SIST EN ISO 16827:2025

SLOVENSKI STANDARD
01-oktober-2025
Neporušitvene preiskave - Ultrazvočne preiskave - Karakterizacija in velikosti
nezveznosti (ISO 16827:2025)
Non-destructive testing - Ultrasonic testing - Characterization and sizing of discontinuities
(ISO 16827:2025)
Zerstörungsfreie Prüfung - Ultraschallprüfung - Beschreibung und Größenbestimmung
von Inhomogenitäten (ISO 16827:2025)
Essais non destructifs - Contrôle par ultrasons - Caractérisation et dimensionnement des
discontinuités (ISO 16827:2025)
Ta slovenski standard je istoveten z: EN ISO 16827:2025
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 16827
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2025
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN ISO 16827:2014
English Version
Non-destructive testing - Ultrasonic testing -
Characterization and sizing of discontinuities (ISO
16827:2025)
Essais non destructifs - Contrôle par ultrasons - Zerstörungsfreie Prüfung - Ultraschallprüfung -
Caractérisation et dimensionnement des discontinuités Beschreibung und Größenbestimmung von
(ISO 16827:2025) Inhomogenitäten (ISO 16827:2025)
This European Standard was approved by CEN on 7 June 2025.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 16827:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 16827:2025) has been prepared by Technical Committee ISO/TC 135 "Non-
destructive testing" in collaboration with Technical Committee CEN/TC 138 “Non-destructive testing”
the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by December 2025, and conflicting national standards
shall be withdrawn at the latest by December 2025.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 16827:2014.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 16827:2025 has been approved by CEN as EN ISO 16827:2025 without any modification.

International
Standard
ISO 16827
Second edition
Non-destructive testing —
2025-06
Ultrasonic testing —
Characterization and sizing of
discontinuities
Essais non destructifs — Contrôle par ultrasons —
Caractérisation et dimensionnement des discontinuités
Reference number
ISO 16827:2025(en) © ISO 2025
ISO 16827:2025(en)
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 16827:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles of characterization of discontinuities . 1
4.1 General .1
4.2 Requirements for surface condition .2
5 Pulse-echo techniques . 2
5.1 General .2
5.2 Location of discontinuity .2
5.3 Orientation of discontinuity .3
5.4 Assessment of multiple indications .3
5.5 Shape of discontinuity .4
5.5.1 Simple classification .4
5.5.2 Detailed classification .4
5.6 Maximum echo height of indication .5
5.7 Size of discontinuity.5
5.7.1 General .5
5.7.2 Maximum echo height techniques .5
5.7.3 Probe movement sizing techniques .6
5.7.4 Selection of sizing techniques .7
5.7.5 Sizing techniques with focusing probes .8
5.7.6 Use of mathematical algorithms for sizing .8
5.7.7 Special sizing techniques .8
6 Through-transmission technique . 9
6.1 General .9
6.2 Location of discontinuity .9
6.3 Evaluation of multiple discontinuities .9
6.4 Reduction of signal amplitude .10
6.5 Size of discontinuity.10
Annex A (normative) Analysis of multiple indications .12
Annex B (normative) Techniques for the classification of discontinuity shape . 14
Annex C (normative) Sizing technique using the maximum echo height .23
Annex D (normative) Sizing techniques using probe movement .25
Annex E (informative) Iterative sizing technique .37
Annex F (informative) Mathematical algorithms for the estimation of the actual size of a
discontinuity . 41
Annex G (informative) Examples of special sizing techniques . 47
Bibliography .50

iii
ISO 16827:2025(en)
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of 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 www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee
SC 3, Ultrasonic testing, in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 138, Non-destructive testing, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 16827:2012), which has been technically
revised.
The main changes are as follows:
— figures have been updated;
— references have been updated;
— information added in the scope that the technique can also be used with phased array.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO 16827:2025(en)
Introduction
The following documents on ultrasonic testing are linked:
ISO 16810, Non-destructive testing — Ultrasonic testing — General principles
ISO 16811, Non-destructive testing — Ultrasonic testing — Sensitivity and range setting
ISO 16823, Non-destructive testing — Ultrasonic testing — Through transmission technique
ISO 16826, Non-destructive testing — Ultrasonic testing — Testing for discontinuities perpendicular to the surface
ISO 16827, Non-destructive testing — Ultrasonic testing — Characterization and sizing of discontinuities
ISO 16828, Non-destructive testing — Ultrasonic testing — Time-of-flight diffraction technique as a method for
detection and sizing of discontinuities

