ISO 204:2018

INTERNATIONAL ISO
STANDARD 204
Third edition
2018-08
Metallic materials — Uniaxial creep
testing in tension — Method of test
Matériaux métalliques — Essai de fluage uniaxial en traction —
Méthode d'essai
Reference number
©
ISO 2018
© ISO 2018
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Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and designations . 6
5 Principle . 8
6 Apparatus . 8
7 Test pieces .12
7.1 Shape and dimensions .12
7.1.1 Shape and dimension of smooth test pieces .12
7.1.2 Shape and dimension of notched test pieces .13
7.2 Preparation .13
7.3 Determination of the original cross-sectional area .14
7.4 Marking of the original gauge length, L .
o 14
7.5 Determination of the reference length, L .
r 14
8 Test procedure .15
8.1 Heating of the test piece.15
8.2 Application of the test force .15
8.3 Test interruptions .16
8.3.1 Planned interruptions of the test .16
8.3.2 Multiple test piece machine with several test pieces in line .16
8.3.3 Combined test.16
8.3.4 Accidental interruption of the test .16
8.4 Recording of temperature and elongation or extension .16
8.4.1 Temperature .16
8.4.2 Elongation and extension .16
8.4.3 Elongation-time diagram or extension-time diagram .17
9 Determination of results .17
10 Test validity .17
11 Accuracy of the results .17
11.1 Expression of the results .17
11.2 Final uncertainty .18
12 Test report .18
Annex A (informative) Information concerning drift of thermocouples .23
Annex B (informative) Information concerning methods of calibration of thermocouples .26
Annex C (normative) Creep testing using test pieces with V or blunt circumferential notches .27
Annex D (informative) Method of estimating the uncertainty of the measurement in
accordance with the Guide to the expression of uncertainty in measurement (GUM) .31
Annex E (informative) Representation of results and extrapolation.38
Annex F (informative) Computer compatible representation of standards .48
Bibliography .49
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
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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.
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expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 1, Uniaxial testing.
This third edition cancels and replaces the second edition (ISO 204:2009), which has been technically
revised. The main changes compared to the previous edition are as follows:
— Some of the symbols have been changed to achieve harmonization with the ISO 6892 series.
— For the purpose of this document, the terms “fracture” and “rupture” are interchangeable.
— The term “indicated temperature”, T , has been replaced by “corrected measured temperature”,
i
T , with errors from all sources being taken into account and any systematic errors having been
c
corrected. The terms “elongation” and “extension” have been clarified and aligned with the terms
used in the ISO 6892 series. Elongation refers to the test piece deformation measured manually
either during deliberate test interruptions or after fracture, whilst extension is determined by
continuous measurement using an extensometer.
— Some information relating to the calibration of thermocouples has been transferred from an
informative annex into the main body of the document.
— Some changes have been made to Table 1 and formulae have been amended using reference length, L .
r
— Equation E.1 (now Formula C.1) has been corrected.
— A new informative annex relating to computer compatible representation of standards has been added.
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 2018 – All rights reserved

Introduction
Creep is the phenomenon exhibited by materials which slowly deform when subjected to loading at
elevated temperature. This document is concerned with the method used to measure such material
behaviour.
Annexes are included concerning temperature measurement using thermocouples and their
calibration, creep testing test pieces with circumferential V and blunt (Bridgman) notches, estimation
of measurement uncertainty, methods of extrapolation of creep rupture life and information about
computer compatible representation of standards.
NOTE 1 Information is still sought relating to the influence of off-axis loading or bending on the creep
properties of various materials. Based on the future availability of quantitative data, consideration might be given
as to whether the maximum amount of bending should be specified and an appropriate calibration procedure be
[43]
recommended. The decision will need to be based on the availability of quantitative data .
NOTE 2 Information concerning the benefit of standards being produced in a computer compatible format is
given in Annex F.
This document incorporates many recommendations developed through the European Creep
Collaborative Committee (ECCC).
NOTE 3 Several different gauge lengths and reference lengths are specified in this document. These lengths
reflect custom and practice used in different laboratories throughout the world. In some cases, the lengths are
physically marked on the test piece as lines or ridges; in other cases, the length can be a virtual length based
upon calculations to determine an appropriate length to be used for the determination of creep elongation. For
some test pieces, L , L and L are the same length (see 3.1, 3.2 and 3.3). “Extension” is used for uninterrupted
r o e
creep test with continuous measurement of the increase of the length of the test piece by using an extensometer.
