ISO/TS 16829:2017

Non-destructive testing — Automated ultrasonic testing — Selection and application of systems

The information in ISO/TS 16829:2017 covers all kinds of ultrasonic testing on components or complete manufactured structures for either correctness of geometry, for material properties (quality or defects), and for fabrication methodology (e.g. weld testing). ISO/TS 16829:2017 enables the user, along with a customer specification, or a given test procedure or any standard or regulation to select: - ultrasonic probes, probe systems and controlling sensors; - manipulation systems including controls; - electronic sub-systems for the transmission and reception of ultrasound; - systems for data storage and display; - systems and methods for evaluation and assessment of test results. With regard to their performance, ISO/TS 16829:2017 also describes procedures for the verification of the performance of the selected test system. This includes - tests during the manufacturing process of products (stationary testing systems), and - tests with mobile systems.

Essais non destructifs — Contrôle automatisé par ultrasons — Sélection et application des systèmes

General Information

• Status: Published
• Publication Date: 17-Oct-2017
• ICS: 19.100 - Non-destructive testing
• Technical Committee: ISO/TC 135/SC 3 - Ultrasonic testing
• Drafting Committee: ISO/TC 135/SC 3 - Ultrasonic testing
• Current Stage: 9092 - International Standard to be revised
• Start Date: 17-Jun-2025
• Completion Date: 13-Dec-2025

Standards Content (Sample)

TECHNICAL ISO/TS
SPECIFICATION 16829
First edition
2017-10
Non-destructive testing — Automated
ultrasonic testing — Selection and
application of systems
Essais non destructifs — Contrôle automatisé par ultrasons —
Sélection et application des systèmes
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Basic system description . 2
4.1 Systems . 2
4.2 System schematic . 3
4.3 Levels of automation . 5
5 Examination of technical objectives and conditions of the testing . 6
5.1 Test task . 6
5.2 Other important conditions . 6
5.2.1 General. 6
5.2.2 Scanning density, test speed, extent and coverage of testing . 6
5.2.3 Environment . 7
5.2.4 Material properties . 7
5.2.5 Complex component geometry . 8
5.3 Test data . 8
5.4 Reference blocks . 8
6 Components and features of an automated test system . 8
6.1 General . 8
6.2 Test mechanics and positioning systems . 9
6.2.1 General. 9
6.2.2 Grade of mechanisation/automation required . 9
6.2.3 Test object . 9
6.2.4 Scale of testing . 9
6.2.5 Test speed/speed along the scanning path .10
6.2.6 Precision of positioning .10
6.2.7 Coupling.10
6.2.8 Additional system requirements.10
6.2.9 Health and safety requirements .10
6.3 Coupling techniques .11
6.3.1 General.11
6.3.2 Selection of couplant with regard to the testing environment .11
6.3.3 Selection of couplant with regard to the ultrasonic requirements.11
6.3.4 Liquid couplants .12
6.3.5 Gaseous couplants .12
6.3.6 Solid couplants .12
6.4 Probes .12
6.4.1 General.12
6.4.2 Piezo-electric probes .13
6.4.3 Electro-magnetic ultrasonic probes (EMAT) .17
6.4.4 Laser ultrasonics .18
6.4.5 Special requirements for probes and cable connections .18
6.5 Testing of electronics and signal digitization .20
6.5.1 Transmission and reception system .20
6.5.2 Digitization.20
6.6 Data acquisition, processing and storage.23
6.6.1 General.23
6.6.2 Hardware .23
6.6.3 Software.23
6.6.4 Probe position and orientation .23
6.6.5 Data acquisition and data reduction .23
6.6.6 Data storage .25
6.7 Presentation and evaluation of data .25
6.7.1 Presentation of data .25
6.7.2 Evaluation of data .25
6.8 System check .26
7 Execution of test .26
7.1 System set-up .26
7.2 Performing the test.27
7.3 Management of NDT data .27
Bibliography .28
iv © ISO 2017 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing,
Subcommittee SC 3, Ultrasonic testing.
ISO/TS 16829 is based on technical report CEN/TR 15134:2005.
TECHNICAL SPECIFICATION ISO/TS 16829:2017(E)
Non-destructive testing — Automated ultrasonic testing —
Selection and application of systems
1 Scope
The information in this document covers all kinds of ultrasonic testing on components or complete
manufactured structures for either correctness of geometry, for material properties (quality or defects),
and for fabrication methodology (e.g. weld testing).
This document enables the user, along with a customer specification, or a given test procedure or any
standard or regulation to select:
— ultrasonic probes, probe systems and controlling sensors;
— manipulation systems including controls;
— electronic sub-systems for the transmission and reception of ultrasound;
— systems for data storage and display;
— systems and methods for evaluation and assessment of test results.
With regard to their performance, this document also describes procedures for the verification of the
performance of the selected test system.
This includes
— tests during the manufacturing process of products (stationary testing systems), and
— tests with mobile systems.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO 5577, Non-destructive testing — Ultrasonic testing — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
4 Basic system description
4.1 Systems
There are two major applications for automated ultrasonic testing systems:
a) detection and evaluation of material defects (e.g. cracks, porosity, geometry);
b) measurement and evaluation of material properties (e.g. sound velocity, scattering).
Essential components of an automated test system are:
a) mechanically positioned and controlled ultrasonic probes and/or test objects;
b) automatic data acquisition for the ultrasonic signals;
c) acquisition and storage of probe positions in relation to the ultrasonic signals;
d) storage of test results.
A test system usually consists of several individually identifiable components. These are:
a) manipulators for probes or test objects;
b) probes and cables;
c) supply (pre-wetting), application and removal of the couplant;
d) electronic ultrasonic sub-systems;
e) data acquisition and processing devices;
f) data evaluation and display devices;
g) system controls;
h) sorting and marking of tested objects.
The complexity of a test system depends on the scope of the test and application of the system.
Test systems may be divided into stationary and mobile devices.
Examples of stationary test systems are testing machines:
— for the continuous testing of steel products, e.g. billets, plates, tubes, rails;
— for the testing of components, e.g. steering knuckles, rollers, balls, bolts, pressure cylinders;
— for the testing of composite materials, such as aerospace structures, e.g. complete wings made of
composite materials, CRFP and GFRP components;
— for the testing of random samples (batch test) in a process accompanying production checks, e.g.
testing for hydrogen induced cracking in steel samples.
Examples of mobile test systems are test rigs:
— for pre-service and in-service testing of components, e.g. valves, vessels, bolts, turbine parts;
— for pre-service and in-service inspection of vehicles;
— for pre-service and in-service testing of pipelines, e.g. oil or gas pipelines;
— ultrasonic testing of rails in railway tracks.
The test systems can be single or multichannel systems.
2 © ISO 2017 – All rights reserved

