Time of flight diffraction ultrasonics
Encyclopedia
Time of Flight Diffraction (TOFD) method of Ultrasonic inspection
is a very sensitive and accurate method for nondestructive testing
of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).
Bray and Stanley (1997) summarized TOFD as Tip-diffraction techniques which utilized the principle that the tips of a crack when struck by a wave will diffract the signals back to the other location on the surface. The depth of these tips can be determined from the diffracted energy.
TOFD was invented in the UK in the 1970s initially as a research tool. 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.
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.
(ISO)
European Committee for Standardization
(CEN)
Ultrasonic inspection
In ultrasonic testing , very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials...
is a very sensitive and accurate method for nondestructive testing
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....
of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997).
Bray and Stanley (1997) summarized TOFD as Tip-diffraction techniques which utilized the principle that the tips of a crack when struck by a wave will diffract the signals back to the other location on the surface. The depth of these tips can be determined from the diffracted energy.
TOFD was invented in the UK in the 1970s initially as a research tool. 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.
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
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
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.
Standards
International Organization for StandardizationInternational Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
(ISO)
- ISO/DIS 10863, Welding - Use of time-of-flight diffraction technique (TOFD) for examination of welds
European Committee for Standardization
European Committee for Standardization
The European Committee for Standardization or Comité Européen de Normalisation , is a non-profit organisation whose mission is to foster the European economy in global trading, the welfare of European citizens and the environment by providing an efficient infrastructure to interested parties for...
(CEN)
- EN 583-6, Non-destructive testing - Ultrasonic examination - Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities
- EN 15617, Non-destructive testing of welds - Time-of-flight diffraction technique (TOFD) - Acceptance levels
Other Sources
- Engineering Applications of Ultrasonic Time-of-Flight Diffraction, 2nd ed., J. P. Charlesworth and J. A. G. Temple, Research Studies Press, 2002.
External links
- TOFD Inspection Systems
- TOFD: - An Alternate Non-Destructive Testing Procedure to Replace Traditional Methods
- NDT.net Database Search results in over 400 hits with the search term 'TOFD'
- TOFD Principles on NDT.net