When new NDT methods are developed and introduced, such criteria often do not exist. Most criteria are based on "good old" radiography. This problem has been encountered during the long-lasting introduction of mechanized ultrasonic techniques for e.g. welds in vessels and pipelines. Industry has already been struggling for decades to draft and introduce adapted criteria for such techniques, with varying levels of success. Still no internationally acepted criteria exist, not even a draft.
In recent years, it has become clear that the Tirne Of Flight Diffraction (TOFD) Technique.developed in the UK in the seventies, is a most powerful technique not only for accurate sizing of known defects but also for defect detection in routine NDT. But, whereas TOFD may have been successfully introduced for recurrent inspections and fingerprinting, using Engineering Critical Assessment (ECA) or Fitness For Purpose (FFP) approaches, the lack of adequate acceptance criteria still prohibits its introduction for routine inspections.
Favorable results with the use of TOFD in various evaluation projects have demonstrated that this technique combines a high Probability Of Detection (POD) with a low False Call Rate (FCR). In addition, the simplicity of its scanning concept enables application on many different components, including complex geometries such as nozzles and nodes: routine inspection results thereby provide information on through-thickness extent and flaw length. Thus, one would say that the TOFD method should be ideally suited for routine inspection if only acceptance criteria existed.
This paper also addresses a Dutch project that aims for the development of defect acceptance criteria for TOFD inspection. Examples of TOFD applications are given, ranging from pipeline girth welds to vessels with wall thicknesses up to 400 man. Some experiences on complex geometries as well as high-alloy steels and Titanium will also be highlighted.
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