Time Of Flight Diffraction – TOFD

TOFD method of ultrasonic testing is a sensitive and accurate method for the non-destructive testing of welds for defects. 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.

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 ultrasonic probes sits on opposite sides of a weld. One of the probes, the transmitter, emits an ultrasonic pulse that is picked up by the probe on the other side, the receiver. 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 beam 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. Due to its sensitivity and sizing accuracy, TOFD is also an excellent tool for in-service material and flaw monitoring.

Advantages Of TOFD

  • Full record of examined weld sections in A-Scan, B-Scan, C-Scan, D-Scan and TOFD Scan presentation. Storage of full examination results
  • Real Time vision on quality of welds by the operator during examination
  • Accuracy in sizing the through thickness dimensions
  • High sensitivity in detecting planar, vertical or oriented defects like cracks or lack of fusion not visible by radiography and hardly to detect by standard ultrasonic pulse echo technique. For these reasons ASME Code with the Code Case 2235 has permitted to perform on pressure vessel welds TOFD examination in lieu of radiography
  • Full control on contact effectiveness and coverage by examining the TOFD map in any time after the examination
  • Supervision of the examination in the dynamic way as during the scanning on your PC in the office or far away the job