Plastics - Determination of flexural properties (ISO 178:2019)

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.
In agreement with ISO 10350-1[5] and ISO 10350-2[6], this document applies to fibre-reinforced compounds with fibre lengths ≤7,5 mm prior to processing. For long-fibre-reinforced materials (laminates) with fibre lengths >7,5 mm, see ISO 14125[7].
The method is not normally suitable for use with rigid cellular materials or sandwich structures containing cellular material. In such cases, ISO 1209-1[3] and/or ISO 1209-2[4] 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.

Kunststoffe - Bestimmung der Biegeeigenschaften (ISO 178:2019)

Dieses Dokument legt ein Verfahren zur Ermittlung der Biegeeigenschaften von steifen und halbsteifen Kunststoffen unter definierten Bedingungen fest. Ein bevorzugter Probekörper wird festgelegt, jedoch sind die Parameter für andere gegebenenfalls anzuwendende Probekörpergrößen enthalten. Ein Bereich von Prüfgeschwindigkeiten ist enthalten.
Das Verfahren wird für die Untersuchung des Biegeverhaltens der Probekörper und für die Bestimmung der Biegefestigkeit, des Biegemoduls und anderer Gesichtspunkte der Beziehung Biegespannung/Biegedehnung unter den definierten Bedingungen eingesetzt. Es bezieht sich auf einen frei unterstützten Biegebalken, der mittig belastet wird (Dreipunkt-Biegeversuch).
Das Prüfverfahren ist zur Anwendung auf folgende Werkstoffgruppen geeignet:
—   thermoplastische Formmassen, Extrusions- und Vergussmassen, einschließlich gefüllter und verstärkter Compounds in Ergänzung zu ungefüllten Sorten; steife thermoplastische Platten;
—   duroplastische Formmassen, einschließlich gefüllter und verstärkter Compounds; duro¬plastische Platten.
In Übereinstimmung mit ISO 10350 1 [5] und ISO 10350 2 [6] gilt dieses Dokument für faserverstärkte Compounds mit Faserlängen ≤ 7,5 mm vor der Verarbeitung. In Bezug auf langfaserverstärkte Werkstoffe (Laminate) mit Faserlängen > 7,5 mm siehe ISO 14125 [7].
Das Verfahren ist üblicherweise nicht für Hartschäume oder Schicht Verbundwerkstoffe geeignet, die geschäumte Strukturen enthalten. In solchen Fällen können ISO 1209 1 [3] und/oder ISO 1209 2 [4] angewendet werden.
ANMERKUNG 1   Für bestimmte Sorten textilfaserverstärkter Kunststoffe wird ein Vierpunktbiegeversuch angewendet. Dies ist in ISO 14125 beschrieben.
Das Verfahren wird mit Probekörpern durchgeführt, die direkt in den festgelegten Maßen gegossen, aus dem Mittelteil eines Standard Vielzweckprobekörpers (siehe ISO 20753) herausgearbeitet oder aus fertigen oder halbfertigen Produkten wie Formteilen, Laminate oder extrudierten oder gegossenen Platten aus¬gearbeitet werden können.
Das Verfahren legt die bevorzugten Maße für den Probekörper fest. Prüfungen an Probekörpern mit abweichenden Maßen oder Probekörpern, die unter anderen Bedingungen hergestellt wurden, können zu Ergebnissen führen, die nicht vergleichbar sind. Andere Einflüsse, wie z. B. die Prüfgeschwindigkeit und das Konditionieren der Probekörper können ebenfalls die Prüfergebnisse beeinflussen.
ANMERKUNG 2   Besonders bei spritzgegossenen teilkristallinen Polymeren wirkt sich die Dicke der orientierten Außenhaut, die von den Spritzgießbedingungen abhängt, auch auf die Biegeeigenschaften aus.
Das Verfahren ist nicht für die Bestimmung von Konstruktionseigenschaften geeignet, kann jedoch für Materialprüfungen und in der Qualitätskontrolle eingesetzt werden.

