camera field of view so that the camera records the sample surface temperature 2-5 s after it is heated, depending on the scanning speed and on the field of view of the camera. The recorded images are digitally shifted to remove the effects of motion between consecutive images, so that the sequence is equivalent to that of a stationary sample that has been heated by a short, homogeneous pulse [9]. The power of the 10 kW generator was adjusted for each scan speed so that a net temperature increase of 3-10 C° was achieved in the samples.…”

Whole-electronic line-focus light-scanner for active thermography

“…Active-thermography with a line-focus heat source, or line scan thermography (LST) has been developed due to its rapid inspecting ability of large areas with minimum time. Recently mechanically-scanned LSTs with both optical and electromagnetic heating scheme have been developed as non-destructive evaluation (NDE) tools [1][2][3] and even sold as a product [4].…”

Temperature reconstruction of infrared images with motion deblurring

“…for steel. It can be excellently carried out in a scanning way, where the objects are moved continuously below or through the induction coil and the camera records the temperature distribution shortly afterwards (Oswald-Tranta et al, 2010b;Oswald-Tranta and Sorger, 2012). Figure 12 shows a small bell, which is moved below a coil, positioned on the left side, already outside of the image.…”

“…Using this a priori knowledge, an additional step is necessary to transform the image first into a linear motion. For example, a rotation can be transformed into a linear motion (Oswald-Tranta and Sorger, 2012), which can be deblurred, and then by applying the inverse transformation an image with high contrast is obtained. Figure 15 shows the infrared image of two small balls with the same temperature, fixed to a stick which is rotating around one of its ends.…”