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| South Wales Branch
Exploring the benefits of TFM

Colin Macfarlane reports

For our first webinar (virtual) meeting of 2021, the South Wales Branch was very pleased to welcome Harry Brittin of Sonatest, with a presentation titled: ‘Benefits of using the total focusing method (TFM) for ultrasonic inspection’.

The presentation began by explaining that the TFM is an ultrasonic phased array technique, which uses the data from full matrix capture (FMC) to produce an image that is focused at every specified point in the image. TFM is implemented by applying an algorithm to the full dataset collected by FMC.

Harry moved on to identify the pulse-echo (direct) and self-tandem (indirect) wave sets/propagation modes that give TFM the advantage in finding indications non-perpendicular to the beam direction. The importance of using the correct propagation mode was illustrated with the example of a 5 mm-deep slot in a 50 mm-thick carbon steel block. With the slot positioned on the top surface, a 5T mode was necessary to fully detect the slot, while the same slot on the bottom surface required a 3T mode.

The presentation continued with some examples of the use of TFM. The first involved a 7 mm carbon steel plate with corrosion pits. Images showing a clear profile of the corrosion pits were obtained using a 10 MHz/64-element standard phased array transducer. It was noted that the FMC technique does not require the use of specialised phased array transducers. In introducing the next example, Harry stated that: “a big advantage of using FMC/TFM is the ability to detect defects in multiple orientations.” Successful detection of corrosion/erosion was shown on a carbon steel flange in a groove and inner diameter using a 5 MHz/ 64-element standard phased array transducer.

Moving on to weld inspection, it was highlighted that there can be situations where it is only possible to inspect from one side of a weld, making it impossible to detect lack of sidewall fusion (LSWF) on the opposite side of the weld using conventional ultrasonic inspection. The use of TFM, however, as Harry noted, “greatly increases” the possibility of detecting LSWF on either side of the weld. Other examples of TFM providing more accurate sizing and positioning data for common weld defects were given, including centreline cracks, slag inclusions and lack of root fusion. The use of TFM for bolt inspection was next covered, with the improved detectability of defects within the bolt thread region using TFM. The final example showed improved defect detectability with the use of TFM over standard phased array ultrasonic testing (PAUT) in the inspection of glass-reinforced plastic.

The presentation concluded with some advantages and disadvantages of TFM compared to phased array.

Advantages given included:
  • Improved sensitivity to flaws non-perpendicular to beam direction
  • Increased ‘resolution imagery’ provides enhanced defect characterisation
  • Total zone of interest is in focus
  • Increased index resolution, as TFM is not bound by probe pitch.

The disadvantages noted were:
  • Correct wave sets/propagation modes must be selected, otherwise defects may be missed
  • Significantly slower scanning speeds compared to phased array
  • Large data files.

A short question and answer session concluded the presentation.

The South Wales Branch would like to express its thanks to Harry Brittin for an interesting and enjoyable presentation. In addition, special thanks go to Karen Cambridge of BINDT for once again making all the necessary arrangements and facilitating the webinar meeting.