ISO 23296:2022
(Main)Metallic materials – Fatigue testing – Force controlled thermo-mechanical fatigue testing method
Metallic materials – Fatigue testing – Force controlled thermo-mechanical fatigue testing method
This document applies to stress and/or force-controlled thermo-mechanical fatigue (TMF) testing. Both forms of control, force or stress, can be applied according to this document. This document describes the equipment, specimen preparation, and presentation of the test results in order to determine TMF properties.
Matériaux métalliques – Essai de fatigue – Méthode d'essai de fatigue thermomécanique à force contrôlée
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 23296
First edition
2022-01
Metallic materials – Fatigue testing –
Force controlled thermo-mechanical
fatigue testing method
Matériaux métalliques – Essai de fatigue – Méthode d'essai de fatigue
thermomécanique à force contrôlée
Reference number
© ISO 2022
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ii
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test methods . 4
4.1 Apparatus . 4
4.1.1 Testing machine . 4
4.1.2 Testing machine calibration . 4
4.1.3 Cycle counting . 4
4.1.4 Waveform generation and control . 4
4.1.5 Force measuring system . 6
4.1.6 Test fixtures . 6
4.1.7 Alignment verification . 7
4.1.8 Heating device . 7
4.1.9 Cooling device . 7
4.2 Specimens . 7
4.2.1 Geometry . 7
4.2.2 Specimen preparation . 9
4.2.3 Specimen measurement . 9
4.2.4 Circular or rectangular sections . 9
4.2.5 Sampling, storage and handling . 9
4.2.6 Specimen insertion . . 10
4.2.7 Thermocouple attachment . 10
4.2.8 Spot welding of thermocouples . 10
4.2.9 Heating the specimen . . 11
4.2.10 Cooling the specimen . 11
5 Test preparatory issues .11
5.1 Temperature measurement . 11
5.1.1 General . 11
5.1.2 Temperature control . 11
5.2 Verification of temperature uniformity - Thermal profiling.12
5.2.1 General .12
5.2.2 Maximum permissible temperature variation along the specimen .12
5.2.3 Data recorders .13
5.2.4 Furnace positioning . 13
5.3 Force waveform optimisation . 13
5.4 Temperature force optimisation. 14
5.5 The application of an extensometer to measure maximum and minimum
mechanical strain to observe the effects of ratcheting . 14
6 Test execution .15
6.1 Test start . 15
6.1.1 General .15
6.1.2 Data recording . 15
6.1.3 Test termination . 15
6.1.4 Test validity . 15
6.1.5 During the test. 15
6.2 Test monitoring . 16
6.3 Termination of test. 16
6.3.1 General . 16
6.3.2 Accuracy of control parameters . 16
7 Analysis and reporting .17
iii
7.1 Validation of analysis software . 17
7.2 Test report . 17
7.2.1 General . 17
7.2.2 Essential information . 17
7.2.3 Additional information . 18
7.2.4 Examination of fracture surface . 18
Annex A (informative) Guidelines on specimen handling and degreasing .20
Annex B (informative) Thermocouple arrangement for a specimen containing a notch
feature .21
Annex C (informative) Thermal imaging for thermal profiling .26
Annex D (informative) Measurement of strain during force controlled TMF testing .27
Annex E (informative) Measurement uncertainty .28
Bibliography .30
iv
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
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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).
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals,
Subcommittee SC 4, 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.
v
Introduction
Thermo-mechanical fatigue (TMF) test method was developed in the early 1970’s to simulate, in the
laboratory, loading behaviour of materials under conditions experienced in their service environment,
such as turbine blades and vanes. The TMF test belongs to one of the most complex mechanical testing
methods that can be performed in the laboratory. TMF is cyclic damage induced under varying thermal
and mechanical loadings. When a specimen is subjected to temperature and mechanical strain phasing
it is called strain controlled TMF. ASTM E2368 and ISO 12111 concern strain controlled TMF. However,
these do not allow for specimens where no compensation for free thermal expansion and contraction
is required. Therefore, this document addresses the need for a separate procedure for force controlled
TMF testing.
This document covers the determination of TMF properties of materials under uniaxial loaded force-
controlled conditions. A thermo-mechanical fatigue cycle is defined as specimen tests where both
temperature and force amplitude waveform are simultaneously varied and independently controlled
over the specimen gauge or test section. A series of such tests allows the relationship between the
applied force and the number of cycles to failure to be established.
