Ultrasonic Laser Excitation
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Ultrasonic laser excitation
General
In non-destructive testing using ultrasound, longitudinal or transverse waves can be introduced into the test object using standard, angle or transmitter(S)-receiver(E) sensors by means of direct coupling, air coupling or water coupling (immersion bath or squirter technique). If contact between the test piece and water or coupling agent is not possible due to possible damage or excessive temperature of the surface, and if air-ultrasound does not produce satisfactory results, there is also the option of contactless direct generation of ultrasound in the test object [1−3].
Contactless electrodynamic ultrasonic generation
One method of generating and detecting ultrasound is to use electromagnetic ultrasonic transducers (EMUS), which, however, only work on electrically conductive test objects [4]. These EMUS transducers use the so-called LORENTZ force to generate ultrasound, which acts on current-carrying areas in an existing permanent magnetic field. The current in the desired direction is generated by high-frequency coils located approximately 1 mm above the surface of the test object (Fig. 1) [5]. The charge carriers in the static magnetic field are simultaneously deflected (eddy current) and oscillate at the frequency of the applied alternating current, generating ultrasound with frequencies of up to 5 MHz in the test volume. The test object is therefore the oscillator, whereby the effect is reversible. The electrodynamic excitation is limited to the area close to the surface by the skin effect (i.e. the displacement of the current from the interior), whereby the penetration depth depends in particular on the material of the test object. At a measurement frequency of 5 MHz, the penetration depth is approximately 4 µm for steel and approximately 40 µm for aluminium, but it is particularly suitable for high temperatures [6].
| Fig. 1: | Contactless electrodynamic ultrasonic generation of (a) transverse waves and (b) longitudinal waves |
Contactless laser-based ultrasonic generation
Another variant of contactless ultrasonic generation in a test object is the use of lasers, which generate coherent laser light with vibration characteristics comparable to electromagnetic vibrations [4, 5]. The laser pulses used in laser-excited ultrasound (LASUS), comparable to Dirac pulses, typically have a pulse duration in the nanosecond range and can be focused onto small focal spots. The pulse laser powers used are in the range of up to 500 mJ with a pulse length of approx. 10 ns, which means that pulse frequencies of up to 100 MHz can easily be achieved [4].
When irradiated with low laser powers, a local but temporary slight heating of the surface of the test object occurs, leading to thermoelastic expansion and contraction transverse to the direction of the surface normal (Fig. 2a). The excited mechanical vibrations of the volume elements parallel to the surface generate ultrasonic pulses in the test object, which propagate as transverse and surface waves.
If the energy of the laser is increased or the laser beam is strongly focused at the same power, atoms near the surface are vaporised in the plasma generated, causing a local recoil on the test object.
In this case, the volume elements on the surface are subjected to vertical stress, which is why ultrasonic pulses in the form of longitudinal waves are generated in the test object, which have significantly higher amplitudes than in the thermoelastic method (Fig. 2b) [4, 5].
| Fig. 2: | Ultrasonic laser excitation of (a) transverse and surface waves and (b) longitudinal waves by (a) low or (b) high laser energy |
Device systems
For contactless detection or reception of ultrasonic echoes from conventional or laser-excited ultrasound, high-resolution laser extensometers, laser anemometers (laser Doppler scanner) or laser vibrometers are generally used, which operate on the Doppler principle, for example. The particular advantage here is that these measuring systems operate without inertia, can be used at high temperatures, and often also allow direction-dependent detection of vibrations in different axes. These laser measuring systems require a high bandwidth with sufficient sensitivity to be able to detect the vibrations of the surface caused by the high-frequency ultrasonic pulses (20 to 100 MHz) and display them directly in units of length. This method of receiving ultrasound is also suitable for calibrating piezoelectric, capacitive, magnetostrictive or electromagnetic ultrasonic sensors and for characterising the spatial and temporal sound fields of ultrasonic transducers [5].
Laser anemometers (Fig. 3a) and laser vibrometers or laser interferometers (Fig. 3b) are particularly well suited for contactless detection of induced ultrasonic vibrations on the surface of test objects. Depending on the configuration of the measuring system, the deformation of the surface can be recorded in the x, y and z directions at a local position. This requires a reference laser beam and an object laser beam, which are superimposed on a detector surface. The difference between the two beam positions when changes occur on the object surface is recorded and displayed. In the case of laser vibrometers and interferometers, the direct positional or length changes (amplitudes) are determined, while in the case of anemometers (LDA), the Doppler velocity of moving surface particles is evaluated and converted into amplitudes.
| Fig. 3: | Schematic ultrasound reception using (a) laser vibrometer and (b) laser anemometer (LDA) |
The advantage of laser excitation over the EMUS process is that its range of applications is not limited to metallic materials [7, 8], but also includes plastics and ceramics [9].
See also
References
| [1] | Langenberg, K.-J., Marklein, R., Mayer, K.: Theoretische Grundlagen der zerstörungsfreien Materialprüfung mit Ultraschall. Oldenbourg Publishing, Munich (2009), (ISBN 978-3-486-59859-9) |
| [2] | Hirschberg, H. G.: Handbuch Verfahrenstechnik und Anlagenbau: Chemie, Technik und Wirtschaftlichkeit. Springer, Berlin (2013), (ISBN 978-3-642-58357-5) |
| [3] | DIN EN 15042-1 (2006-06): Thickness Measurement of Coatings and Characterization of Surface with Surface Waves − Part 1: Guide to the Determination of Elastic Constants, Density and Thickness of Films by Laser Induced Surve Acoustic Waves |
| [4] | Gevatter, H.-J., Grünhaupt, U. (Hrsg.): Handbuch der Mess- und Automatisierungstechnik in der Produktion. Springer, Berlin, 2nd Edition (2006), (ISBN 978-3-540-21207-2) |
| [5] | Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (2012), (ISBN 978-3-642-63864-0) |
| [6] | Schrüfer, E.: Elektrische Messtechnik. Carl Hanser, Munich, 9th Edition (2007), (ISBN 978-3-446-40904-0) |
| [7] | Deppe, G.-J., Hüls, F., Sauerland, M., Schneider, H.: Laser-Ultraschall-Wanddickenmessung an heißen Rohren in einem Walzwerk. DGZfP-Jahrestagung, Berlin (2001), Berichtsband 75-CD, Download |
| [8] | Schlawne, F., Deppe, G.-J., Graff, A., Schneider, H.: Moderne Verfahren zur Qualitätssicherung und Prozesssteuerung in Rohrwerken. DGZfP-Jahrestagung, Weimar (2002), Berichtsband, Download |
| [9] | Pohl, J., Willberg, C., Gabbert, U., Mook, G.: Analyse der Lambwellenerzeugung durch Piezoaktoren für Structural Health Monitoring-Systeme. DGZfP-Jahrestagung, Bremen (2011), Proceedings A.2 Download as pdf |