“Elongation” is mainly used for interrupted creep test with the manual measurement of the increase of the length
of the test piece.
NOTE 4 For many applications, the term “strain” is synonymous with extension.
INTERNATIONAL STANDARD ISO 204:2018(E)
Metallic materials — Uniaxial creep testing in tension —
Method of test
1 Scope
This document specifies the methods for
a) uninterrupted creep tests with continuous monitoring of extension,
b) interrupted creep tests with periodic measurement of elongation,
c) stress rupture tests where normally only the time to fracture is measured,
d) a test to verify that a predetermined time can be exceeded under a given force, with the elongation
or extension not necessarily being reported.
NOTE A creep test can be continued until fracture has occurred or it can be stopped before fracture.
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 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature
ISO 6892-2, Metallic materials — Tensile testing — Part 2: Method of test at elevated temperature
ISO 7500-2, Metallic materials — Verification of static uniaxial testing machines — Part 2: Tension creep
testing machines — Verification of the applied force
ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing
3 Terms and definitions
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:
— IEC Electropedia: available at http: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
3.1
reference length
L
r
base length used for the calculation of either percentage elongation or percentage extension
Note 1 to entry: A method to calculate this value is given in 7.5.
3.2
original gauge length
L
o
length between gauge length marks on the test piece measured at ambient temperature before the test
Note 1 to entry: In general, L ≥ 5D.
o
3.3
extensometer gauge length
L
e
distance between the measuring points of the extensometer
3.4
parallel length
L
c
length of the parallel reduced section of the test piece
3.5
final gauge length after fracture
L
u
length between gauge length marks on the test piece measured after fracture, at ambient temperature,
with the pieces carefully fitted back together with their axes in a straight line
3.6
original cross-sectional area
S
o
cross-sectional area of the parallel length as determined at ambient temperature prior to testing
3.7
minimum cross-sectional area after fracture
S
u
minimum cross-sectional area of the parallel length as determined at ambient temperature after
fracture, with the pieces carefully fitted back together with their axes in a straight line
3.8
initial stress
R
o
applied force divided by the original cross-section area, S , of the test piece
o
3.9
extension
ΔL
e
increase of extensometer gauge length, L , at time t and at test temperature
e
Note 1 to entry: For further information, see 6.2.
3.10
elongation
ΔL
o
increase of original gauge length, L , at time t
o
Note 1 to entry: For further information, see 6.2.
3.11
percentage extension
e
extension at test temperature expressed as a percentage of the reference length, L , as given in Formula (1)
r
ΔL
e
e =×100 (1)
L
r
Note 1 to entry: See Figure 1.
2 © ISO 2018 – All rights reserved

3.12
percentage elongation
A
elongation expressed as a percentage of the reference length, L , as given in Formula (2)
r
ΔL
o
A =×100 (2)
L
r
3.13
percentage elastic extension
e
e
extension at test temperature expressed as a percentage of the reference length, L , which is proportional
r
to the initial stress, R
o
Note 1 to entry: This value can be calculated from the stress/percentage extension values during loading. See 8.4.2.
Note 2 to entry: See Figure 1.
3.14
percentage initial total extension
e
ti
extension at test temperature expressed as a percentage of the reference length, L , at end of loading
r
with the initial stress, R
o
Note 1 to entry: See Figure 1.
3.15
percentage initial plastic extension
e
i
extension at end of loading and at test temperature with the initial stress, R , expressed as a percentage
o
of the reference length, L , and determined as the difference between the percentage initial total
r
extension, e , and the percentage elastic extension, e , as given in Formula (3)
ti e
e = e − e (3)
i ti e
Note 1 to entry: See Figure 1.
Note 2 to entry: This value represents the plastic extension during the loading phase.
3.16
percentage total extension
e
t
extension at the test force at time t and at test temperature, expressed as a percentage of the reference
length, L
r
Note 1 to entry: See Figure 1.
3.17
percentage plastic extension
e
p
extension at time t and at test temperature determined as the difference between the percentage total
extension, e , and the percentage elastic extension, e , expressed as a percentage of the reference length,
t e
L , as given in Formula (4)
r
e = e − e (4)
p t e
Note 1 to entry: See Figure 1.
