TOFD (Time-of-Flight Diffraction) Ultrasonic Testing is an advanced NDT method used for the high-precision detection and sizing of internal discontinuities, particularly in heavy-wall welded structures. Unlike conventional UT, which relies on analyzing the amplitude of reflected waves, TOFD utilizes the diffraction phenomenon of ultrasonic waves.
The inspection employs two broadband angled longitudinal wave probes in a transmitter-receiver configuration. The probes detect diffracted echoes generated at the edges of material discontinuities. In defect-free material, two waves are received: the lateral wave (traveling just below the surface) and the back-wall reflection. When a discontinuity is present, it generates additional diffracted echoes from its edges. By analyzing the time-of-flight of these signals, our inspectors can determine the exact position and vertical extent (height) of the flaw with exceptional accuracy.
TOFD is primarily used for inspecting butt welds in steel structures, such as pipelines, tanks, and load-bearing structures. Its suitability for automated scanning makes it ideal for high-volume repetitive inspection of welded joints in serial production. TOFD detects typical welding defects, including cracks (including those perpendicular to the surface), incomplete fusion, lack of penetration, and internal inclusions (e.g., slag). It is used both for new-build quality control and in-service diagnostics to monitor defect growth over time.
TOFD offers superior sensitivity compared to conventional UT, enabling the detection of even small discontinuities and precise measurement of defect dimensions (e.g., crack height) by calculating the time-of-flight difference between waves from the top and bottom edges of the defect. Its independence from defect orientation ensures reliable results even for flaws positioned at unfavorable angles to the ultrasonic beam—providing a significant advantage over conventional UT. The scanning process is digitally recorded.