The TOFD technique relies on the interaction of ultrasonic waves with the tips of flaws. When an ultrasonic wave interacts with a planar flaw it results in the production of diffracted waves from the tips of the flaw in addition to the normal reflected wave. This diffracted energy is emitted over a wide angular range and is assumed to originate from the flaw extremities.

Recognition of these diffracted signals makes it possible to establish the presence of flaws, and, as signals may be received from a range of flaws of different orientations using a single probe pair, the technique may be used for flaw detection purposes.

The technique is also useful for flaw sizing since the time separation of the diffracted waves is directly related to the through-wall extent of the flaw. All measurements of flaw height and through-wall position are made from the timing of the received signals; signal amplitude, which is notoriously variable, is not used.

The TOFD technique normally makes use of compression waves, as (i) these arrive first at the receiving probe, and (ii) they diffract favourably. Their use simplifies the received waveform and therefore interpretation of the resultant images. The basic configuration for the TOFD technique (Fig.1) consists of a transmitter and receiver probe pair held at a fixed separation straddling the volume of material to be inspected, e.g. a weld.

Probes having a wide beam divergence angle are generally used, since the diffraction of ultrasonic waves from flaw tips is less dependent on the orientation of the flaw than in the case of a reflection from the face of a flaw. Fig.2 shows a typical received A-scan signal from an embedded planar flaw.


The first signal to arrive at the receiver is the lateral wave which travels just beneath the upper (scan) surface of the specimen. In the absence of any flaws, the second signal to arrive is that reflected from the far (backwall) surface, commonly referred to as the backwall echo.

When a planar flaw exists, diffracted energy from the upper tip or extremity will be received before that from its lower tip. These two compression wave signals from the flaw tips will occur between the lateral wave and backwall echo responses. Therefore it is possible to determine not only the through-wall height of the flaw, but also its location within the thickness of the specimen. Note that Fig.2 shows a phase reversal between the lateral wave and backwall echo, and between the upper and lower flaw tip signals. This characteristic effect can greatly assist interpretation of TOFD data and is why TOFD A-scans are acquired unrectified, i.e. they are RF waveforms.