Time of Flight Diffraction

Time of Flight Diffraction measures where a flaw ends rather than 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 determines whether a weld is fit for service and whether a crack is growing.

How it works

Conventional ultrasonics uses shear waves and relies on the amplitude of the signal reflected back from a flaw. TOFD uses longitudinal waves and a pair of probes, one either side of the weld: one transmits, the other receives. 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 those tip signals locates the top and bottom of the flaw directly. Because the method depends on diffraction rather than 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 are collected along the weld and assembled into a B-scan side view. Post-acquisition analysis measures flaw length and through-wall height from the recorded data, which means the inspection can be reviewed independently long after the technician has left site.

Advantages over conventional UT

• Orientation independentDiffraction-based, so relatively indifferent to weld bevel angle and flaw orientation — the defects pulse-echo misses are found.
• Accurate sizingTip signal timing gives precise through-wall height rather than an amplitude estimate, which is what fitness-for-service assessment requires.
• Crack monitoringSizing repeatable enough to track growth between inspections, supporting a run-or-repair decision rather than forcing a shutdown.
• Wide coverage, fastBeam spread covers a large volume from a single pass, so setup and scanning are quick relative to the coverage achieved.
• Fully recordedEncoded data with imaging, retained for independent review and comparison against future inspections.
• EconomicalSingle pulser and receiver with conventional probes, so equipment 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. At the opposite face, the backwall echo masks a similar region. Defects within 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 represent a significant proportion of the wall, we will say so before the inspection rather than 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
• Pre-service baseline recording for later comparison

Standards applied

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

In place of radiography

TOFD, alone or combined with phased array, is widely accepted where radiography was previously specified. It requires no radiation source, so no exclusion zone and no stoppage of surrounding work; it produces data immediately rather than film requiring 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 accreditation

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. ITMAD is accredited to ISO/IEC 17020 as an inspection body, is an ADNOC approved inspection contractor, and is listed on the Abu Dhabi City Municipality register of Approved Third Party Inspection Bodies. Certificates are issued in the format required by the governing authority, including Dubai Municipality and the Dubai free zone authorities, and inspection is carried out across Dubai, Abu Dhabi, Sharjah and the northern Emirates.

Related methods

TOFD is normally applied alongside phased array ultrasonics, which provides the near-surface and backwall coverage TOFD cannot, and conventional ultrasonic testing where amplitude-based detection or thickness measurement is required.

What you receive

• The procedure and technique applied, with justification for the parameters chosen
• Encoded TOFD data retained and available for independent review
• B-scan imagery supporting each finding
• Flaws reported with location, length and through-wall height
• Statement of the dead zone extent and how those regions were covered
• Assessment against the acceptance criteria of the governing code
• Technician qualification level and calibration traceability