What is Time of Flight Diffraction Inspection

Time of flight diffraction (TOFD) is one of the most reliable non-destructive testing methods in testing welds for both pre-service and in-service inspection. An accurate assessment of welds is vital to ensure that the stability and integrity of welded structures is not compromised by defects.

A pair of ultrasonic probes is placed on opposite sides of the weld or tested item. One of the probes acts as a transmitter and sends out an ultrasonic pulse that is picked up by the other probe which is the receiver.  With an undamaged item, the signals picked up by the receiver probe are from two waves – one wave that travels along the surface (lateral wave) and one that reflects off the far wall (back wall reflection).  If there is a flaw, such as crack, there is a diffraction of the ultrasound pulse from the tips of the crack.  By measuring the time of flight of the pulse, the depth of the crack can be calculated by trigonometry.

 

Why is Phased Array Ultrasonic Inspection used

The benefits of time-of-flight diffraction include:

  • Can take place during production work – no shutdown is required.
  • No radiation with this method.
  • Inspection data is recorded and can be stored digitally for future reference.
  • Inspection results are immediately available.
  • Quick inspection method.
  • Sensitivity to all weld flaw types including cracks, slag and lack of fusion.
  • Accurate sizing of defects.
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