SIST-TP CEN/TR 15134:2006
(Main)Non-destructive testing - Automated ultrasonic examination - Selection and application of systems
Non-destructive testing - Automated ultrasonic examination - Selection and application of systems
Automatic ultrasonic scanning inspection systems are becoming more and more popular. There is a growing dependence on these systems, the data (both ultrasonic signals and probe location) and the automatic or manual evaluation of the data.
Stationary and mobile test systems are discussed, as used for pre-service testing (testing during manufacture) and in-service testing (testing after manufacture, including regular safety assurance testing).
The information in this Technical Report covers all tests and testing on all component parts or complete manufactured systems for either correctness of geometry, material properties (quality or defects) and fabrication methodology (e.g. welds).
This Technical Report can be used for training purposes.
This Technical Report is aimed at suppliers and users of automatic scanning systems.
The scope of this Technical Report is to permit the user, along with a customer specification or test description and any national or international standards or regulations to specify:
- ultrasonic probes, probe systems and mechanical controlling sensors;
- manipulation systems including controls;
- ultrasound electronic sub-systems;
- data storage and display systems;
- evaluation and assessment methods or techniques
with regard to their performance and suitability for purpose.
This Technical Report also defines a means of verifying the performance of any specified system.
This includes:
- tests during the manufacturing process on parts and completed items (stationary testing systems)
and also
- tests with mobile systems.
Zerstörungsfreie Prüfung - Automatisierte Ultraschallprüfung - Auswahl und Anwendung von Systemen
Automatisierte Ultraschall-Prüfsysteme finden immer größere Verbreitung. Das Vertrauen in diese Systeme, die Daten (sowohl Ultraschallsignale als auch Prüfkopfortung) und die automatische oder manuelle Bewertung der Daten nimmt zu.
Es werden ortsfeste und mobile Prüfsysteme behandelt, wie sie für die Prüfung vor Inbetriebnahme (Prüfung während der Herstellung) und für die Prüfung während des Betriebes (Prüfung nach der Herstellung, einschließlich regelmäßiger Prüfungen zur Zusicherung der Sicherheit) verwendet werden.
Die Angaben in diesem Dokument umfassen alle Prüfungen und das Prüfen an allen Bauteilen oder komplett hergestellten Systemen auf Richtigkeit der Geometrie, auf Werkstoffeigenschaften (Qualität oder Mängel) und bei Fertigungsverfahren (z. B. Schweißnähte).
Dieses Dokument kann für Schulungszwecke verwendet werden.
Dieses Dokument richtet sich an Lieferanten und an Anwender von automatisierten Abtastsystemen.
Der Anwendungsbereich dieses Dokuments soll den Anwender in die Lage versetzen, anhand einer Kundenspezifikation oder einer Prüfvorschrift und nationalen oder internationalen Normen oder Vorschriften:
- Ultraschallprüfköpfe, Prüfkopfsysteme und mechanische Steuersensoren;
- Handhabungssysteme einschließlich Steuereinrichtungen;
- Elektronische Ultraschall-Subsysteme;
- Datenspeicherungs- und Anzeigesysteme;
- Auswerte- und Beurteilungsverfahren oder -techniken
hinsichtlich ihrer Gebrauchstauglichkeit und ihrer Eignung für den Einsatzzweck festzulegen.
Dieses Dokument definiert weiterhin einen Weg zum Nachweis der Gebrauchstauglichkeit eines festgelegten Prüfsystems.
Dies beinhaltet:
- Prüfungen während des Herstellungsprozesses an Teilen und kompletten Einheiten (ortsfeste Prüfsysteme) und außerdem
- Prüfungen mit mobilen Systemen.
Essais non destructifs - Examen automatisé par ultrasons - Sélection et application des systemes
Les systemes automatiques d'inspection par contrôle par ultrasons deviennent de plus en plus populaires. On accorde de plus en plus d'importance a ces systemes, aux données (signaux ultrasons et emplacement de la sonde) et a l'évaluation automatique ou manuelle des données.
Le présent Rapport technique traite des systemes d'essai fixes et mobiles avant l'utilisation (essais pendant la fabrication), ainsi que des essais effectués pendant l'utilisation (essais apres la fabrication, y compris des essais d'assurance de sécurité réguliers).
