Technologies
- NDT and Asset Reliability
  - Advanced NDT

Time of Flight Diffraction

$Time of Flight Diffraction$

In Time of Flight Diffraction (TOFD) system, a pair of ultrasonic probes is used, sitting on opposite sides of a weld-joint or area of interest. A transmitter probe emits an ultrasonic pulse which is picked up by the receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are from two waves: one that travels along the surface (lateral wave) and one that reflects off the far wall (back-wall reflection). When a discontinuity such as crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the pulse, the depth of the crack tips can be calculated automatically by trigonometry application. This method is even more reliable than traditional radiographic, pulse echo manual UT and automated UT weld testing methods.

The TOFD offers great accuracy for measuring the critical through-wall size of ‘crack-like-defects’. The accuracy of better than ±1mm can be obtained in a wide range of material thickness from which pressurised components are constructed.

This TOFD was initially developed as a method of accurate monitoring and sizing of through-wall height of in-service discontinuities in the nuclear industry. It has now been independently validated as the one of the most effective technique for locating and sizing of discontinuities in ferritic welds.

TWI Resources

TWI acquired the following equipment which is capable of performing TOFD;

Olympus MX
Olympus MX2
Zetec Dynaray
Zetec Tomoscan III 128
GE Phasor XS
Sonotron NDT’s Isonic 2005
Force Technology P-Scan 4+8

TWI has a range of versatile scanners specially designed for manual and mechanical applications on site.

Benefits

The advantages of this technique are;

Speed of detecting discontinuities
Accurate positioning of discontinuities
Accurate sizing of discontinuities
A single pass can be sufficient to obtain the inspection coverage
Reproducibility and accuracy is proven <0.5mm
Permanent digital records with graphic images
Detection is independent of type and orientation of discontinuities
Compliant to ASME Code Case 2235
Consistent results between pre- and post-heat treatment
Consistent results between pre- and in-service inspection
Accurate methods available for measuring the growth of discontinuities
Offers a higher probability of detection
Data is collected in B-scan or D-scan format to improve the detection of discontinuities
Planar discontinuities, which are not perpendicular to measured surfaces can be detected
Higher POD which means, improving risk reduction and calculation
No evacuation of areas during operation because no radiation is used
Inspection results are immediately available
Inspection can be performed above 200° C