·Table of Contents
·Workshop - Reliability
Human Factors: The NDE Reliability of Routine Radiographic Film EvaluationFerenc Fücsök, Budapest Power Plant Ltd, Hungary
Christina Müller(Nockemann), Martina Scharmach, BAM, Berlin, Germany
In order to estimate the performance of inspectors and laboratories involved in NDE, volunteer laboratories from Croatia, Hungary and Poland conducted a series of round-robin inspections using X-ray films of welds. A set of well-characterised X-ray films was sent to each laboratory for their individual assessment. The selected films were initially evaluated by BAM's Reliability Laboratory with the assistance of an image-processing computer program. This established a baseline of "true values" for each weld image. Each image, in turn, was evaluated by individuals at the participating laboratories.
Using the results obtained from the round-robin study ROC diagrams were established to give an overview about the overall reliability of routine inspections.
The paper is organized as follows: The next section will give a short background of the ROC method. Then the practical procedure of the RRT is described. Finally the results in terms of the ROC diagrams are presented and discussed.
|Fig 1: The four cases of NDT-diagnosis|
|Fig 2: Creation of an ROC curve (theory)|
|Fig 3: Practical creation of an ROC curve|
So - in the lower part of the curve the highest signals (correct indications) are included and only a small amount of noise (false calls). In the higher part more and more all of the defects are taken into account but also a greater amount of false calls has to be paid as price. In practice it is not possible to apply continuously growing signal thresholds and to count correct and false call rates for each. Therefore different discrete categories of signal counting are defined to be applied by the inspectors during the non-destructive testing evaluation as indicated in Fig. 3. These categories might correspond to the visibility of defects on a radiographic film or to an echo height in an ultrasonic A-scan. We call it detectability later on. So we yield five different experimental points in the ROC diagram - in the current RRT investigation we reduced it to four. The maximum point represents the actual possible operating point. From the whole curve shape - which can be obtained by using a special regression method on the basis of the binormal model - the overall capability of the system is indicated. There is e.g. a forecast possible what will happen when the sensitivity of the system will be raised: Is there a gain in defect finding or is only the false alarm rate increasing? Considering the area under the ROC-curve (see Fig. 4) it may vary from 0.5 (pure chance curve 1) up to 1.0 which corresponds to an ideal NDT system belonging to the left corner's step curve. For the fictive systems shown in Fig. 4 the performance of the system increases from curve 1 to curve 7.
|Fig 4: Differentiating NDT-systems by ROC curves|
To try to estimate the reliability of the human factor in radiographic film evaluation an international Round-robin test (RRT) was organised. The RRT is in progress in Croatian, Hungarian and Polish laboratories by voluntary attendees. For the purpose of the RRT a set of films was selected in the Reliability Laboratory of BAM in Germany which provides the scientific and technical support.
The selected 38 films contain more than 200 defect indications of different types and dimensions.
The selected films were scanned by a state of the art film digitizer (LS85 SDR, Lumisys). The digital images were evaluated with the help of a dedicated image processing computer program of BAM. The results of this evaluation are taken as the true values of the discontinuities. The types of the discontinuities were discussed and agreed by a small group of Hungarian experts. At the same time the X-ray films were copied with a laser printer (AGFA Scopix LR5200), for further information see . Four sets of films were printed into AGFA Scopix Laser films which were sponsored by AGFA. In this way all of the participants of the RRT evaluate exactly the same films.
The voluntary evaluators were provided with clear instructions of the procedure and specific forms for the support of the evaluation work and to aid the pre-processing of the results with the computer.
The forms contain the columns of identification, the code of defects/imperfections according to EN 26520:1991, the co-ordinates, the dimensions and the detectability of the discontinuities and for completeness the IQI and the optical density of the films
The inspectors were asked to evaluate and identify each weld image cm by cm which is a very strict prescription. Additionally they were required to fill in a form for the circumstances of the film evaluation including the length of evaluation time. The evaluation work of the 38 films took 5 to 8 hours. The longer the evaluation time the more detailed discontinuities were indicated.
The evaluation results were filled in an Excel table to support the data processing of the indication results for the ROC statistics.
From the international RRT test with 19 human inspectors we learned about the value of years of professional experience for high testing performance and smalll scatter of results. We learned further about the influence of the individual capability of each person. From this level of knowledge we could recommend ROC tests for film evaluators to exclude pure chance results. For a final confirmation of these results we shall wait until the end of our RRT when a higher statistical basis will be available.
We thank Dr. Uwe Zscherpel and Gisela Malitte from BAM for supporting the digitization and film copy work. We thank AGFA for providing the film material for the copies. We thank further all the voluntary inspectors for the effort of film evaluation.
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