Les informations du présent Rapport technique couvrent tous les essais et tests d'exactitude de géométrie, de propriétés des matériaux (qualité ou défauts) et de méthodologie, de fabrication (par exemple soudures) de tous les composants ou systemes entierement construits.
Le présent Rapport technique peut etre utilisé en vue de formation.
Il est destiné aux fournisseurs et utilisateurs des systemes de contrôle automatiques.
L'objet du présent Rapport technique est de permettre a l'utilisateur de spécifier les éléments ci-dessous en s'appuyant sur la spécification ou sur la description d'essais d'un client ou sur les normes ou réglementations nationales ou internationales :
- sondes ultrasoniques, systemes de sondes et capteurs de commande mécaniques ;
- systemes de manipulation y compris les commandes ;
- sous-systemes électroniques a ultrasons ;
- systemes de mise en mémoire et d'affichage des données ;
- méthodes ou techniques d'évaluation et d'estimation
eu égard a leur performance et adéquation pour l'application en question.
Le présent Rapport technique définit également un moyen de vérifier la performance de tout systeme spécifié.
Ceci comprend :
- les essais des pieces et composants terminés (systemes d'essai fixes) effectués pendant le processus de fabrication ;
et également
- les essais effectués avec des systemes mobiles.
Neporušitveno preskušanje – Avtomatsko ultrazvočno preskušanje – Izbor in uporaba sistemov
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP CEN/TR 15134:2006
01-marec-2006
1HSRUXãLWYHQRSUHVNXãDQMH±$YWRPDWVNRXOWUD]YRþQRSUHVNXãDQMH±,]ERULQ
XSRUDEDVLVWHPRY
Non-destructive testing - Automated ultrasonic examination - Selection and application of
systems
Zerstörungsfreie Prüfung - Automatisierte Ultraschallprüfung - Auswahl und Anwendung
von Systemen
Essais non destructifs - Examen automatisé par ultrasons - Sélection et application des
systemes
Ta slovenski standard je istoveten z: CEN/TR 15134:2005
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
SIST-TP CEN/TR 15134:2006 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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TECHNICAL REPORT
CEN/TR 15134
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
October 2005
ICS 19.100
English Version
Non-destructive testing - Automated ultrasonic examination -
Selection and application of systems
Essais non destructifs - Examen automatisé par ultrasons - Zerstörungsfreie Prüfung - Automatisierte
Sélection et application des systèmes Ultraschallprüfung - Auswahl und Anwendung von
Systemen
This Technical Report was approved by CEN on 24 April 2005. It has been drawn up by the Technical Committee CEN/TC 138.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15134:2005: E
worldwide for CEN national Members.
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Contents Page
Foreword .3
1 Scope .4
2 Normative references .4
3 Terms and definitions.4
4 Basic system description.5
4.1 Systems .5
4.2 System schematic.6
4.3 Levels of automation .9
5 Examination of technical objectives and conditions .9
5.1 Task.9
5.2 Other controlling conditions.10
5.3 Examination data .11
5.4 Reference blocks .12
6 Components and features of a test system .12
6.1 General .12
6.2 Test mechanics and positioning systems.12
6.3 Coupling technique.14
6.4 Probes.16
6.5 Testing electronics and signal digitisation.23
6.6 Data acquisition, processing and storage .26
6.7 Presentation and evaluation of data.28
6.8 System check .29
7 Execution of test .29
7.1 System set-up .29
7.2 Performing the test .30
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Foreword
This CEN Technical Report (CEN/TR 15134:2005) has been prepared by Technical Committee CEN/TC 138
“Non-destructive testing”, the secretariat of which is held by AFNOR.
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1 Scope
Automatic ultrasonic scanning inspection systems are becoming more and more popular. There is a growing
dependence on these systems, the data (both ultrasonic signals and probe location) and the automatic or
manual evaluation of the data.
Stationary and mobile test systems are discussed, as used for pre-service testing (testing during manufacture)
and in-service testing (testing after manufacture, including regular safety assurance testing).
The information in this Technical Report covers all tests and testing on all component parts or complete
manufactured systems for either correctness of geometry, material properties (quality or defects) and
fabrication methodology (e.g. welds).
This Technical Report can be used for training purposes.