3.18
percentage total ultimate extension
e
u
total extension at rupture and at test temperature, expressed as a percentage of the reference length, L
r
3.19
percentage creep extension
e
f
extension at loading determined and at test temperature as the difference between the percentage
plastic extension, e , and the percentage initial plastic extension, e , expressed as a percentage of the
p i
reference length, L , as given in Formula (5)
r
e = e − e (5)
f p i
Note 1 to entry: See Figure 1.
Note 2 to entry: Suffix f originates from “fluage”, “creep” in French.
3.20
percentage anelastic extension
e
k
negative extension at end of unloading at test temperature, expressed as a percentage of the reference
length, L
r
Note 1 to entry: See Figure 1 and 8.4.
3.21
percentage permanent extension
e
per
extension at end of unloading and at test temperature determined as the difference between the
percentage total extension, e , and the sum of percentage elastic extension, e , plus the percentage
t e
anelastic extension, e , expressed as a percentage of the reference length, L , as given in Formula (6)
k r
e = e – (e + e) (6)
per t e k
Note 1 to entry: In the case of e ≈ 0, the following relationship may be used: e ≈ e .
k per p
Note 2 to entry: See Figure 1.
3.22
percentage permanent elongation
A
per
elongation expressed as a percentage of the reference length, L , at end of unloading and at room
r
temperature
3.23
percentage elongation after creep fracture
A
u
permanent elongation after fracture, L − L , expressed as a percentage of the reference length, L , as
u o r
given in Formula (7)
LL−
uo
A = ×100 (7)
u
L
r
Note 1 to entry: A may have the specified temperature, T, in degrees Celsius as superscript and the initial stress,
u
R , in megapascals as subscript; see the example in Table 1.
o
4 © ISO 2018 – All rights reserved

3.24
percentage reduction of area after creep fracture
Z
u
maximum change in cross-sectional area measured after fracture, S − S , expressed as a percentage of
o u
the original cross-sectional area, S , as given in Formula (8)
o
SS−
ou
Z = ×100 (8)
u
S
o
Note 1 to entry: Z may have the specified temperature, T, in degrees Celsius as superscript and the initial stress,
u
R , in megapascals as subscript; see the example in Table 1.
o
3.25
creep extension time
t
fx
time required for a strained test piece to obtain a specified percentage creep extension, x, at the
specified temperature, T, and the initial stress, R
o
EXAMPLE t
f0,2
3.26
plastic extension time
t
px
time required to obtain a specified percentage plastic extension, x, at the specified temperature, T, and
the initial stress, R
o
Note 1 to entry: An example for t is given in Figure E.2 a) (t = 100 000 h corresponds to e = 1 % at
p1 p1 p
R = 120 MPa).
o
3.27
creep rupture time
t
u
time to rupture for a test piece maintained at the specified temperature, T, and the initial stress, R
o
Note 1 to entry: The symbol t can have as superscript the specified temperature, T, in degrees Celsius and as
u
subscript the initial stress, R , in megapascals; see the example in Table 1.
o
3.28
single test piece machine
testing machine that permits straining of a single test piece
3.29
multiple test piece machine
testing machine that permits straining of more than one test piece simultaneously at the same
temperature
4 Symbols and designations
The symbols and corresponding designations are given in Table 1.
Table 1 — Symbols and designations
a
Symbol Unit Designation
D mm Diameter of gauge length of a cylindrical test piece
D mm Diameter of gauge length containing a notch
n
d mm Diameter of gauge length without a notch in a combined notched/unnotched
test piece (see Figure C.1)
d mm Diameter across root of circumferential notch
n
For a combined notched/unnotched test piece d = d
n
b mm Width of the cross-section of the parallel length of a test piece of square or rec-
tangular cross-section
L mm Reference length
r
a mm Thickness of a test piece of square or rectangular cross-section [see Figure 2 b)]
ΔL Increase of extensometer gauge length at time t
et
ΔL mm Increase of original gauge length at time t
ot
L mm Original gauge length
o
L mm Parallel gauge length containing a notch
n
L mm Final gauge length after fracture
u
L mm Parallel length
c
L mm Extensometer gauge length
e
r mm Transition radius
t
r mm Notch root radius
n
S mm Original cross-sectional area of the parallel length
o
S mm Minimum cross-sectional area after fracture
u
R MPa Initial stress
o
e % Percentage extension
e % Percentage elastic extension
e
e % Percentage creep extension:
f
ΔL
ot
e =×100
f
L
r
NOTE  As an example, the symbol can be completed as follows:
e : percentage creep extension with an initial stress of 50 MPa after
f550/ 000
5 000 h at the specified temperature of 375 °C.
