ISO 12493:2023
(Main)Rubber, vulcanized or thermoplastic — Determination of stress in tension under non-isothermal conditions
Rubber, vulcanized or thermoplastic — Determination of stress in tension under non-isothermal conditions
This document describes two methods for measuring stress in tension under non-isothermal conditions. — Method A: The thermal stress is measured for various pre-strain and temperature conditions as a function of time. — Method B: The change of stress is measured in a test piece at a given strain and under variation of temperature at a given heating rate as a function of temperature. In this way, the determination of the thermal-mechanical behaviour of a rubber can be accelerated, e.g. for the purpose of comparative testing of aging or estimating the upper limit of the operating temperature. The measurement device, which is equipped with a suitable heating chamber, is used to record the stress as a function of time or temperature until the sample breaks or the stress has approached zero or for a certain time.
Caoutchouc vulcanisé ou thermoplastique — Détermination de la contrainte en traction dans des conditions non isothermes
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 12493
Third edition
2023-01
Rubber, vulcanized or
thermoplastic — Determination
of stress in tension under non-
isothermal conditions
Caoutchouc vulcanisé ou thermoplastique — Détermination de la
contrainte en traction dans des conditions non isothermes
Reference number
ISO 12493:2023(E)
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ISO 12493:2023(E)
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ISO 12493:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
5.1 Test machine . 2
5.1.1 General . 2
5.1.2 Thermal-stress testing machine . 2
5.1.3 Stress relaxometer . 3
5.2 Oven. 4
5.3 Thickness- and width-measuring devices . 4
6 Calibration .4
7 Test piece . 4
7.1 Dimensions . 4
7.1.1 Method A: . 4
7.1.2 Method B: . 4
7.2 Number of test pieces . 5
7.3 Time lapse between moulding and testing . 5
7.4 Conditioning. 5
8 Test conditions .5
8.1 Heating rate and temperature interval under investigation. 5
8.2 Strain of the sample . 5
9 Procedure .6
9.1 Method A . 6
9.2 Method B . 7
9.2.1 Isothermal test period . . 7
9.2.2 Non-isothermal test period (TSSR, temperature scanning stress relaxation
test) . 7
10 Evaluation of the data and expression of results . 8
10.1 Method A . 8
10.2 Method B . 9
11 Precision .13
12 Test report .13
12.1 Test report Method A . 13
12.2 Test report Method B . 14
Annex A (normative) Calibration schedule . .15
Bibliography .17
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ISO 12493:2023(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 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 45, Rubber and rubber products,
Subcommittee SC 2, Testing and analysis.
This third edition cancels and replaces the second edition (ISO 12493:2017), which has been technically
revised.
The main changes are as follows:
— another method (method B) for measuring tensile stress under non-isothermal conditions has been
added. In this method, the change of stress is measured at a given strain and under variation of
temperature at a given heating rate as a function of temperature.
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.
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ISO 12493:2023(E)
Introduction
Usually, stress relaxation tests (see ISO 6914) are performed at constant temperature conditions
because temperature has a strong impact on the relaxation time constants of rubber materials.
Depending on the respective relaxation time constant or, more realistic, relaxation time spectrum,
those measurements are more or less time consuming and can require testing times of several days,
weeks or even months. Accelerated stress relaxation tests can be performed under non-isothermal
conditions, if the temperature is increased at a certain constant heating rate, because relaxation
processes are thermally activated and therefore occur faster at higher temperatures. Thus, the entire
stress relaxation behaviour of the material can be scanned within a short period of time, typically a few
hours. This method is designated as the temperature scanning stress relaxation (TSSR) test method and
offers the ability to characterize rubber ─ vulcanized or thermoplastic ─ by short-term measurements.
TSSR tests cannot replace conventional isothermal stress relaxation measurements, but are considered
as a comparative test method, e.g. for the purpose of material pre-selection or in order to determine the
state of crosslink density ─ which is an important reason for ageing phenomena ─ of a sample.
During non-isothermal testing, the material undergoes not only stress relaxation, but additional
phenomena occur which need to be considered by adequate corrections. Most important in this case
is an increase of retractive forces due to the Gough-Joule effect and thermal expansion of the sample.
Whereas the latter can be numerically compensated by considering a proper value of the coefficient
of thermal expansion (CTE), the increase of retractive forces offer the opportunity to calculate the
crosslink density of the material, based on fundamental theory of rubber elasticity. Furthermore,
an enhanced sensitivity of the test, with respect to specific relaxation processes is achieved by
determination of the first derivative of the measured stress–temperature curve. Similar to stress
relaxation measurements in the time domain, the latter can be used to calculate a corresponding
relaxation temperature spectrum, instead of a relaxation time spectrum.
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INTERNATIONAL STANDARD ISO 12493:2023(E)
Rubber, vulcanized or thermoplastic — Determination of
stress in tension under non-isothermal conditions
WARNING 1 — Users of this document should be familiar with normal laboratory practice. This
document does not purport to address all of the safety problems, if any, associated with its use. It
is the responsibility of users to establish appropriate safety and health practices and determine
the applicability of any other restrictions.
WARNING 2 — Certain procedures specified in this document can involve the use or generation
of substances, or the generation of waste, that could constitute a local environmental hazard.