This Technical Report is aimed at suppliers and users of automatic scanning systems.
The scope of this Technical Report is to permit the user, along with a customer specification or test description
and any national or international standards or regulations to specify:
- ultrasonic probes, probe systems and mechanical controlling sensors;
- manipulation systems including controls;
- ultrasound electronic sub-systems;
- data storage and display systems;
- evaluation and assessment methods or techniques
with regard to their performance and suitability for purpose.
This Technical Report also defines a means of verifying the performance of any specified system.
This includes:
- tests during the manufacturing process on parts and completed items (stationary testing systems)
and also
- tests with mobile systems.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 1330-2:1998, Non-destructive testing - Terminology - Part 2: Terms common to the non-destructive testing
methods.
EN 1330-4:2000, Non-destructive testing - Terminology - Part 4: Terms used in ultrasonic testing.
3 Terms and definitions
For the purposes of this Technical Report, the terms and definitions given in EN 1330-2:1998 and EN 1330-
4:2000 apply.
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4 Basic system description
4.1 Systems
There are two major applications for automated ultrasonic inspection systems:
- for the detection and evaluation of material defects (e.g. cracks, porosity, geometry);
- for the measurement and evaluation of material properties (e.g. sound velocity, scattering).
Essential components of an automatic inspection system are:
- mechanically positioned and controlled ultrasonic probes and/or test objects;
- automatic data acquisition for the ultrasound signals;
- acquisition and storage of transducer location in relation to ultrasonic signals;
- test results.
A system usually consists of several individually identifiable components. These are:
- manipulators for probes or test objects;
- probes and cables;
- couplant supply and removal;
- ultrasonic sub-system;
- data acquisition and processing;
- data evaluation and display;
- system control.
The complexity of a system depends on the scope of the test and application of the system.
Test systems may be divided into stationary and mobile.
Examples of stationary systems are:
- continuous inspection of steel products, e.g. billets, plate material, tubes, rails;
- component testing, e.g. steering knuckles, rollers, balls, bolts, pressure cylinders;
- composite materials e.g. aerospace structures , e.g. complete wings made of composite materials, CRFP
and GFRP components;
- random sample control (batch test) for process accompanying checks, e.g. testing for hydrogen induced
cracking in steel samples.
Examples of mobile systems are:
- pre-service and in-service inspection of components, e.g. valves, vessels, bolts, turbine parts;
- pre-service and in-service inspection of vehicles;
- pre-service and in-service inspection of pipelines e.g. oil or gas pipelines;
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- inspection of railway tracks.
The test systems can be single or multichannel systems.
The manipulator complexity of the system depends on the examination task.
The complexity of the data acquisition and evaluation system depends on the number of test channels, the
test velocity and the test requirements.
4.2 System schematic
The essential components of an automatic scanning system are shown in Figure 1. More detailed descriptions
can be found elsewhere in this document. A detailed description of the individual functions is given in Clause 5.
Key
1 probe 1 4 data lines
2 probe 2 5 control line
6 control line/location data
3 signal lines
Figure 1 — System schematic
The probe position shall be known and be recorded together with the ultrasonic data. This can be achieved by
using encoders, ultrasound or video techniques.
If the probe motion is in one axis only, the probe position can be determined by measuring elapsed time
compared to the motion velocity.
The most simple ultrasonic system consists of one probe, see Figure 2.
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Figure 2 — Simple system with one probe
In order to fulfil any test requirement the system can include several hundred probes e.g. in a pig for pipeline
testing, see Figure 3.
The ultrasonic sub-system is the main component of the overall system. Figure 4 shows a block diagram of
the basic electronic components of the ultrasonic sub-system. Depending on the required complexity, the
ultrasonic sub-system can be made from one module for a single channel system or multiple modules for
multi-channel systems. These can be self-contained modules, computer plug-in cards or rack mounted
systems.
Figure 3 — The probe assembly of an intelligent pig for use on a 40 inch diameter pipeline
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Figure 4 — Block diagram of the electronics of the ultrasonic sub-system
Some digital systems used for testing provide acquisition and storage of the full RF ultrasonic signals. This
method offers the most information compared to other acquisition methods.
In order to reduce the testing time, data processing and storage requirements, other methods use data
reduction techniques such as peak testing. For many applications this provides a perfectly adequate level of
data for the purposes of the inspection.
