Ultrasonic Composite Sensors
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Ultrasonic composite sensors
General
Ultrasonic sensors containing composite transducers are increasingly being used in testing practice when low acoustic impedance and high acoustic efficiency for thickness vibrations are required [1−4]. At the same time, such composite sensors have low cross-coupling and high internal damping with sufficient mechanical stiffness and strength, as well as high flexibility of the surface [1], i.e., they offer very good sensitivity and damping.
Selection of suitable ultrasonic sensors
Since conventional ultrasonic transducers with piezoelectric ceramics or piezoelectric polyvinylidene difluoride (abbreviation: PVDF) foils (see also: piezoelectric ceramic transducer) have high damping and low sensitivity [5] and lead zirconate titanate (PZT) piezoceramic transducers have high sensitivity with low damping, choosing a suitable sensor for a specific testing task is always a compromise due to these opposing tendencies [1, 6]. In particular, for volume testing of sound-attenuating (plastics) or sound-scattering (filled and reinforced plastics) components, including areas close to the surface, a high amplitude of the signals used is therefore required in order to obtain clearly identifiable defect or back wall echoes. At the same time, a high measurement frequency with a short pulse duration, i.e., high damping, with a modifiable pulse repetition frequency is necessary in order to be able to detect small defects near the surface (e.g., pores) [1].
Piezo composite sensors
As a result of the testing requirements, composite transducers were developed to optimize the piezoelectric ceramics as a composite of a polymer matrix and a piezoceramic (Fig. 1).
| Fig. 1: | Schematic diagram of 1-3-piezo composite sensor |
Highly compacted PZT ceramic rods aligned parallel to the plane are usually embedded in an epoxy resin (abbreviation: EP) matrix, resulting in a comparatively low density and acoustic impedance. The two-dimensionally arranged sensor elements (ceramic rods) can transmit and receive individually or in groups, thus generating a two-dimensional image. The rods must be optimally mechanically decoupled to prevent mutual interference. The composite sensors are manufactured using the so-called dice-and-fill technique, whereby orthogonal cuts are sawn into a ceramic disc to a depth of approx. 80 %, creating a fine grid field. The grid field, known as a 1-3-arrangement, is then filled with casting resin, ground and contacted at the surfaces. The acoustic and mechanical properties of such composite sensors depend on the ceramic properties (see also: piezo ceramic) but also significantly on the type of plastic and the filler content (15–90 wt.-%) [7]. Due to the relatively free choice of combination, geometric conditions, and spacing, these probes can be specifically modified and adapted to special testing tasks. If very flexible plastics are used for the filling, the oscillator can also be shaped so that it can be adapted to curved surfaces of test pieces. At the same time, this also allows for the focusing of line or point-shaped oscillators [1]. The maximum resonance frequency of such test sensors is approximately 10 MHz, but with a very broad frequency response, whereby test sensors with 2 to 5 MHz are typically used. Due to the plastics used, the operating temperature is limited to 100 °C, with up to 150 °C possible for short periods.
Application of ultrasonic composite sensors
Composite sensors can be used for different applications in direct coupling, in immersion bath technique, and in tests using air-ultrasound, although in this case it is essential to adjust the test frequency [8]. When comparing a composite sensor with a standard sensor (Fig. 2) in immersion technology with an HFUS 2000 testing system from Hillger, Braunschweig, on a composite made of glass fibre-reinforced plastics and polypropylene (abbreviation: PP), it can be seen that, due to the higher damping of the composite sensor (Fig. 2b), the interface and back-wall echoes are more clearly detectable despite the low measuring frequency.
| Fig. 2: | HF-scans of a GFRP with 70% GF fabric (UP resin Derakene 411) and a PP inliner using (a) a 2.25 MHz standard probe A 106S from Panametrics GmbH, Hofheim, and (b) a 1.0 MHz composite probe from Fa. SONOTEC Ultraschallsensorik Halle GmbH, Halle (Saale) |
When evaluated using the “FreqScan” method from Polymer Service GmbH Merseburg, the standard sensor exhibits significantly stronger transient behaviour (Fig. 2a) with lower sensitivity and resolution. The broadband composite sensors are particularly suitable for testing plastics, composite materials, and other materials that strongly dampen or scatter sound. Composite sensors can offer up to 20 dB higher sensitivity with significantly shorter sound pulses than standard sensors with conventional piezo ceramics. Due to their low acoustic impedance, composite transducers are particularly well suited as the basis for transducers with plastic lead-in sections, such as transmitter (S)-receiver (E) sensors or angle beam sensors, as well as immersion transducers.
See also
- Ultrasound testing
- Ultrasound − Elastic parameters
- Ultrasonic phased array sensors
- Ultrasonic immersion bath sensors
- Ultrasonic transmitter(S)-receiver(E) sensors
- Ultrasonic angle beam sensors
References
| [1] | Splitt, G.: Prüfköpfe mit Composite-Schwingern − Ein Meilenstein für die Ultraschallprüfung. NDTnet 7 (1996) 1 |
| [2] | 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) |
| [3] | Ultraschallprüfköpfe und -sensoren, Fraunhofer-Institut für Keramische Technologien und Systeme IKTS https://www.ikts.fraunhofer.de/en/industrial_solutions/ultrasound/ultrasonic_probes_sensors.html (access on November 19, 2025) |
| [4] | Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (2012), (ISBN 978-3-642-63864-0) |
| [5] | Schrüfer, E.: Elektrische Messtechnik. Carl Hanser, Munich, 9th Edition (2007) (ISBN 978-3-446-40904-0) |
| [6] | Schuster, V., Lach, M., Platte, M.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz. DGZfP-Jahrestagung „Zerstörungsfreie Werkstoffprüfung“ 2004, Salzburg, Austria, Sonderdruck Karl Deutsch, SD 1/51 |
| [7] | Lerch, R., Sessler, F. M. Wolf, D: Technische Akustik: Grundlagen und Anwendungen. Springer, Berlin (2009), (ISBN 978-3-540-23430-2) |
| [8] | Bühling, L., Hillger, W., Ilse, D.: Modulare Ultraschallprüfsysteme für Forschung, Entwicklung und Qualitätssicherung. DGZfP-Jahrestagung 2.–4.05.2005, http://www.ndt.net/article/dgzfp05/v01.pdf (access on November 20, 2025) |
| [9] | Sirch, C., Oluschinski, A., Bierögel, C., Grellmann, W., Rufke, B., zur Horst-Meyer. S.: Ultraschall-Untersuchungen an Grenzflächen in GFK-Thermoplast-Verbunden. Tagungsband 12. Internationale Fachtagung "Polymerwerkstoffe 2006", 27.–29.09.2006, Halle/Saale |