Time of Flight Diffraction Ultrasonic (TOFD)

Definition - What does Time of Flight Diffraction Ultrasonic (TOFD) mean?

Time of flight diffraction is an ultrasound testing method used primarily to detect welding defects. When using the time of flight diffraction method in its simplest form, two ultrasound probes are placed on each side of the completed weld joint. One of the probes sends signals that travel across the volume of the weld and are picked up by the other probe.

Corrosionpedia explains Time of Flight Diffraction Ultrasonic (TOFD)

Time of flight diffraction ultrasonic testing is excellent for detecting weld discontinuities and defects. To do this, at least two ultrasound probes are moved about the weld. One probe is known as the transmitter, which sends ultrasonic waves along the top of the weld joint and the backside of the weld joint. The transmitted waves are collected by the other probe, known as the receiver. There are systems with multiple transmitters and receivers, as opposed to just one of each.

If there is a discontinuity, such as a lack of fusion, the ultrasound waves will diffract and will appear on a viewing monitor. The technician will observe this and investigate further.

Time of flight diffraction is excellent at detecting discontinuities and defects with very sharp geometrical features. Discontinuities such as weld cracks, lack of fusion and lack of penetration are excellent examples of discontinuities that are easily detected by the time of flight diffraction process. Discontinuities near the surface, such as weld undercut, are more difficult for time of flight diffraction to detect. Furthermore, discontinuities without sharp geometrical features, such as porosity, can be slightly more difficult to detect than weld cracks with the time of flight diffraction process.

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