v
International Standard ISO 16827:2025(en)
Non-destructive testing — Ultrasonic testing —
Characterization and sizing of discontinuities
1 Scope
This document specifies the general principles and techniques for the characterization and sizing of
previously detected discontinuities in order to ensure their evaluation against applicable acceptance
criteria.
This document is applicable, in general terms, to discontinuities in those materials and applications covered
by ISO 16810.
Phased array techniques can also be applied but additional steps or verifications can be needed.
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
ISO 16810, Non-destructive testing — Ultrasonic testing — General principles
ISO 16811, Non-destructive testing — Ultrasonic testing — Sensitivity and range setting
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principles of characterization of discontinuities
4.1 General
Characterization of a discontinuity involves the determination of those features which are necessary for its
evaluation with respect to specified acceptance criteria.
Characterization of a discontinuity can include:
a) determination of basic ultrasonic parameters (echo height, time of flight);
b) determination of its basic shape and orientation;
c) sizing, which may take the form of either:
1) the determination of one or more dimensions (or area/volume), within the limitations of the
methods; or
ISO 16827:2025(en)
2) the determination of some specified parameter, e.g. echo height, where this is taken as representative
of the physical size of the discontinuity;
d) location, e.g. the proximity to the surface or to other discontinuities;
e) determination of any other parameters or characteristics that may be necessary for complete evaluation;
f) assessment of probable nature, e.g. crack or inclusion, where adequate knowledge of the test object and
its manufacturing history makes this feasible.
g) The requirements of ISO 16810 apply unless stated otherwise.
Where the test in accordance with the principles of ISO 16810 yields sufficient data on the discontinuity
for its evaluation against the applicable acceptance criteria, no further characterization is necessary.
h) The techniques used for characterization shall be specified in conjunction with the applicable acceptance
criteria.
4.2 Requirements for surface condition
a) The surface finish and profile shall be such that it permits sizing of discontinuities with the desired
accuracy.
In general, the smoother and flatter the surface the more accurate the results will be.
For most practical purposes a surface finish of R = 6,3 μm for machined surfaces and 12,5 μm for shot-
a
blasted surfaces are recommended.
NOTE Determination of the surface roughness is out of the scope of this document.
b) The gap between the probe and the surface shall not exceed 0,5 mm.
c) The above surface requirements shall normally be limited to those areas from which sizing is to be
carried out as, in general, they are unnecessary for discontinuity detection.
d) The method of surface preparation shall not produce a surface that gives rise to a high level of noise.
5 Pulse-echo techniques
5.1 General
The principal ultrasonic characteristics/parameters of a discontinuity that are most commonly used for
evaluation by the pulse-echo techniques are described in 5.2 to 5.7 inclusive.
The characteristics/parameters to be determined shall be defined in the applicable standard or any relevant
contractual document, and shall meet the requirements on the characterization of discontinuities given in
ISO 16810.
5.2 Location of discontinuity
The location of a discontinuity is defined as its position within a test object with respect to an agreed system
of reference co-ordinates.
a) It shall be determined in relation to one or more datum points and with reference to the index point and
beam angle of the probe, and measurement of the probe position and sound path length at which the
maximum echo height is observed.
b) Depending on the geometry of the test object and the type of discontinuity, it may be necessary to
confirm the location of the discontinuity from another direction, or with another beam angle, to ensure
that the echo is not caused e.g. by a wave mode conversion at a geometrical feature of the test object.

