ISO/TS 21913:2022
(Main)Temperature verification method applied to dynamic fatigue testing
Temperature verification method applied to dynamic fatigue testing
This document establishes verification procedures to determine the accuracy, speed of response, and stability of temperature measurement for materials testing equipment. These procedures are specified for the expected use in fatigue tests on metals where these characteristics are important to the fidelity of tests at high or varying temperature. The principles set out include sufficient provision for both contacting and non-contacting methods of temperature measurement. This document is for the end-to-end verification of registered value compared with “true” specimen temperature at the point of measurement. It cannot be used to specify the correct method or location of measurement. NOTE: The methodologies could be found applicable to test types beyond mechanical fatigue of metals, but that is outside the remit of this document.
Méthode de vérification de la température appliquée aux essais de fatigue dynamique
General Information
Standards Content (Sample)
TECHNICAL ISO/TS
SPECIFICATION 21913
First edition
2022-06
Temperature verification method
applied to dynamic fatigue testing
Méthode de vérification de la température appliquée aux essais de
fatigue dynamique
Reference number
ISO/TS 21913:2022(E)
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ISO/TS 21913:2022(E)
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ISO/TS 21913:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement behaviour . 3
5 Representative verification equipment . 4
5.1 Reference measurements . . 4
5.2 Direct physically connected thermocouples (welded to specimen) . 5
5.3 Contacting thermocouples and other contacting devices not welded to specimen . 5
5.4 Non-contacting thermo-optical devices . 6
5.4.1 Constant emissivity coatings . 6
5.4.2 Native surfaces . 6
6 Basic isothermal verification methods . 6
6.1 Equipment set up . 6
6.2 Measurement of system resolution . 7
6.3 Summation of component bias errors . 7
6.4 Single point measurement of system bias error . 7
6.5 Multiple point assessment of resolution and bias error . 7
7 Evaluation of long term measurement drift . 8
7.1 Single point method (Post-test measurement) . 8
7.2 Time profile method (System assessment) . 8
8 Verification of dynamic temperature measurement . 8
8.1 Time lag . 8
8.1.1 Continuous ramp method . 8
8.1.2 Turning point method . 9
8.2 Stabilisation time . 9
9 Reporting . 9
Annex A (informative) Specific considerations for specimen temperature measurement
during fatigue tests .10
Bibliography .13
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ISO/TS 21913:2022(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
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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).
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URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical Testing of Metals,
Subcommittee SC 05, Fatigue, fracture and toughness testing.
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/TS 21913:2022(E)
Introduction
It is the aim of this document to provide methodologies to verify the error in indicated measurement
relative to the actual temperature of the specimen test piece. Therefore, it is essential to account for all
factors, inclusive of environmental effects; not limiting the assessment to, for example, the performance
of a recording system and the thermoelectric coefficient of a batch of thermocouple wire.
Certain types of test and advanced simulation rely on accurately controlled and rapidly changing
temperature during the test, usually synchronised with control of mechanical loading. Within the scope
of this document, that would usually be a thermo-mechanical fatigue test.
Where temperature varies deliberately and rapidly during the test, it is appropriate to verify the degree
of time lag in system temperature reading. Without this evaluation (and implicitly a correction) then
either the apparent temperature accuracy or the phase accuracy may need to be unnecessarily reduced.
This document has been written with the intention of using congruent language and approach to that
[1] [2][3]
used for calibration of extensometers and verification of force measurement .
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TECHNICAL SPECIFICATION ISO/TS 21913:2022(E)
Temperature verification method applied to dynamic
fatigue testing
1 Scope
This document establishes verification procedures to determine the accuracy, speed of response, and
stability of temperature measurement for materials testing equipment. These procedures are specified
for the expected use in fatigue tests on metals where these characteristics are important to the fidelity
of tests at high or varying temperature.
The principles set out include sufficient provision for both contacting and non-contacting methods of
temperature measurement.
This document is for the end-to-end verification of registered value compared with “true” specimen
temperature at the point of measurement. It cannot be used to specify the correct method or location of
measurement.
NOTE The methodologies could be found applicable to test types beyond mechanical fatigue of metals, but
that is outside the remit of this document.
2 Normative references
There are no normative references in this document.
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/
3.1
test system
equipment used to perform the (fatigue) test during which this temperature measurement is to be
utilised.