The complexity of the manipulator of the system depends on the examination task.
The complexity of the data acquisition and evaluation system depends on the number of test channels,
on the required test speed, and on other test requirements.
4.2 System schematic
The essential components of an automated ultrasonic 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 no 1 4 data lines
2 probe no 2 5 control line
3 signal lines 6 control line/position data
Figure 1 — System schematic
The probe position shall be determined and be recorded together with the ultrasonic data. This can be
achieved by using encoders, ultrasound, or video techniques.
The most simple ultrasonic system uses only one probe (Figure 2).
Figure 2 — Simple system with one probe
In order to fulfil more complex 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 complete test 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 electronic systems.
Figure 3 — Probe assembly of an intelligent pig for use on a 40-inch-diameter pipeline
Figure 4 — Block diagram of the electronics of the ultrasonic sub-system
Some digital systems used for testing provide acquisition and storage of full RF ultrasonic signals. This
mode offers the most information compared to other acquisition methods.
In order to reduce the time for testing, data processing and storage, other methods use data reduction
techniques such as signal peak evaluation. For many applications, this provides a perfectly adequate
level of data for the purposes of the testing.
Methods for data reduction are described in 6.6.5.2.
The data, which are transferred from the ultrasonic unit to the data acquisition unit, are referred to as
test data.
In the data processing unit, the test data are processed in a way which enables them to be visualized on
a display for the interpreter (user) performing the evaluation.
The data can be assessed and the test verified automatically during automated testing of objects.
In certain industrial sectors, the evaluation has to be performed by experienced test personnel, e.g. for
welds on vessels and pipelines, or for safety-critical components in the aerospace industry. In these
4 © ISO 2017 – All rights reserved

