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 criteria still prohibits its Introduction for routine inspections.
Although the present Good Workmanship approach actually gives conventional NDT methods the credit they deserve (their capabilities are well-used), there should also be a certainty beyond reasonable doubt that an accepted weld is strong enough for service. Many years of industrial experience have demonstrated that this certainty statistically exists. But there is a price to be paid. Good Workmanship acceptance criteria for conventional technique must, to a certain extent, be conservative, in order to compensate for the inherent "limitations" of conventional NDT. And, what is worse, the degree of conservatism is more or less unknown. Therefore the question can be asked: "if the historic background of present acceptance criteria would not exist, what would we like to know about a weld to be able to accept or reject it?"
In the ideal situation we would like to have a balanced combination between evaluation of the welders' performance on the one hand and a fracture mechanics basis on the other hand. The second aspect could be regarded as a safety net with a balanced conservatism. This requires not only the use of modified criteria, but also the use of a NDT technique that can provide the necessary information: a technique that can detect small defects for evaluating weld quality as well as estimate the through thickness extent of planar defects, if present, both with a high reliability (Probability Of Detection or POD). On the other hand, a low false call rate is required. Criteria should thereby be formulated in such a way that full advantage of capabilities of the NDT method is taken, without leading to unnecessarily high repair rates. It is the compatibility between the NDT technique and the criteria that counts: a balanced combination of certainty and a degree of conservatism not higher than needed.
Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00
Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:
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