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 many years to draft and introduce such criteria, with varying levels of success.

In recent years, it has become clear that the Time 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, enabling the application of Engineering Critical Assessment (ECA) or Fitness For Purpose (FFP) approaches, again the lack of adequate acceptance criteria still prohibits its introduction for routine inspections.

Favourable results with the use of TOFD in various evaluation projects have demonstrated that this technique is indeed very powerful, and combines a high Probability Of Detection (POD) with a low false call rate. In addition, the simple scanning concept enables application on almost any component, 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 suitable for Good Workmanship inspection, if only acceptance criteria existed.

The presentation also describes a Dutch project that aims for the development of defect acceptance criteria for TOFD inspection, and summarizes the first results of the evaluation of draft criteria. Examples of TOFD applications are given, ranging from pipeline girth welds to vessels with wall thicknesses up to 400 mm. Some experiences on complex geometries as well as high-alloy steels and Titanium will also be highlighted.