cases, the data processing unit has to provide images from the test data as a projection or sectional
image. Other tasks are possible by filtering of 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 moments during the signal processing, as shown in Figure 1. If this is a
simple go/no go test, only the final test result needs be recorded. In contrast, during testing of safety-
critical components, the test data are stored together with any assessment result.
The control and synchronization of the individual system components is achieved by the system control.
This ensures that the proper test sequence is performed.
The system control also synchronizes the storage of the probe positioning data and the ultrasonic data.
In-process testing can provide automated sorting or marking of unacceptable test objects.
A practical example of a basic system for automated scanning is shown in Figure 1. The set-up of a
multi-channel test system is shown in Figure 5. This system has an XY-manipulator, and can be used for
testing of vessels and pipes.
Key
1 sector of testing 3 manipulator control
— online survey 4 ultrasonic electronics
— data acquisition 5 probe cable
2 sector of evaluation 6 position data
— test planning 7 motor control, encoder signals
— data acquisition 8 optional network link to ultrasonic electronics
— display 9 network interface
— assessment
— documentation
Figure 5 — Set-up of a multi-channel test system
4.3 Levels of automation
Various levels of automated testing are possible, ranging from simple probe movement assisted by
mechanical means through to fully automated acquisition and assessment of test data, and marking or
sorting of test objects.
5 Examination of technical objectives and conditions of the testing
5.1 Test task
The test task specifies the discontinuities or material properties that the test is intended to detect or to
measure.
The specification for the test system shall be designed within practical and economical viable limits,
with due consideration to the properties of the test object.
Any existing relevant normative documents shall be taken into consideration.
The technical limits of the test system are governed, by amongst other things, the following parameters:
a) overall signal-to-noise ratio of the ultrasonic sub-system;
b) bandwidth of the probe(s) and the ultrasonic sub-system;
c) spatial resolution of the sound beam(s).
The most important factor in all methods of automated scanning is the system’s dynamic lateral
resolution. The scanning pattern and the scanning speed shall be specified in accordance with the
sound beam dimensions as determined by a relevant reflector.
5.2 Other important conditions
5.2.1 General
The following conditions shall be considered for the specification of the test system:
a) requirements governed by the material properties, e.g. surface conditions and coupling
requirements;
b) standards, guidelines and other specifications;
c) limitations to perform the testing, e.g. by test environment, accessibility, weather conditions, and
power restrictions.
5.2.2 Scanning density, test speed, 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 gap between points of testing, speed of probe motion, pulse
repetition frequency, and speed of data acquisition. This relationship shall also consider the number of
channels.
If the probe is moved in a direction x and test data have to be taken equidistantly (either amplitude or
time-of-flight), the following condition shall be satisfied:
v< Δx * f/ n (1)
()
r
6 © ISO 2017 – All rights reserved