Note 1 to entry: This includes the gripping or fixtures, a representative specimen, the heating system (all parts
which influence the measurement), measurement conditioning device (e.g. thermocouple conditioning system),
data recording device and software.
Note 2 to entry: This verification is expected to take place on a complete test system, but it does not strictly
require the presence of the load frame provided that all parts influencing the specimen environment are held in
a representative configuration.
Note 3 to entry: The operating environment of the test system should be considered as part of the verification,
because changes in temperature of conditioning electronics can affect the measurement, especially if the cold
junction of a thermocouple or the detector array of a thermo-optical device is not at constant temperature.
Note 4 to entry: to entry:: Good laboratory conditions would typically be maintained by some form of climate
control, but that is not always possible and it does not guarantee to prevent local problems for specific
instruments.
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3.2
representative specimen
test piece to be used in the verification process
Note 1 to entry: It should be of the same dimensions and material as the specimen(s) to be tested. In certain cases
this may be unfeasible so a very similar geometry to that expected in the final test schedule, with comparable
conductivity and emissivity should be used. Ideally, this would be an actual test specimen, but some deviation is
acceptable, provided it does not significantly affect the heat transfer characteristics of the system.
3.3
reference system
independent measurement system to be used to verify the test system (3.1)
Note 1 to entry: This should have a resolution at least 3 times smaller than that which will be published for the
verification (preferably more than 5 times) and should be traceable to a certified constant reference.
3.4
resolution
resolution of the temperature measurement system
fluctuation amplitude (half of the difference between maximum and minimum indicated values) of the
noise on the indicated temperature, over a period of 30 s or 30 consecutive readings at the intended
rate of data acquisition (whichever is larger)
Note 1 to entry: expressed in K or °C.
Note 2 to entry: Stated for specific temperature test point(s),or may be described as a function of indicated
temperature.
3.5
bias error
bias error of the temperature measurement system
difference between indicated temperature and reference measurement, for the mean average of values
measured over 30 s or 30 consecutive readings at the intended rate of data acquisition (whichever is
larger) at constant temperature
Note 1 to entry: expressed in K or °C.
Note 2 to entry: Alternatively, the average of values during one complete loading cycle may be used for slower
isothermal cycles, whose duration exceeds 30 s.
Note 3 to entry: Stated for specific temperature test point(s), or may be described as a function of indicated
temperature.
3.6
measurement drift
maximum variation between indicated temperature and reference measurement, during a
representative test period
Note 1 to entry: expressed in K or °C.
Note 2 to entry: Stated for specific temperature test point(s), or may be described as a function of indicated
temperature.
3.7
time lag
delay, between a known change in specimen temperature and the resultant change in indicated
temperature
Note 1 to entry: expressed in seconds.
Note 2 to entry: This determination is made on the basis of delay in rate of temperature change, thus requires a
method of heating capable of significant changes in specimen heating rate within a few seconds.
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3.8
stabilisation time
amount of time after a change in temperature ramp rate, during which the combination of heating
system and measurement device leads to an additional level of error in reading
Note 1 to entry: This is pertinent to variable temperature tests (typically thermo-mechanical fatigue) where
certain combinations of temperature control system and measurement device can lead to a temporarily unstable
or significantly inaccurate measurement, at points when there is a significant change in heat flow into or out of
the specimen. It will generally not be relevant to isothermal test scenarios.
3.9
in situ verification
verification performed with the complete test system (3.1) in a fully assembled state, with all system
components which will be present in the test
Note 1 to entry: including (but not limited to):
— Mechanical grips or other fixtures used to introduce loads to the specimen
— Furnace or temperature chamber or other environmental enclosure
— Transparent or open ports used to allow access to for other monitoring or measurement devices
— Baffles or wadding used to prevent unintended convective heat loss
— Shields or sheathing or other materials used to protect transducers
— Cooling systems such as fans or water circulators
— Transducer extension cables
Note 2 to entry: The purpose of using a fully representative verification is to capture errors which may be
introduced by “parasitic” heating or cooling effects on the measurement device.
4 Measurement behaviour
Figure 1 provides a schematic representation summarising the characteristic behaviours of interest
in this document, as described by the terms defined in 3.4 to 3.9, in the context of how they would be
observed during a ramp-dwell test.
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Key
Vertical axis: temperature
Horizontal axis: time
1 model specimen temperature
2 indicated specimen temperature corresponding to model
3 time lag
4 resolution (of the temperature measurement system)
5 isothermal bias error
Figure 1 — illustration of temperature measurement characteristics
Note 1 to entry a separate time lag will also be present between the command signal of the temperature control
system and the specimen temperature. This is not the same as the time lag to be measured by the methods in this
document, although a similar quantification method is possible.