The specific aim of this document is to provide recommendations and guidance for harmonized
procedures for preparing and performing force controlled TMF tests using various specimen
geometries. The document serves only as a guideline for users and does not form any basis for legal
liability neither of its authors nor of the TMF-Standard project partners. The purpose of this document
is to ensure the compatibility and reproducibility of test results. It does not cover the evaluation or
interpretation of results. Health safety issues, associated with the use of this Standard, are solely the
responsibility of the user.
The following clauses of this document are intended to provide the steps to be implemented in sequence,
during the process of carrying out force controlled TMF tests.
vi
INTERNATIONAL STANDARD ISO 23296:2022(E)
Metallic materials – Fatigue testing – Force controlled
thermo-mechanical fatigue testing method
1 Scope
This document applies to stress and/or force-controlled thermo-mechanical fatigue (TMF) testing. Both
forms of control, force or stress, can be applied according to this document. This document describes
the equipment, specimen preparation, and presentation of the test results in order to determine TMF
properties.
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 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 12111, Metallic materials — Fatigue testing — Strain-controlled thermomechanical fatigue testing
method
ISO 23788, Metallic materials — Verification of the alignment of fatigue testing machines
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
force
F
force applied to the test section, in kN
Note 1 to entry: Tensile forces are considered to be positive and compressive forces negative.
3.2
maximum force
F
max
highest algebraic value of force applied, in kN
3.3
minimum force
F
min
lowest algebraic value of force applied, in kN
3.4
force range
ΔF
algebraic difference between the maximum and minimum forces, in kN
Note 1 to entry: ΔF = F – F
max min
3.5
force amplitude
F
a
half the algebraic difference between the maximum and minimum forces, in kN
Note 1 to entry: F = (F – F )/2
a max min
3.6
mean force
F
m
half the algebraic sum of the maximum and minimum forces, in kN
Note 1 to entry: F = (F + F )/2
m max min
3.7
force ratio
R
algebraic ratio of the minimum force to the maximum force
Note 1 to entry: R = F /F
min max
Note 2 to entry: See Figure 2 for examples of different force ratios.
3.8
stress ratio
R
s
ratio of minimum stress to maximum stress during a fatigue cycle
Note 1 to entry: R = σ /σ
s min max
3.9
stress range
Δσ
arithmetic difference between maximum stress and minimum stress, in MPa
Note 1 to entry: Δσ = σ - σ
max min
3.10
stress
σ
force divided by the nominal cross-sectional area, in MPa
Note 1 to entry: It is the independent variable in a stress-controlled fatigue test.
Note 2 to entry: The nominal cross-sectional area (engineering stress) is that calculated from measurements
taken at ambient temperature and no account is taken for the change in section as a result of expansion at
elevated temperatures.
3.11
fatigue strength at N cycles
σ
N
value of the stress amplitude at a stated stress ratio under which the specimen would have a life of at
least N cycles with a stated probability, in MPa
3.12
maximum stress
σ
max
highest algebraic value of stress applied, in MPa
3.13
minimum stress
σ
min
lowest algebraic value of stress applied, in MPa
3.14
number of force cycles
N
number of loading and unloading sequences applied
3.15
time per cycle
t
time applied per loading and unloading sequence
3.16
maximum temperature
T
max
highest algebraic value of temperature applied, in °C
3.17
minimum temperature
T
min
lowest algebraic value of temperature applied, in °C
3.18
fatigue life
N
f
number of cycles to failure
3.19
theoretical stress concentration factor
K
t
ratio of the notch tip stress to net section stress, calculated in accordance with defined elastic theory, to
the nominal section stress
Note 1 to entry: Different methods used in determining K may lead to variations in reported values.
t
3.20
phase angle
Φ
angle between temperature and mechanical force, defined with respect to the temperature as reference
variable
Note 1 to entry: The phase angle is expressed in degrees. A positive phase angle (0°< ɸ <180°) means that the
maximum of load lags behind the maximum temperature.
