Application of digital image processing to radiographs for the interpretation of corrosion cracks

At in-service inspections in German nuclear power plants radiographic testing (RT) methods had been applied at a larger scale after. cracks had been detected at the media. affected inner surface of austenitic welds. The defect indications which follow the heat affected zone are difficult to evaluate in radiographs due to their. low er 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 sufficiant high spatial resolution (35 mu m pixel width) and a grey-scale resolution of 12 bit (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 the wall 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 definded.