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{{Language_sel|LANG=ger|ARTIKEL=Ultraschall-Winkel-Prüfköpfe}}
{{PSM_Infobox}}
Ultrasonic angle beam sensors
__FORCETOC__

==General remarks==

If defects occur in a test piece that are not oriented perpendicular to the [[Ultrasonic Sensors|sensor]], then no [[Ultrasonic Standard Sensors|standard sensor]] with longitudinal waves can be used due to the [[Ultrasonic Waves Reflection|reflection behaviour]] at the defect [[Phase Boundary Surface|boundary surface]]. In this case, transverse wave sensors or angle sensors must be used to generate a defect indication.

The transverse wave probe (Y-cut sensor) resembles the normal sensor in appearance, but emits transverse or shear waves due to its vibration characteristics. Since these waves cannot be transmitted by liquid coupling agents, highly viscous pastes or putty are used in this case for coupling to the test piece, whereby strong pressure must be applied to the [[Surface|surface]] at the same time. Due to these disadvantages, this type of sensor has only become established in laboratory use [1, 2].

==Schematic structure==

Ultrasonic angle sensors, on the other hand, are widely used in testing practice, especially for [[Ultrasonic Weld Inspection|defectoscopy on weld seams]]. Similar to standard sensors, angle beam sensors also contain a single-element vertical sensor, which, however, has a certain inclination to the normal or to the test piece surface ('''Fig. 1'''). This causes a longitudinal wave to be transmitted obliquely to the test piece surface in the sound field axis. The reflections of the transmitted longitudinal wave generated inside the transducer by the wedge geometry are not returned to the transducer, but are converted into heat by the damper. The wedge is usually made of [[Plastics|plastics]] such as polystyrene ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PS), polycarbonate ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PC) or polymethyl methacrylate ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PMMA). Between the transducer and the wedge there is usually an acoustic matching layer whose acoustic impedance lies between that of the wedge and transducer materials and whose thickness is ''λ''/4 [1]. This ensures good sound transmission into the wedge, produces high damping of the [[Piezoelectric Ceramic Transducer|transducer]] and results in minimal vibration with a high bandwidth of the angle beam sensor [1]. Since the emitted longitudinal wave strikes the [[Phase Boundary Surface|interface]] between the sensor and the test piece surface at an angle, a transmitted and reflected portion of the ultrasound is registered, but in addition, the transmitted and reflected wave is converted into a different type of wave.

[[File:US_Angle_Beam_Fig1.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px"|Schematic illustration of an ultrasonic angle beam sensor (a) without and (b) with electrical matching
|}

==Wave types and propagation of ultrasound==

The conversion of ultrasonic waves causes both longitudinal and transverse waves to be reflected, transmitted and refracted, with their angles of incidence defined by SNELLIUS' law of refraction ('''Fig. 2a'''). The following wave types arise from the obliquely incident longitudinal wave:

{|
|-
|width="400px" height="75px"|• a reflected longitudinal wave with
|width="200px"|<math>\frac{\sin \alpha _{0}}{\sin \alpha _{R}}=\frac{c_{L1}}{c_{L1}}</math>
|(1)
|-
|width="400px" height="75px"|• a reflected transversal wave with
|width="200px"|<math>\frac{\sin \alpha _{R}}{\sin \beta _{R}}=\frac{c_{L1}}{c_{T1}}</math>
|(2)
|-
|width="400px" height="75px"|• a transmitted longitudinal wave with
|width="200px"|<math>\frac{\sin \alpha _{0}}{\sin \alpha _{D}}=\frac{c_{L1}}{c_{L2}}</math>
|(3)
|-
|width="400px" height="75px"|• a transmitted transversal wave with
|width="200px"|<math>\frac{\sin \alpha _{D}}{\sin \beta _{D}}=\frac{c_{L2}}{c_{T2}}</math>
|(4)
|}

with:

{|
|-
|Characteristic impedance longitudinal ''W''L1 = ''ρ''1 ''c''L1 of medium 1
|-
|Characteristic impedance transverse ''W''T1 = ''ρ''1 ''c''T1 of medium 1
|-
|Characteristic impedance longitudinal ''W''L2 = ''ρ''2 ''c''L2 of medium 2
|-
|Characteristic impedance transverse ''W''T2 = ''ρ''2 ''c''T2 of medium 2
|-
|[[Density]] ''ρ''1/2 of medium 1 or 2
|-
|Longitudinal wave velocity ''c''L1/2 of medium 1 or 2
|-
|Transverse wave velocity ''c''T1/2 of medium 1 or 2
|-
|''P''L0 – [[Sound Pressure|Sound pressure]] of the incident longitudinal wave
|-
|''P''LR – Sound pressure of the reflected longitudinal wave
|-
|''P''LD – Sound pressure of the transmitted longitudinal wave
|-
|''P''TR – Sound pressure of the reflected transverse wave
|-
|''P''TD – Sound pressure of the transmitted transverse wave
|}