Annex A provides general information on common temperature measurement techniques for specimens
in fatigue test systems. These are intended to assist in consideration of techniques for both the test
system and the reference system,
5 Representative verification equipment
5.1 Reference measurements
The reference system should be capable of providing a measurement whose centre can be localised
within < 2 mm from the central measurement point to be verified and should be < 1 mm where possible.
Where possible, any aspect of area-averaging should be comparable between the reference and the test
system.
The verified accuracy and resolution of the test system cannot exceed that of the reference system.
(That is to say, an isothermal accuracy of 0,1 °C could not be verified using a reference system whose
resolution is only 0,5 °C).
The reference system should be stable (or have a proven, repeatable, drift correction) to better than
1 % of reading in °C, over the duration of the verification process at the selected temperature(s). For
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thermocouple-based references, the cold junction temperature should be appropriately controlled, for
example using a “triple point” bath or similar device.
NOTE Detailed discussion of laboratory equipment, resolution and uncertainties in the context of
[7]
thermocouple verification can be found in ASTM E2846 .
5.2 Direct physically connected thermocouples (welded to specimen)
At the time of writing, an unsheathed thermocouple, with thin wires (<0,25 mm), welded to the
surface of a specimen is generally considered the most reliable, practical method of measuring the skin
[6]
temperature of a metallic specimen and there is a published European code of practice for use in
[7]
fatigue testing .
For that reason, it should be acceptable to verify this type of measurement on the basis of a summation
of errors, which should include (but not be limited to) errors generated by:
— thermocouple conductor composition
— use of thermocouple compensating cables
— conditioning electronics calibration
— cold junction temperature stability
[7]
ASTM E2846 provides some guidance on use of this approach and developing an expanded
[10]
uncertainty. ASTM E220 provides a normative calibration method for an individual thermocouple
before integration into the test system.
This method of determination is not infallible and great care should be taken in ensuring correct
placement of wires and good, clean, conductive weld points. It is necessary to eliminate sources of stray
EMF, by ensuring close interlinking of earth between specimen and test system (note that a mechanical
test frame does not always have an earth link to the temperature measurement electronics in use),
and avoiding unbalanced induced current effects from passing thermocouple conductors near to strong
electromagnetic fields.
If at all possible, the system should be subject to a full in situ verification, even if frequent partial
verifications are performed on system components off-line.
5.3 Contacting thermocouples and other contacting devices not welded to specimen
A full in situ verification should be performed if a contacting device is used (as opposed to a welded
thermocouple).
Many fatigue tests are performed using temperature “probes” contacting the specimen, but not welded
to the surface (see Annex A). This can take the form of:
— a sheathed thermocouple, whose tip is gently spring-loaded against the surface
— an unsheathed thermocouple, bead welded and tied onto the specimen
— an unsheathed thermocouple, with its junction point held against the specimen by tensioning the
wires
— other devices and placement methods
This can be a more practical solution than welding, due to the fact that it allows the measurement
location to be placed within the parallel length or gauge length, without, in theory, interfering with
the surface condition of the specimen (which the normative references mentioned in this document
require to be tightly controlled). Furthermore, performing a good, clean thermocouple weld demands
considerable skill on the part of the operator. However, in this case, environmental effects could add
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significantly to the total error. For example, the insulating effect may be measurable if a binding of
ceramic cord is used to hold the probe in place.
5.4 Non-contacting thermo-optical devices
A full in situ verification should be performed if a non-contacting device is used. If not, then the
verification should be performed with an optically equivalent path (i.e. apertures and lenses) between
source or target and detector.
[11]
Calibration methods and guidance on use of this type of equipment may be found in ISO 10880 and
[12] [13]
ISO 18251-1 . ASTM E1933 defines a normative method for measuring emissivity.
5.4.1 Constant emissivity coatings
In the case that an infrared camera or pyrometer system is in use with a constant emissivity coating,
a similar methodology to that given in 5.2 for welded thermocouple systems is acceptable. In this case
a summation of errors should be used, which should include (but not be limited to) errors identifiable
from:
— variability in coating emissivity and thickness
— black body transducer calibration
— transducer element temperature stability
— viewing angle of surface
— reflected radiation from other sources
Nonetheless, care should be taken, since problems can occur with coatings due to disbonding from
the surface and flaking under high strains or prolonged exposure to very high temperature. When
this happens, the coating will no longer retain the same relationship to the bulk temperature of the
specimen.