Plastiques - Détermination des propriétés en flexion (ISO 178:2019)

Le présent document spécifie une méthode pour la détermination des propriétés en flexion des plastiques rigides et semi-rigides dans des conditions définies. Une éprouvette recommandée est définie, mais des paramètres sont inclus pour d'autres dimensions d'éprouvettes lorsque l'usage est approprié. Une gamme de vitesses d'essai est incluse.
La méthode est utilisée pour l'étude du comportement en flexion des éprouvettes et pour la détermination de la résistance en flexion, du module en flexion et d'autres aspects des relations entre la contrainte et la déformation en flexion dans les conditions définies. Elle s'applique à une poutre supportée sans contrainte, chargée au milieu de sa portée (essai de chargement en trois points).
La méthode est adaptée à la gamme de matériaux suivants:
— matériaux thermoplastiques pour moulage, extrusion et coulée, y compris les compositions chargées et renforcées en plus des types non chargés; feuilles thermoplastiques rigides;
— matériaux thermodurcissables pour moulage, y compris les compositions chargées et renforcées; feuilles thermodurcissables.
En accord avec l'ISO 10350-1[5] et l'ISO 10350-2,[6] le présent document s'applique aux compositions renforcées de fibres dont les longueurs avant mise en œuvre sont inférieures ou égales à 7,5 mm. Pour les matériaux renforcés de fibres longues (stratifiés) avec des longueurs de fibres supérieures à 7,5 mm, voir l'ISO 14125[7].
La méthode n'est normalement pas adaptée pour utilisation avec des matériaux alvéolaires rigides ou à structures sandwich contenant des matériaux alvéolaires. Dans ces cas, les normes ISO 1209-1[3] et/ou ISO 1209-2[4] peuvent être utilisées.
NOTE 1 Pour certains types de plastiques renforcés de fibres textiles, un essai de flexion en quatre points est utilisé. Ce dernier est décrit dans l'ISO 14125.
La méthode est réalisée à l'aide d'éprouvettes qui peuvent être soit moulées aux dimensions spécifiées, soit usinées à partir de la partie centrale d'une éprouvette normalisée à usages multiples (voir l'ISO 20753) ou usinées à partir de produits finis ou semi-finis, tels que des pièces moulées, des stratifiés ou des feuilles extrudées ou coulées.
La méthode spécifie les dimensions recommandées pour les éprouvettes. Des essais réalisés avec des éprouvettes de dimensions différentes ou avec des éprouvettes préparées dans des conditions différentes peuvent donner des résultats qui ne sont pas comparables. D'autres facteurs, tels que la vitesse d'essai et le conditionnement des éprouvettes, peuvent également influer sur les résultats.
NOTE 2 En fonction des conditions de moulage, pour les polymères semi-cristallins moulés par injection en particulier, l'épaisseur de la couche (peau) orientée affecte les propriétés en flexion.
La méthode n'est pas adaptée pour la détermination des paramètres de calcul mais elle peut être utilisée pour les essais de matériaux et comme essai dans un contrôle qualité.

Polimerni materiali - Določanje upogibnih lastnosti (ISO 178:2019)

