DoaWise

2026-07-07 · EN

TOFD weld testing: principle and practice

TOFD (time-of-flight diffraction) is an ultrasonic technique that delivers highly accurate flaw sizing on weld seams. Unlike conventional amplitude-based techniques, it works with the sound waves diffracted from the tips of a flaw. That is what makes it so accurate at measuring crack height.

1. How it works: diffraction

TOFD uses one transmitter probe and one receiver probe facing each other across the weld. Both probes generate angled longitudinal (compression) waves, with typical wedge angles between 45° and 70°. Two reasons drive that choice: longitudinal waves travel faster than shear waves, and they let the lateral wave separate cleanly from the diffraction signals on the time axis. When a flaw is present, part of the sound diffracts from its upper and lower tips and reaches the receiver at different times.

The difference in arrival time between these two diffraction signals gives the depth and height of the flaw directly. Because the method relies on time of flight rather than amplitude, it is far less sensitive to flaw orientation than conventional techniques.

2. The signals: lateral wave and back-wall echo

Expect two reference signals in every TOFD scan:

  • Lateral wave: the signal that travels the shortest path between the two probes along the top surface, so it arrives first on screen. Its amplitude is weak, and it defines the near-surface dead zone.
  • Back-wall echo: the signal returning from the far surface of the material.

The diffraction signals from a flaw appear between these two references. A surface-breaking flaw interrupts the lateral wave. A flaw in the root area sits so close to the back wall that it can fall inside the back-wall dead zone. For that reason, back up root testing with PAUT or an angle-beam probe.

One detail matters: the lateral wave and the back-wall echo are of opposite polarity, and the upper- and lower-tip diffractions are likewise out of phase with each other. An experienced operator reads this phase relationship to tell the upper tip from the lower one and measure crack height.

3. Display: the gray-scale cross section (D-scan)

A TOFD instrument normally presents the data as a gray-scale cross-sectional image (depth versus scan position, commonly called a TOFD D-scan or B-scan). An encoder tracks the probe pair along the seam, and the instrument records every trace against position. An experienced operator recognizes point-like flaws by the characteristic arc (parabolic) trace they leave. Height, however, does not come from the shape of that pattern — it comes from the time difference between the diffraction signals.

4. Advantages

  • High sizing accuracy: on the order of ±1 mm for crack height under good conditions, a clear step up from the typical few-millimeter uncertainty of amplitude-based techniques.
  • Speed: a single pass covers most of the seam.
  • Orientation independence: because the method relies on diffraction, flaw angle has little effect on detection.
  • Full recording: auditable data that a second specialist can re-evaluate later.

5. Limitations: dead zones and positioning

TOFD has two dead zones you need to know about:

  • Near-surface region (under the lateral wave): the first few millimeters below the surface sit in the shadow of the lateral wave, and TOFD resolves them poorly. The depth of this dead zone depends on the pulse width of the lateral wave. A higher-frequency, short-pulse probe reduces it.
  • Region near the back wall: diffraction signals overlap with the strong back-wall echo, and you cannot separate them from it.

TOFD measures depth precisely, but on its own it cannot resolve the lateral position of a flaw across the width of the seam. Point-like flaws such as porosity all leave the same parabolic trace, which makes them hard to tell apart and hard to size.

For these reasons TOFD is, in practice, mostly used together with phased array ultrasonic testing (PAUT). PAUT covers the surface regions and the lateral position; TOFD takes on the volumetric sizing. Each method closes the other's blind spot.

6. Related standards

  • ASME BPVC Section V, Article 4: the relevant Mandatory Appendix provisions for TOFD (the appendix number varies with the edition in use).
  • EN ISO 10863: TOFD technique and testing levels (A–D) for welds; contains no acceptance criteria.
  • EN ISO 15626: acceptance levels for TOFD.

Acceptance criteria come from the construction or in-service code that governs the testing.

From the field

TOFD is at its best in sizing and at its worst in the dead zones. Its repeatability is worth a great deal when you are tracking whether a crack has grown between outages. But leaving surface-breaking flaws to TOFD alone is asking for trouble. The right setup runs TOFD alongside PAUT or an angle-beam scan.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we apply TOFD and phased array ultrasonic testing (PAUT) together on weld seams. The combination delivers reliable flaw detection and precise sizing to international standards, and it produces recordable, auditable test results.