Volumetric & Advanced NDT · TOFD
Time-of-Flight Diffraction
Crack tips whisper — TOFD times the whisper.
Time-of-flight diffraction (TOFD) is an ultrasonic technique that sizes defects from the faint waves diffracted by their edges, rather than the strong reflections used by conventional UT. A transmitter and receiver straddle the weld; a crack’s top and bottom tips each scatter energy, and the precise arrival times of those tip signals measure the defect’s through-wall height with exceptional accuracy.
The explainer
Watch how it works.
Schematic · loops automatically
1Straddle the weld
Transmitter and receiver sit either side of the joint at a fixed separation, wide beams flooding the full thickness between them.
2Two reference waves bracket the wall
The lateral wave skims the surface first; the backwall echo arrives last — every depth in between is on the clock.
3Defect tips diffract
A crack’s upper and lower edges each scatter faint waves toward the receiver — orientation barely matters, tips radiate everywhere.
4Arrival times give height
The gap between top-tip and bottom-tip arrivals converts by triangulation into through-wall size — sizing to fractions of a millimetre.
5One pass, whole weld
Encoded scanning assembles the D-scan: length and depth of the weld in a single recorded image for assessment to ISO 15626.
The physics
When an ultrasonic wave strikes the sharp edge of a discontinuity, the edge re-radiates energy in all directions — diffraction, the same physics that bends light at a slit. TOFD listens for these tip-diffracted waves instead of specular reflections, which makes it largely indifferent to defect orientation: a crack angled away from a conventional beam still has tips, and tips always answer.
The geometry is a fixed pair: wide-beam transmitter and receiver facing each other across the weld. Four arrivals matter — first the lateral wave racing just beneath the surface, last the backwall reflection, and between them any diffracted signals from defect tips. Because arrival time maps directly to depth through triangulation, the separation of top-tip and bottom-tip signals measures defect height, typically to within fractions of a millimetre.
Scanned along the weld with an encoder, TOFD builds a D-scan — a grayscale map of the weld’s length and depth in one image, acquired in a single pass. Its blind spots are near-surface zones (masked by the lateral wave) and its images demand practised interpretation, which is why TOFD is normally paired with phased array or conventional UT: detection and characterisation from one, precision sizing from the other.
Where it earns its keep
Typical applications
- Precision height sizing of weld flaws for engineering critical assessment
- Thick-section pressure vessel and pipeline weld examination
- Baseline-and-monitor programmes tracking defect growth in service
- Combined PAUT+TOFD premium examination of critical joints
Standards & schemes
| Designation | Covers |
|---|---|
| ISO 10863 | Use of time-of-flight diffraction technique for weld testing |
| ISO 15626 | TOFD acceptance levels, aligned to ISO 5817 quality levels |
TOFD acquisition and analysis are performed by advanced-certified technicians with Level 3 technical oversight, reporting sized defects suitable for fitness-for-service assessment.
Why is TOFD sizing so much better than amplitude methods?
Conventional UT infers size from echo amplitude, which depends on orientation, surface roughness and coupling as much as on the defect. TOFD measures time, not loudness — and clocks are far more reliable than volume knobs. Height accuracy of fractions of a millimetre is routine on suitable geometry.
If TOFD is so accurate, why not use it alone?
Its lateral-wave dead zone can hide near-surface defects, and its grayscale images characterise defect type less intuitively than PAUT sectorial scans. The industry answer is the combined scanner — PAUT for detection and characterisation, TOFD for sizing — one pass, both datasets.
When does a client actually need TOFD?
Whenever a defect decision hinges on through-wall height: engineering critical assessments, run-repair-replace calls on pressure plant, and monitoring known flaws for growth. If the question is “how tall is it, exactly?”, TOFD is the instrument built to answer.
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