EN ISO 178:2019

SLOVENSKI STANDARD
01-julij-2019
Nadomešča:
SIST EN ISO 178:2011
SIST EN ISO 178:2011/A1:2014
Polimerni materiali - Določanje upogibnih lastnosti (ISO 178:2019)
Plastics - Determination of flexural properties (ISO 178:2019)
Kunststoffe - Bestimmung der Biegeeigenschaften (ISO 178:2019)
Plastiques - Détermination des propriétés en flexion (ISO 178:2019)
Ta slovenski standard je istoveten z: EN ISO 178:2019
ICS:
83.080.01 Polimerni materiali na Plastics in general
splošno
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 178
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2019
EUROPÄISCHE NORM
ICS 83.080.01 Supersedes EN ISO 178:2010
English Version
Plastics - Determination of flexural properties (ISO
178:2019)
Plastiques - Détermination des propriétés en flexion Kunststoffe - Bestimmung der Biegeeigenschaften (ISO
(ISO 178:2019) 178:2019)
This European Standard was approved by CEN on 23 March 2019.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
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Turkey and United Kingdom.
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COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 178:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 178:2019) has been prepared by Technical Committee ISO/TC 61 "Plastics" in
collaboration with Technical Committee CEN/TC 249 “Plastics” the secretariat of which is held by NBN.
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 November 2019, and conflicting national standards
shall be withdrawn at the latest by November 2019.
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 178:2010.
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, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 178:2019 has been approved by CEN as EN ISO 178:2019 without any modification.

INTERNATIONAL ISO
STANDARD 178
Sixth edition
2019-04
Plastics — Determination of flexural
properties
Plastiques — Détermination des propriétés en flexion
Reference number
ISO 178:2019(E)
©
ISO 2019
ISO 178:2019(E)
© ISO 2019
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
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CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 178:2019(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3  Terms, definitions and symbols . 2
4 Principle . 5
5 Test machine . 5
5.1 General . 5
5.2 Test speed . 6
5.3 Supports and loading edge . 6
5.4 Force- and deflection-measuring systems . 6
5.4.1 Introductory remarks . 6
5.4.2 Definition of precision and accuracy requirements . 6
5.4.3 Deflection measurement . 8
5.5 Equipment for measuring the width and thickness of the test specimens . 9
6 Test specimens. 9
6.1 Shape and dimensions . 9
6.1.1 General. 9
6.1.2 Preferred specimen type . 9
6.1.3 Other test specimens .10
6.2 Anisotropic materials .10
6.3 Preparation of test specimens .11
6.3.1 From moulding, extrusion and casting compounds .11
6.3.2 From sheets .11
6.4 Specimen inspection .11
6.5 Number of test specimens .12
7 Atmosphere for conditioning and testing .12
8  Procedure.12
9  Calculation and expression of results .16
9.1 Flexural stress .16
9.2 Flexural strain .16
9.3 Flexural modulus .16
9.4 Statistical parameters .17
9.5 Significant figures .17
10 Precision .17
11 Test report .17
Annex A (informative) Precision statement .19
Annex B (informative) Influence of changes in test speed on the measured values of
flexural properties .21
Annex C (normative) Compliance correction for Type III-tests .22
Annex D (informative) Relation between tensile and flexural modulus: Theoretical
expectations and experimental observations .24
Bibliography .25
ISO 178:2019(E)
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 ISO/TC 61, Plastics, Subcommittee SC 2, Mechanical properties.
This sixth edition cancels and replaces the fifth edition (ISO 178:2010), which has been technically
revised. It also incorporates the Amendment ISO 178:2010/Amd.1:2013. The main changes compared to
the previous edition are as follows:
— differentiating calibration requirements according to the type of test;
— the introduction of deflectometers;
— the reinstatement of procedures for compliance correction;
— the addition of a new Annex D showing the relation between tensile and flexural modulus.
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 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 178:2019(E)
Plastics — Determination of flexural properties
1 Scope
This document specifies a method for determining the flexural properties of rigid and semi-rigid
plastics under defined conditions. A preferred test specimen is defined, but parameters are included for
alternative specimen sizes for use where appropriate. A range of test speeds is included.
The method is used to investigate the flexural behaviour of the test specimens and to determine the
flexural strength, flexural modulus and other aspects of the flexural stress/strain relationship under the
conditions defined. It applies to a freely supported beam, loaded at midspan (three-point loading test).
