NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

At in-service inspections in German nuclear power plants radiographic testing methods had been applied at a large scale after cracks had been detected at the media affected inner surface of austenitic welds. The defect indications which follow the HAZ are difficult to evaluate in radiographs due to their low contrast and the presence of undercut and large density differences which occur as a result of mismatches and excess penetration.

For the interpretation of crack-like indications an evaluation procedure has been developed which applies digital image processing with a scanning system of sufficient high spatial resolution (35µm pixel width) and a gray-scale resolution of 12 bits (4096 grey levels). The procedure evaluates the half width, the maximum contrast of the RT indication and its length above a defined contrast threshold. This threshold takes into account the optical density, the signal-to-noise ratio and thewall thickness. By comparison with results from metallography of weld specimens for a large number of radiographic crack indications (true and false calls) criteria for a quantitative indication evaluation were defined.

With the help of special algorithms an objective and fast computer based evaluation of crack-like indications is possible, even when the indications lack contrast and are in an area of wide density variation. In the case of plant monitoring radiographic indications can be compared quantitatively in terms of contrast, half width and length to establish evidence in the case of no or possible crack growth.

Figure 1. computer based evaluation: 1.) indication search, 2.) noise supression, 3.) density profile

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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