Time of Flight Diffraction
Time of Flight Diffraction (TOFD) is an excellent tool for the rapid evaluation of welds but is a complimentary technique that must be combined with other inspection methods (e.g., PA-AUT and AUT).
TOFD inspections should only be used when there is access from both sides of the weld.
When used correctly, TOFD has been shown to provide good through wall sizing accuracy as the flaw's orientation is not as critical to providing good results as conventional methods.
TOFD insonifies the examination volume with energy and relies on the diffracted signals to generate satellite pulses from the flaw's extremities or tips. The satellite pulses are depicted as curved images on the display and once linearized, provide information as to the circumferential position of the flaw along the weld length as well as its position within the weld volume.
The image below shows the resulting near surface energy (lateral wave), the back wall echo created from the inside surface and the linear relationship for flaws contained in the weld volume. Satellite pulses emanating from the flaw tips are also depicted.
TOFD should never be utilized as a stand alone inspection method due to the inherent "blind spots" created by the lateral wave and back wall echo.