ISO 16827:2025(en)
5.3 Orientation of discontinuity
The orientation of a discontinuity is defined as the direction or plane along which the discontinuity has its
major axis (axes) with respect to a datum reference on the test object.
a) The orientation can be determined by a geometrical reconstruction analogous to that described for
location, with the difference that more beam angles and/or scanning directions are generally necessary
than for a simple location.
b) The orientation may also be determined from observation of the scanning direction at which the
maximum echo height is obtained.
c) In several applications, the precise determination of the discontinuity orientation in space is not
required, only the determination of the projection of the discontinuity onto one or more specified planes
and/or sections within the test object.
5.4 Assessment of multiple indications
The method for distinguishing between single and multiple discontinuities may be based on either
qualitative assessment or quantitative criteria.
a) The qualitative determination consists of ascertaining, through the observation of the variations
of the ultrasonic indications, whether or not such indications correspond to one or more separate
discontinuities. Figure 1 shows typical examples of signals from grouped discontinuities in a forging or
casting.
b) Where acceptance criteria are expressed in terms of maximum allowable dimensions, preliminary
quantitative determinations shall be made in order to determine whether separate discontinuities are
to be evaluated individually or collectively according to pre-established rules governing the evaluation
of the group.
c) Such rules may be based on the concentration of individual discontinuities within the group, expressed
in terms of the total of their lengths, areas or volumes in relation to the overall length, area or volume of
the group.
Alternatively, the rules may specify the minimum distance between individual discontinuities, often as
a ratio of the dimensions of the adjacent discontinuities.
d) Where a more accurate characterization of a group of indications is required, an attempt shall be made to
determine whether the echoes arise from a series of closely spaced but separate discontinuities, or from
a single continuous discontinuity having a number of separate reflecting facets, using the techniques in
accordance with Annex A.
a) Resolvable grouped discontinuities b) Unresolvable grouped discontinuities

ISO 16827:2025(en)
Key
s sound path
Y signal amplitude
Figure 1 — Examples of A-scan signals from grouped discontinuities
5.5 Shape of discontinuity
5.5.1 Simple classification
There is a limited number of basic reflector shapes that can be identified by ultrasonic testing.
In many cases, evaluation against the applicable acceptance criteria only requires a relatively simple
classification in accordance with B.1.
a) According to this, the discontinuity shall be classified as either:
1) point, i.e. having no significant extent in any direction;
2) elongated, i.e. having a significant extent in one direction only;
3) complex, i.e. having a significant extent in more than one direction.
b) When required, the classification “complex” may be sub-divided into:
1) planar, i.e. having a significant extent in 2 directions only, and
2) volumetric, i.e., having a significant extent in 3 directions.
c) Depending upon the requirements of the acceptance standard, either:
1) separate acceptance criteria may apply to each of the above classifications; or
2) the discontinuity, independently of its point, elongated or complex configuration, is projected onto
one or more pre-established sections, and each projection is conservatively treated as a crack-like
planar discontinuity.
d) Simple classification will normally be limited to the use of those probes and techniques specified in the
test procedure.
e) Additional probes or techniques may only be used where agreed.
5.5.2 Detailed classification
a) In order to correctly identify the discontinuity types specified in the acceptance criteria, or to make a
correct fitness-for-purpose evaluation, it can be necessary to make a more detailed assessment of the
shape of the discontinuity.
Guidance on the methods that may be used for a more detailed classification is contained in B.2.
A classification can require the use of additional probes and scanning directions to those specified in
the test procedure for the detection of discontinuities, and can also be aided by the use of the special
techniques in Annexes E, F and G.
b) Classification of discontinuity shape will be limited to the determination of those discontinuity shapes
which are necessary for the correct evaluation of a discontinuity against the acceptance criteria or other
requirements.
The validity of such a classification should be proven for the specific application, e.g. materials and
configuration of the test object, test procedure, type of instrument and probes used.