The method is suitable for use with the following range of materials:
— thermoplastic moulding, extrusion and casting materials, including filled and reinforced compounds
in addition to unfilled types; rigid thermoplastics sheets;
— thermosetting moulding materials, including filled and reinforced compounds; thermosetting sheets.
[5] [6]
In agreement with ISO 10350-1 and ISO 10350-2 , this document applies to fibre-reinforced
compounds with fibre lengths ≤7,5 mm prior to processing. For long-fibre-reinforced materials
[7]
(laminates) with fibre lengths >7,5 mm, see ISO 14125 .
The method is not normally suitable for use with rigid cellular materials or sandwich structures
[3] [4]
containing cellular material. In such cases, ISO 1209-1 and/or ISO 1209-2 can be used.
NOTE 1 For certain types of textile-fibre-reinforced plastic, a four-point bending test is used. This is described
in ISO 14125.
The method is performed using specimens which can be either moulded to the specified dimensions,
machined from the central section of a standard multipurpose test specimen (see ISO 20753) or machined
from finished or semi-finished products, such as mouldings, laminates, or extruded or cast sheet.
The method specifies the preferred dimensions for the test specimen. Tests which are carried out on
specimens of different dimensions, or on specimens which are prepared under different conditions, can
produce results which are not comparable. Other factors, such as the test speed and the conditioning of
the specimens, can also influence the results.
NOTE 2 Especially for injection moulded semi-crystalline polymers, the thickness of the oriented skin layer,
which is dependent on the moulding conditions, also affects the flexural properties.
The method is not suitable for the determination of design parameters but can be used in materials
testing and as a quality control test.
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 291, Plastics — Standard atmospheres for conditioning and testing
ISO 293, Plastics — Compression moulding of test specimens of thermoplastic materials
ISO 294-1:2017, Plastics — Injection moulding of test specimens of thermoplastic materials — Part 1:
General principles, and moulding of multipurpose and bar test specimens
ISO 178:2019(E)
ISO 295, Plastics — Compression moulding of test specimens of thermosetting materials
ISO 2602, Statistical interpretation of test results — Estimation of the mean — Confidence interval
ISO 2818, Plastics — Preparation of test specimens by machining
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
ISO 10724-1, Plastics — Injection moulding of test specimens of thermosetting powder moulding compounds
(PMCs) — Part 1: General principles and moulding of multipurpose test specimens
ISO 16012, Plastics — Determination of linear dimensions of test specimens
ISO 20753, Plastics — Test specimens
3  Terms, definitions and symbols
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org
3.1
test speed
v
rate of relative movement between the specimen supports and the loading edge
Note 1 to entry: It is expressed in millimetres per minute (mm/min).
3.2
flexural stress
σ
f
nominal stress at the outer surface of the test specimen at midspan
Note 1 to entry: It is calculated from the relationship given in Formula (5).
Note 2 to entry: It is expressed in megapascals (MPa).
3.3
flexural stress at break
σ
fB
flexural stress at break of the test specimen
Note 1 to entry: It is expressed in megapascals (MPa).
Note 2 to entry: See Figure 1, curves a and b.
3.4
flexural strength
σ
fM
maximum flexural stress (3.2) sustained by the test specimen during a bending test
Note 1 to entry: It is expressed in megapascals (MPa).
Note 2 to entry: See Figure 1, curves a and b.
2 © ISO 2019 – All rights reserved

ISO 178:2019(E)
3.5
flexural stress at conventional deflection
σ
fc
flexural stress at the conventional deflection, s (3.7)
C
Note 1 to entry: It is expressed in megapascals (MPa).
Note 2 to entry: See also Figure 1, curve c.
3.6
deflection
s
distance over which the top or bottom surface of the test specimen at midspan deviates from its original
position during flexure
Note 1 to entry: It is expressed in millimetres (mm).
3.7
conventional deflection
s
C
deflection (3.6) equal to 1,5 times the thickness, h, of the test specimen
Note 1 to entry: It is expressed in millimetres (mm).
Note 2 to entry: Using a span, L, of 16h, the conventional deflection corresponds to a flexural strain (3.8) of 3,5 %.
3.8
flexural strain
ε
f
nominal fractional change in length of an element of the outer surface of the test specimen at midspan
Note 1 to entry: It is expressed as a dimensionless ratio or a percentage (%).
