Ultrasonic Shock Wave Sensors

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Ultrasonic shock wave sensors

Criteria for the use of sensors

Conventionally used and distributed standard sensors are employed in many testing tasks where sufficient measurement resolution and accuracy are required for most of the components under investigation. Due to the dimensions of the near and far fields, these sensors cannot usually be used for defectoscopy on components with low thickness or surface-related defects. Another reason is the pulse shape and frequency characteristics of these sensors, which are characterized by relatively low attenuation and a comparatively narrow frequency band (Fig. 1) [1]. Figure 1a shows the HF-scan of a sensor with medium attenuation, which exhibits a clear decay behaviour of the ultrasonic signal. Weak or unmatched attenuation results in strong oscillation behavior and a narrow frequency band of the sensor, as shown in Fig. 1b.

Fig. 1:

HF-Scan and frequency spectrum of a standard sensor (a) with short pulse shape and high bandwidth, (b) with wide pulse shape and low bandwidth based on [2]

However, the vibration mode and sensitivity of the transducer are also influenced by the electrical adaptation to the measuring device electronics, which also acts as a frequency filter. The bandwidth Δf of the sensor is calculated from the difference between the upper f_o and lower f_u cut-off frequencies, i.e. the frequency at which the amplitude of the frequency spectrum has decreased by 3 dB.

Applications in defectoscopy

The sensors shown in Fig. 1 are typically used in defectoscopy or wall thickness measurement of thick-walled test pieces. Due to the ultrasonic signal (transmission pulse) decaying over a longer period of time, problems naturally arise when defects in thin-walled components (sheet metal, plastic laminates) are to be detected or when wall thickness measurements are to be performed on such test specimens.

For such thin-walled test pieces, shock wave sensors with significantly reduced decay behaviour and a very broadband frequency response should be used, especially for wall thickness measurement or defectoscopy (Fig. 2). These highly damped shock wave sensors with low acoustic impedance allow accurate separation of the start and stop signals during runtime measurement, i.e., there is no interference between the transmitted signal and the backwall or fault echo, as the pulse shape is close to the ideal “Dirac impulse.”

Fig. 2:

HF-scan and frequency spectrum of a shock wave sensor with very short pulse shape and extremely high bandwidth based on [2]

The waveform of the sensor signal depends largely on the ratio of the acoustic impedances of the transducer and damper materials. If the impedances are identical, the sensor is highly damped. The waveform is very short and consists of only a few sine waves (Fig. 1a). Extremely short pulses with one to one and a half sinusoidal oscillations are referred to as shock wave sensors, which have broadband transducer characteristics (Fig. 2). The less the impedance of the damper is matched to that of the transducer, the poorer the damping and the narrower the frequency characteristic (Fig. 1b).

Lead metaniobate transducers (PbNb₂O₆) have the lowest acoustic impedance Z (20.5⋅10⁶ kg/m²s) of all piezoelectric ceramics and are the easiest to dampen. These damping bodies usually consist of mixtures of heavy metal powder and plastics. The higher the acoustic impedance of the sensor is to be, the greater the proportion of heavy metal in the damping body must be. Lead metaniobate, like polyvinylidene fluoride (abbreviation: PVDF), is particularly suitable for constructing high-resolution sensors with extremely short pulses for shock wave sensors. In PVDF transducers, the damping body consists only of highly absorbent plastics. At identical frequencies and with similar transducer diameters, shock wave transducers made of PVDF and lead metaniobate, for example, show approximately comparable sensitivity at very short ultrasonic pulses (see Fig. 2) [2]. Due to their good impedance matching, lead metaniobate transducers are almost exclusively used for shock wave sensors for direct coupling to steel and other metallic and ceramic materials, allowing the manufacture of very small and high-frequency sensors (finger tips).

