Time of Flight Diffraction (TOFD)
TOFD is usually performed using longitudinal waves as the primary detection method. Ultrasonic sensors are placed on each side of the weld. One sensor sends the ultrasonic beam into the material and the other sensor receives reflected and diffracted ultrasound from anomalies and geometric reflectors. TOFD provides a wide area of coverage with a single beam by exploiting the ultrasonic beam spread theory inside the wedge and the inspected material. When the beam comes in contact with the tip of a flaw, or crack, diffracted energy is cast in all directions. Measuring the time of flight of the diffracted beams enables accurate and reliable flaw detection and sizing, even if the crack is off-oriented to the initial beam direction. During typical TOFD inspections, A-scans are collected and used to create B-scan (side view) images of the weld. Analysis is done on the acquisition unit or in post-analysis software, positioning cursors to measure the length and through-wall height of flaws.
Application of TOFD:
- Weld Inspection (pressure vessel and piping)
- Weldment integrity assessment (During maintenance)
Advantages of Time of Flight Diffraction:
- Based on diffraction, so relatively indifferent to weld bevel angles and flaw orientation.
- Uses time of arrival of signals received from crack tips for accurate defect positioning and sizing.
- Rapid scanning with imaging and full data recording.
- Highly sensitive to all weld flaw types.
- Data and image recorded