Note 2 to entry: It is calculated in accordance with the relationships given in Formulae (6) and (7).
ISO 178:2019(E)
Key
curve a specimen that breaks before yielding
curve b specimen that gives a maximum and then breaks before the conventional deflection, s
C
curve c specimen that neither gives a maximum nor breaks before the conventional deflection, s
C
Figure 1 — Typical curves of flexural stress, σ, versus flexural strain, ε, and deflection, s
f f
3.9
flexural strain at break
ε
fB
flexural strain at which the test specimen breaks
Note 1 to entry: It is expressed as a dimensionless ratio or a percentage (%).
Note 2 to entry: See Figure 1, curves a and b.
3.10
flexural strain at flexural strength
ε
fM
flexural strain at maximum flexural stress
Note 1 to entry: It is expressed as a dimensionless ratio or a percentage (%).
Note 2 to entry: See Figure 1, curves a and b.
3.11
modulus of elasticity in flexure
flexural modulus
E
f
ratio of the stress difference, σ − σ , to the corresponding strain difference,
f2 f1
ε (= 0,002 5) − ε (= 0,000 5)
f2 f1
Note 1 to entry: It is expressed in megapascals (MPa).
Note 2 to entry: The flexural modulus is only an approximate value of Young's modulus.
Note 3 to entry: See Formula (9).
4 © ISO 2019 – All rights reserved

ISO 178:2019(E)
3.12
rigid plastic
plastic that has a modulus of elasticity in flexure (3.11) or, if that is not applicable, then in tension, greater
than 700 MPa under a given set of conditions
[SOURCE: ISO 472:2013, 2.884, modified — Note to entry has been omitted.]
3.13
semi rigid plastic
plastic that has a modulus of elasticity in flexure (3.11) or, if that is not applicable, then in tension,
between 70 MPa and 700 MPa under a given set of conditions
[SOURCE: ISO 472:2013, 2.909, modified — Note to entry has been omitted.]
3.14
span between specimen supports
L
distance between the points of contact between the test specimen and the test specimen supports
Note 1 to entry: It is expressed in millimetres (mm).
Note 2 to entry: See Figure 2.
3.15
flexural strain rate
r
rate at which the flexural strain (3.8) increases during a test
−1
Note 1 to entry: It is expressed in percent per minute (% ⋅ min ).
4 Principle
A test specimen of rectangular cross-section, resting on two supports, is deflected by means of a
loading edge acting on the specimen midway between the supports. The test specimen is deflected in
this way at a constant rate at midspan until rupture occurs at the outer surface of the specimen or until
a maximum strain of 5 % (see 3.8) is reached, whichever occurs first. During this procedure, the force
applied to the specimen and the resulting deflection of the specimen at midspan are measured.
This document specifies two methods: method A and method B. Method A uses a strain rate of
1 %/min throughout the test. Method B uses two different strain rates: 1 %/min for the determination
of the flexural modulus and 5 %/min or 50 %/min, depending on the ductility of the material, for the
determination of the remainder of the flexural stress-strain curve.
NOTE 1 The strain rates mentioned above are to be interpreted as nominal ones. Nominal test speeds are
calculated using Formula (4). For the machine settings the best fitting ones are selected from Table 1.
NOTE 2 For materials exhibiting nonlinear stress/strain behaviour, the flexural properties are only
nominal. The formulae given have been derived assuming linear elastic behaviour and are valid for deflections
of the specimen that are small compared to its thickness. With the preferred specimen (which measures
80 mm × 10 mm × 4 mm) at the conventional flexural strain of 3,5 % and a span-to-thickness ratio, L/h, of 16, the
deflection is 1,5h. Flexural tests are more appropriate for stiff and brittle materials showing small deflections at
break than for very soft and ductile ones.
5 Test machine
5.1 General
The machine shall comply with ISO 7500-1 and ISO 9513 and the requirements given in 5.2 to 5.4.
ISO 178:2019(E)
5.2 Test speed
The test machine shall be capable of maintaining the test speed, as specified in Table 1.
Table 1 — Recommended values of the test speed, v
Test speed, v Tolerance
mm/min %
a
1 ±20
2 ±20
5 ±20
10 ±20
20 ±10
50 ±10
100 ±10
200 ±10
500 ±10
a
The lowest speed is used for specimens with thicknesses between
1 mm and 3,5 mm (see also 8.5).
