Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

TOFD is generally recognised as the most accurate ultrasonic technique for
measuring the through-wall height of planar flaws that lie perpendicular to the
surface and as a method for detecting and quantifying crevice corrosion at the
weld root. At present, national standards for the application of TOFD exist,
however, no acceptance criteria have been agreed upon.
The TOFD technique is suited for the detection and sizing of all types of
embedded flaws, especially those planar in nature. However, the detection of
small near the scan surface flaws can be more difficult due to the presence of
the lateral wave response which often occupies several millimeters of the
depth axis on images.