Time of Flight Diffraction (TofD)
Time-of-flight diffraction (TOFD) is a reliable method of ultrasonic examination that is a very sensitive and accurate method for non-destructive examination. Primarily, it is used for rapid weld testing of circumferential and axial weld seams, which is referred to as perpendicular TOFD scanning. It can also be used for weld overlays and inspection of the heated areas of other components such as piping, clad material, pressure vessels, storage tanks, and structural steel.
Similar to other forms of Ultrasonic Testing (UT), a pair of probes are positioned 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 probes 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
Advantages of TOFD
Time-of-flight diffraction is widely accepted as among the fastest of NDT methods. One of its key advantages is being able to find and assess the size of multiple defects, even those of different types, over a short period of time. TOFD is able to determine damage that typically would only be seen through other methods such as radiography.
As TOFD is based on diffraction, it is relatively indifferent to weld bevel angles and orientation of any flaws. It makes use of time of arrival signals that are reflected back from crack tips to determine accurate defect positioning and sizing. A single beam can cover a wide area and the entire process is quick to set up and deploy. The procedure is highly sensitive to all weld flaw types and can be used for corrosion inspections.