Reference should be made to appropriate documentation on safe handling and disposal after
use.
1 Scope
This document describes two methods for measuring stress in tension under non-isothermal conditions.
— Method A: The thermal stress is measured for various pre-strain and temperature conditions as a
function of time.
— Method B: The change of stress is measured in a test piece at a given strain and under variation of
temperature at a given heating rate as a function of temperature. In this way, the determination of
the thermal-mechanical behaviour of a rubber can be accelerated, e.g. for the purpose of comparative
testing of aging or estimating the upper limit of the operating temperature.
The measurement device, which is equipped with a suitable heating chamber, is used to record the
stress as a function of time or temperature until the sample breaks or the stress has approached zero or
for a certain time.
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 188:2011, Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests
ISO 5893, Rubber and plastics test equipment — Tensile, flexural and compression types (constant rate of
traverse) — Specification
ISO 18899:2013, Rubber — Guide to the calibration of test equipment
ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test
methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
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ISO 12493:2023(E)
3.1
thermal stress
σ
T
force per initial unit area that is developed in the test piece upon heating
2
Note 1 to entry: It is expressed in N/m or Pa.
3.2
maximum thermal stress
σ
T,max
peak value of the thermal stress recorded during the test
3.3
thermal stress after a specified time
σ
T,t
stress induced in the test piece upon heating for a specified time t
3.4
pre-strain
strain to which the test piece is subjected at the beginning of the test
Note 1 to entry: It is expressed as:
ll−
fi
*100
l
i
where
l is the initial length;
i
l is the length after strain.
f
3.5
pre-stress
force per initial unit area that results from the pre-strain
2
Note 1 to entry: It is expressed in N/m or Pa.
4 Principle
A test piece is held at a constant pre-strain in a tensile mode at a constant temperature. When the pre-
stress resulting from the given pre-strain has reached an apparent equilibrium value, the temperature
of the test piece is increased. In method A, the test temperature is reached as quickly as possible,
in method B, heating is carried out at a constant rate. The thermal stress developed at the elevated
temperature is measured.
5 Apparatus
5.1 Test machine
5.1.1 General
The setup of the measuring devices can be done in different ways. Two possibilities are described below.
5.1.2 Thermal-stress testing machine
An example of a test machine for measuring the thermal stress developed in rubbery materials when
heated is shown in Figure 1. Two clamps hold the test piece in a temperature-controlled chamber, with
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ISO 12493:2023(E)
the upper clamp connected to a load cell and the bottom clamp connected to a crosshead. The crosshead
is moved using a screw driven by a motor to impose a pre-strain on the test piece. The thermal stress
developed when the temperature is raised is transmitted to the load cell and the output is recorded to
give the variation in stress as a function of time.
The test machine shall be in accordance with ISO 5893 with force measurement to class 1 and the
machine shall be capable of setting the pre-strain to within ±0,1 at a speed of (20 ± 2,5) mm/min.
Key
1 temperature-controlled chamber
2 clamps
3 load cell
4 rod
5 linear variable differential transformer
6 screw
7 crosshead
8 motor
Figure 1 — Example of thermal-stress testing machine
The temperature-controlled chamber shall be capable of raising the temperature at a rate of at least
30 K/min and maintaining the test piece at the required temperature as specified in ISO 23529. A
suitable volume for the chamber is 3 l to 5 l. A temperature-sensing device shall be located within the
chamber near the test piece.
Load cell and crosshead shall be in accordance with ISO 18899:2013.
5.1.3 Stress relaxometer
Another example of a test machine consists of an electrically driven linear actuator equipped with two
grips, which hold the test piece without slipping at a fixed extended length (to within ±1 %) together
with a means of measuring and recording the force on the test piece.
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ISO 12493:2023(E)
The grips shall be arranged such that the test piece can be positioned in an oven. The force-measuring
system may be, for example, a calibrated spring or electronic load cell, but it shall be accurate and stable
to within ±1 % of the force reading throughout the duration of the test.
The force measuring system and strain measuring system shall be in accordance with the requirements
specified for ISO 18899:2013.
5.2 Oven
The oven shall be in accordance with ISO 188:2011, cell-type oven, for testing of a single test piece. For
the heating chamber, other shapes than cylindrical are acceptable.
5.3 Thickness- and width-measuring devices
Instruments for measuring the thickness and width of the test piece shall be in accordance with
ISO 23529.
6 Calibration
The test apparatus shall be calibrated in accordance with the schedule given in Annex A.
7 Test piece
7.1 Dimensions
7.1.1 Method A:
The test piece shall be prepared by cutting from moulded flat sheet and shall have the shape and
dimensions shown in Figure 2. In addition, the thickness shall be (2 ± 0,2) mm. The test piece shall have
a smooth surface and be free from irregularities.
Dimensions in millimetres
Figure 2 — Test piece for thermal-stress measurements
7.1.2 Method B:
Wherever possible, the test specimens shall be dumb-bell-shaped, e.g. types 2 or 4 in accordance
with ISO 37:2017. The test piece shall be prepared by cutting from moulded flat sheet. In addition,
the thickness shall be (2 ± 0,2) mm. The test piece shall have a smooth surface and be free from
irregularities. For certain applications, both smaller and larger samples can be used.
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