Methods for data reduction are described in 6.6.4.2.
The data, which are transferred from the ultrasonic unit to the data acquisition unit, is referred to as
measurement data.
In the data processing unit the measurement data is processed in a way, which permits it to be visualized on a
display for the interpreter (user) performing the evaluation.
The data can be assessed and the test verified automatically during automatic component testing.
In certain areas, the evaluation has to be performed by experienced test personnel, e.g. welds on vessels and
pipelines or safety-critical components in aerospace. In these cases, the data processing unit has to provide
images from the measurement data as a projection or sectional image. Other tasks are possible by filtering
the data to remove unwanted information. This can be achieved by software in a computer or by special
hardware.
Data can be stored at different points during the measurement signal processing as shown in Figure 1. If this
is a simple go/no go test only the test result need be recorded. In contrast, during testing of safety critical
components the measurement data is stored together with any assessment result.
The control and synchronisation of the individual system components is achieved by the system control, this
ensures that the proper test sequence is performed.
The system control also synchronises the storage of the probe location data and ultrasonic data.
In-process inspection can provide automatic sorting or marking of defective parts.
A practical example for a basic automatic scanning system is shown in Figure 1. The set-up of a multi-channel
test system is shown in Figure 5. This has an XY manipulator and can be used for testing vessels and pipes.
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Key
3 manipulator control
1 testing location
4 ultrasound electronics
- on-line survey
5 probe cable
- data acquisition
2 evaluation location 6 position data
- test planning 7 motor control, encoder signals
- data acquisition 8 optional network link to ultrasound
device
- display
9 network
- assessment
- documentation
Figure 5 — Set-up of a multi-channel test system
4.3 Levels of automation
Various levels of automated inspection are possible, ranging from simple mechanical assisted probe
movement through to fully automated examination of data, marking or sorting of test objects.
5 Examination of technical objectives and conditions
5.1 Task
The examination task defines the discontinuities or material properties that the test is intended to detect or to
measure.
The specification shall be designed within practical and economical viable limits with due consideration to the
test object.
Any existing relevant normative documents shall be taken into consideration.
The technical limit of the test system is governed, by amongst other things, the following parameters:
- the overall signal-to-noise ratio in the ultrasound sub-system;
- the band width of the probe(s) and ultrasound sub-system;
- the positional resolution of the probes.
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The most important factor in all automatic scanning methods is the system’s dynamic lateral resolution. The
scanning pattern and speed shall be designed in accordance with the beam profile dimensions as determined
by a relevant reflector.
5.2 Other controlling conditions
The following conditions shall be considered:
- the requirements governed by the material properties, e.g. surface conditions and coupling requirements;
- standards, directives and other specifications;
- the application limitations, e.g. test environment, access, weather conditions, power restrictions.
5.2.1 Testing density, test speed and extent and coverage of testing
High speed testing is typical in automated scanning. This generates large amounts of data. If this is to be
automatically assessed processing speed is a key issue.
There is a relationship between the distance between measurement points, speed of probe motion, pulse
repetition rate and data acquisition speed. This relationship shall also consider the number of channels.
If the probe is moved in a direction x and measurement data are required equidistantly (either amplitude or
time-of-flight) the following condition shall be satisfied:
v < (∆x * f ) / n (1)
where :
v = relative speed between probe and test specimen (mm/s)
∆x = distance between measurement points (mm)
f = pulse repetition rate (Hz)
n = number of pulses required per measurement point.
If the complete A-scan has to be acquired at each spot the following equation applies:
v ≤ ∆x / t (2)
s
where :
v = relative speed between probe and test specimen (mm/s)
∆x = distance between measurement points (mm)
t = acquisition and storage time of an A-scan
s
Normally, the transfer time of an A-scan to a storage medium (e.g. hard disk) is longer than the duration
(length) of an A-scan. In this case t shall be equal to the slowest process step in the system.
s
5.2.2 Environmental considerations
Special consideration shall be given when the design is to be used in harsh environments e.g.:
- ionising radiation;
- extreme temperature of the test object or the environment it is in;
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- very high or low pressure in the environment (air or water pressure);
- aggressive atmosphere;
- areas at risk of explosion.
