Ultrasonic Standard Sensors

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Ultrasonic standard sensors

Fundamentals

Ultrasonic standard sensors, also known as single-element vertical sensors or longitudinal wave sensors, essentially consist of a piezoelectric ceramic transducer that generates mechanical stress waves and thus longitudinal waves as a result of electrical excitation by voltage pulses, a damping material, and the associated electrical adaptation (Fig. 1) [1–3]. The generation of ultrasound is based on the piezoelectric effect, whereby the oscillator material and its geometry (thickness d, diameter D) essentially determine the sensor properties such as resonance frequency f_R and near-field geometry. A short voltage pulse or an alternating voltage is applied to the transducer via a capacitor/inductance circuit, causing the transducer to convert the voltages into mechanical vibrations in accordance with the piezoelectric effect (see also: piezoelectric ceramic). When subjected to a voltage pulse, it performs a damped vibration, which depends on the damper material, at its resonance frequency. In the alternating voltage field, forced vibrations occur at the excitation frequency [4–6].

Schematic setup

Electrical matching is achieved, for example, with a coil (inductivity) and capacitors in order to optimize the appropriate frequency band and increase the power of the ultrasonic transmitter. In this case, the capacity of the oscillator and the cable capacities (electrical adjustment) can be largely compensated for by an inductance, so that the transducer converts almost only active power. This results in an oscillating circuit whose frequency is tuned to the resonance frequency of the oscillator and to specific testing requirements (Fig. 1a). In order to achieve high pulse repetition frequencies of up to 10 kHz in pulse-echo mode during ultrasound testing, the signal must be well attenuated, as otherwise the received signal will be influenced by the transmitted pulse.

Fig. 1:

Schematic setup of an ultrasonic standard sensor (a) without and (b) with lead-in distance for the ultrasonic signal

Plastics such as polymethyl methacrylate (abbreviation: PMMA), polycarbonate (abbreviation: PC), polyamide imide (abbreviation: PAI), epoxy resins (abbreviation: EP), and soft rubber are most commonly used as damping materials. Further adjustments are necessary to increase the damping effect, broaden the frequency band, and minimize the swing-out time. The acoustic adjustment and mechanical protective film consists of a λ/4 layer that is tuned to the nominal frequency of the oscillator in order to reduce reflections on the top and bottom of the damping layer.

Examples of implementation

Designs of ultrasonic standard sensors with different acoustic properties are shown in Fig. 2.

Fig. 2:

Examples of standard sensors (handheld sensors) from Inspection Technologies GmbH, Ahrensburg (B, MB – wear protection film, 1, 2, 4, 5 – nominal frequency)

If sound waves from a standard sensor strike an interface (surface) of a test piece perpendicularly, no wave conversion [7] occurs. Part of the ultrasound emitted by the transducer is reflected back into the sensor and its intensity is reduced by the damping material (Fig. 3a).

Fig. 3:

Perpendicular incidence of ultrasonic waves on (a) a flat and (b) a rugged interface

Depending on the properties of the sensor (medium 1) and the test piece (medium 2), part of the longitudinal wave penetrates the material being tested and another part is also reflected toward the sensor. This property also depends on the contact between the sensor and the test piece and is described by the reflection (R) and transmission factor (T or D) (Eq. 1 and 2).

$R={\frac {W_{2}-W_{1}}{W_{2}+W_{1}}}$

(1)

$T={\frac {2\cdot W_{2}}{W_{2}+W_{1}}}$

(2)

with:

Sound impedance W1 = ρ1 cL1 of the medium 1

Sound impedance W2 = ρ2 cL2 of the medium 2

Density ρ1/2 of the medium 1 or 2

Longitudinalwellengeschwindigkeit cL1/L2 of the medium 1 or 2

P0 – Sound pressure of the incident sound wave

PR – Sound pressure of the reflected sound wave

PD – Sound pressure of the transmitted sound wave

With identical media, i.e. W₁ = W₂, R = 0 and T = 1, sound transmission is unimpeded, whereas reflection at the sound-soft material causes a phase reversal and the reflection factor then has a negative sign. However, this requires unimpeded sound entry into medium 2 or the test piece, which is why coupling media (water, oil, sonogel) are used, especially if the surface quality is poor (Fig. 3b), as even thin layers of air prevent the coupling of ultrasound.

