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I understand that the ultrasonic time-of-flight diffraction (TOFD) technique can be used to rapidly inspect welds for the presence of planar flaws. Is this the case and what do I need to be aware of?


Frequently Asked Questions

Yes, the TOFD technique can be used to rapidly inspect welds for the detection of a variety of flaw types, including planar flaws. In addition, the TOFD technique has been shown to demonstrate accurate flaw sizing, in particular the through-wall height.

Like all NDT techniques TOFD does have some limitations. For example, it is best suited for the detection of flaws that do not lie close to, or break, a surface. In the past it has been shown that during initial detection scans, surface breaking planar flaws can (depending on their through-wall extent) fail to be detected by TOFD. Flaws that are open to the scanned surface can often be obscured by the presence of the lateral wave response which makes identification and sizing difficult. However, modern TOFD equipment has come some way in combating this particular problem by:

  • Use of shock-wave transducers (high energy, short pulse) reducing the lateral wave effect to 2-3mm
  • Signal processing, lateral wave straightening and lateral wave removal
  • Equipment can be configured to target specific zones within a component under examination.

Because of these control measures the near surface indication detection is significantly improved, however if very near surface critical sizing is called for it may be necessary to ally TOFD with a NDT surface inspection technique.

Other considerations that need be taken into account when looking at the suitability of TOFD are:

  • It is difficult to obtain good results on thinner materials. Even with 10 or 15MHz probes, it is not recommended for use on thicknesses of less than 13mm ( 1/ 2").
  • For initial detection purposes, a 2-dimensional TOFD image is usually produced (scanned length versus depth). To fully characterise a flaw indication, it will be necessary to perform additional scans to be able to position the indication in the remaining dimension.
  • Although standards exist for the application of TOFD, specific acceptance criteria for defects have yet to be established.
  • A high degree of operator skill/experience is required to correctly interpret and report the findings.

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