e % Percentage creep extension at creep rupture time
fu
e % Percentage initial plastic extension
i
e % Percentage anelastic extension
k
a
The main subscripts (r, o and u) of the symbols are used as follows:
r  corresponds to reference;
o  corresponds to original;
u  corresponds to ultimate (after rupture).
NOTE  For the purposes of creep testing in this document, the terms “fracture” and “rupture” are interchangeable and are
used to describe when a test piece breaks.
6 © ISO 2018 – All rights reserved

Table 1 (continued)
a
Symbol Unit Designation
e % Percentage plastic extension
p
e % Percentage permanent extension
per
e % Percentage plastic extension at creep rupture time
pu
e % Percentage total extension
t
e % Percentage total extension at creep rupture time
u
A % Percentage permanent elongation
per
NOTE  As an example, the symbol can be completed as follows:
A : percentage permanent elongation with an initial stress of 50 MPa
per550/ 000
after 5 000 h at the specified temperature of 375 °C.
A % Percentage elongation after creep fracture:
u
LL−
uo
A = ×100
u
L
r
NOTE  As an example, the symbol can be completed as follows:
A : percentage elongation after creep fracture with an initial stress of
u 50
50 MPa at the specified temperature of 375 °C.
Z % Percentage reduction of area after creep fracture:
u
SS−
ou
Z = ×100
u
S
o
NOTE  As an example, the symbol can be completed as follows:
Z : percentage reduction of area after creep fracture with an initial stress of
u 50
50 MPa at the specified temperature of 375 °C.
t h Elapsed time from end of loading
t h Creep extension time
fx
t h Plastic extension time
px
t h Creep rupture time
u
NOTE  As an example, the symbol can be completed as follows:
t : creep rupture time with an initial stress of 50 MPa at the specified
u 50
temperature of 375 °C.
t h Creep rupture time of a notched test piece
un
T °C Specified temperature
T °C Corrected measured temperature
c
x % Specified percentage creep or plastic extension
n Norton creep exponent
a
The main subscripts (r, o and u) of the symbols are used as follows:
r  corresponds to reference;
o  corresponds to original;
u  corresponds to ultimate (after rupture).
NOTE  For the purposes of creep testing in this document, the terms “fracture” and “rupture” are interchangeable and are
used to describe when a test piece breaks.
5 Principle
The test consists of heating a test piece to the specified temperature and of straining it by means of
a constant tensile force or constant tensile stress (see Note) applied along its longitudinal axis for a
period of time to obtain any of the following:
— a specified creep extension (uninterrupted test) with continuous extension measurement;
— values of permanent elongation at suitable intervals throughout the test (interrupted test);
— the creep rupture time (uninterrupted and interrupted test).
NOTE “Constant stress” or ”true stress” means that the ratio of the force to the instantaneous cross-section
remains constant throughout the test. The results obtained with constant stress are generally different from
[47]
those obtained with constant force .
6 Apparatus
6.1 Testing machine.
The testing machine shall apply a force along the axis of the test piece while keeping inadvertent bending
or torsion of the test piece to a minimum. Prior to test, the machine should be visually examined to
ensure that loading bars, grips, universal joints and associated equipment are in a good state of repair.
The force shall be applied to the test piece without shock.
The machine should be isolated from external vibration and shock. The machine should be equipped
with a device which minimizes shock when the test piece fractures.
NOTE At present, there appears to be insufficient quantitative data in the literature demonstrating the
influence of bending upon creep and stress rupture life. Any organization with such information is encouraged to
[43]
forward it to ISO/TC 164 .
The machine shall be verified and shall meet the requirements of at least class 1 in ISO 7500-2.
6.2 Extension and elongation measuring devices.
6.2.1 Extension measuring device.
In uninterrupted tests, the extension shall be measured using an extensometer which meets the
performance requirements of class 1 or better of ISO 9513 or by other means which ensure the same
accuracy without interruption of the test. The extensometer can either be directly attached to the test
piece or be non-contacting (e.g. a non-contacting optical or laser extensometer).
