In Time of Flight Diffraction (TOFD) two ultrasonic transducers are mounted in an assembly that straddles the area to be inspected (usually a weld). The transmitter probe floods the material with a wide beam of ultrasound. The receiver probe collects a number of signals.
The first signal to arrive is the lateral wave travelling along the material’s surface. If any flaw is present, its tips (upper and lower) diffract the ultrasound and the receiver collects signals from these tips. Finally, a signal is received from the back wall. The times of flight of the lateral and back-wall echo signals serve to calibrate the speed of ultrasound in the material. The time of flight of the Upper and Lower Flaw Tip signals enable the positions of these tips and hence the position and through wall height of the flaw to be accurately determined.
TWI's TOFD Equipment
TWI has five systems capable of TOFD inspections: two phased array and three more conventional advanced computerised UT systems:
- Olympus NDT's Tomoscan Focus phased array system.
- Zetec Tomoscan III 128 channel phased array flaw detector.
- Sonotron NDT’s Isonic 2005.
- Force Technology P-Scan 4+ 8 channel flaw detector.
- Inspection Solutions USB Sector Scan 2 channel flaw detector
To complement these computerised flaw detectors TWI has a range of versatile scanners, manual and mechanised, which can be used to manipulate the probes for TOFD data collection.
TOFD Applications
Examples of the successful application of TOFD by TWI are:
- Inspection of seam welds in gas storage bullets in Saudi Arabia
- Monitoring crack growth in a reactor vessel over a period of years using high temperature probes at an Exxon/Mobil refinery
For more information, please contact us.
