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{{Language_sel|LANG=ger|ARTIKEL=Ultraschall-Normal-Prüfköpfe}}
{{PSM_Infobox}}
Ultrasonic standard sensors
__FORCETOC__

==Fundamentals==

Ultrasonic standard sensors, also known as single-element vertical sensors or longitudinal wave sensors, essentially consist of a [[Piezoelectric Ceramic Transducer|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 & Werkstoff|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 [[Piezoelectric Ceramic Transducer|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|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 Ultrasonic Technique|pulse-echo mode]] during [[Ultrasound Testing|ultrasound testing]], the signal must be well attenuated, as otherwise the received signal will be influenced by the transmitted pulse.

[[File:US_Standard_Sensors-Fig1.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px"|Schematic setup of an ultrasonic standard sensor (a) without and (b) with lead-in distance for the ultrasonic signal
|}

[[Plastics]] such as polymethyl methacrylate ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PMMA), polycarbonate ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PC), polyamide imide ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PAI), epoxy resins ([[Plastics – Symbols and Abbreviated Terms|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 [[Ultrasonic Waves Reflection|reflections]] on the top and bottom of the damping layer.

==Examples of implementation==

Designs of ultrasonic standard sensors with different [[Acoustic Properties|acoustic properties]] are shown in '''Fig. 2'''.

[[File:Normalpruefkoepfe-2.JPG|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px"|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 [[Phase Boundary Surface|interface]] ([[Surface|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''').

[[File:US Standard Sensors-Fig3.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px"|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 & Werkstoff|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 [[Ultrasonic Waves Reflection|reflection]] (R) and [[Transmission Sound Waves|transmission]] factor (''T'' or ''D'') ('''Eq. 1''' and '''2''').

{|
|-
|width="20px"|
|width="500px"|<math>R= \frac{W_{2}-W_{1}}{W_{2}+W_{1}}</math>
|(1)
|}

{|
|-
|width="20px"|
|width="500px"|<math>T= \frac{2 \cdot W_{2}}{W_{2}+W_{1}}</math>
|(2)
|}

with:
{|
|-
|Sound impedance ''W''1 = ''ρ''1 ''c''L1 of the medium 1
|-
|Sound impedance ''W''2 = ''ρ''2 ''c''L2 of the medium 2
|-
|[[Density]] ''ρ''1/2 of the medium 1 or 2
|-
|Longitudinalwellengeschwindigkeit ''c''L1/L2 of the medium 1 or 2
|-
|''P''0 – [[Sound Pressure|Sound pressure]] of the incident sound wave
|-
|''P''R – Sound pressure of the reflected sound wave
|-
|''P''D – Sound pressure of the transmitted sound wave
|}

With identical media, i.e. ''W''1 = ''W''2, ''R'' = 0 and ''T'' = 1, sound transmission is unimpeded, whereas [[Ultrasonic Waves Reflection|reflection]] at the sound-soft [[Material & Werkstoff|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|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, [[Heterogeneity|heterogeneities]], and [[Anisotropy|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''').

[[File:US Standard Sensors-Fig4.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px"|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 [[Piezoelectric Ceramic Transducer|transducer]] and the wavelength ''λ'' in the test medium and is calculated according to '''Eq. (3)''' [1, 2].

{|
|-
|width="20px"|
|width="500px"|<math>a_{N}\approx \frac{D^{2}}{4\lambda}</math>
|(3)
|}

Standard sensors can be used for [[Ultrasonic Wall Thickness Measurement|wall thickness measurement]] and defectoscopy in transmission mode or in [[Pulse-Echo Ultrasonic Technique|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|ultrasonic sensors]] with a different nominal frequency or [[Ultrasonic Transmitter(S)-Receiver(E) Sensors|transmitter(S)-receiver(E) sensors]] should be used.

To use the [[Ultrasonic Transmission Technique|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 Ultrasonic Technique|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''').

[[File:US_Standard_Sensors-Fig5.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 5''':
|width="600px"|Use of standard sensors in the [[Ultrasonic Transmission Technique|ultrasonic transmission technique]]
|}

[[File:US_Standard_Sensors-Fig6.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 6''':
|width="600px"|Use of standard sensors heads in the [[Pulse-Echo Ultrasonic Technique|pulse-echo ultrasonic technique]]
|}

The detectability of defects in both [[Ultrasonic Transmission Technique|transmission]] testing and [[Pulse-Echo Ultrasonic Technique|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|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 [[Ultrasonic Angle Beam Sensors|angle beam sensors]] or [[Ultrasonic Phased Array Sensors|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 [[Sound Velocity|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 Technique|A-scan]].

==See also==

* [[Ultrasound Testing|Ultrasound testing]]
* [[Ultrasonic Sensors|Ultrasonic sensors]]
* [[HF-Scan|HF-scan]]
* [[Non-destructive Polymer Testing|Non-destructive polymer testing]]
* [[Ultrasonic Direct Coupling|Ultrasonic direct coupling]]

==References==

{|
|-valign="top"
|[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)
|-valign="top"
|[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)
|-valign="top"
|[3]
|Krautkrämer, J., Krautkrämer H.: Werkstoffprüfung mit Ultraschall. Springer, Berlin (1986), (ISBN 978-3-662-10909-0)
|-valign="top"
|[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
|-valign="top"
|[5]
|Diederichs, R.: Ultraschall Prüfkopf. Ultraschallprüfkopf und Schallfeld. (1995) (access on November 23, 2025)
|-valign="top"
|[6]
|Höfler, A.: Ultraschallprüfung, tec-science, July 13, 2018 (access on November 23, 2025)
|-valign="top"
|[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))

[[Category:Acoustic Test Methods_Ultrasonics]]

Ultrasonic Standard Sensors - Revision history