It is recommended that the extensometer be calibrated over an appropriate range based upon the
expected creep strain.
The extensometer shall be calibrated at intervals not exceeding 3 years, unless the test duration
is longer than 3 years. If the predicted test duration exceeds the date of the expiry of the calibration
certificate then the extensometer shall be recalibrated prior to commencement of the creep test.
The extensometer gauge length shall not be less than 10 mm.
The extensometer shall be capable of measuring extension of one side of the test piece or, preferably, on
opposite sides of the test piece.
The type of extensometer used (e.g. single-sided, double-sided, axial, diametral) should be reported.
When the extension is measured on the opposite sides, the average extension should be reported.
8 © ISO 2018 – All rights reserved

When the extension is measured with an extensometer attached to the grip ends of the test piece, the
ends shall be of such shape and size that it can be assumed that the observed extension has occurred
completely within the reference length of the test piece. Percentage creep extension is measured over L .
r
The extensometer gauge length should normally be as near as possible to the reference length. In the
case of accurate creep measurements, a gauge length as long as possible should be used to improve the
accuracy of measurements.
Care should be taken to avoid spurious negative creep when using nickel base alloy extensometers. See
[42]
the Code of Practice .
For low creep strain measurements, e.g. ≤1 % strain, on test pieces with short gauge lengths, careful
consideration should be given to ensure that the measuring device used has sufficient resolution and
accuracy over the range of use.
NOTE 1 Information on the long-term stability of transducers used for creep testing and accreditation issues
is given in References [40] and [41].
NOTE 2 If only the percentage elongation after creep fracture or the percentage creep elongation for a
specified test duration is determined, the use of an extensometer is not necessary.
6.2.2 Elongation measuring device.
In interrupted tests, periodically unload the test piece, cool it to ambient temperature and measure the
permanent elongation on the gauge length with an appropriate device. The precision of this device shall
be 0,01 ΔL or 0,01 mm, whichever is the greater. After this measurement, the test piece may be first
r
reheated and then reloaded.
6.3 Heating device, temperature measuring equipment and calibration.
6.3.1 Permissible temperature deviations.
The heating device shall heat the test piece to the specified temperature, T. The permitted deviations
between the corrected measured temperature, T , and the specified temperature, T, and the permitted
c
maximum temperature variation along the test piece shall be as given in Table 2.
Table 2 — Permitted deviations between T and T and maximum permissible temperature
c
variation along the test piece
Specified temperature Permitted deviation between Maximum permissible temperature
T and T variation along the test piece
c
T
°C °C
°C
T ≤ 600 ±3 3
600 < T ≤ 800 ±4 4
800 < T ≤ 1 000 ±5 5
1 000 < T ≤ 1 100 ±6 6
For specified temperatures greater than 1 100 °C, the permitted values, including drift, shall be defined
by agreement between the parties concerned.
The corrected measured temperatures, T , are the temperatures measured at the surface of the parallel
c
length of the test piece, errors from all sources, including drift (see Annex A), being taken into account
and any systematic errors having been corrected.
It is permitted to carry out indirect measurement of the temperature of each heating zone of the furnace
provided that it is demonstrated that the tolerance defined above is fulfilled on the test piece instead of
measuring the temperature at the surface of each individual test piece.
If an extensometer is used, the parts of this instrument outside the furnace shall be designed and
protected in such a way that the temperature variations in the air around the furnace do not significantly
affect the measurements of the variations in length.
Variations in temperature of the air surrounding the test machine should not exceed ±3 °C.
In the interrupted test, the variation of the room temperature during all measurements of the gauge
length should not exceed ±2 °C. If this range is exceeded, corrections for ambient temperature variations
shall be applied.
6.3.2 Temperature measurement.
6.3.2.1 General.
The temperature indicator shall have a resolution of at least 0,5 °C. The temperature measuring
equipment shall have an accuracy equal to or better than ±1 °C.
For thermocouples, in the absence of measuring instruments with cold junction compensation, cold
junction temperatures, normally at 0°C, shall be measured to within 0,5 °C.
Many laboratories maintain the cold junction above ambient temperature. Whatever its temperature,
it shall remain stable and appropriate compensation shall be applied to determine the temperature
measured by the thermocouple.
