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Currently at Deutsche Bahn (DB) ultrasonic inspections in maintenance procedures for wheelset axles with a bore hole are, to the greatest possible extent, carried out using automated ultrasonic inspection system. Although the acceptance levels are in accordance with DIN 27201 part 7, the testing results have shown in recent years that in the case of true indications, the effective defect sizes were far below the level of acceptance. Due to this experience it can be assumed that the automated ultrasonic inspection systems are testing substantially more sensitively than required. This increased sensitivity leads to an increase in false indications, generally resulting in the unnecessary demounting of wheelsets. In Research cooperation between Federal Institute for Materials Research and Testing (BAM) and DB the effective flaw detection sensitivity of existing automated ultrasonic inspection system will be determined and the true/false indication ratio optimised. Through systematic investigations with the Probability of Detection method on the existing automated ultrasonic inspection system the results could be directly applied to the optimisation of existing ultrasonic inspections of wheelset axles with a bore hole in maintenance procedures and the level of reliability can be considerably increased.
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 it is looked for life damaging objects in the case of mine seeking and when it is even more difficult to distinguish between the different suppliers of methods concerning their actual capability.
In accordance with the conception of EFNDT WG5 and CEN BT 126 CW 07 the authors have the aim to transfer the knowledge in reliability measurement of NDE systems to the reliability measurement of mine detection systems. For this the three basic ways to investigate the 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. 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 theory of systems where the reliability of the total system is composed of 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. A further essential parameter especially for demining systems are the repeatability and reproducibility. Their affect on the reliability and its scatter will be investigated. Examples for the ROC/POD approach in terms of the investigation of the reliability of ultrasonic manual testing and for first attempts for the reliability investigation of demining data will be presented.
It will be strongly recommended to apply the ROC/POD and Modular Conceptions to the assessment and optimization of mine seeking systems.
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 total reliability of a mine searching system is driven by the triple of intrinsic
capability, which describes the physical-technical basis capability, the application and
environmental factors and the human factors. Some of them can be determined in
laboratory measurements but the human factor and a part of environment conditions
and their interaction with the device need to be treated statistically. That is why the
test & evaluation procedure described in CEN CWA 14747:2003, includes in addition
to parameter tests of metal detectors also the reliability or blind field tests under local
conditions with local personnel. A series of three big field trials had been
accomplished in the ITEP-project 2.1.1.2 Reliability Model for Test & Evaluation of
Metal Detectors to specify the optimum conditions for reliable trial results with
affordable effort. For each set of specific working conditions, characterized in terms
of a combination of one mine type in one soil with one detector handled by local
personnel, the searching system will show up a working performance in mine
detection rates as a function of mine depth and show up a certain overall false call
rate. During the ITEP-trials in Benkovac and Oberjettenberg the authors learnt to
determine this function separately for each mine type in each soil, which is especially
important for low metal mines in uncooperative soil which will be illustrated for the
mine PMA2 in different types of soil. The question of representativeness for field
conditions on the one hand and necessary statistical set up for the possibility to
distinguish between individual detector performances is still under discussion.