5.3  Supports and loading edge
Two supports and a central loading edge shall be arranged as shown in Figure 2. The supports and the
loading edge shall be parallel to within ±0,2 mm over the width of the test specimen.
The radius, R , of the loading edge and the radius, R , of the supports shall be as follows:
1 2
R = 5,0 mm ± 0,2 mm;
R = 2,0 mm ± 0,2 mm for test specimen thicknesses ≤3 mm;
R = 5,0 mm ± 0,2 mm for test specimen thicknesses >3 mm.
The span, L, shall be adjustable.
5.4  Force- and deflection-measuring systems
5.4.1  Introductory remarks
Flexural tests, according to the specific requirements on the data to be obtained, can be differentiated in
several classes, comprising different complexity and requirements on accuracy. This starts with simple
tests for obtaining flexural strength only on the one hand and on the other hand necessitates the use of
a deflectometer to obtain the deflection accurately and free of compliance effects of the machine. The
compliance of flexural testing machines has several possible sources (play and deformations in fixtures,
deformations in the load train, and deformations of the load cell). Precise and true determination of
deflection is especially important for the determination of the flexural modulus, for which the use of
uncorrected crosshead displacement is not suitable. For a repeatable determination of flexural modulus
results a compliance correction shall be applied or, preferably, a deflectometer shall be used.
5.4.2  Definition of precision and accuracy requirements
Table 2 defines objectives of testing in increasing order of test complexity and appertaining need for
accuracy. A good precision without absolute accuracy as indicated in type III-tests can be sufficient in
many quality control environments when properties are to be supervised over periods of time only.
Accurate, meaning true and precise, results as indicated in type IV-tests are needed if the results are to
be compared between laboratories. Different types of deflection measurement and different accuracy
6 © ISO 2019 – All rights reserved

ISO 178:2019(E)
requirements for the deflection measurement are therefore defined, based on the needs on precision
and trueness of the test results.
Key
1 test specimen h thickness of specimen
2 support base plate F applied force
3 deflectometer position l length of specimen
4 supports L length of span between supports
R radius of loading edge R radius of supports
1 2
Figure 2 — Position of test specimen and deflectometer at start of test
ISO 178:2019(E)
Table 2 — Types of tests and calibration requirements
Types (I-IV)
of tests in increasing order of complexity and requirements for accuracy
Required objective Stress/strength only Stress/strength/ Stress/strength/ Stress/strength/
of testing strains > 1 % strains/repeat- strains/true and
able and precise precise = accurate
modulus modulus
Property I II III IV
σ × × × ×
fB
σ × × × ×
fM
σ × × ×
fC
s × × ×
C
ε × × ×
fB
ε × × ×
fM
E  × ×
f
Calibration requirement
Force ISO 7500-1, class 1
Deflection measure- — ISO 9513, Class 2 ISO 9513, Class 2 plus ISO 9513, Class 1 plus
ment condition set in 5.4.3 condition set in 5.4.3
Type of deflection — Crosshead displace- Crosshead displace- Direct measurement
measurement ment ment with compli- using a deflectometer
ance correction
5.4.3  Deflection measurement
The machine shall be capable of continuously recording the crosshead displacement with an accuracy
conforming to the class of ISO 9513 indicated in Table 2. This shall be valid over the whole range of
deflections to be measured. Non-contact systems may be used provided they meet the accuracy
requirements stated above. The measurement system shall not be influenced by machine compliance.
When determining the flexural modulus as indicated in type IV, the deflection-measuring system, in
accordance with ISO 9513 Class 1, shall be capable of measuring the change in deflection to an accuracy
of 1 % of the relevant value or better, corresponding to ± 3,4 µm for a support span, L, of 64 mm and a
specimen thickness, h, of 4,0 mm (see Figure 3).
For type III tests the deflection-measuring system, in accordance with ISO 9513 Class 2, shall be
capable of measuring the change in deflection to an accuracy of 2 % of the relevant value or better,
corresponding to ±6,8 µm for a support span, L, of 64 mm and a specimen thickness, h, of 4,0 mm.
Other support spans and specimen thicknesses will lead to different requirements for the accuracy of
the deflection-measuring system.
For the determination of the flexural modulus using the crosshead displacement a
...