where
v is the scanning speed on the test object (mm/s);
Δx is the distance between test points (mm);
f is the pulse repetition frequency (Hz);
r
n is the number of pulses required per test point.
If the complete A-scan has to be acquired at each test point, Formula (2) applies:
vx≤Δ /t (2)
s
where
v is the relative speed between probe and test object (mm/s);
Δx is the distance between test points (mm);
t is the 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
5.2.3 Environment
Special consideration shall be given when the test system has to be used in harsh environments, e.g.:
— ionizing radiation;
— extreme temperature of the test object or the environment it is in;
— very high or low pressure in the environment (air or water pressure);
— aggressive atmosphere;
— areas at risk of explosion.
5.2.4 Material properties
Material properties may cause problems in performing an ultrasonic test. The following material
properties may cause 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
welds, and in fibre-reinforced composites);
— interfaces (dissimilar welds, composites, hardened zones).
These properties are often combined. They interfere with the propagation of the sound waves and may
cause the following problems:
— spurious indications;
— errors in locating of indications;
— wave mode conversion;
— sensitivity variations;
— local zones, which are not tested.
By selecting suitable techniques, these problems may be reduced or eliminated. An example is the
testing of austenitic welds where the evaluation of the test results is often possible only after processing
of B-scans and C-scans, which may be compared with the pattern of indications from previous tests or
from tests on test blocks containing known reference reflectors.
5.2.5 Complex component geometry
On complex geometries, an A-scan alone is insufficient for the evaluation. In such cases, position related
B-scans or C-scans, as well as imaging, e.g. synthetic aperture focusing techniques (SAFT), holography
or tomography, shall be considered.
These images, produced from time-of-flight and amplitude information (data), enable differentiation
between indications caused by geometry and those caused by discontinuities. Pattern recognition may
also be used to detect items of interest.
EXAMPLE For the testing of the spherical heads and bottom sections of nuclear pressure, vessels, very
often array probes, are used. These run on the spherically curved surface on predetermined tracks between
the nozzles for control and measuring rods. Testing for cracks on the inner surfaces is done by varying the skew
and beam angles of the probes. B-scans from these tracks running parallel to each other are then compared. It is
simple to differentiate between indications from cracks and from geometry.
5.3 Test data
The collected data shall be extensive enough to enable an assessment according to the required test
specification.
5.4 Reference blocks
It is recommended to use a set of reference blocks representative of the test object to ensure the
sensitivity and suitability of the overall system. The reference blocks shall have acoustical properties
same or similar to the tested material. The blocks shall contain reference reflectors that represent the
various discontinuities that need to be detected.
The reference reflectors allow to determine the detectability of a particular type of discontinuity and
the limits of detection.
The overall system should be checked with the reference blocks at regular intervals to maintain the
reliability of testing.
6 Components and features of an automated test system
6.1 General
The requirements on 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 technique and all prevailing
conditions.
Selection of any single component of the system is based on the test task and its ability for achieving the
desired test results.
Major characteristics of a test system are discussed in the following subclauses.
8 © ISO 2017 – All rights reserved

6.2 Test mechanics and positioning systems
6.2.1 General
Automated ultrasonic testing requires a movement of probe(s) and test object relative to each other. The
probe guidance mechanism provides the spatial relationship between the probe(s) and the test object.
The probe position is usually determined by electro-mechanical and/or electronic means (position
encoders).
The control of the mechanism may also provide:
a) control/guidance of other sensors (manually or mechanically);
b) control/guidance of the test object;
c) synchronization between other sensors and the test object;
d) feeding and removal of the test objects;
e) supply, application, and removal of the couplant.
In mobile test systems. the mechanics usually move the probe in relation to the test object. In stationary
(fixed) test systems, the mechanics usually move the test object in relation to the probe. Stationary test
systems are usually integrated into the production process of the test object.
Some of the parameters that shall be considered when designing a test system:
6.2.2 Grade of mechanisation/automation required
Different grades might be:
a) mechanized or hand-operated guidance of the probes;
b) machine-operated mechanized guidance of the probes;
c) manual feeding and removal of the test objects;
d) mechanized or automated feeding and removal of the test objects;
e) automated guidance of the sensors when the test object is outside the production process;
f) automated guidance of the sensors when the test object is in the production process;
g) marking or sorting of the test objects after semi- or fully automated assessment.
6.2.3 Test object
a) shape;
b) material;
c) surface condition;
d) temperature.
6.2.4 Scale of testing
a) testing of the whole test object or only parts of it;
b) single or multiple tests on each test object.
6.2.5 Test speed/speed along the scanning path
The test speed is determined by the following parameters:
a) approach and reset period;
b) pulse repetition frequency;
c) sound path length;
d) single or multi-channel operation.
6.2.6 Precision of positioning
The following requirements determine the accuracy of positioning:
a) detection of specified discontinuities;
b) characterization of discontinuities (position and size);
c) reproducibility of the test results (precision of access).
For some applications, the accuracy of positioning is stipulated by standards and specifications.
6.2.7 Coupling
The mechanical system shall provide suitable and appropriate coupling for the ultrasonic waves (5.2),
with particular concern to:
a) compatibility of couplant and test object (corrosion);
b) pressure;
c) temperature of the test object;
d) distance between the probe(s) and the test object;
e) viscosity of couplant;
f) supply application and removal of the coupling medium.
6.2.8 Additional system requirements
Consideration shall be given to the system’s actual mechanical condition:
a) environmental conditions;
b) availability (wear resistance);
c) life cycle;
d) maintainability/repairability;
e) long-term availability of the control software and firmware.
6.2.9 Health and safety requirements
All relevant health and safety regulations shall be observed.
10 © ISO 2017 – All rights reserved