5.2.3 Material properties
Material properties may cause problems in performing an ultrasonic examination. The following properties can
result in problems:
- coarse grain structure (castings, austenitic steel, and concrete);
- inhomogeneous structure (varying structure in the same object);
- anisotropy (texture of wrought and forged products, columnar crystalline structure in austenitic weld
metal, and in fibre reinforced composites);
- interfaces (dissimilar welds, composites, hardened zones).
Often these properties are combined. They interfere with the propagation of the sound waves and may result
in the following problems:
- spurious indications;
- errors in locating indications;
- wave mode conversion;
- sensitivity variations;
- local zones, which are not examined.
By selecting suitable techniques these problems may be reduced or eliminated. An example is the
examination of austenitic welds where the evaluation of the examination results is often possible only after
processing of B- and C-scans, which may be compared with the pattern of indications from previous
examinations or from the examination of test blocks containing reference reflectors.
5.2.4 Complex component geometry
On complex geometries A-scan alone is insufficient for the evaluation. In such cases location oriented B-
scans or C-scans as well as imaging e.g. synthetic aperture focussing techniques (SAFT), holography or
tomography etc. shall be considered.
These images, produced from time-of-flight and amplitude information (data), enable differentiation between
geometry and discontinuity. Pattern recognition may also be used to detect items of interest.
EXAMPLE:
In the examination of the heads (or lids) and spherical bottoms of nuclear reactor pressure vessels transducer
array probes are often used. These are mounted on predetermined tracks between the control and measuring
rod nozzles on the spherically curved surface. Crack testing on the inner surfaces is done by varying the skew
and incident angles of the probes. The B-scans from tracks running parallel to each other are then compared.
It is simple to differentiate between the cracks and the geometry indications.
5.3 Examination data
The acquisition of data shall be complete so that an assessment according to a predetermined test
specification is possible.
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5.4 Reference blocks
It is recommended that a set of reference blocks representative of the test object is used to ensure the
sensitivity and suitability of the overall system. The blocks should contain test reflectors that represent the
various defects that need to be detected.
The reference reflectors allow a relationship between the detectability of a particular type of defect and the
limits of detection to be determined.
The system should be checked against the reference blocks at regular intervals to maintain testing confidence.
6 Components and features of a test system
6.1 General
The requirements of any individual test system are dictated by the application. The set-up and the operation
mode are determined by the technical objectives of the test and all the prevailing conditions.
Selection of any single component is based on the examination task and that item’s performance in achieving
the desired result.
Major system characteristics are as follows:
6.2 Test mechanics and positioning systems
Automatic ultrasonic testing requires that the probe and test object are moved in relation to each other. The
probe guidance mechanism provides the spatial relationship between the probe and the test object. The probe
position is usually determined by electro-mechanical and/or electronic means.
The probe controlling mechanism also provides:
- control/guidance of other sensors (manually or mechanically);
- control/guidance of the test object;
- synchronisation between other sensors and the test object;
- feeding and removal of the test objects;
- supply and disposal of the ultrasound couplant.
In mobile test systems the system mechanics usually move the probe in relation to the test object. In
stationary (fixed) systems the system mechanics usually move the test object in relation to the probe. Fixed
testing systems are usually integrated into the production process of the object.
The following are some of the parameters that shall be considered in designing a system:
• Degree of mechanisation / automation required
Different levels might be:
- mechanised hand-operated guidance of the probes;
- manual feeding and removal of the test objects;
- machine-operated mechanised guidance of the probes;
- manual or mechanised or automatic feeding and removal of the test objects;
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- automatic guidance of sensors and the test object removed from the production process;
- automatic guidance of the sensors and the test object in the production process;
- marking or sorting of the test objects after semi- or full automatic test assessment.
• Test object
- shape;
- material;
- surface condition;
- temperature.
• Test scale
- inspection of the whole test object or parts thereof;
- single or multiple tests on each object.
• Scanning path velocity
The scanning path velocity is determined by the following parameters:
- approach and reset period;
- pulse rate;
- sound path length;
- single or multi-channel operation.
• Precision of positioning
The following requirements determine the positioning accuracy:
- detection of discontinuities;
- description of discontinuities (location and size);
- reproducibility of the test result (precision of access).