4 Test methods
4.1 Apparatus
4.1.1 Testing machine
The tests shall be carried out on a tension-compression machine designed for a smooth start-up. All
test machines are used in conjunction with a computer or controller to control the test and log the
data obtained. The test machine shall permit cycling to be carried out between predetermined limits
of force to a specified waveform and for R < 0 tests there shall be no discernible backlash when passing
through zero. In order to minimise the risk of buckling of the specimen, the machine should have great
lateral rigidity and accurate alignment between the test space support references. The machine force
indicator shall be capable of displaying cyclic force maxima and minima for applied waveforms to a
resolution consistent with the calibration requirement.
During elevated temperature tests the machine load cell shall be suitably shielded and/or cooled such
that it remains within its temperature compensation range.
Machines employing closed loop control systems for force and temperature shall be used.
4.1.2 Testing machine calibration
Machines shall be force calibrated to class 1 of ISO 7500-1.
4.1.3 Cycle counting
The number of cycles applied to the specimen shall be recorded such that for tests lasting less than
10 000 cycles, individual cycles can be resolved, while for longer tests the resolution should be better
than 0,01 % of indicated life.
4.1.4 Waveform generation and control
The force cycle waveform shall be maintained consistent and is to be applied at a fixed frequency
throughout the duration of a test programme. The waveform generator in use shall have repeatability
such that the variation in requested force levels between successive cycles is within the calibration
tolerance of the test machine as stated in ISO 7500-1, for the duration of the test.
Terms have been identified relative to the trapezoidal waveforms in Figure 1 and Figure 2. Other
waveform shapes may require further parameter definition although nomenclature should be retained
where possible. Often, Force-controlled TMF loading waveforms do not follow standard trapezoidal
patterns.
The phase angle between temperature and force is defined by the parameter Φ. Typical phase angles to
characterize a TMF test are Φ = 0° which is called “in phase” and Φ = 180° which is called “out of phase”.
Any other phase angle may be possible and permitted.
Key
X time
Y force
a
Mean force.
b
Minimum force.
c
Maximum force.
d
Force range.
e
Force amplitude.
f
One cycle.
Figure 1 — Trapezoidal fatigue force cycle
Key
X time
Y force
a
Cyclic tension.
b
Reversed.
c
Cyclic compression.
Figure 2 — Varying force ratio
4.1.5 Force measuring system
The force measuring system, consisting of a load cell, amplifier and display, shall meet the requirements
of ISO 7500-1 over the complete range of dynamic forces expected to occur during the TMF test series.
The load cell should be rated for fully-reversed tension-compression fatigue testing. Its overload-
capacity should be at least twice as high as the forces expected during the test. The load cell shall
be temperature compensated and should not have a zero drift and temperature sensitivity variation
greater than 0,002 % (Full scale/°C). During the test duration the load cell should be maintained within
the range of temperature compensation and suitably protected from the heat applied during the test.
4.1.6 Test fixtures
An important consideration for specimen grips and fixtures is that they can be brought into good
alignment consistently from test to test. Good alignment is achieved from very careful attention to
design details, i.e. specifying the concentricity and parallelism of critical machined parts.
In order to minimise bending strains the gripping system should be capable of alignment such that the
major axis of the specimen coincides closely with the force axis throughout each stress cycle and in the
case of through zero tests (Rε ≤ 0) shall also be free from backlash effects.
A parallelism error of less than 0,2 mm/m, and an axial error of less than 0,03 mm for a specimen of
less than 300 mm in length, and of less than 0,1 mm for a test space of more than 300 mm in length,
should allow the alignment requirements described in 4.1.7 to be achieved. A further benefit can be
realised by minimising the number of mechanical interfaces in the load train and the distance between
the machine actuator and crosshead.
4.1.7 Alignment verification
Alignment of the load train assembly shall be conducted and verified in accordance with ISO 23788 to a
minimum of class 10.
4.1.8 Heating device
Testing will generally be conducted in air at elevated temperatures, although there may be a
requirement to test in vacuum or in a controlled atmosphere. Where additional apparatus is used such
as vacuum chambers etc. it is essential that the full force indicated by the force indicator is being applied
to the specimen and is not being diverted through the auxiliary apparatus (e.g. by friction). The heating
device employed shall be such that the specimen can be uniformly heated to the specified temperature
and maintained for the duration of the test. Radiant lamp furnaces are ideally suited to apply a dynamic
temperature change. Induction furnaces are also suitable for rapid temperature change. However, the
specimen geometry (thickness or diameter) and the temperature rate can be a limiting factor in their
application.