NOTE Use of a coating over the entire specimen surface will affect the result of a mechanical test and the
practitioner needs to consider whether it affects how representative or comparable the data generated will be.
5.4.2 Native surfaces
For some tests, a native surface may be usable, typically oxidised prior to mechanical testing by pre-
conditioning at test temperature under an oxidising atmosphere. This can provide a stable oxide layer,
resulting in constant emissivity needed for reliable readings. However, this is not always reliable for
reasons including: change in chemical composition; change in layer thickness; roughening or spalling
due to high strains or prolonged exposure to very high temperature. Pre-oxidised surfaces may be
beneficial for some tests, but in general, it cannot be assumed that it provides guaranteed stability
during a test.
For a pre-oxidised or otherwise native specimen surface, a full in situ verification is required.
6 Basic isothermal verification methods
6.1 Equipment set up
For isothermal verification in situ, the test system should be set up with a representative specimen,
in the intended test configuration. This should include any data acquisition features in hardware or
software, which could affect the measurement performance, such as expected signal range, data
acquisition rate, or signal filtering.
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The temperature control system should be set to maintain the temperature at the required target,
the specimen heated to that set point and a suitable soak period allowed before commencing the
measurement procedure.
6.2 Measurement of system resolution
At a stable temperature, data should be collected from the test system for not less than 30 s or
30 consecutive readings (whichever provides the larger sample).
System resolution should be declared as the amplitude [(maximum – minimum)/2] of this data set.
6.3 Summation of component bias errors
Identify the measurement system components or blocks which can be measured and attribute a
component bias error to each and a component resolution for the digital signal conditioning and data
acquisition. These may be measured directly off-line, or in situ, or it is acceptable to use a documented
manufacturer’s specification. Data acquisition system error may be assessed by use of a high precision
voltage source to simulate the designated thermocouple output at the temperature(s) of interest, then
following the procedures in 6.2 and 6.4 of this document.
The estimated system resolution and system bias error should be the summation of all the identified
component values at the temperature(s) of interest.
For thermocouple-based measurement systems, it is acceptable to use the procedures for verification
[7]
in a calibration laboratory that are laid out in ASTM E2846, but note that this does not mandate the
use of the same signal conditioning and data acquisition electronics. These should be accounted for, as
should the influence of thermocouple attachment to the specimen.
6.4 Single point measurement of system bias error
At a stable temperature, data should be collected concurrently from the test system and the reference
system for not less than 30 s or 30 consecutive readings (whichever provides the larger sample) on both
systems.
System bias error should be declared as the difference between the mean of the test system data from
the mean of the reference system data.
6.5 Multiple point assessment of resolution and bias error
For some measurement systems, both resolution and bias error vary as a continuous function of
specimen temperature. Therefore, for verification of systems intended to run at varying temperatures,
a series of no fewer than 3 single-point readings may be taken, as detailed above, and a curve fitted to
the relationship with temperature.
If more than 5 single point readings are collected and a function can be identified with a correlation
coefficient better than 0,98 then this relationship may be declared. If fewer than 5 points are available
or if a good correlation cannot be identified, then a simple straight line should be applied such that all
data points fall below it (i.e. the resolution or bias deduced from this line for any given temperature will
ensure a conservative value in an uncertainty budget).
This pair of functions may be used to estimate the system resolution and bias error at intermediate
temperatures. They should not be used to extrapolate to temperatures lying outside the range of single-
point data available.
This clause should not be used where measurement drift (as per 6.2) exceeding 2 °C or 2 % of reading
in °C (whichever is greater) is expected to take place during the time required to conduct the full series
of verifications.
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7 Evaluation of long term measurement drift
7.1 Single point method (Post-test measurement)
For the best confidence in the achieved accuracy of measurement during a test (single or series) of any
significant duration, the isothermal bias error verification (Clause 5) should be performed immediately
before and after the test.
Note 1 In the context of isothermal fatigue testing, the consideration of a significant duration typically
would be exceeding 100 h. However, some combinations of test piece and measurement device might only cause
readings to drift significantly with a much longer or a much a shorter time span.
The isothermal temperature achieved for the post-test verification should be within ±5 °C of that used in
the
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