For some applications the positioning accuracy is controlled by directives.
• coupling
The mechanical system shall provide suitable and appropriate coupling for the ultrasound (see 5.2), with
particular concern to:
- pressure;
- distance between the probe(s) and the test object;
- viscosity of couplant;
- supply and removal of the coupling medium.
• Additional system requirements
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Consideration shall be given to the system’s actual mechanical condition:
- environmental conditions;
- availability (wear resistance);
- life cycle;
- maintainability/repairability respectively conditions;
- long-term availability of the control soft- and firmware.
• Health and safety requirements
All relevant safety regulations shall be observed.
6.3 Coupling technique
6.3.1 General
A coupling medium (couplant) is necessary to allow the transfer of mechanical energy (vibration) from the
electro-mechanical transducer in the probe to the test object and back to the transducer.
There are other ultrasonic test techniques which operate without a coupling medium, e.g. ultrasound
generated electro-magnetically (EMAT) or by laser. In these techniques, the elastic vibrations are produced in
the test object itself (see 5.3).
Media in all states can be used for couplant. Usually:
gaseous air
liquid water, oil, gel, etc.
solid metal foils, rubber, low melting crystals
However, not all of these are suitable for automatic scanning systems. The most common couplants are water,
oil and emulsions, air if applicable or combined liquid/solid coupling (as in an ultrasonic wheel probe).
6.3.2 Selection of couplant with regard to the testing environment
Conditions regarding the testing environment shall be considered when selecting couplant types, e.g.:
- compatibility with the test object (e.g. avoidance of corrosion);
- contamination of the test object by the couplant and decontamination/cleaning after testing where
necessary;
- surface of the test object (e.g. flatness or roughness of the surface);
- contour (complex geometry) and accessibility of the test object;
- temperature of the test object with respect to the testing environment;
- testing speed;
- contamination of the couplant by the test object (e.g. radioactivity, dangerous chemicals);
- environmental compatibility and disposal of the couplant.
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6.3.3 Selection of couplant with regard to the ultrasonic requirements
The ultrasonic test itself shall be considered when selecting a couplant, e.g.:
- wave type used;
- transferability of the ultrasound by the couplant (distortion by bubbles or other scatters);
- frequency and bandwidth;
- sound beam diameter;
- sound path in the delay line and the test object;
- technique (through transmission or pulse echo).
6.3.4 Liquid couplants
Suggestions for coupling techniques and applications are given in Table 1.
Table 1 — Various coupling techniques using liquids
Technique Description Guidance of the probes
a) Immersion technique test object completely immersed in liquid external mechanics
b) Partial immersion liquid chamber or basin as immersion external mechanics,
technique vessel for a part of the test object test object
c) Squirter technique sound is conducted over a long, free liquid external mechanics
jet
d) Jet technique liquid column is guided by nozzles close external mechanics,
to the test object test object
e) Contact technique probe mounted in a shoe with a couplant test object
filled slot between probe, shoe and test
object
f) Flow gap coupling thin liquid film between probe and test test object
object; probe floats
g) Direct contact direct contact of the probe shoe with wear test object
technique shoe with coupling pressure onto the test
object while surface kept moist
Immersion techniques are particularly useful for testing single items, the others are more suitable for testing in
a continuous production process.
6.3.5 Gas couplants
Using gaseous couplants, air for instance, offers particular flexibility on complex geometries and high testing
speeds. It greatly simplifies couplant handling. The other problems arising with liquid couplants are removed.
Enormous differences of acoustic impedance between the gas and the test object result in high signal losses,
this necessitates the use of low frequency ultrasound (up to 1 MHz), usually in through transmission mode.
Due to the low acoustic velocity of air testing, this is also restricted to low pulse repetition rates.
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6.3.6 Solid couplants
If required, a solid couplant can be used. Silicone film offers dry coupling through a soft solid.
The wheel probe has an oil-filled tyre containing a probe emitting radially, this offers a combination of liquid
and dry coupling. A slight wetting of the test object surface is advantageous.
In practice using solid couplants is problematic since there can be undesirable air/gas interface layers
produced between the test object and the probe.
6.4 Probes
6.4.1 General
Probes contain transducers which are usually piezo-electric devices which convert electrical energy into
mechanical energ
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