Ta dokument določa metodo za določanje upogibnih lastnosti togih in poltogih polimernih materialov pri določenih pogojih. Določen je prednostni preskušanec, vendar so dodani parametri za druge velikosti preskušancev za uporabo po potrebi. Vključen je nabor preskusnih hitrosti.
Metoda se uporablja za preučevanje upogibnih lastnosti preskušancev in za določanje
upogibne trdnosti, modula upogibnosti in drugih vidikov razmerja upogibna napetost/deformacija pri določenih pogojih. Uporablja se za prosto podprt nosilec, obremenjen na sredini (preskus z obremenitvijo v treh točkah).
Metoda je ustrezna za naslednje vrste materialov:
– plastomerni materiali za oblikovanje, ekstrudiranje in ulivanje, vključno s polnjenimi, nepolnjenimi in ojačenimi spojinami; trde plastomerne plošče;
– termoreaktivni materiali za oblikovanje, vključno s polnjenimi in ojačenimi spojinami; termoreaktivne plošče.
V skladu s standardoma ISO 10350-1[5] in ISO 10350-2[6] se ta dokument nanaša na spojine, ojačene z vlakni dolžine ≤ 7,5 mm pred predelavo. Za z dolgimi vlakni ojačene materiale (laminate) z dolžino vlaken > 7,5 mm glej ISO 14125[7].
Metoda običajno ni primerna za trde penjene materiale ali strukture tipa »sendvič«, ki vsebujejo penjeni material. V takih primerih se uporablja ISO 1209-1[3] in/ali ISO 1209-2[4].
OPOMBA 1: Za nekatere vrste polimernih materialov, ojačenih s tekstilnimi vlakni, se uporablja štiritočkovni preskus upogibanja. Ta je opisan v standardu ISO 14125.
Metoda se izvaja s preskušanci, ki so lahko oblikovani na določene mere,
strojno obdelani iz osrednjega dela standardnega večnamenskega preskusnega vzorca (glej ISO 20753) ali strojno obdelani iz končnih izdelkov ali polizdelkov, kot so oblikovanci, laminati ali ekstrudirane oziroma lite plošče.
Metoda določa prednostne dimenzije preskusnih vzorcev. Preskusi, ki se opravljajo
na preskušancih drugih velikosti ali na preskušancih, ki so pripravljeni pod drugačnimi pogoji, lahko dajo rezultate, ki niso primerljivi. Na rezultate lahko vplivajo tudi drugi dejavniki, kot sta hitrost preskušanja in priprava preskušancev.
OPOMBA 2: Debelina usmerjenega vrhnjega sloja, ki je odvisna od pogojev oblikovanja, vpliva tudi na upogibne lastnosti, zlasti za polkristalinične polimere, oblikovane z brizganjem.
Metoda ni primerna za ugotavljanje parametrov zasnove, lahko pa se uporabi za preskušanje materialov
in kot preskus za nadzor kakovosti.

General Information

Status
Published
Public Enquiry End Date
31-Jul-2017
Publication Date
26-May-2019
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
20-May-2019
Due Date
25-Jul-2019
Completion Date
27-May-2019

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SLOVENSKI STANDARD
SIST EN ISO 178:2019
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
SIST EN ISO 178:2019 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 178:2019
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SIST EN ISO 178:2019
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,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey 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

© 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.
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SIST EN ISO 178:2019
EN ISO 178:2019 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 178:2019
EN ISO 178:2019 (E)
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.

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SIST EN ISO 178:2019
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SIST EN ISO 178:2019
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
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SIST EN ISO 178:2019
ISO 178:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© 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
CP 401 • Ch. de Blandonnet 8
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
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SIST EN ISO 178:2019
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 2019 – All rights reserved iii
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SIST EN ISO 178:2019
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
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SIST EN ISO 178:2019
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 2019 – All rights reserved 1
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SIST EN ISO 178:2019
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
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
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
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

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
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SIST EN ISO 178:2019
ISO 178:2019(E)
3.5
flexural stress at conventional deflection
flexural stress at the conventional deflection, s (3.7)
Note 1 to entry: It is expressed in megapascals (MPa).
Note 2 to entry: See also Figure 1, curve c.
3.6
deflection

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
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

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).

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Key
curve a specimen that breaks before yielding

curve b specimen that gives a maximum and then breaks before the conventional deflection, s

curve c specimen that neither gives a maximum nor breaks before the conventional deflection, s

Figure 1 — Typical curves of flexural stress, σ, versus flexural strain, ε, and deflection, s

f f
3.9
flexural strain at break
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
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
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).
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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

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
rate at which the flexural strain (3.8) increases during a test
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.

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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 %
1 ±20
2 ±20
5 ±20
10 ±20
20 ±10
50 ±10
100 ±10
200 ±10
500 ±10
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

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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 sup
...

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