Quantitative NDT and Fitness for Purpose

The concepts of Fitness for Purpose (FFP), now popular in engineering, have placed greater demands on Non-Destructive Testing/Evaluation (NDT) to provide quantitative results. No NDT method seems better adapted to provide quantitative results than ultrasonic testing. However, acceptance criteria presented to the ultrasonic operator are sometimes difficult to comply with, as they seem not to consider the limitations and tolerances intrinsic in the techniques used. Overly optimistic expectations of sizing accuracy could result in under-sizing of flaws, but overly stringent acceptance criteria to “compensate” for sizing errors can result in more repairs than would be required by radiographic workmanship acceptance criteria in spite of much higher detection rates by ultrasonic methods. This paper considers some ultrasonic sizing techniques and their associated accuracies and application limitations. Examples from early work in Time-Of-Flight Diffraction (TOFD) and tip diffraction are considered as well as statistical studies from girth weld inspection techniques.