Time of flight diffraction (TOFD) inspection is an advanced ultrasonic technique which allows for the location and sizing of defects. Two probes are used in a transmitter-receiver arrangement, with the transmitter introducing ultrasonic sound waves at an angle into the material, and the receiver picking up diffracted signals from any defects present.

TOFD is one of the fastest methods of non-destructive because, unlike other types of UT methods, generally only one scan is required to find any defect information within the weld. It can locate and measure the size of many different types of defects with incredible precision. It also has a high degree of repeatability. Because of this, the growth of any flaws can be tracked over time.

The use of ultrasonic diffraction has distinct advantages in the sizing of defects, particularly crack like planar defects. During a ToFD inspection, diffracted waves effectively originate at the crack tips.

ToFD is a single pass operation, not amplitude dependent and very accurate for sizing of vertical defects.

The ToFD method is popular because of its high probability of detection, low false call rate, portability and most importantly, the intrinsic accuracy in flaw sizing, especially in depth.

The technique requires access to both sides from one surface. It requires a probe to run on both sides of the weld so occasionally access can be a constraint.

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