# Inductive Thermography as Non-Destructive Testing for Railway Rails
## 6 Measurements of Head Checks from Rails in Service
### Inspection of Long Rail Pieces  
For the inspection of long rail pieces two different kinds of measurement were carried out and compared, a'stop-and-go' measurement and a scanning technique:  
* In a'stop-and-go' procedure a linear table moves the rail for a certain distance between two static measurements. In one measurement a rail piece of about 25 cm is inspected, and each static measurement is separately evaluated to a phase image. For the calculation of the panoramic view of the entire rail piece, in first step a rectification of the phase images is necessary. This means that perspectivity and distortion due to the IR camera optics has to be eliminated. This was carried by usage of a calibration object, for which a chessboard similar pattern was taken. This technique will be described in more detail in a separate publication. Figure 17 shows a panorama image, which was created out of 5 overlapping static measurements for RP02. One static measurement covers an area of 440 \(\times\) 1273 pixels with a resolution of 45.225 pixels/mm. Since the size of each image and the movement of the linear table is known, the images can be shifted horizontally to their approximate position. In the next step, the exact position was calculated by means of phase correlation in the overlapping area of two consecutive images, and the images were merged to form one panorama image. This step was repeated for each consecutive images to create one panorama image from the 5 separate measurements. As in this figure the cracks are not well visible due to the limited pixel resolution of a print-out, we have created a video, which is available as additional online material in Supplementary Materials to this paper. Figure 16: Two options for counting head checks for RP02; (**a**) distances of head checks with profile of the phase image; (**b**) distances of head checks with edge detection. The previously presented characterization technique of the head checks can also be applied to the panorama images. Figure 18 demonstrates this. This was done for the same image as in Figure 17, but here only a shorter part is shown, as the head checks can be better seen in this way. Figure 19 shows the same edge detection technique applied to the rail piece RP04. It is of note that the head checks on this rail piece have another orientation along NE-SW, as the travel direction was from the right to the left. * In the second measurement technique the rail piece is continuously moved by a linear table and it is'scanned' in the region heated by the induction coil. The top section of the rail is heated with a single thread coil and the IR camera is positioned in a way that the areas before and after the heated area, as well as the area under the coil, are recorded. The recorded IR sequence is re-ordered in such a way that each location of the rail has its temperature value assigned according its distance behind the heating position [25]. Likewise, in this case a rectification of the recorded sequence is necessary to eliminate the perspectivity in the image created by the IR camera. The transformed and re-ordered sequence shows a heating process which is similar to a static measurement and an evaluation to a phase image is possible. In Figure 20 the resulting phase image of such a scan can be seen, performed on RP02. Although the scan was carried out over the whole length of 25 cm of the rail, only 10 cm of the phase image is shown in Figure 20, due to the otherwise too large size of the image. Comparing the two measurement techniques, the following points can be stated:  
* With the'stop-and-go' method the phase images have better contrast than with the scanning technique, and the head checks can be better located and characterized in higher quality. * The'stop-and-go' inspection method is slower and more difficult to realize for inspection of long rails than the scanning method. Figure 17: A panorama image from five measurements of RP02 (heating pulse duration: 0.3 s).