ISO 23296:2022

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 23296:2022(E)
© ISO 2022

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ISO 23296:2022(E)
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© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 23296:2022(E)
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
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ISO 23296:2022(E)
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
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ISO 23296:2022(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
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 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.
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ISO 23296:2022(E)
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.
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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
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ISO 23296:2022(E)
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
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ISO 23296:2022(E)
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.
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ISO 23296:2022(E)
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.
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ISO 23296:2022(E)
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
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ISO 23296:2022(E)
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.
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ISO 23296:2022(E)
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 radiu
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