T-T / TT-T / T-TT / TT-TT

T-T, TT-T, T-TT and TT-TT are the notations used to describe the shear wave imaging paths used in the generation of total focusing method (TFM) images. The ‘T’ denotes transverse waves. Accurate TFM images are reliant on the beam paths to the various pixels in the image. The letters to the left of the hyphen denote the transmitted leg of the path and the letters to the right of the hyphen denote the received path. So, T-T is a shear beam down to the defect, which is reflected directly back to the probe.

TT-T is the transmitted shear beam down to the backwall reflected back up to the defect and then reflected back to the probe.

T-TT is the reverse: the transmitted shear beam down to the defect, reflected down to the backwall and then back up to the probe.

TT-TT is the transmitted beam down to the backwall and up to the defect, where it is reflected back down to the backwall and then back up to the probe.

Longitudinal wave imaging paths are denoted by ‘L’ and there are similar combinations of L imaging paths. Where there are mode conversions at the backwall or at the defect, then the imaging paths become combinations of T and L, for example TT-L is a transmitted shear wave beam down to the backwall, reflected up to the defect where it is mode-converted to a longitudinal wave beam, which travels straight back to the probe.

For diagrams and more information on T-T, TT-T, T-TT and TT-TT, see:
BS ISO 23865:2021, ‘Non-destructive testing – Ultrasonic testing – General use of full matrix capture/total focusing technique (FMC/TFM) and related technologies’, 2021.
or
ASME Section V, Article 4, Non- Mandatory Appendix F.
or
View the websites of the original equipment manufacturers (OEMs) of FMC/TFM equipment.

WHAT THE HEC?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org