Time-of-flight diffraction, or TOFD, is an ultrasonic inspection technique that sizes flaws by timing the ultrasonic waves that diffract from the tips of a defect rather than by measuring the strength of a reflected echo. A pair of angled probes, one transmitting and one receiving, is placed on either side of a weld, and the instrument records how long it takes signals to travel from the transmitter, around the flaw tips, and back to the receiver. Because the height of a flaw determines the difference in travel time between its upper and lower tips, TOFD can measure crack height with a level of accuracy that amplitude-based methods struggle to match. This makes it a favored tool for finding and tracking cracks in welds.
Time-of-Flight Diffraction (TOFD) in one line: Time-of-flight diffraction is an ultrasonic method that sizes flaws by measuring the travel time of waves diffracted from a defect's tips rather than the amplitude of reflected echoes. The timing difference between the upper and lower tips gives an accurate flaw height, which makes TOFD excellent for detecting and monitoring cracks in welds.
TOFD relies on a physical effect that most ultrasonic methods treat as noise: when an ultrasonic wave strikes the sharp tip of a crack, it diffracts, sending out a weak wave in many directions. In a TOFD setup, a transmitter probe on one side of the weld sends a broad ultrasonic beam through the material, and a receiver probe on the other side picks up the signals that arrive, including the faint diffracted waves from the top and bottom tips of any flaw in the path.
The instrument measures the arrival time of each of these signals. A wave diffracted from the upper tip of a crack travels a shorter path and arrives sooner than one from the lower tip, and the time difference between them corresponds directly to the through-wall height of the flaw. Because this is a timing measurement rather than an amplitude one, it is largely insensitive to the flaw's orientation and to the exact coupling of the probe, which are the factors that make amplitude sizing unreliable.
A typical TOFD display shows the reference signals that bracket the inspection: a lateral wave that travels just under the surface and a back-wall echo from the far side, with any flaw indications appearing between them. Reading the position of the tip signals against these references lets an operator locate a flaw through the thickness and measure its height, and repeated scans of the same location let that height be tracked over time.
The standout strength of TOFD is accurate height sizing and sensitivity to crack growth. Because it measures tip diffraction, it detects planar flaws such as cracks and lack of fusion well and can measure how much a flaw has grown between inspections, which is precisely what an engineer needs when deciding whether a known crack remains acceptable. A single pair of probes can also cover a substantial thickness of weld in one pass, making it efficient for screening.
TOFD has well-known blind regions. Near the scanning surface, the strong lateral wave masks shallow flaws, creating a dead zone at the top of the weld where near-surface defects can be hidden. There is a corresponding uncertainty near the back wall. Because of these dead zones, TOFD is not used entirely on its own for critical near-surface regions, and inspectors account for them when planning coverage.
There are other limitations to keep in mind. The diffracted signals are weak, so interpretation requires skill and good equipment, and flaws with unfavorable geometry can be harder to characterize. TOFD is excellent at telling you the height of an indication but less directly informative about its exact nature, so it is often combined with other techniques to both find and classify defects rather than relied on for every aspect of an inspection.
TOFD is most powerful when combined with complementary methods that cover its weaknesses. Pairing it with phased array ultrasonics is common practice: the phased array technique provides angled imaging and good coverage of the near-surface region where TOFD has a dead zone, while TOFD contributes its accurate tip-to-tip height sizing. Together they give both reliable detection across the full thickness and precise measurement of flaw height.
Radiography plays a complementary role as well, being strong at revealing volumetric flaws such as porosity and slag that ultrasonic tip diffraction is not aimed at. Using radiography alongside the ultrasonic methods gives a fuller picture of a weld, with each technique contributing what it does best, so the combined result is more complete than any single method could provide on its own.
For crack monitoring, the value of TOFD is that its measurements are repeatable and quantitative, which lets a known flaw be tracked over successive inspections. When those height measurements are stored in an integrity data system alongside continuous monitoring and other inspection records, an engineer can see whether a crack is stable or growing and how fast. Bringing periodic TOFD sizing into the same platform that holds field monitoring data supports fitness-for-service decisions grounded in the actual growth history of the defect.
Pulse-echo sizing relies on the amplitude of reflected echoes, which changes with a flaw's orientation and the probe coupling, so it can misjudge height. TOFD instead measures the travel time of waves diffracted from the crack tips, and that timing directly gives the through-wall height regardless of orientation. Because timing is far more stable than amplitude, TOFD produces more reliable height measurements.
TOFD has a dead zone near the scanning surface, where the strong lateral wave masks shallow, near-surface flaws, and a region of reduced reliability near the back wall. These blind areas mean TOFD alone cannot be trusted for near-surface critical inspection, which is why it is usually combined with another method such as phased array ultrasonics that covers those regions.
Not entirely; the two are complementary. TOFD excels at detecting and sizing planar flaws such as cracks through tip diffraction, while radiography is better at revealing volumetric defects like porosity and slag. Many weld-integrity programs use ultrasonic methods and radiography together so each technique contributes what it does best, rather than treating one as a full replacement for the other.
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