ISO 16827:2025(en)
5.6 Maximum echo height of indication
The maximum echo height from a discontinuity is related to its size, shape and orientation. It is measured by
comparison with a specified reference level according to the methods described in ISO 16811.
Depending on the application and acceptance criteria the maximum echo height can be:
a) compared directly with a reference level that is specified in the acceptance standard;
b) used to determine the equivalent size of a discontinuity by comparison with the echo from a reference
reflector at the same sound path length in the material under test, or in a reference block having the
same acoustic properties, as described in 5.7.2;
c) used in probe movement sizing techniques based on a specified echo drop (e.g. 6 dB) below the
maximum, as described in 5.7.3.
5.7 Size of discontinuity
5.7.1 General
The sizing of a discontinuity consists in determining one or more projected dimensions/areas of the
discontinuity onto pre-established directions and/or sections.
If sizing is required, sizing techniques according to Annex C or Annex D shall be applied.
A short description of these techniques is found in Annex F and further details are given in ISO 16811.
5.7.2 Maximum echo height techniques
These techniques are based on a comparison of the maximum echo height from a discontinuity with the
echo height from a reference reflector at the same sound path length.
They are only meaningful if:
a) the shape and orientation of the discontinuity are favourable for reflection, hence the need to perform
echo height determinations from several directions or angles, unless the shape and orientation are
already known; and
b) the dimensions of the discontinuity, perpendicular to the beam axis, are less than the beam width in
either one or both directions;
c) the basic shape and orientation of the reference reflector are similar to those of the discontinuity to be
evaluated.
d) The reference reflector may be either a disc-shaped reflector, e.g. flat-bottomed hole, or an elongated
reflector, e.g. a side-drilled hole or notch.
e) Discontinuities subject to sizing may be classified as follows:
1) discontinuities whose reflective area has dimensions less than the beam width in all directions;
2) discontinuities whose reflective area shows a narrow, elongated form, i.e. having a length greater
than the beam width and a transverse dimension less than the beam width.
f) For discontinuities corresponding to e) 1), the area of the discontinuity, projected onto a section normal to
the ultrasonic beam axis, is assumed to be equivalent to the area of a disc-shaped reflector, perpendicular
to the beam axis, producing a maximum echo of the same height at the same sound path length.
g) For discontinuities corresponding to e) 2), the reference reflectors are generally of elongated form,
transverse to the ultrasonic beam axis, and having a specified transverse profile.
Such reflectors may be notches with rectangular, U- or V-shaped profile, or side-drilled holes.

ISO 16827:2025(en)
5.7.3 Probe movement sizing techniques
a) When using a straight-beam probe the dimensions generally determined are l and l , in directions
1 2
parallel to the scanning surface, by probe movement in two mutually perpendicular directions (see
Figure 2).
b) When using an angle-beam probe, the dimensions generally determined are:
1) dimension, l, parallel to the lateral scanning direction, determined by lateral movement of the probe
(see Figure 3);
2) dimension, h, perpendicular to the scanning surface, determined by transverse movement of the
probe (see Figure 3).
c) The techniques are classified into three categories, as follows:
1) fixed amplitude level techniques where the ends of a discontinuity are taken to correspond to the
plotted positions at which the echo height falls below a specified evaluation level;
2) techniques where the edges of the discontinuity are taken to correspond to the plotted positions
at which the maximum echo height at any position along the discontinuity has fallen by a specified
number of decibels.
The edges of the discontinuity may be plotted along the beam axis or along a predetermined beam edge;
3) techniques which aim to position the individual echoes from the tips of the discontinuity, or from
reflecting facets immediately adjacent to the edges.
The principal probe movement sizing techniques are described in Annex D.
Key
1 probe movement in direction x
2 probe movement in direction y
3 straight-beam probe
l extent in direction x
l extent in direction y
Figure 2 — Parameters l and l for the conventional sizing of a discontinuity by a straight-beam probe
1 2
ISO 16827:2025(en)
Key
1 transverse movement
2 lateral movement
3 angle-beam probe
l extent parallel to the lateral scanning direction
h extent perpendicular to the scanning surface
Figure 3 — Projected parameters l and h for the conventional sizing of a discontinuity by an angle-
beam probe
5.7.4 Selection of sizing techniques
The selection of sizing technique(s) depends upon the specific application and product type, and on the size
and nature of the discontinuity.
The following general rules apply:
a) maximum echo height techniques (see 5.7.2) may be applied only if the dimension to be measured is less
than the 6 dB beam width;
b) fixed amplitude level techniques [see 5.7.3, c)1)] may be applied to discontinuities of any dimension,
but since the measured size is an arbitrary value dependent on the particular amplitude level selected,
these techniques should only be used when specifically called for in the acceptance standard;
c) techniques based on probe movement at a specified dB drop below the maximum echo height from the
particular discontinuity [see 5.7.3, c)2)] may be applied only where the measured dimension is greater
than the beam width at the same dB drop.
d) If condition c) is not fulfilled the dimension of the discontinuity shall be assumed to be equal to the
applicable beam width;
e) techniques based on positioning the individual edges of a discontinuity [see 5.7.3, c)3)] can only be applied
when the ultrasonic indication from the discontinuity shows two or more resolvable echo maxima;
f) if the dimension to be determined is measured by more than one technique of 5.7.3, that value
determined by the technique whose reliability and accuracy can be demonstrated to be the highest shall
be assumed to be correct.
g) Alternatively, the highest measured value shall be assumed.

