NDTnet 1998 Aug, Vol.3 No.8

Abstract

Projection Radiography is used since a long time for pipe inspection and corrosion monitoring. The distance of border lines of the projected wall shadows of a pipe onto the film is a direct measure for the wall thickness after correction by a magnification factor. A software based analysis tool can be used for a quick and simple interactive estimation on the digitised radiograph. The automatic determination of the projected wall thickness avoids the problems of correct visual recognition by human inspection.The direct digital access to film densities after film digitisation allows even more: A correlation of the film density change with the corresponding wall thickness based on Beer's absorption law for the penetrating radiation. A two point calibration (nominal wall thickness and a known change in wall thickness) is sufficient for most practical applications. After this calibration step the corresponding wall thickness change can be calculated directly from the density change. This works well for step wedges or steel plates.

The more practical case of pipe inspection is more complicated by the double wall penetration of the pipe. The penetrated wall thickness is changing rapidly in radial direction of the pipe. One possibility to take this into account is a complex geometrical transformation of the estimated wall thickness in projection direction into real wall thickness. For this the centre line of the pipe and the exact source position must be known by the algorithm. But a much more simpler algorithm was developed which is based on a local calibration. No other geometrical information on the pipe beside its rotational symmetry is needed.

The developed software and user interface will be presented. It is integrated into the "Scan Manager System" [1,2] a digitisation, archiving and analysis work stand developed for the needs of a computer based industrial NDT film inspection.Data are presented from the investigation of the application range (wall thickness range depending on the energy, achieved accuracy). The influence of the scattered radiation, medium and insulation is discussed.

The proposed algorithms can be adapted easily for other digital detectors (radioscopy, imaging plates or direct capturing devices). This requires the correct model for the detector response, which is realised by a Lookup table (LUT). This LUT translates the stored digital values of the image into a signal proportional to the detected radiation dose.

[1] http://www.dba-sys.com/ndscan.html
[2] http://www.kb.bam-berlin.de/trend.html

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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