[[File:US_Angle_Beam_Fig2.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px"|Directions of propagation of ultrasound incident obliquely at an interface (a) in an unlimited medium and (b) at an angle beam sensor
|}

If medium 1 is a shear stress-free [[Material & Werkstoff|material]], such as water or air, then no transverse waves are separated off, which is particularly advantageous in defectoscopy when using the [[Ultrasonic Immersion Bath Technique|immersion bath technique]] or [[Air-Ultrasound|air-ultrasound]]. For the use of angle beam sensors under the conditions shown in '''Fig. 2a''', no meaningful application for defect detection is conceivable, since, for example, in the [[A-Scan Technique|A-scan]], the transmitted longitudinal wave results in a broken longitudinal and transverse wave, which always appear as two time-delayed echo displays. This is due to the different sound propagation velocities of longitudinal and transverse waves (''c''L ≈ 2 ''c''T). The effect that the longitudinal wave is refracted more strongly from the perpendicular than the transverse wave due to its higher [[Velocity|velocity]] is exploited technically to allow only a transverse wave to penetrate the test piece. If the angle of incidence ''α''0 is increased, the refracted longitudinal wave propagates along the [[Surface|surface]] below a critical angle and, if the angle is increased further, it can no longer penetrate the test piece (sin ''α''D > 1) ('''Fig. 2b'''). From '''Eq. (3)''', it can be seen for ''α''D = 90° that the angle of incidence depends on the ratio of the longitudinal wave velocities (e.g. PMMA: ''c''L1 = 2,730 m/s and steel: ''c''L1 = 6,000 m/s).

==Design variants of angle sensors==

Commercially available angle beam sensors are therefore available with fixed angles for ''β''D of 35, 45, 60, 70 and 80° or with adjustable angles of incidence, making them suitable for a wide range of materials. Angle sensors with an angle of 90° are referred to as surface wave sensors and are intended for special applications. A selection of different angle sensors is shown in '''Fig. 3'''.

[[File:Winkelpruefkoepfe-3.JPG|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px"|Ultrasonic angle sensors (a) from Inspection Technologies GmbH, Ahrensburg, and (b) from [https://www.sonotec.eu/en/ Fa. SONOTEC Ultraschallsensorik Halle GmbH, Halle (Saale)]
|}

In contrast to [[Ultrasonic Standard Sensors|standard sensors]], the [[Sound Pressure|sound pressure]] of angle sensors depends on the angle of incidence, which makes it much more difficult to represent the reflection (R) and transmission factors (T or D) [4]. While the emitted ultrasound in [[Ultrasonic Standard Sensors|standard sensors]] can be described by a line, the angle sensor emits an ultrasound beam at the sound exit point, which, however, has analogue relationships for the near and far field as well as the divergence as the standard sensor.

==Application examples==

Angle sensors can be used for wall thickness measurement and defectoscopy in [[Ultrasonic Transmission Technique|transmission mode]] or in [[Pulse-Echo Ultrasonic Technique|pulse-echo technique]], whereby in both cases the jump or projection distance must be taken into account ('''Fig. 4'''). When the angle sensor is placed on a test piece of thickness ''d'', the ultrasound is reflected on the rear side and reaches the [[Surface|surface]] at the so-called jump distance as. The jump distance depends on the angle of incidence ''γ'' and is calculated according to '''Eq. (5)''', whereby this value is often indicated on the sensor ('''Fig. 4a''').

{|
|-
|width="20px"|
|width="500px"|<math>a_{s}=2 d\cdot \tan \gamma </math>
|(5)
|}

A test piece edge therefore always produces a reciprocal echo display when the sensor is at an integer distance of 1/2 ''a''s from the top edge (1/2, 3/2, 5/2, etc.). If the ultrasonic beam does not reach the back wall due to a defect in the beam path, a so-called projection distance ''a''p is defined, which indicates the projected distance between the centre of the sensor and the defect position on the surface according to '''Eq. (6)''' ('''Fig. 4b'''). If referring to the front of the sensor, the value ''a’''p is used. In the case of curved surfaces, such as pipes, the crack distance depends on the radius and the calculation rule becomes more complicated [3, 5–7].

{|
|-
|width="20px"|
|width="500px"|<math>a_{p}=s \cdot \sin \gamma </math>
|(6)
|}

[[File:Winkelpruefkoepfe-4.JPG|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px"|Definition of (a) the jump distance and (b) the projection distance for the ultrasonic angle beam sensor
|}

==Tandem testing technology==

To use the transmission or intensity method, two identical angle sensors (pairs) are required, the test object must be accessible from both sides, and the tandem testing technique must be used ('''Fig. 5'''). In this case, impulse or continuous wave excitation can be used in analogy to the [[Ultrasonic Standard Sensors|standard sensors]], but care must be taken to ensure that the sensors are precisely aligned in terms of gap distance and acoustic axis, and that identical coupling media are used.