Application in defectoscopy

In addition to material-specific attenuation or extinction (polycrystallinity, heterogeneities, and anisotropies), diffraction at material inhomogeneities and the divergence of the sound field at standard sensors affect the sound pressure available for defectoscopy [1, 7] (Fig. 4).

Fig. 4:

Sound field of the centre axis of a circular piston probe

The near-field length and the properties of the far field can be significantly influenced by the use of lead-in sections (Fig. 1b), as the near-field resolution is impaired, particularly in thin test specimens [2]. This prevents the transmission pulse from interfering with the backwall or error echo. In simplified terms, the near-field length aN depends on the diameter of the transducer and the wavelength λ in the test medium and is calculated according to Eq. (3) [1, 2].

$a_{N}\approx {\frac {D^{2}}{4\lambda }}$

(3)

Standard sensors can be used for wall thickness measurement and defectoscopy in transmission mode or in pulse-echo technique, whereby certain minimum requirements must be met in terms of test piece thickness, depending on the material properties and the test frequency or wavelength λ. Otherwise, ultrasonic sensors with a different nominal frequency or transmitter(S)-receiver(E) sensors should be used.

To use the transmission or intensity method, two identical standard sensors (pairs) are required and the test object must be accessible from both sides. Pulse or continuous sound can be used for testing, but care must be taken to ensure that the sensors are aligned exactly parallel to the acoustic axis and that the same coupling agent is used (Fig. 5). If the ultrasound can pass freely to the backwall of the test object, the intensity reduction I compared to the initial intensity I0 consists only of the attenuation caused by the path length and the material. In the case of an extended defect, the ultrasound is reflected back and there is no indication signal. If the defect is only partially exposed to the ultrasound, an intensity reduction occurs that depends on the degree to which the defect covers the receiver probe. The size of the defect can be approximately estimated by simultaneously moving the sensors horizontally (Fig. 5). In the pulse-echo method, there is no need for the test object to be accessible from both sides, as the sensor serves as both transmitter and receiver (Fig. 6).

Fig. 5:

Use of standard sensors in the ultrasonic transmission technique

Fig. 6:

Use of standard sensors heads in the pulse-echo ultrasonic technique

The detectability of defects in both transmission testing and pulse-echo testing depends on the orientation of the defect relative to the incident ultrasound. Only defects oriented perpendicular to the ultrasound wave are detected, and in the case of round defect structures such as air bubbles (see: gas bubbles), only the front side will produce an echo. Very narrow defects that lie in the direction of sound cannot be detected using a normal sensor, but only with angle beam sensors or phased array technology. In the case of discontinuities or the backwall, the signal is reflected to the sensor (Fig. 6) and the defect depth or wall thickness can be determined from the runtime if the longitudinal wave velocity is known. A defect that is only partially hit by the ultrasonic wave leads to a reduction in the intensity of the defect echo as well as the backwall signal in the A-scan.

See also

• Ultrasound testing
• Ultrasonic sensors
• HF-scan
• Non-destructive polymer testing
• Ultrasonic direct coupling

References

[1]	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)
[2]	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)
[3]	Krautkrämer, J., Krautkrämer H.: Werkstoffprüfung mit Ultraschall. Springer, Berlin (1986), (ISBN 978-3-662-10909-0)
[4]	Schuster, V., Lach, M., Platte, M.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz. DGZfP-Jahrestagung „Zerstörungsfreie Werkstoffprüfung“ 2004, Salzburg, Österreich, Sonderdruck Karl Deutsch, SD 1/51
[5]	Diederichs, R.: Ultraschall Prüfkopf. Ultraschallprüfkopf und Schallfeld. (1995) (access on November 23, 2025)
[6]	Höfler, A.: Ultraschallprüfung, tec-science, July 13, 2018 (access on November 23, 2025)
[7]	Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Ultraschallprüfung. Springer, Berlin (2014), (ISBN 978-3-662-44699-7)

Standard references

• ISO 16810 (2024-10): Non-destructive Testing – Ultrassonic Testing – General Principles
• DIN EN 1330-4 (2010-05): Non-destructive Testing – Terminology – Part 4: Terms used in Ultrasonic Testing (withdrawn; replaced by DIN EN ISO 5577 (2026-02))