Time of flight diffraction ultrasonics

Time of Flight Diffraction (TOFD) method of Ultrasonic inspection is a very sensitive and accurate method for nondestructive testing of welds for defects. TOFD is a computerised system that was invented in the UK in the 1970s for the nuclear industry by Dr. Maurice Silk. The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure.

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Principle of operation

Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, TOFD uses the time of flight of an ultrasonic pulse to determine the position of a reflector.

In a TOFD system, a pair of probes sit on opposite sides of a weld. One of the probes emits an ultrasonic pulse that is picked up by the probe on the other side. In undamaged pipe, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tip can be calculated automatically by simple trigonometry.This method is even more reliable than Radiographic testing of a weld.

Features of TOFD

• A computerised and automated system for weld inspection.
• Probes are mounted on a buggy that travels along a weld, recording data as it moves.
• Compared to conventional methods of ultrasonic inspection, TOFD is sensitive to cracks and measures their dimensions accurately.
• TOFD has two blind zones where it is not sensitive to defects, hence TOFD ought always to be supplemented by a conventional pulse-echo examination of the near and far walls. The pulse echo probes are commonly mounted on the same buggy as the TOFD probes.
• Requires ultrasound technicians with advanced training.

For Further Reading

• TOFD: - An Alternate Non-Destructive Testing Procedure to Replace Traditional Methods
• TOFD: An Advanced NDT technique for inspection of welds for heavy-walled pressure vessels
• TOFD Principles

References

Silk, M.G., "Sizing crack like defects by ultrasonic means", in Research Techniques in Non-destructive Testing, vol. 3, ed. by R.S. Sharpe, Academic Press, London, 1977.

Engineering Applications of Ultrasonic Time-of-Flight Diffraction, 2nd ed., J. P. Charlesworth and J. A. G. Temple, Research Studies Press, 2002.