[[File:US_Angle_Beam_Fig5.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 5''':
|width="600px"|Application of tandem technology for transmission testing with the angle beam sensor
|}

With the tandem technique, the sound emission and entry points must be mounted precisely at the jump distance of the ultrasonic sensors, taking into account the thickness dependence. If the tandem is moved across the test piece surface, either fixed defect depths (e.g. delaminations in the middle of laminates or bonded panels) or the back-wall of the test piece are detected. If there is no defect, no indication is given ('''Fig. 5a''') or the defect at a specified depth is detected ('''Fig. 5b'''). An adjustable tandem must be used for different defect depths and manual testing. In the pulse-echo method, there is no need for both sides of the test object to be accessible, as the sensor serves as both transmitter and receiver ('''Fig. 6'''). In this case, edge defects or discontinuities at different depths can be detected by the tester ('''Fig. 6a''').

==Weld seam testing==

However, the most important area of application for the angle beam sensor is the weld seam inspection, as the standard sensor cannot be used here due to the surface roughness and unevenness of the top layer.

[[File:US_Angle_Beam_Fig6.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 6''':
|width="600px"|Application of the angle beam sensor for (a) error detection and (b) weld seam inspection using the [[Pulse-Echo Ultrasonic Technique|pulse-echo technique]]
|}

When testing weld seams with oblique incidence, it is important to note whether the cover layer ('''Fig. 6b, left''') or the root ('''Fig. 6b, right''') is to be tested for defects [8–10]. In any case, the thickness-dependent jump distance for such tests and good coupling conditions on the test piece surface must be observed. The weld seam is characterised as free of defects by scanning the test piece width in a meandering pattern between half and full crack spacing or between ''a''s/4 and 5 ''a''s/4. The most important defects occurring in solid bodies (e.g. castings) are cavities or inclusions with rough or smooth inner contours, [[Crack|cracks]] and separations, as well as surface defects that can occur in different positions and represent mechanical weak points. The weld seam defects that occur in practice are gas pores, burn-off and foreign body inclusions, root and bond defects, longitudinal and transverse cracks, as well as surface irregularities and burn-in notches, which can occur at different positions depending on the type of welded joint (V-seam, double V-seam, butt and fillet weld) and also represent weak points ('''Fig. 7''').

[[File:US_Angle_Beam_Fig7.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 7''':
|width="600px"|Schematic illustration of significant weld defects
|}

==See also==

* [[Acoustic Properties|Acoustic properties]]
* [[Air-Ultrasound – Device Technology|Air-Ultrasound – Device technology]]
* [[Ultrasonic Immersion Bath Technique|Ultrasonic immersion bath technique]]
* [[Ultrasonic Weld Inspection|Ultrasonic weld inspection]]
* [[Ultrasonic Waves Reflection|Ultrasonic waves reflection]]

==References==

{|
|-valign="top"
|[1]
|Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (2012), (ISBN 978-3-642-63864-0; see [[AMK-Library]] under M 45)
|-valign="top"
|[2]
|Deutsch, V., Vogt, M.: Die Ultraschallprüfung – Teil 1: Akustische Grundlagen. Sonderdruck Karl Deutsch, SD 1/1
|-valign="top"
|[3]
|Deutsch, V., Vogt, M.: Die Ultraschallprüfung – Teil 2: Apparative Grundlagen. Sonderdruck Karl Deutsch, SD 1/1
|-valign="top"
|[4]
|Krautkrämer, J., Krautkrämer H.: Werkstoffprüfung mit Ultraschall. Springer-Verlag, Berlin (1986), (ISBN 978-3-662-10909-0)
|-valign="top"
|[5]
|Lach, M., Platte, M., Schuster, V.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz. DGZfP-Jahrestagung „Zerstörungsfreie Werkstoffprüfung“ 2004, Salzburg, Austria, Sonderdruck Karl Deutsch, SD 1/51 [https://www.ndt.net/search/docs.php3?id=2626 Download as pdf]
|-valign="top"
|[6]
|Lexikon der Ultraschallsensoren: [http://www.pulsotronic.de/index.php?option=com_content&view=category&id=305&Itemid=100582&lang=de http://www.pulsotronic.de/index.php?option=com_content&view=category&id=305&Itemid=100582&lang=de] (last access on November 27, 2025)
|-valign="top"
|[7]
|Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Ultraschallprüfung. Springer, Berlin (2014), (ISBN 978-3-662-44699-7)
|-valign="top"
|[8]
|ISO 11666 (2018-01): Non-destructive Testing of Welds – Ultrasonic Testing – Acceptance Levels
|-valign="top"
|[9]
|DIN EN ISO 23279 (2017-08): Non-destructive Testing of Welds – Ultrasonic Testing – Characterization of Discontinuities in Welds
|-valign="top"
|[10]
|DIN EN ISO 17640 (2018-10): Non-destructive Testing of Welds – Ultrasonic Testing – Techniques, Testing Levels, and Assessment
|}

[[Category:Acoustic Test Methods_Ultrasonics]]

Ultrasonic Angle Beam Sensors - Revision history