ISO 16827:2025(en)
5.7.5 Sizing techniques with focusing probes
If focusing probes are used for sizing, the techniques described in 5.7.2 and 5.7.3 can also be used, provided
that the discontinuity falls within the focal zone of the beam.
In general, the rules given under 5.7.4 also apply to focusing probes.
Where a higher accuracy of sizing is requested, an alternative technique can be used that is based on the
construction of a series of C-scan presentations of the discontinuity.
These shall be plotted through an iterative process of 6 dB steps, starting from an initial plot corresponding
to the 6 dB step from the maximum echo of discontinuity, down to the step where the evolution of the plot
corresponding to a 6 dB drop step is equal to, or less than, the 6 dB half-width of the ultrasonic beam.
In principle, this iterative technique can be used with both focused and unfocused ultrasonic beams, but
where high accuracy is required, it is particularly suitable for use with focused beams.
Annex E illustrates this technique in detail.
5.7.6 Use of mathematical algorithms for sizing
The main purpose of the sizing techniques illustrated in 5.7.2 and 5.7.3 is to compare the determined
discontinuity size with acceptance levels expressed in terms of maximum allowable dimensions (or areas/
volumes).
Where a higher accuracy is required in order to better estimate the actual size of a discontinuity, but only
data from the techniques described in 5.7.2 and 5.7.3 are available, mathematical algorithms may be of help.
Annex F illustrates in detail algorithms that can be used for the estimation of the actual size of discontinuities
that are either larger or smaller than the diameter of the ultrasonic beam.
5.7.7 Special sizing techniques
Special sizing techniques are additional to those described in 5.7.2 to 5.7.6 and may be used in particular
applications where higher levels of reliability and accuracy are called for.
a) When required, the reliability and accuracy of a special technique, applied to meet specified acceptance
criteria, shall be demonstrated on the same configuration and type of material using the same test
procedure and type of instrument and probes.
b) The following list of special techniques is not comprehensive due to the large number available and their
continuous development. Those described are the most commonly applied and their use is sufficiently
well established.
1) Tip diffraction techniques
These techniques can be used for the confirmation of the planar nature of a discontinuity (if this is
the case) and for sizing the transverse dimension (“h” of Figure 3) of a planar discontinuity. They
are based on the detection and location of pulses diffracted by discontinuity edges. See Annex G.1.
2) Mode conversion techniques
Where applicable these techniques can be used for the detection and characterization of planar
discontinuities.
They make use of mode conversion to generate an additional ultrasonic beam at a different reflected
angle and velocity when the plane of the discontinuity is oriented at the appropriate angle to the
incident beam.
In certain cases, these techniques can also be used for sizing, but require the use of special reference
blocks representative of the test object and containing planar reflectors of different sizes.

