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How Does a TOFD Flaw Detector Work?

Nov. 08, 2022

The Time of Flight Diffraction (TOFD) technique for ultrasonic inspection of welds was first introduced in 1977. The method was widely reported in English publications and was also introduced in Germany; however, it was more or less ignored by German NDT experts. Finally, in 1996, a European pre-standard was announced and, as a result of this standard and some new publications, TOFD seems to be replacing radiography and other UT techniques. a paper published in 1995 referred to the widespread acceptance of the method with the article 'TOFD Comes of Age'.

The Time of Flight Diffraction (TOFD) method of ultrasonic inspection is a sensitive and accurate method for the non-destructive detection of defects in welds, which originated from the cutting edge diffraction technique first published by Silk and Liddington in 1975, paving the way for TOFD. Later work on the technique is given in many sources including Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).

HS 811 TOFD Flaw Detector

HS 811 TOFD Flaw Detector           

Bray and Stanley (1997) summarise TOFD as the technique of tip diffraction, which uses the principle that cracked tips diffract the signal back to other locations on the surface when struck by a wave. The depth of these tips can be determined from the diffraction energy.

TOFD was invented in the UK in the 1970s, initially as a research tool. The use of TOFD allows for more accurate measurement of crack dimensions, thus allowing expensive components to operate for as long as possible and minimising the risk of failure.

The following key principles describe TOFD.

1. Two angular beam probes (usually 45°) are arranged and connected together as transmitter-receiver. The distance between the probes is calculated according to the wall thickness.

2. Longitudinal waves are usually applied. The sound beam spread is large in order to maximise the scanning area...

3. The A-Scan shows so-called transverse waves, back wall echoes and other signals that may occur between the two signals, which may be due to inhomogeneities.The A-Scan is not corrected for in the TOFD technique.

4. The TOFD technique is always applied in conjunction with an imaging method.

HS 810 TOFD Flaw Detector

 HS 810 TOFD Flaw Detector

Principle of operation

Measuring the amplitude of the reflected signal is a relatively unreliable method of determining the size of a defect, as the amplitude depends heavily on the direction of the crack. instead of using amplitude, TOFD uses the time of flight of the ultrasonic pulse to determine the position and size of the reflector.

In a TOFD system, a pair of ultrasonic probes are located on opposite sides of the weld. One of the probes (the transmitter) emits an ultrasonic pulse, which is received by the probe on the other side (the receiver). In an undamaged pipe, the receiver probe receives signals from two types of waves: one propagating along the surface and the other reflected from the distant wall. When cracks are present, the ultrasonic waves from the crack tip are diffracted. Using the measured pulse time of flight, the depth of the crack tip can be automatically calculated by a simple triangulation method.

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