4.1.9 Cooling device
In order to reduce the specimen temperature to the required cooling rate it is recommended to pass
compressed air over the surface of the specimen. There are a number of devices which are able to
satisfactorily perform this task. For induction systems a range of air jets that can be independently
directed at the specimen are adequate for this task. For radiant lamp furnaces the use of an air amplifier
is recommended. This is best positioned at the top of the furnace. Coupled to a compressed air supply
the air amplifier accelerates a curtain of air through the centre of the furnace.
4.2 Specimens
4.2.1 Geometry
Subject to the objectives of the test programme, the type of specimen geometry used will depend on the
equipment capacity, the type of equipment and the form in which the material is available. Consideration
should be given to the interface to the test machine i.e. the gripping system and any possible test area
envelope caused by the furnacing.
The gauge portion of the specimen in a TMF test should, under ideal conditions, represent a volume
element of the investigated material contained within the thermally loaded component. Therefore,
the geometry of the specimen should not affect the resulting lifetime behaviour, e.g. due to stress
inhomogeneities, undesired stress deviations etc. Failure shall occur within the gauge section for the
test to be considered valid.
Generally, a specimen having a fully machined test section is of the type shown in Figure 3 for a smooth
cylindrical-type gauge section. The specimens may be of the following:
— circular cross-section with tangentially blending fillets between the test section and the ends, or
with a continuous radius between the ends (i.e. “hourglass” specimen);
— rectangular cross-section of uniform thickness over the test section with tangentially blending
fillets between the test section and the gripped ends (see Figure 4).
Specimens commonly known as “hourglass” specimens may be employed for testing with caution. In
such specimens, there is a continuous radius between grip ends with a minimum diameter or width of
the test section centrally located between these ends for cylindrical and flat specimens respectively.
Unlike a smooth, constant diameter or width, gauge section where a volume of material is equally
under stress, the hourglass specimen permits sampling only of a thin planar element of material at
the minimum cross section. Thus, the fatigue results produced may not represent the response of the
bulk material where, particularly in the long-life fatigue regime, inclusions govern behaviour in high
hardness metals and there is a duality in crack initiation from surface to subsurface. In fact, such results
may be non-conservative particularly in the longer life regime where the largest micro discontinuity
may not lie in the planar section of greatest stress.
It is important to note that for specimens of rectangular cross-section, it may be necessary to reduce
the test section in both width and thickness. If this is necessary, then blending fillets will be required in
both the width and thickness directions. Also, for a rectangular-section specimen, where it is desired
to take account of the surface condition in which the metal will be used in actual application, then at
least one surface of the test section of the test piece should remain unmachined. It is often the case, for
fatigue tests conducted using a rectangular-section piece, that the results are not always comparable to
those determined on cylindrical specimens because of the difficulty in obtaining an adequate surface
finish or because fatigue cracks initiate preferentially at the corner(s) of the rectangular test piece.
Dimensions in millimetres
NOTE 1 For definitions of symbols for geometrical tolerances, see ISO 5459.
NOTE 2 The perpendicularity requirement applies to any gripping parts used for alignment.
Figure 3 — Recommended geometry of cylindrical specimen
Dimensions in millimetres
NOTE 1 For definitions of symbols for geometrical tolerances, see ISO 5459.
NOTE 2 The perpendicularity requirement applies to any gripping parts used for alignment.
Figure 4 — Possible geometry of flat-sheet specimen
The gauge portion of the specimen represents a volume element of the material under study, which
implies that the geometry of the specimen shall not affect the use of the results. The width of the
specimen is reduced in the gauge length to avoid failures in the grips. In some applications it might be
necessary to add end-tabs to increase the grip and thickness, as well as to avoid failure in the grips.
Geometric dimensions in Table 1 are recommended.
Table 1 — Geometric dimensions
Parameter Dimensions
Diameter of cylindrical gauge length d ≥ 3 mm
Transition radius (from parallel section to grip end) r ≥ 2d
External diameter D ≥ 2d
Length of reduced section L ≤ 8d
p
Other geometric cross-sections and gauge lengths may be used. It is important that general tolerances
of the specimens respect the three following properties:
— Parallelism // ≤0,005d
— Concentricity ⊚ ≤0,005d
— Perpendicularity ⊥ ≤0,005d
These values are expressed in relation to the axis or reference plane.
