What Is TOFD? Time-of-Flight Diffraction

Quick Answer

TOFD (Time-of-Flight Diffraction) is an advanced ultrasonic testing technique that uses diffracted signals from flaw tips to determine precise flaw height, length, and position within a weld or material. Unlike conventional UT which relies on reflected amplitude, TOFD measures the time difference between diffracted waves from the upper and lower edges of a flaw. This makes TOFD largely independent of flaw orientation, providing accurate through-wall sizing critical for fitness-for-service assessments per ASME V Article 4 and EN ISO 10863.

How TOFD Works

Two angled ultrasonic probes are placed on opposite sides of the weld—one transmitter, one receiver. The transmitter emits a broad, unfocused beam that covers the full weld thickness. Three types of signals arrive at the receiver:

Signal	Path	Information
Lateral wave	Surface-skimming wave, direct path	Time reference; surface condition
Back-wall reflection	Reflects off the opposite wall	Wall thickness reference
Tip-diffracted signals	Diffract from upper and lower flaw tips	Flaw position and through-wall height

The time difference between the upper and lower tip-diffracted signals gives the flaw height. The time relative to the lateral wave gives the depth below the surface. Results are displayed as a D-scan (gray-scale image showing the weld cross-section along the scan length).

TOFD vs PAUT vs RT

Parameter	TOFD	PAUT	RT
Flaw sizing (height)	Excellent (+/- 1 mm accuracy)	Good (+/- 2-3 mm)	Poor (no through-wall sizing)
Flaw detection	Good (misses some near-surface flaws)	Excellent (multi-angle coverage)	Good for volumetric defects
Dead zones	Near OD and ID surfaces (~2-5 mm)	Minimal	None
Permanent record	Full encoded D-scan	Full encoded S/B/C-scan	Film or digital image
Setup complexity	Moderate (probe separation, gain calibration)	High (focal law design)	Low (place source and film)
Radiation	None	None	Yes
Inspection speed	Fast (single pass for full volume)	Fast (electronic scanning)	Moderate to slow
Best for	Flaw height sizing, ECA input	Detection + characterization	Porosity, volumetric flaws

TOFD Dead Zones

TOFD has inherent dead zones near the outer and inner surfaces where the lateral wave and back-wall signals mask tip-diffracted signals:

Zone	Typical Depth	Mitigation
Near-surface (OD)	2-5 mm below surface	Supplement with PAUT cap scan or MT/PT
Near-ID surface	2-5 mm above ID	Supplement with PAUT root scan
Lateral wave ring time	Depends on probe frequency/damping	Use higher frequency probes (10-15 MHz) to reduce ring time

For this reason, TOFD is almost always used in combination with PAUT rather than as a standalone technique.

Key Standards

Standard	Scope
ASME Section V, Article 4	UT examination of welds (includes TOFD)
EN ISO 10863	TOFD technique for weld testing
EN ISO 15626	TOFD acceptance criteria
API 1104 Annex S	Mechanized UT (including TOFD) for pipeline welds
BS 7706	TOFD calibration and characterization of indications
DNV-ST-F101	Subsea pipeline requirements (TOFD + PAUT)

Typical TOFD Setup Parameters

Parameter	Typical Range
Probe frequency	5-15 MHz (higher for thin wall)
Probe angle	45-70 degrees (60 degrees most common)
Probe center separation (PCS)	Calculated per wall thickness and beam angle
Scan speed	20-50 mm/s (mechanized)
Digitization rate	20-80 MHz
Wall thickness range	6-300 mm

Common Mistake

Relying on TOFD alone for near-surface flaws. The lateral wave dead zone means TOFD can miss shallow surface-breaking cracks at the weld cap or root. Always combine TOFD with PAUT surface scans and/or surface NDT methods (VT, MT, PT) for complete weld coverage.

TOFD records are stored digitally and archived with mill test certificates and hydrostatic test data in the project weld documentation package.

Read the full guide to non-destructive testing