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Proposals for Performance Demonstration and Modular Reliability Assessment for Humanitarian Demining
(2003)
For safe and reliable demining it is necessary to determine the actual true performance of mine searching equipment in detecting mines. The subject to detect a hidden subject by penetrating physical interaction with the target is similar to that of non-destructive testing where it is looked for hidden cracks etc. in material via waves and rays. The non destructive testing profession is now about 100 years old and developed some procedures to check reliability of testing. Those principles like the performance demonstration where the successful detections are statistically evaluated against false calls rates and their implementation in an industrial standard (ASME section XI appendix VIII) are analysed. A first adoption to demining was accomplished in the prescription for blind trials in the CEN workshop agreement CEN BT 126
CWA07 for test and evaluation of metal detectors. A number of blind trials were accomplished within an ITEP project to learn about the necessary statistical layout of those trials to achieve true, reproducible and repeatable results to give guidance to selection and improvement of metal detectors. The special focus in these investigations was on the influence of the human factor due to the degree of experience of the operators and the infulence of uncooperative soil. The correlation of the results of the physical parameter measurement and the statistical results is analysed in a first attempt.
Because the demining process is always connected with a danger for human beings, it is necessary to make sure the proper functioning of mine searching equipment in detecting mines. Detecting a hidden target by penetrating physical interaction of rays or waves is similar to non-destructive testing. The non-destructive testing profession developed procedures to check the reliability of testing. These procedures, like the performance demonstration where the successful detections are statistically evaluated against false call rates and their implementation in an industrial standard (ASME section XI appendix VIII) are used as a template. A first adoption to demining was accomplished in the prescription for blind trials in the CEN workshop agreement CWA 14747: 2003 for test and evaluation of metal detectors. Within an ITEP project a number of blind trials were carried out to learn about the necessary statistical basis of those trials to achieve true, reproducible and repeatable results to give guidance for the selection and improvement of metal detectors. The special focus in this first investigation was on the influence of the human factor due to the different degree of experience of the operators and the influence of the soil.
The reliability investigations are dedicated to throw light on the performance of the NDE system with
respect to the required aim. This is especially of interest when digitized and processed signals are
involved where it becomes hard to assess whether the quality of the system has raised or not. Three
different ways to investigate reliability of NDE signals will be described. The first way of investigation,
the performance demonstration, is preferred e.g. in the US American nuclear power industry. This is an
integral consideration of the non destructive test as a system where the whole NDE system is packed in a
black box and only the input in terms of the real existing flaws in the component is considered and
compared to the output in terms of the indications of the human inspector or of the automated system.
The second the European tradition relies on a standardized description of physical/technical
parameters of the NDE system which are preconditions for successful system performance. An example
for such a standardized set of performance parameters is given in the recently released standard about Xray
film digitization CEN EN 14096. The third approach the modular conception is a marriage of
both: The signal chain is cut into main modules. Each module is assessed in a most appropriate
individual way e.g. via modeling calculations. The single results are joint together according to the
reliability of the subsystems. Separating criteria for the system were proposed through a reliability
formula developed during a series of European-American workshops on NDE reliability. Examples of
all three approaches will be given.
Das Basisanliegen - Einleitung
Mit der zunehmenden Zahl akkreditierter ZfP-Firmen und zunehmender Anwendung risikobasierender Konzepte ergibt sich die Notwendigkeit, internationale Standards wie die DIN EN ISO 17025 und darüber hinausgehende Richtlinien bei der Verfahrensauswahl in der ZfP zu berücksichtigen. Dieser Aufsatz soll aufzeigen, welche internationalen Aktivitäten gegenwärtig für die Zuverlässigkeitsbetrachtungen von Bedeutung sind (siehe auch [1]). In Übereinstimmung mit den Vereinbarungen und Definitionen der European-American Workshops zur Zuverlässigkeit [2, 3] gilt: Das Basisanliegen der Zuverlässigkeitsuntersuchungen beliebiger diagnostischer Systeme besteht darin, den Grad festzustellen mit dem das Prüfsystem in der Lage ist, seinen vorgesehenen Zweck zu erfüllen. Im Falle der ZfP besteht dieser Zweck zumeist in der Aufgabe, Bauteildefekte aufzufinden und zu beschreiben bzw. die Defektfreiheit von Bauteilen zu bestätigen, sowie Wanddicken zu messen oder Material zu charakterisieren.