6.3 Coupling techniques
6.3.1 General
A coupling medium (couplant) is necessary to enable 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. with
ultrasound generated by electromagnetic transducers (EMAT) or by lasers. With these techniques, the
elastic vibrations are produced in the test object itself (5.3).
Media in all states can be used as couplant:
— gaseous state air
— liquid state water, oil, gel, etc.
— solid state metal foils, polymer foils, low melting crystals
However, not all of these are suitable for automated scanning systems. The most common couplants
are water, oil and emulsions, air if applicable or combined liquid/solid coupling (as by 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 couplants:
a) compatibility with the test object (e.g. avoidance of corrosion);
b) contamination of the test object by the couplant and decontamination/cleaning after testing where
necessary;
c) surface of the test object (e.g. flatness or roughness of the surface);
d) contour (complex geometry) and accessibility of the test object;
e) temperature of the test object with respect to the testing couplant and probe(s);
f) testing speed;
g) contamination of the couplant by the test object (e.g. radioactivity, dangerous chemicals);
h) environmental compatibility and disposal of the couplant.
6.3.3 Selection of couplant with regard to the ultrasonic requirements
The ultrasonic test itself shall be considered when selecting a couplant:
a) wave type used;
b) transferability of ultrasound by the couplant (distortion by bubbles or other scatterers);
c) frequency and bandwidth;
d) sound beam dimensions;
e) sound path in the delay line and in the test object;
f) test 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 by external mechanics
b)  Partial immersion liquid chamber or basin as immersion vessel for a by external mechanics,
technique part of the test object
by test object
c)  Squirter technique sound is conducted via a long, free liquid jet by external mechanics
d)  Jet technique liquid column is guided by nozzles close to the by external mechanics,
test object
by test object
e)  Contact technique probe mounted in a shoe with a couplant filled slot by test object
between probe, probe shoe and test object
f)  Flow gap coupling thin liquid film between probe and test object; by test object
probe floats
g)  Direct contact technique direct contact of the probe shoe (with wear sole) by test object
with coupling pressure onto the test object while
the surface is wettened
Immersion techniques are particularly useful for testing single objects, the others are more suitable for
testing in a continuous production process.
6.3.5 Gaseous couplants
Using gaseous couplants, air for instance, offers particular flexibility on complex geometries and high
testing speed. It greatly simplifies couplant handling. Other problems arising with liquid couplants are
removed.
By the 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.
6.3.6 Solid couplants
If required, a solid couplant can be used. A silicone foil offers dry coupling through a soft solid.
The wheel probe has an oil-filled tyre containing a probe transmitting sound waves radially, this offers a
combination of liquid and dry coupling. A slight wetting of the surface of the test object is advantageous
when using a wheel probe.
In practice using solid couplants is problematic since undesirable air/gas interface layers may arise
between the test object and the probe.
6.4 Probes
6.4.1 General
Probes contain transducers, usually piezo-electric devices, which convert electrical vibrations into
mechanical onces and vice-versa. Probes can therefore transmit ultrasonic waves as well as receive.
Other principles of ultrasound generation and reception are also available for non-destructive testing.
Examples are electro-magnetic ultrasonic (EMAT) probes and laser sources. With both techniques, the
surface layer of the test object forms part of the acoustic transducer.
12 © ISO 2017 – All rights reserved

