TOFD Testing | Time of Flight Diffraction | Weld Volumetric Inspection | Atlantis NDT

Complete guide to TOFD testing (time of flight diffraction): diffraction physics, D-scan interpretation, lateral wave and back wall reference signals, dead zone, ASME V Article 4 Mandatory Appendix III, EN ISO 10863, BS 7706. TOFD vs PAUT vs radiographic testing comparison. Specialist TOFD training and ASME Appendix III procedure development from Atlantis NDT.

How Time-of-Flight Diffraction (TOFD) works

TOFD places a transmitter and receiver either side of a weld and measures the arrival time of sound diffracted from the tips of a discontinuity, rather than the amplitude reflected from its face. Because sizing depends on diffraction timing rather than reflection amplitude, TOFD is largely insensitive to flaw orientation — the weakness that limits conventional pulse-echo on planar defects that are not favourably aligned to the beam.

What it detects

Planar defects regardless of orientation — lack of fusion, lack of penetration, and cracks that pulse-echo can miss entirely
Through-wall height with accuracy typically far better than amplitude-based sizing, which is what fitness-for-service assessment actually needs
Flaw growth between inspections, because the same scan geometry reproduces closely
Embedded defects across the full weld volume in a single pass along the weld

Limitations worth knowing before you specify it

TOFD has a near-surface dead zone from the lateral wave and a back-wall zone that both require complementary coverage — normally pulse-echo or phased array. It also produces a grey-scale image that demands a trained interpreter; TOFD data reviewed by someone without specific TOFD qualification is a common source of both missed and over-called indications.

When to choose it

Choose TOFD where through-wall sizing drives a fitness-for-service or acceptance decision, where radiography is impractical for access or radiation-safety reasons, or where weld volume must be covered quickly on long seams.

Governing codes

ASME Section V Article 4 Mandatory Appendix III, ASME Code Case 2235 for ultrasonic examination in lieu of radiography, ISO 10863, ISO 15626 for acceptance, and API 1104 Annex A for pipeline girth welds.

What TOFD does that pulse-echo cannot

Time-of-flight diffraction sizes flaws from the diffracted waves at their tips rather than reflected amplitude — which removes the orientation lottery that makes amplitude sizing unreliable on planar flaws. A mis-oriented crack that returns almost nothing to a pulse-echo probe still diffracts at its extremities, and the tip-signal timing gives through-wall size with accuracy amplitude methods cannot match. That is why TOFD anchors critical weld examinations and why codes accept it for detection and sizing in a single pass.

What it will not do

TOFD has a near-surface dead zone under the lateral wave — the top few millimetres are its blind spot, which is why code-compliant examinations pair TOFD with pulse-echo or phased array covering the surface zones. Coarse-grained austenitic materials degrade it, transverse flaws need supplementary scans, and interpretation demands real training: a TOFD image is a different language from an A-scan.

How Atlantis delivers it

Encoded TOFD to ASME Section V Article 4 mandatory appendices and ISO 10863 — scan plans engineered per joint, paired PA/PE coverage for the dead zones, analysis by Level IIs qualified in the technique under Level III oversight, and deliverables that include the data, not just the verdict. Common scopes: new-construction girth and long-seam welds, in-service crack monitoring with repeatable encoded baselines, and HTHA-adjacent examinations paired with specialised techniques. Scope a TOFD examination · phased array services · TOFD in the glossary.