|NDT.net - August 2000, Vol. 5 No. 08|
Staatliche Materialprüfungsanstalt, Universität Stuttgart
Pfaffenwaldring 32, D-70569 Stuttgart, Germany
Technischer Überwachungsverein Nord e.V.
Große Bahnstraße 31, D-22525 Hamburg, Germany
Corresponding Author Contact:
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|TABLE OF CONTENTS|
The test pieces consisted of cut-out austenitic weld specimens containing service induced cracks. After the non-destructive testing metallographic analysis was performed in order to verify the NDT results. With these data the performance of the NDT could be assessed using POD and ROC curves.
Welds produced since the 1980's normaly do not have these geometric constraints. In new power plants or in those, where pipes have been replaced it is far more easy to produce good NDT-results. Except for geometric constraints, the ultrasonic inspection is also affected by the dendritic structure of the weld material which may lead to skewing or tilting of the ultrasonic beam, to scatter or high attenuation. The reliability of the NDT-result is influenced by these factors. This has been a topic in many investigations and research programmes in the past. Examples that should be mentioned here are performance demonstration methods developed in the USA and the international Programme for the Inspection of Steel Components (PISC). The European Network for Inspection Qualification (ENIQ) has developed a methodology for qualification that shall lead to standardized NDT qualifications in the future. In Germany the requirements for non-destructive testing of austenitic and dissimilar welds are listed in the KTA-code. The testing techniques have to be according to the state of science and technology, which is envigilated by independent technical experts like TÜV ( Technischer Überwachungsverein i.e. Organisation for Technical Envigilation). The qualification of non-destructive testing methods in Germany is done by
Furthermore, service experiences with the components are also considered in the qualification. The qualification of NDT methods including the written procedures is envigilated by technical experts of the regulatory bodies. This method has been applied with good results in the past. In the future, in Germany qualification of NDT-methods will be done according to the ENIQ methodology. A research programme sponsored by German VGB and performed by MPA Stuttgart (Materialprüfungsanstalt , Institute for Materials Testing) had the objective to evaluate methods to assess the performance of non-destructive testing methods. For this programme, material with real IGSCC originating from replacement of pipe systems in several German boiling water reactors could be used.
The research programme consisted of the following steps:
Fig 1: Drawing of typical weld used in the round robin test
Fig 2: Typical shape of a weld produced in the 1970's
Fig 3: Typical shape of aweld produced in the 1980's
The round robin test was performed as a real blind test, i.e. the inspectors did not have any information about the flaws in the test specimens and did not get any feedback about their performance until all teams had finished their inspection task. The following NDT-methods were applied:
A comparison of radiographic film and digitized films was also performed. This paper concentrates on radiographic and ultrasonic testing.
The NDT result for each grading unit as stated by the inspection vendor was compared to the results of the metallographic investigation. The detection rate as a function of the flaw depth could be displayed in a POD-diagram (probability of detection). As the number of defects is still limited, it is more appropriate to speak of a detection rate rather than of detection probability. With the POD-diagram, it is possible to compare the detection rates of inspection teams and also to compare different inspection methods. The detection rate is a good means for this purpose, but it does not state how sure and how reliable the NDT result is.
The NDT of a weld can have four possible results:
With these values, the flaw detection detection rate FDR can be defined as
FDR = S (TP) / S ( all sections with defects ),(1)
whereas the false call rate FCR is defined as
FCR = S (FP) / S ( all sections without defects ).(2)
The flaw detection rate FDR can be displayed as a function of the false call rate FCR. A diagram of this type reveals the relative operating characteristics and is therefore called ROC-diagram. It allows a better assessment of NDT methods and inspection teams than a POD diagram.
In order to have a clear assessment of the NDT methods it was necessary to define regions in the ROC diagram that can be used for evaluating the curves. In Fig. 4 the respective regions in the ROC diagram are marked. An excellent performance would be given in the upper left corner of the diagram. A high detection rate combined with a low false call rate gives excellent knowledge about the integrity of a weld with reasonable costs. The regions marked 1-4 are varying degrees of acceptable NDT performances.
Fig 4: Evaluation zones in ROC-diagram
Fig 5: ROC diagram for radiographic testing
Fig 6: ROC-diagram for ultrasonic testing
In Fig. 6 the ROC curve of one of the better ultrasonic teams is displayed. The left curve again reveals the detection performance achieved at new welds. The flaw detection rate is more than 80% for defects with a TWE of 25% of the wall thickness.
For both UT and RT the improved geometry of the welds produced in the 1980's (see Fig. 3) leads to a increased inspection performance. The flaw detection rate is higher ( note that the curves for 'old' geometry cover 30% TWE instead of 25% ) and the false call rate is considerably reduced.
The transfer from the results of a round robin test to results that can be achieved when using the NDT-method in a power plant is possible and may be useful. Nevertheless, some aspects have to be considered:
If there is a doubt about the intgrity of a weld, additional NDT-techniques (like using a different incident angle or focussed probes) are applied. In such a case, often diversitary NDT-methods ( e.g. eddy current or radiographic testing, when the first method was UT) are used to get a better understanding of the integrity of a weld.
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