In most cases, the pulse-echo technique is used. Short ultrasonic pulses are transmitted into the test
object. They are reflected, diffracted or scattered by discontinuities in the test object creating signals
which are then received as echoes. Some parameters of the received signals are extracted and used
for evaluation of the discontinuity. The amplitude of the signals can be used to evaluate the size of the
discontinuity. The time-of-flight of the signals enables the location of the discontinuity.
6.4.2 Piezo-electric probes
6.4.2.1 General
Piezo-electric probes can be specifically designed to suit the application. The geometry of the test
object, the area to be covered, the required resolution and the required wave type [longitudinal
(compressional) or transverse (shear)] shall be considered.
Besides the active piezo-electric transducer other important elements of a probe for pulse-echo testing
are a damping element, protective or matching layers, electrical matching circuits and the housing.
6.4.2.2 Piezo-electric transducer materials
Usually the piezo-electric transducer is a thin plate. The nominal frequency, f , of a transducer is
n
determined by the thickness, d, of this plate:
c
f= (3)
n
2d
where c is the sound velocity (longitudinal wave) in the piezo-electric material.
Because the frequency is inverse proportional to the thickness of the transducer plate the required
thickness decreases with increasing test frequency which in turn reduces the mechanical resistance of
the plate.
The piezo-electric transducer in most cases is made from ceramic material. The most common ceramics
is lead zirconate titanate (PZT), lead titanate (PbTiO ) and lead metaniobate (PbNb O ) are also used.
3 2 6
These ceramics are used for test frequencies up to 30 MHz.
The energy transmitted into the test object is determined by this quantity, but also by the acoustic
impedances of couplant and test object.
Piezo-electric foils made of polyvinylidene fluoride (PVDF) are also used as transducer material. Their
low acoustic impedance provides efficient transmission, particularly for the immersion technique.
These foils are rarely applied in the contact techniques. High frequencies can be achieved using PVDF
foils. Due to their flexibility, curved focusing transducers can easily be produced. However, their
disadvantage is their poor resistance to temperature (up to about 80 °C).
Composite piezo-materials are ceramic sticks or particles embedded in an epoxy resin matrix whose
properties as transducers are determined by the structure and the ratio of the components ceramic
and resin. They also exhibit low acoustic impedance, like foil transducers, but combined with the high
efficiency of piezo-ceramics. However, they have poor resistance to mechanical load and poor resistance
to temperature (up to about 100 °C).
6.4.2.3 Layout of piezo-electric probes
The transducer is coated with electrically conducting layers on both sides and basically offers a
capacitive load. The transmitted and received signals are conducted via wires connected to these
conductive layers.
There is usually a damping block (mass) behind the transducer which influences the vibrational
behaviour and the bandwidth of the probe housing the transducer. Mechanically attached to the piezo-
electric plate, the damping block also absorbs sound waves being emitted backwards. Interfering
reflections from within the damping mass are suppressed by specific shaping of the damping block.
When using piezo-electric materials with low acoustic impedances (foil transducers, composites, both
in contact techniques) with a plastic delay line located in front of the transducer (delay-line probes,
angle-beam probes with wedges, dual-element probes), damping blocks may be omitted.
Key
1 transducer 3 electrical matching circuit
2 transducer backing (damping block) 4 matching layer/protective layer
Figure 6 — Schematic of the set-up of a piezo-electric ultrasonic probe
The exterior probe face provides mechanical protection and sometimes acoustic matching.
Electrical impedance can be included in the probe to provide matching to the transmission and
receiving circuitry.
6.4.2.4 Probes for the contact technique
The simplest probe type is the normal-beam probe for the emission and reception of longitudinal waves
in the axial direction of the probe (Figure 6).
Angle-beam probes contain a plastic wedge allowing by refraction the transmission of sound waves at
a predetermined angle. The transducers of angle-beam probes very often are rectangular and usually
generate transverse waves in the test object. Angle-beam probes are used for the detection of reflectors
not detectable by a normal-beam probe. Weld testing is a typical field of application. The plastic wedges
necessary for the refraction of waves are wear parts, so angle-beam probes are built to enable easy
replacement of the wedge (Figure 7).
14 © ISO 2017 – All rights reserved

Key
1 transducer 5 wedge
2 absorber block 6 longitudinal wave
3 electrical matching circuit 7 refracted transverse wave
4 damping block
Figure 7 — Schematic of an ultrasonic angle-beam probe
Dual-transducer probes are designed with separate transducers for transmission and reception. Both
transducers are electrically independent and are separated acoustically by a barrier and a delay line.
This avoids cross-talk from transmitter to receiver. The transducers can be inclined towards one
another (the interior angle is known as roof angle) to achieve a focusing effect and an improvement
of re
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