The comparatively low impedance of lead metaniobate is also important for ultrasonic transmitters(S)-receivers(E), and angle sensors if a wide frequency range is required at the same time. When the acoustic impedance of the transducer and the attachment wedge are identical, the wedge acts in conjunction with the matching layer as a damper, but also as a frequency filter for the transducer. In this case, the energy transfer into the test object is optimal. Lead metaniobate is often used when very small transducer dimensions are required, e.g., for finger-tip sensors, as in this case there are no disturbing transverse vibrations that can influence the signal shape and frequency (signal distortions and frequency shifts) [2]. Piezoelectric plastic films made of PVDF provide highly effective sound radiation, especially in liquids and plastics. PVDF is therefore suitable for high-frequency standard sensors with and without a delay line (delay line or immersion bath technique sensors) (f up to 150 MHz). Due to their acoustic impedance, piezo composites are also very well suited for sound radiation in liquids and plastics, whereby the plastic filler of the damper should only be moderately filled (impedance). This also allows the production of very broadband immersion bath sensors with a higher sensitivity than piezoelectric ceramic transducers. For angle beam and S/E sensors, simply bonding 1-3 piezo composites to the attachment wedge is sufficient to achieve broadband transmission characteristics similar to those of piezo ceramic sensors with an adaptation layer [2].

The shock wave sensors described above can be used, for example, to assess the bonding quality of composites (plastic coatings on metals, rubberized metal rollers) based on the position of the intermediate echoes and their phase position, as well as the polarity of the reflection factor. An ultrasonic device for evaluating HF-scans is required when materials or composites with very different acoustic impedances are to be tested. The amplitude of the intermediate echo then allows conclusions to be drawn about the quality of the bond (no air) or in the case of delamination (coupling fluctuations). If the echoes are very close together (defect echo 1 and subsequent echoes or backwall echoes), they can overlap or interfere with each other, shifting the reference point of the zero crossing of a wall thickness measurement. This occurs when the zero crossings are used instead of the amplitude to evaluate the runtime measurements, which are then used as the start and stop signals of the discriminator (threshold). This can occur with coarse-grained or highly scattering materials, where interference with the noise level occurs. These errors can be minimized by using shock wave sensors instead of standard sensors.

Use for wall thickness measurement

When using digital wall thickness measurement systems, particular attention must be paid to the absolute component thickness, the geometry of the test piece, the material being examined, and the required accuracy or resolution limit. While comparatively low test frequencies are used for large or thick components, coarse-grained materials, and plastics, high frequencies are preferable for low thicknesses or fine-grained structures when using S/E or shock wave sensors. High test frequencies, especially when using shock wave sensors, provide a high level of absolute accuracy that is not achieved at low test frequencies and when using S/E sensors [3].

Except for spectroscopic examinations, it is therefore necessary to work with very short, i.e., broadband ultrasonic pulses for many testing problems, whereby the following applications are possible for shock wave sensors [4]:

• wall thickness measurement on thin metal components
• wall thickness measurement on plastic components, including glass fibre-reinforced and filled plastics
• detection of surface-related and remote defects
• immersion bath testing on components in the aerospace industry
• testing of welded, soldered, and bonded small components for bonding defects (see: weld line)
• accurate measurements of sound velocity through the evaluation of successive echo signals [5].

See also

• Non-destructive polymer testing
• Imaging ultrasonic testing
• Ultrasonic immersion bath sensors
• Ultrasonic time-of-flight diffraction (TOFD)
• Ultrasonic runtime measurement

References

[1]	Krautkrämer, J., Krautkrämer H.: Werkstoffprüfung mit Ultraschall. Springer, Berlin, (1986), (ISBN 978-3-662-10909-0)
[2]	Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (1997), (ISBN 3-540-62072-9; see AMK-Library under M 45)
[3]	Matthies, K. u. a.: Dickenmessung mit Ultraschall. DVS-Verlag GmbH, Berlin, 2nd Edition, (1998), (ISBN 3-87155-940-7; see AMK-Library under M 44)
[4]	Klein, M.: Untersuchungen des Schallfeldes breitbandiger Ultraschall-Prüfköpfe. Technische Forschung Stahl, Abschlussbericht zum Forschungsvertrag 6210-GA/101, (1977)
[5]	Heeling, A.: Ein Vierteljahrhundert Spundwanddickenmessung mittels Ultraschall bei der Bundesanstalt für Wasserbau. DGZfP Fachtagung Bauwerksdiagnose, February 18–19, 2010, Berlin (http://www.ndt.net/search/docs.php3?MainSource=97) (access on November 30, 2025)