ISO 16827:2025(en)
3) Other special techniques
Other examples of ultrasonic techniques for the sizing of volumetric and planar discontinuities are:
— acoustical holography;
— acoustical tomography;
— array techniques using beams of variable angle;
— synthetic aperture focusing techniques (SAFT, see G.2); and
— construction of B-scan presentation or S-scan presentation, if applicable.
6 Through-transmission technique
6.1 General
The general principles and requirements of the through-transmission technique are given in ISO 16823.
6.2 to 6.5 describe some of the ultrasonic parameters and characteristics of the received signals that may be
used, either alone or in combination, to evaluate a discontinuity by this technique.
6.2 Location of discontinuity
When using straight-beam probes as shown in Figure 2, the location of a discontinuity is defined as the
position on the surface of the test object, with respect to a two-dimensional co-ordinate system, at which
the maximum reduction in received signal amplitude is observed.
If it is practicable to direct ultrasonic beams in two different directions through the area to be tested, e. g by
the use of pairs of angle-beam probes, the location of the discontinuity may be determined in 3 directions as
illustrated in Figure 4.
The discontinuity lies at the intersection of the two beam paths A A and B B , at which the maximum
1 2 1 2
reduction in received signal amplitude is observed (see Figure 4).
Key
A , A pair of angle beam probes
1 2
B , B pair of angle beam probes
1 2
1 artificial discontinuity represented by side drilled holes
Figure 4 — Location of discontinuities by the through-transmission technique using angle-beam probes
6.3 Evaluation of multiple discontinuities
a) Whether a discontinuity is continuous or intermittent shall first be determined qualitatively by
observing variations in signal amplitude as the probe is scanned over the discontinuity.

ISO 16827:2025(en)
b) If the signal amplitude remains relatively constant the discontinuity shall be classified as continuous
and evaluated as such against the acceptance criteria.
c) Conversely, if the signal amplitude shows marked maxima and minima the discontinuity may be
classified as intermittent.
In this case, it is necessary to determine quantitatively whether the concentration of discrete
discontinuities within the affected area is sufficiently high to apply the size/area limitations imposed by
the acceptance criteria.
d) The concentration of discontinuities within the affected area may be expressed, e. g. in terms of the
ratio between:
1) the dimensions (or area) of individual discontinuities and the distance between them;
2) the total length of discontinuities and a specified overall length; and
3) the total area of individual discontinuities and a specified overall area.
6.4 Reduction of signal amplitude
a) The reduction of signal amplitude shall be taken into account whenever the signal amplitude falls below
the specified evaluation level.
b) If the signal is lost completely, the limits of the zone on the scanning surface over which this occurs shall
be determined.
c) If there is only partial loss of the signal, the position on the scanning surface corresponding to the
maximum amplitude reduction shall be determined, together with the dB value of the reduction
compared to the signal obtained in a zone free from discontinuities.
d) If the area on the scanning surface, affected by the signal reduction, is less than the cross-sectional area
of the ultrasonic beam, the size of the discontinuity normal to the beam shall be estimated by matching
the reduction in amplitude with that due to a known reference reflector, e.g. a flat-bottomed hole, in a
representative sample of discontinuity-free material (see 6.5 a)).
e) Where a relatively constant partial reduction in signal amplitude is observed over a zone significantly
greater than the area of the sound beam, it is probable that the discontinuity can take the form of, e.g. a
band of numerous small inclusions, an area of abnormal grain structure, a layer of ultrasonically semi-
transparent material, or a large discontinuity under high compressive stress.
6.5 Size of discontinuity
The sizing of a discontinuity consists in determining one or more dimensions (or the area) of the projection
of the discontinuity onto the scanning surface. In particular, the dimensions (or areas) so determined are
compared with the applicable acceptance standards, whenever these standards are expressed in terms of
maximum allowable dimensions (or areas), in order to assess the acceptability or unacceptability of the
discontinuity.
Sizing techniques can be classified essentially in the following two categories.
a) Techniques based on the comparison of the maximum amplitude reduction of the signal with respect to
the maximum amplitude reduction of an equivalent reflector.
Adoption of these techniques for sizing is limited to the case where the dimension (or area) of the zone
on the scanning surface corresponding to the signal amplitude reduction below the evaluation level is
less than the probe dimension (or area) projected onto the scanning surface.
In this case, the maximum amplitude reduction of the signal with respect to the signal amplitude in a
zone free of discontinuities shall be determined, together with the reflector, generally a flat-bottomed
hole perpendicular to the beam axis located at a given depth (e.g. half thickness), producing the same
maximum reduction in the received signal amplitude.

ISO 16827:2025(en)
The dimension (or area) of the discontinuity, projected onto a plane perpendicular to the beam axis,
shall be assumed to be the same as the dimension (or area) of the flat-bottomed
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