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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.
Additional recommended knowledge
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
For Further Reading
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Time_of_flight_diffraction_ultrasonics". A list of authors is available in Wikipedia.|