TWI Frequently asked questions
by Mark Kirby
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 provide extremely accurate estimates of flaw size, in particular the through-wall height.
However, the TOFD technique 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. 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. Depending on the coupling variations and probe characteristics, this feature can occupy a significant proportion of the material thickness on TOFD images. Likewise, small flaws (<2mm in height) that break the far (or backwall) surface are often obscured by the large backwall echo response, which can be very irregular depending on the condition of the surface/weld root. It is therefore recommended that automated pulse-echo ultrasonics is used forinitial detection scans, unless a TOFD procedure has been validated for the detection of all flaw types and sizes of interest. If appropriate, TOFD should then be used to specifically target any such flaws in an attempt to determine their through-wall height. An ideal solution would be the use of a combined pulse-echo/TOFD system, e.g. phased arrays.
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.
