Time of Flight Diffraction: Accurate Flaw Sizing in Welds

Time of flight diffraction measures where a flaw ends and not how strongly it reflects. That single difference makes it the most accurate ultrasonic method for sizing a defect and measuring its through-wall height, which is what decides whether a weld is fit for service and whether a crack is growing. itmad provides TOFD inspection, encoded, imaged and fully recorded, to ISO 10863 and ASME V Article 4. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

How it works

Conventional ultrasonics uses shear waves and relies on the amplitude of the signal that a flaw reflects back. TOFD uses longitudinal waves and a pair of probes, one either side of the weld, one transmitting and the other receiving. When the beam meets the tip of a crack, it diffracts in all directions, and a small amount of that diffracted energy reaches the receiver.

Measuring the time of flight of the tip signals locates the top and bottom of the flaw directly. Because the method depends on diffraction and not reflection, it detects and sizes flaws regardless of their orientation to the beam, the case that most often defeats conventional pulse-echo, where a crack lying parallel to the beam returns almost nothing. A-scans collected along the weld are assembled into a B-scan side view, and post-acquisition analysis measures flaw length and through-wall height from the recorded data, so the inspection can be reviewed independently long after the technician has left site.

Advantages over conventional UT

TOFD is orientation independent, being diffraction-based and so relatively indifferent to weld bevel angle and flaw orientation, which means that the defects pulse-echo misses are found. Tip signal timing gives precise through-wall height and not an amplitude estimate, which is what fitness-for-service assessment requires, and the sizing is repeatable enough to track crack growth between inspections, supporting a run-or-repair decision and not forcing a shutdown.

Beam spread covers a large volume in a single pass, so setup and scanning are quick relative to the coverage achieved. The data are encoded with imaging and retained for independent review and for comparison with future inspections, and the equipment is economical, a single pulser and receiver with conventional probes, so its cost is lower than phased array for the sizing capability delivered.

Dead zones and how they are covered

TOFD has two blind regions, and any competent application accounts for them. Near the scanning surface, the lateral wave travelling directly between the probes masks indications in the top few millimetres, and at the opposite face the backwall echo masks a similar region, so defects in those zones can be missed by TOFD alone. For that reason TOFD is normally applied with complementary pulse-echo or phased array coverage of the near-surface and backwall regions. Where a procedure specifies TOFD as the sole method on a thickness where the dead zones are a significant proportion of the wall, itmad says so before the inspection and does not qualify the report afterwards.

Applications

Butt welds in pressure vessels, piping and storage tanks; pipeline girth welds; structural and heavy section welds; in-service crack detection and growth monitoring; weld overlay and clad interface examination; corrosion inspection, including detection of irregularly shaped areas of metal loss; and pre-service baseline recording for later comparison.

Standards applied

ISO 10863 covers the use of TOFD for testing of welds and ISO 15626 the acceptance levels for TOFD examination of welds. ASTM E2373 is the standard practice for ultrasonic examination by TOFD, and ASME Boiler and Pressure Vessel Code Section V, Article 4, applies including the mandatory appendix covering TOFD, with acceptance to the relevant construction code section. Client, project and manufacturer specifications apply where they govern.

In place of radiography

TOFD, alone or combined with phased array, is widely accepted where radiography was previously specified. It needs no radiation source, so there is no exclusion zone and no stoppage of surrounding work, it produces data immediately and not film that needs processing, and it measures through-wall height, which radiography cannot. Substitution requires the governing code to permit it and the technique to be qualified for the application.

Personnel and related methods

TOFD is carried out by technicians qualified to ISO 9712 and ASNT SNT-TC-1A Level II with specific TOFD qualification, under Level III oversight of procedure approval and technique validation. It is normally applied alongside phased array ultrasonics, which provides the near-surface and backwall coverage that TOFD cannot, and conventional ultrasonic testing where amplitude-based detection or thickness measurement is needed.

What the client receives

The procedure and technique applied, with justification for the parameters chosen, and the encoded TOFD data retained and available for independent review. B-scan imagery supports each finding, and flaws are reported with location, length and through-wall height. The report states the extent of the dead zones and how those regions were covered, assesses the findings against the acceptance criteria of the governing code, and gives the technician's qualification level and calibration traceability.

Frequently asked questions

What does TOFD measure that conventional ultrasonics does not?

Where a flaw ends and not how strongly it reflects. It times the signals diffracted from the tips of a flaw, giving accurate through-wall height and detecting flaws regardless of their orientation to the beam.

Why is TOFD used for crack monitoring?

Its sizing is repeatable enough to track growth between inspections. That supports a run-or-repair decision and does not force a shutdown.

What are TOFD dead zones?

Two blind regions: near the scanning surface, where the lateral wave masks indications in the top few millimetres, and at the opposite face, where the backwall echo masks a similar region. TOFD is therefore normally combined with pulse-echo or phased array coverage of those regions.

Can TOFD replace radiography?

It is widely accepted where radiography was previously specified, alone or combined with phased array. It needs no radiation source, produces data immediately and measures through-wall height, but the governing code must permit substitution and the technique must be qualified.

Which standards apply?

ISO 10863, ISO 15626, ASTM E2373 and ASME Section V Article 4 including its mandatory TOFD appendix, with client, project and manufacturer specifications where they govern. Acceptance is to the relevant construction code section.

What does a TOFD report include?

The procedure and justification for the parameters, B-scan imagery, flaws with location, length and through-wall height, and a statement of the dead zone extent and how it was covered. The encoded data are retained for independent review.