NOTE Information concerning drift of thermocouples is given in Annex A and methods of calibration of
thermocouples are given in Annex B.
For indirect methods of temperature measurement e.g. pyrometry, thermal cameras or resistivity
techniques, it shall be demonstrated that traceability is provided to the SI System of temperature
measurement and that the above criteria for accuracy and resolution can be achieved.
6.3.2.2 Calibration of the temperature measuring equipment.
The calibration of the temperature measuring equipment (including the cable, the connection, the cold
junction, the indicator or the recorder, the data line, etc.) shall be carried out by a method traceable to
the international unit (SI) of temperature.
If practicable, this calibration should be carried out annually over the range of temperatures measured
by the equipment and the readings shall be given in the calibration report.
6.3.2.3 Single test piece machines.
In single test piece machines, with thermocouples used for temperature measurement, at least two
thermocouples should be used for test pieces with a parallel length less than or equal to 50 mm. For test
pieces with a parallel length greater than 50 mm, at least three thermocouples should be used. In all
cases, a thermocouple should be placed at each end of the parallel length and, if a third thermocouple is
used, it should be placed in the middle region of the parallel length.
The number of thermocouples may be reduced to one if it can be demonstrated that the conditions
of the furnace and the test piece are such that the variation of temperature of the test piece does not
exceed the values specified in 6.3.1.
6.3.2.4 Multiple test piece machines.
In multiple test piece machines, with thermocouples used for temperature measurement, at least one
thermocouple should be used for each test piece. If only one thermocouple is used, it shall be positioned
at the middle of the parallel length. Three thermocouples may only be used if located at appropriate
positions within the furnace, and if there is supporting data to demonstrate that for all test pieces the
temperature conforms to the requirements of 6.3.1.
10 © ISO 2018 – All rights reserved

In the case of indirect temperature measurement, regular control measurements are required to
determine differences between the thermocouple(s) of each heating zone and a significant number of
test pieces within a given zone. The non-systematic components of the temperature differences shall
not exceed ±2 °C up to 800 °C and ±3 °C above 800 °C.
6.3.2.5 Notched test pieces.
Temperature measurement of notched test pieces shall be in accordance with either 6.3.2.3 or 6.3.2.4. It
is recommended that one thermocouple be placed close to the notch.
NOTE Details about testing notched test pieces are given in Annex C.
6.3.3 Thermocouples.
The thermocouple junctions shall make good thermal contact with the surface of the test piece and
shall be screened from direct radiation from the heating source. The remaining portions of the wires
within the furnace shall be thermally shielded and electrically insulated.
Precautions shall be taken to minimize contamination and physical damage of thermocouples.
Insulators and/or insulation shall be maintained in a clean state to also minimise contamination and
prevent conduction.
[7]
Information concerning different types of thermocouples is given in IEC 60584-1 .
The use of rare metal thermocouples, preferentially of type S or R, is recommended for temperatures
[44]
equal to or greater than 400 °C .
Base metal thermocouples of type K should only be used either for temperatures lower than 400 °C
or for times less than 1 000 h at higher temperatures and should not be re-used without cutting back
exposed wire and re-calibrating.
Base metal thermocouples of type N may be used either for temperatures lower than 600 °C or for
times less than 3 000 h at higher temperatures and should not be re-used. The use of type N base metal
[44]
thermocouples is not permitted at temperatures above 760 °C .
Where the drift exceeds the following values within the calibration period, either more frequent
calibrations should be carried out or a correction be applied to the temperature indicated by the
thermocouple. See Annex A and References [48] to [52].
— ±1 °C for T ≤ 600 °C;
— ±1,5 °C for 600 °C < T ≤ 800 °C;
— ±2 °C for 800 °C < T ≤ 1 100 °C.
Records of drift shall be recorded and available on request.
NOTE 1 Thermocouple drift is dependent on the type of thermocouple used and the exposure duration at
temperature; see Annex A.
[8]
NOTE 2 Reference can be made to ASTM E633 .
NOTE 3 This clause is not applicable in the case of indirect temperature measurement.
NOTE 4 Other types of thermocouples are available but their suitability will be demonstrated.
6.3.4 Calibration of the thermocouples.
NOTE 1 Further information relating to thermocouples calibration is given in Annex B.
