Ultrasound-guided Waves

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Ultrasound guided waves

Definition of guided waves

By definition, ‘guided waves’ are types of waves that propagate in the main axis direction of a comparatively thin medium, i.e. they are conducted or guided through the medium. The distance or thickness d of the confining surface layers in the normal direction is approximately in the order of magnitude of the wavelength λ, i.e. d ≈ λ. The term ‘guided waves’ is therefore justified by the ‘guidance’ of the waves through the geometry of the tested component, although Lamb or plate waves are generally a vibration-related property of thin plates or membranes, as is the hat wave.

Types of waves and their propagation

The plate waves or guided waves, which propagate differently from Rayleigh waves (surface waves, see Fig. 1a), Stoneley waves and Love waves (Fig. 1b) in the presence of interface surfaces, are divided into symmetric and asymmetric plate or Lamb waves (Fig. 2) and represent their vibration modes. Stoneley and Love waves, which are also known from seismology [1], have so far been of little significance in non-destructive testing (NDT), but are already being used for special applications in ultrasound testing [2−4]. Surface waves (Fig. 1a) are very important for sound emission testing (SEP) and acousto-ultrasonics testing, as these waves are only slightly attenuated in their direction of propagation and can therefore be received over long distances, whereby special AE probes are used [5, 6]. The symmetric plate wave (Fig. 2a) is also referred to as the S-type and the asymmetric wave as the A-type (Fig. 2b) with different vibration modes (index 1, 2, 3) if the wavelength becomes very small. With such geometric wave types, the propagation velocity generally depends on the wavelength and leads to dispersion phenomena in the material.

Fig. 1:

Direction of propagation (AR) and direction of vibration (SR) of (a) Rayleigh waves and (b) Love waves

Fig. 2:

Examples of guided waves: (a) symmetric plate wave, (b) asymmetric plate wave (Lamb waves) with the direction of propagation (AR) and direction of vibration (SR)

Plate waves with limited geometries, as shown in Figure 2a, are also referred to as expansion or compression waves, whereby it can be seen that the neutral fibre of the plate does not change its geometric position (pure longitudinal vibration), in contrast to the asymmetrical wave in Figure 2b, where the neutral fibre performs a transverse vibration. It is typical for Lamb waves that also harmonics can occur, although their amplitude decreases significantly with increasing order.

Requirements for component testing

The guided surface waves do not emerge from the material but remain ‘attached’ to the geometry, similar to light waves in glass fibres. This means that possible geometric changes, such as component curvatures or edges, do not pose an obstacle for these waves and (due to d ≈ λ) no reflection of sound waves occurs at them. As a result, the loss of sound energy into the adjacent media is considerably lower than with established methods of ultrasound testing, such as the pulse-echo method. The guided waves therefore propagate almost undamped in the tested medium and can therefore be transmitted over relatively long distances to perform tests on components that are inaccessible in places, such as pipe penetrations under roads or buildings [7]. Guided waves are actively excited in the component to be tested by means of a suitable transmitter and received by the identical receiver (echo mode) or an additional ultrasonic probe (transmission mode).

When propagating guided waves, however, it should be noted that different wave modes can be excited depending on the excitation frequency and the component thickness, and dispersions can occur [8], which can sometimes make it considerably more difficult to interpret structural damage that has occurred.

Application examples for guided waves

However, guided waves have a wide range of applications and are particularly useful when sections of large components, such as pipes or tanks, are difficult to access. Current technical literature cites the following areas of application [9−13]:

Inspection of plates,
Pipe inspection,
Inspection of adhesive joints,
Airfoil inspection,
Structural monitoring for early damage detection (permanent),
Inspection of wind turbines (rotor blades),
Container and tank inspection.

References

[1]	Clauser, C.: Einführung in die Geophysik. Springer, Berlin Heidelberg, 2nd Edition (2016), (ISBN 978-3-662-46883-8)
[2]	Malischewsky, P. G., Schnapp, J.-D.: Oberflächenwellen und Materialprüfung aus seismologischer Sicht. DGZfP-Dach Jahrestagung, Salzburg (2004)
[3]	Wendrich, A.: Zerstörungsfreie Ortung von Anomalien in historischem Mauerwerk mit Radar und Ultraschall – Möglichkeiten und Grenzen. (Dissertation), Bauhaus-Universität Weimar (2008)
[4]	Jüngert, A.: Untersuchung von GFK-Bauteilen mit akustischen Verfahren am Beispiel der Rotorblätter von Windenergieanlagen. (Dissertation), Universität Stuttgart (2010) (access on November 30, 2025)
[5]	Meyer, E., Neumann, E.-G.: Physikalische und technische Akustik. Springer Verlag, Berlin, 2nd Edition, (2013), (ISBN 978-3-322-91086-8)
[6]	Grosse, C. U., Ohtsu, M. (Eds.): Acoustic Emission Testing. Springer Verlag, Berlin (2008), (ISBN 978-3-540-69895-1)
[7]	Prager, J., Gravenkamp, H., Rahman, M.-U., Köppe, E.: Einsatz geführter Wellen für die Ultraschallprüfung. tm − Technisches Messen Plattform für Methoden, Systeme und Anwendungen der Messtechnik. 79 (2012) 5, pp. 251–261, DOI: https://www.degruyter.com/document/doi/10.1524/teme.2012.0168/html
[8]	Schmidt, D.: Modenselektive Übertragung von Lambwellen in Faserverbundstrukturen. Dissertation, Technische Universität Braunschweig (2014) (access on November 30, 2025)
[9]	Prager, J., Köppe, E. Bartholmai, M.: Früherkennung von Strukturschäden mittels geführter Lamb-Wellen. GMA/ITG-Fachtagung Sensoren und Messsysteme. Proceedings (2012) pp. 531–540
[10]	Schubert, L., Klesse,T., Weihnacht, B., Schulze, E., Lieske, U., Frankenstein, B.: Aktuelle Entwicklungsarbeiten zu aktiv angeregten geführten Wellen (Acousto-Ultrasonics) für SHM-Anwendungen. 20. Kolloquium Schallemission, DGZfP (2015) https://www.ndt.net/search/docs.php3?id=20941
[11]	Rau, E., Bamberg, J., Berwig, P.: Ultraschallprüfung an Turbinenschaufeln mittels Oberflächenwellen. DGZfP-Seminar FA Ultraschallprüfung (2015), Lecture No. 8; https://www.ndt.net/article/dgzfp-ut-2015/papers/8.pdf
[12]	Prager, J., Brackrock, D., Dohse, E., Gaal, M., Homann, T., Grezeszkowski, M.: Anwendung geführter Ultraschallwellen für die Prüfung von Klebeverbindungen. DGZfP-Dach Jahrestagung, Potsdam (2014)
[13]	Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Ultraschallprüfung. Springer, Berlin (2014), (ISBN 978-3-662-44699-7)

Weblink

Wikipedia – The free encyclopedia: https://en.wikipedia.org/wiki/Lamb_waves (access on February 25, 2026)