4.2.2 Specimen preparation
Utmost importance should be given to the condition of the specimen and method of preparation.
Inappropriate methods of preparation, which may be material specific, can greatly bias the test data
generated. The effect of contaminants such as cutting fluids and degreasing agents shall also be
understood. Whilst it may be the purpose of some commercial tests to establish the effect of a particular
representative surface finish, standard specimens shall have been prepared so that no alteration of the
microstructure or the introduction of residual stresses are applied to the material.
Final surface finishing processes should ensure that all machining marks or scratches on the specimen
test section or end transitions, and any burrs on notches, be completely eliminated as to the prescribed
surface finish of the drawing.
Throughout the test, observe any special handling requirements for the material in question.
4.2.3 Specimen measurement
The specimen dimensions for calculating the cross-sectional area shall be measured prior to the test
to an accuracy of 0,2 % or 0,005 mm, whichever is the greatest value. The surface finish shall not be
jeopardised by this activity.
4.2.4 Circular or rectangular sections
The width or diameter and thickness of the gauge section shall be measured at three positions on
the gauge length. The cross-sectional area is calculated from the average of these three values. For
continuous radius gauge sections the minimum diameter shall be used.
4.2.5 Sampling, storage and handling
Specimens shall be stored in a manner that protects them from mechanical damage such as scratching,
and environmental effects such as extreme humidity, chemical contamination, etc.
Before attaching thermocouples, clean and degrease non-titanium specimens thoroughly. Titanium
specimens should be degreased after final finishing and from then on only be handled using cotton
gloves. Throughout the testing process, any special handling requirements for the material under
investigation should be adhered to. The use of clean cotton gloves is recommended. Annex A describes
default handling and degreasing requirements.
4.2.6 Specimen insertion
Before loading ensure the load cell has been calibrated, in accordance with ISO 7500-1 in compression
and tension, and aligned in accordance to ISO 23788.
The gripping device should transmit the imposed cyclic forces without backlash. Hydraulic gripping is
preferred and the number of mechanical interfaces within the load train should be minimized.
The grips should be water cooled in order to allow quick cyclic stabilisation of the longitudinal
temperature distribution within the gauge length and to provide stable thermal conditions during the
experiment. Therefore, the water cooled gripping device should be designed in a way that allows the
heat of the specimen shaft to be carried away by the cooling water as directly as possible. Heat flow
from the specimen to the load cell shall be avoided.
The method employed to insert the specimen into the test fixture shall not jeopardise the alignment
mechanisms, surface finish integrity or material properties. Excessive twisting should be avoided
and compressive forces limited to a maximum of 500 N or 10% of the intended test maximum force,
whichever is smaller.
Insert the specimen in the upper grips, tighten the grips by applying hydraulic clamping, and zero the
machines load cell output. Move the lower grips up until the specimen is located in the lower grips, and
then tighten the grips by applying hydraulic clamping. If the testing machine does not have an ‘anti-
rotate’ clamp then it is recommended to tighten off the lower grip before the upper grip.
4.2.7 Thermocouple attachment
The position of the thermocouples is dependent on the stage of the test programme, i.e. thermal
profiling versus actual testing. Examples of the thermocouple layouts for a test programme on a feature
based specimen using a radiant lamp furnace can be found in Annex B.
NOTE The use of other forms of thermocouples such as ribbon, beaded, etc., is permitted only when
temperature control can be maintained over the period of the test within in the designated limits set out in the
section on temperature control 5.1.2. For more information regarding the appropriate choice of thermocouples it
is advisable to consult ASTM E220, BS 1041-4, BS 60584-1 and EN 60584-3.
4.2.8 Spot welding of thermocouples
Accurate temperature monitoring is crucial in TMF testing. The best method to control the temperature
can be realised by spot welding thermocouples within the gauge section. However, it shall be ensured
that no crack initiation can occur at the position of the spot welded thermocouple. If this cannot be
ensured and alternative control method can be applied by controlling the temperature in the specimen
radius. This method is described in Annex B. When using the method in Annex B, then a second
temperature measurement should be applied in the gauge section, but risk of crack initiation should
also be avoided. The number of thermocouples needed to control the temperature depends on the
heating device used i.e., the number of heating zones of the furnace.