The calibration period for rare metal thermocouples in repeated use for short duration tests (typically
500 h or less) shall not exceed 13 months. Otherwise the period for calibration shall be as follows:
— 4 years for T ≤ 600 °C;
— 2 years for 600 °C < T ≤ 800 °C;
— 1 year for T > 800 °C.
The re-use of base metal thermocouples is not permitted without re-calibration after each test unless
experience has shown that errors due to drift do not exceed the limits in 6.3.3 for the test conditions in
question, in which case they may be calibrated at intervals not exceeding 500 h use.
If the test duration exceeds the above calibration periods, the thermocouple shall be calibrated upon
completion of the test.
It shall be demonstrated that the error of the thermocouple used has been established at the test
temperature or is typical for a range containing the test temperature. Thermocouples showing errors
in excess of 1 °C may be used provided the appropriate corrections are made.
If a thermocouple is rewelded, the thermocouple shall be recalibrated before use.
NOTE 2 Base metal thermocouples can be cut back removing any exposed wire and will be re-calibrated
before use.
7 Test pieces
7.1 Shape and dimensions
7.1.1 Shape and dimension of smooth test pieces
Normally test pieces with circular cross-sections shall be used; see Figure 2.
In general, the smooth (unnotched) test piece is a machined proportional cylindrical test piece
(L = k√S ) with a circular cross-section (see Figure 2). The value k should be equal to or greater than
r o
5,65. The value used shall be recorded in the test report, i.e. L ≥ 5D.
r
In special cases, the cross-section of the test piece may be square, rectangular or of some other shape.
For these specific test pieces, the provisions specified for cylindrical test pieces with a circular cross
section shall be applied.
In general, L should not exceed L by more than 10 % for circular test pieces, or by more than 15 % for
r c
square or rectangular test pieces.
The parallel length shall be joined by transition curves to the gripped ends, which may be of any shape
to suit the grips of the testing machine. The transition radius (R) should be between 0,25D and 1D for
the cylindrical test pieces, or 0,25b and 1b in the case of rectangular or square test pieces.
Unless the sample size does not permit it, the original cross-sectional area (S ) shall be greater than or
o
equal to 7 mm .
NOTE In some cases, especially for brittle materials, the transition radius can be greater than 1D.
When a test piece having extensometer attachment ridges (collars) in the parallel length is used, the
transition radius of the collars may be less than 0,25d; this should be selected to minimize stress
concentrations and there should be no evidence of undercut when inspected. For test pieces with
collars, the diameter between the collar and the grip end may be up to 10 % larger than the diameter of
the original gauge length; this should ensure that fracture will occur within the gauge length.
12 © ISO 2018 – All rights reserved

The grip ends of test pieces shall have the same axis as the parallel length with a coaxiality tolerance of
— 0,005D or 0,03 mm, whichever is greater, for cylindrical test pieces, and
— 0,005b or 0,03 mm, whichever is greater, for rectangular or square test pieces.
When oxidation is a significant factor, test pieces with a larger original cross-sectional area (S ) should
o
be used.
The original reference length shall be determined to a measurement uncertainty of ±1 %. The final
reference length should be determined to a measurement uncertainty of ±1 %.
7.1.2 Shape and dimension of notched test pieces
When a notched test piece is used, the geometry and the position of this notch should be defined by
agreement and in accordance with Annex C.
7.2 Preparation
The test piece shall be machined in such a way as to minimize any residual deformation or surface
defects.
The shape tolerances shall conform to Table 3 for test pieces with circular cross-sections and to Table 4
for test pieces with square or rectangular cross-sections.
Table 3 — Shape tolerances of test pieces with circular cross-sections
Dimensions in millimetres
a
Nominal dimension Shape tolerances
D
3 < D ≤ 6 0,02
6 < D ≤ 10 0,03
10 < D ≤ 18 0,04
18 < D ≤ 30 0,05
a
Maximum deviation between the measurements of a transverse dimension
[3]
determined along the entire parallel length of the test piece .
Table 4 — Shape tolerances of test pieces with square or rectangular cross-sections
Dimensions in millimetres
a
Nominal dimension Shape tolerances
b
3 < b ≤ 6 0,02
6 < b ≤ 10 0,03
10 < b ≤ 18 0,04
18 < b ≤ 30
...