Time of Flight Diffraction (TOFD)


Most ultrasonic techniques rely on receiving specular reflections from defects, even if only from particular facets.  Time of flight diffraction (TOFD) detects flaws using the signals diffracted from the flaw’s extremities. 

Two angled compression wave probes are used in transmit-receive mode, one on each side of the weld.

The beam divergence is such that the majority of the thickness is inspected, although, for thicker components, more than one probe separation may be required.  When the sound strikes the tip of the crack, this acts as a secondary emitter which scatters sound out in all directions, some in the direction of the receiving probe.  A lateral wave traveling at the same velocity as the compression waves, travels directly from the transmitter to the receiver.  The time difference between the lateral wave and the diffracted signal from the flaw provides a measure of its distance from the scanned surface.  If the flaw is large enough in the through-wall dimension, it may be possible to resolve the tip-diffracted signals from its top and bottom, thereby allowing the through-wall height of the flaw to be measured.

The TOFD technique is well suited for the detection and the sizing of all types of embedded flaws, especially those planar in nature.  However, the detection of small near-the-scan-surface flaws can be more difficult due to the presence of the lateral wave response which often occupies several millimeters of the depth axis on images.

To sort through this technical explanation and to determine the applicability of TOFD Ultrasonic Testing (UT) for your particular non-destructive testing (NDT) needs, please call one of our highly-qualified ultrasonics technicians.  They can readily explain the limitations and the advantages of using TOFD UT.




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