These thermocouples shall remain attached to the specimen surface for the entire duration of the test
which can be as much as three months. The thickness of the thermocouple wires should be kept as
small as possible, to reduce the effects of cold spots on the specimen (the thermocouple effectively
absorbs heat from the specimen surface causing localised cooling). Wrapping the thermocouple wires
approximately one quarter around the specimen also reduces the effects of cold spots.
Alternative methods need to be developed for either:
Pyrometry: This method is non-contacting and could be used for the primary means of temperature
control, to ensure the correct temperature at the centre of the specimen during the test. Pyrometry
does have additional problems such as ensuring correct emissivity etc. and needs to be validated and
proven over the length of a typical TMF test.
Thermal imagery: Similarly to pyrometry, the emissivity is the deciding factor in the correct
quantitative temperature measurement. It is feasible with the use of accurate high resolution infrared
equipment with the necessary frame rate to obtain a thermal profile over the entire gauge length of
the specimen. The camera should be calibrated against thermocouple readings focused at the same
location. Readings should be performed over a range of temperatures to establish the accuracy of
the temperature measurement obtained. Dynamic temperature measurement should be carefully
controlled and verified against thermocouple readings.
4.2.9 Heating the specimen
The specimen shall be heated to the specified temperature at the customer’s prescribed heating
rate. During heating, the temperature difference across the specimen should not exceed the limits
recommended in 5.2.1 and should be held at zero force. If in doubt, advice should be sought from the
customer, as to the temperature sensitivity of the particular material. Monitoring of the specimen
temperature should be carried out using a calibrated temperature logger, which allows a temperature
history to be saved and the parameters of the test to be recorded.
Expansion during the heating process shall not result in compressive forces being applied to the
specimen. The specimen shall therefore be maintained at zero force, throughout the heating process.
4.2.10 Cooling the specimen
The specimen should be cooled using dissipated air to ensure a uniform temperature across the
specimen. The cooling should be controlled to a rate which is in accordance with the test conditions.
As with heating, the specimen temperature should be monitored using a calibrated temperature logger
and not exceed the limits recommended in 5.2.2. Maintaining a constant cooling air flow across the
specimen during both heating and cooling phases of the test cycle has shown to stabilise the furnace by
forcing it to constantly provide power to the heating element.
5 Test preparatory issues
5.1 Temperature measurement
5.1.1 General
The temperature measuring system comprising sensors and readout equipment shall be capable
of operating continuously for the duration of the test and have a resolution of at least 0,5 °C and an
accuracy of ±1 °C. It shall be verified over the working temperature range. The sensors employed shall
not affect the surface properties of the material.
5.1.2 Temperature control
The temperature cycle shall remain constant throughout the duration of the test. The importance of
maintaining constant temperature profiles through the test are discussed in Reference [14].
Throughout the duration of the test, the temperature(s) indicated by the control sensor, e.g.
thermocouple, should not vary by more than the greater of ±5 °C or 1 % of the test temperature range
from the stabilized value(s) (i.e. following the establishment of dynamic equilibrium) at any given
instant in time within the cycle. Throughout the duration of the test, the temperature(s) indicated by
the non-control sensor(s) should not vary.
Furthermore, the reproducibility of the position of the thermocouples and of the specimen with respect
to the heat source and the specimen fixtures and cooling devices should be kept within a tolerance
of ±0,5 mm. Generally, a second temperature measurement system independent of the temperature
control equipment should be used to cross-check the reproducibility of the readout of the control
temperature measuring device. This is applicable for the set-up phases and for the TMF test.
5.2 Verification of temperature uniformity - Thermal profiling
5.2.1 General
The uniformity of temperature along the gauge section and at the shoulders shall be verified before
every series of tests that introduces a new specimen geometry, material or test profile, or in which the
cooling, fixturing or heating device mounting arrangement are adjusted.
This verification may be made by means of a dummy specimen of identical geometry and material
to that to be tested, equipped with several thermocouples fixed along and around the specimen. The
distance between the thermocouples should not exceed the specimen diameter and they should be
suitably screened from direct radiant heat from the heating device.
The thermocouple layout will depend on the specimen geometry and any additional specific customer
requirements. The profiling can consist of several stages. From the initial stage, each additional stage
will contain reduced numbers of thermocouples and this is also customer specific.
Before the thermal profiling begins, temperature paths should be plotted in software which
enables graphical depiction of numerical values. This will aid accurate profiling for each stage
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Questions, Comments and Discussion
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