Table of Contents ECNDT '98
Session: Nuclear Industry
Interpretation of Radiographs by Digital Image ProcessingT. Just*, W. Thale, R. Clausen, TÜV Nord, Hamburg
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fig. 1 Corrosion cracking in an austenitic weld
fig. 2 Contrast, halfwidth and noise level of a RT indication
In order to make evaluation of grey-scale profile plots more objective and faster, we developed an algorithm for a computer-based evaluation of crack-like indications /4/. The automatic evaluation is conducted in four steps: to suppress noise, an averaging routine is performed of a number of scan lines, for example 5 lines or 175 (m, predetermined by a parameter data set. In the second step, the search phase, the scan lines are searched for relative minima above a contrast threshold D = 0.01. To suppress signals with little or no linear coherence in the direction of the expected defect orientation (longitudinal or vertical to the weld axis), all indications are deleted in the next step whose length is less than a pre-set value. Fig. 3 shows the result of automatic evaluation with a length threshold of 10 mm. In the digitized radiograph the areas of indications which meet the pre-set criteria remain (minimum contrast D greater than 0.01 and minimum length 10 mm). The last step is to print the remaining indications with their half width and their contrast.
| 1. indication search 2. noise suppression 3. density profile
fig. 3 Computer based evaluation of digitized RT indications
The printed out indications are evaluated with regard to their pattern and the parameters (maximum contrast, width and length) derived from the grey scale values. They are classified as
fig. 4 Contrast distribution of crack and non-crack indications
fig. 5 Max. contrast and halfwidth of crack and non-crack indications
fig. 6 Max. contrast vs. wall thickness of crack and non-crack indications
fig. 7 Comparison of interpretation results between conventional film viewing and digitized radiographs applying the proposed criteria
The values of these can be quantified in digital radiographs very easily. By comparing RT indications with the results of the destructive examination, the following criteria have been found by us to discriminate the relevant indications e.g. cracks and lack of root fusion against non-relevant indications as superficial shrinkage grooves:
The interpretation results from conventional film viewing and from digitization applying the discussed criteria are shown in fig. 7. Even with digital image processing only those indications can be interpreted as defects which differ from the noise level. Therefore, the radiographic testing parameters have to be optimised when generating a radiograph in order to reach a flaw detection sensitivity as high as possible. With the proposed evaluation criteria a more reliable interpretation of defect indications can be achieved with the help of digital image processing of radiographs.