Ultrasonic Time-of-Flight Diffraction (TOFD) Technique

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Ultrasonic time-of-diffraction (TOFD) technique

General

The time-of-flight diffraction (TOFD) method is an ultrasonic measurement method used in particular to test steel components for cracks and volume defects. It is used for quality control of weld seams both in outgoing goods inspection and in field tests on components during operation. A special feature of this method is the presentation of the test result, which allows the depth and geometry of the defect to be determined immediately. Although this testing technique is also suitable for plastics in principle, no publications on this subject are known in the technical literature to date.

Measurement setup

Two identical angle beam sensors are used, which are aligned with each other in such a way that the sound fields of the sensors overlap in the component volume, allowing all potential defects to be ‘illuminated’ (Fig. 1). The ultrasonic transducers are shown in Fig. 1 as standard sensors glued onto lead wedges. In special angle sensors, this design is arranged in a housing. The angle of incidence requires a corresponding distance between the sensors in order to be able to sound through the test sample volume. In addition to the angle of incidence, this distance depends on the structure of the sound field, which is determined by the combination of transducer and damping body, but also on the sound path in the lead-in distance that realises the angle of incidence.

Fig. 1:

Principle diagram of the TOFD method [1]

Functionality

The measuring method works according to the transmission principle, i.e. one angle sensor transmits the sound (transmitter) and the other receives the sound waves reflected in the volume (receiver). Since the two sensors are aligned so that the reflection signal (back-wall reflection) can be registered by the receiving sensor, this signal limits the measuring range on the time axis upwards (HF images in Fig. 1 below). The lower limit of the measuring range is marked by the lateral waves, whose sound velocity corresponds to that of the longitudinal waves.

The measuring range of the method, in which defects can be detected, lies between these two limits. As is known from wave optics, cracks, delaminations and discontinuities represent diffraction obstacles and starting points for elementary waves. These sound waves can be registered by the receiver and displayed in the HF-scan. As an example, Fig. 1 shows a volume crack running vertically through the tested weld seam.

Evaluation

The TOFD measurement signal shown in Fig. 1, which is recorded from a weld seam, is usually recorded with simultaneous indication of the path information. For this purpose, both ultrasonic test sensors are connected to each other at a fixed distance and moved parallel to the weld seam on a trolley with an incremental encoder (tandem arrangement). This results in a three-dimensional (‘transmission’) image with the representation of the time axis (vertical) and the distance travelled or the horizontal position with a grey value coding of the peak amplitudes (Fig. 2).

Fig. 2:

Example of a three-dimensional TOFD measurement signal [2]

The lateral wave and the reflected back-wall signal in Fig. 2 represent the boundaries of the TOFD image. Between these two signals, the diffraction signal originating from a material defect can be seen in the right half of the image. If the geometries and the angles of incidence are known, the defect depths can then be calculated.

Summary

The ultrasonic time-of-flight diffraction technique is a testing method used to determine the condition of weld seams both qualitatively and quantitatively. It can be used to determine the geometry and size of defects in welds. This method is also suitable for assessing corrosion in pipes [3] and is standardised for other applications in non-destructive testing [4–7].

References

[1]	https://www.olympus-ims.com/de/ultrasonic-transducers/tofd/ (access on January 26, 2026)
[2]	http://www.autsolutions.net/ndt-resources/tofd/ (access on January 26, 2026)
[3]	Spies, M., Dillhöfer, A., Müller, W., Rieder, H., Schmitz, V.: SAFT, TOFD, Phased Array – Klassische Anwendungen und neuere Entwicklungen der Ultraschall-Bildgebung. Seminar des Fachausschusses Ultraschallprüfung der DGZfP (DGZfP UT 2013)
[4]	ISO 16828 (2025-07): Non-destructive Testing – Ultrasonic Testing – Time-of-Flight Diffraction Technique for Detection and Sizing of Discontinuities
[5]	ISO 10863 (2020-09): Non-destructive Testing of Welds – Ultrasonic Testing – Use of Time-of-Flight Diffraction Technique (TOFD)
[6]	DIN EN 583-6 (2009-03): Non-destructive Testing – Ultrasonic Examination – Part 6: Time-of-Flight Diffraction Technique as a Method for Detection and Sizing Discontinuities (wthdrawn; replaced by DIN EN ISO 16828 (2025-07))
[7]	ISO 15626 (2018-07): Non-destructive Testing of Welds – Time-of-Flight Diffraction Technique (TOFD) – Acceptance Levels