Das Basisanliegen der Zuverlässigkeitsuntersuchungen findet seinen Niederschlag bei folgenden international relevanten Aktivitäten:
neue ISO 17025 als Notwendigkeit mit variablen Möglichkeiten der Verfahrensvalidierung
als Kettenglied in RBLM (Risk Based Life Time Management) (r) EPERC-Initiative
CEN/TC138/WG9 "Qualifizierung von ZfP-Verfahren"
Six-Sigma Strategie: als notwendiger Bestandteil
ASNT-Initiative: Standing Committee on NDE Reliability: als Arbeitspaket
Produkthaftung: als notwendige Voraussetzung für die Handhabung
Reliability of Routine Radiographic Film Evaluation - an Extended ROC Study of the Human Factor
(2002)
Many radiographic exposures and film evaluations are made daily in a typical industrial NDT laboratory. But questions remain regarding the precise probability of detecting discontinuities, including the reliability of each individual inspector or laboratory. Although most experienced inspectors of the laboratories evaluate each radiograph, the actual quantitative reliability of each of these inspectors remains unknown.
In order to estimate the performance of representative inspectors and laboratories involved in NDE, volunteer laboratories from Croatia and Hungary conducted a series of round-robin inspections using X-ray films of welds. While the RRT is still in progress, the authors try to summarize the results of the second round and discuss the evaluation method. The results of the first round had been reported already. The focus is here to present the new results in terms of additional inspectors and a new more field oriented evaluation scheme.
So far the results of a total of 35 inspectors of the different laboratories were evaluated. Those inspectors were selected who have done usually the work of radiographic film interpretation. They have from a half up to 35 years professional experience. For better overview we created four groups: maximum 4 year-experience, from 5 to 12 year-experience, from 15 to 25 year- experience and from 26 to 35 year-experience.
Discussing the circumstances of the RRT with the participants, many of them expressed, they could not exactly follow the prescription of the test. According to their professional experience the small discontinuities can be omitted because of the strictest acceptance level accept them. So the most experienced evaluators did not write the small gases and slags into the list, which yield a wrong reliability of detection. As a consequence we should accomplish two types of evaluation: taking into consideration of all discontinuity and only the bigger once they are over the acceptance level.
In addition we analyzed the true values in a critical review. These developments were made and the new results of the evaluation of the testers were more precise and reliable.
The corresponding results are evaluated and presented in a ROC diagram where the p(TP) is plotted against the p(FP). The p(TP) means the probability of a true positive indication (correct defect finding) and p(FP) means the probability of a false positive (false alarm) indication. From the mean value of all inspectors maximum operation points we can estimate a representative overall reliability in terms of a correct indication rate of about 68 % (64 % new) and a false call rate of about 17 % (8 % new). This is a reasonable result within the strict 1 cm score.
During the second European-American Workshop on NDE Reliability, September 99 in Boulder, the term NDE reliability was defined as the degree that an NDT system is capable of achieving its purpose regarding detection, characterization and false calls. The most common but also most expensive - way to determine this degree of capability in e.g. defect detection is to make just a performance demonstration using realistic test samples and to count the correct detections and false calls. For a more efficient way of reliability evaluation we propose to decompose the system into main modules e.g. according to the reliability formula set up on the first workshop: into f(IC) a function of IC the intrinsic capability, determined by the physics and the technique of the NDE method representing an ideal upper bound of the reliability and g(AP) a function of AP the industrial application factor like surface state or limited access to a component in general diminishing the ideal capability and finally h(HF) the function of the human factor which is in general also diminishing the ideal capability. The workshop proposed a rather plain mathematical shape for the formula R = f(IC) g(AP) h(HF) which should be considered merely as a philosophical expression but not as exact mathematical formula to be applied for evaluation of e.g. POD data. We propose to decompose the NDE system into modules in terms of functions of IC, AP and HF if appropriate or additional ones if necessary and then to analyze the mutual relationships of the terms via fault tree analysis. Finally the total reliability of the system is composed of the reliability of the subsystems via the rules of statistical systems theory.
First trials for this approach of data analysis will be presented via examples from NDE systems in the aerospace industry.