Time-of-Flight Diffraction (TOFD)

TOFD is an advanced UT technique using two angled probes in pitch-catch arrangement to detect and accurately size defects from the time of arrival of diffracted tip signals, achieving height-sizing accuracy of ±0.5 mm.

Definition

Time-of-Flight Diffraction (TOFD) uses a pair of longitudinal-wave angle probes, one transmitter and one receiver, positioned on either side of a weld. Diffracted waves from the tips of an embedded discontinuity arrive at the receiver at times that correspond to the through-wall position of those tips, allowing very accurate height sizing.

Technical Context

The TOFD image shows a lateral wave, back-wall echo, and any tip-diffracted signals as grey-scale bands. Phase information distinguishes upper from lower crack tips. TOFD complements PAUT by providing accurate sizing where PAUT's amplitude-based sizing is limited.

When It Is Used

Pressure-vessel and pipeline girth-weld inspection
In-service crack growth monitoring
Fitness-for-service input for API 579 assessments

Related Standards

ASME Section V Article 4 Mandatory Appendix III, ISO 10863, ASTM E2373.

Related terms

Phased Array Ultrasonic Testing (PAUT) — PAUT is an advanced ultrasonic technique using a multi-element transducer (16–256 elements) with electronic beam steering and focusing, producing sectorial, linear, and compound scans for superior weld inspection and defect characterization.
Ultrasonic Testing (UT) — Ultrasonic Testing (UT) is an NDT method that uses high-frequency sound waves (typically 0.5–25 MHz) to detect internal flaws and measure thickness in metals, plastics, and composites by analyzing reflected echoes from a transducer.
Fitness-for-Service (FFS) — Fitness-for-service is a quantitative engineering assessment of in-service equipment with measured damage to determine whether it can continue in service safely, must be repaired, derated, or retired, performed per API 579-1 / ASME FFS-1.
API 579-1 / ASME FFS-1 Fitness-for-Service — API 579-1 / ASME FFS-1 is the recommended practice for quantitative engineering assessment of in-service flaws and damage, providing Level 1, 2, and 3 assessment procedures for brittle fracture, general and local metal loss, pitting, blisters, crack-like flaws

More method terms

Radiographic Testing · Magnetic Particle Testing · Penetrant Testing · Eddy Current Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing · Total Focusing Method

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.