Metallic materials — Fatigue testing — Strain-controlled thermomechanical fatigue testing method

ISO 12111:2011 is applicable to the TMF (thermomechanical fatigue) testing of uniaxially loaded metallic specimens under strain control. Specifications allow for any constant cyclic amplitude of mechanical strain and temperature with any constant cyclic mechanical strain ratio and any constant cyclic temperature-mechanical strain phasing.

Matériaux métalliques — Essais de fatigue — Méthode d'essai de fatigue thermo-mécanique avec déformation contrôlée

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Status
Published
Publication Date
09-Aug-2011
Current Stage
9093 - International Standard confirmed
Start Date
31-Oct-2017
Completion Date
31-Oct-2017
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INTERNATIONAL ISO
STANDARD 12111
First edition
2011-08-15
Metallic materials — Fatigue testing —
Strain-controlled thermomechanical
fatigue testing method
Matériaux métalliques — Essais de fatigue — Méthode d'essai de
fatigue thermo-mécanique avec déformation contrôlée
Reference number
ISO 12111:2011(E)
ISO 2011
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ISO 12111:2011(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2011

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,

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Published in Switzerland
ii © ISO 2011 – All rights reserved
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ISO 12111:2011(E)
Contents Page

Foreword .............................................................................................................................................................v

Introduction........................................................................................................................................................vi

1 Scope......................................................................................................................................................1

2 Normative references............................................................................................................................1

3 Terms and definitions ...........................................................................................................................1

4 Symbols..................................................................................................................................................3

5 Apparatus...............................................................................................................................................4

5.1 Testing machine ....................................................................................................................................4

5.2 Strain measuring system......................................................................................................................5

5.3 Heating system ......................................................................................................................................5

5.4 Instrumentation for test monitoring ....................................................................................................5

5.5 Checking and verification of the apparatus........................................................................................6

6 Specimens..............................................................................................................................................6

6.1 Geometry................................................................................................................................................6

6.2 Preparation of specimens.....................................................................................................................9

6.3 Machining procedure ..........................................................................................................................10

6.4 Sampling and marking........................................................................................................................10

6.5 Surface condition of the specimen....................................................................................................11

6.6 Dimensional check ..............................................................................................................................11

6.7 Storage and handling of specimens..................................................................................................11

7 Procedure.............................................................................................................................................12

7.1 Laboratory environment .....................................................................................................................12

7.2 Specimen mounting ............................................................................................................................12

7.3 Temperature control............................................................................................................................12

7.4 Temperature gradients........................................................................................................................12

7.5 Mechanical strain control ...................................................................................................................13

7.6 Thermal strain compensation ............................................................................................................13

7.7 Temperature/mechanical strain phasing ..........................................................................................15

7.8 Command waveforms .........................................................................................................................16

7.9 Start of test...........................................................................................................................................16

7.10 Monitoring the test ..............................................................................................................................17

7.11 Failure criteria......................................................................................................................................17

7.12 Failure...................................................................................................................................................18

7.13 Test interruption sequence ................................................................................................................18

8 Expression of results..........................................................................................................................18

8.1 Preliminary data...................................................................................................................................18

8.2 Reduction of recorded data................................................................................................................18

8.3 Analysis of results...............................................................................................................................18

9 Test report............................................................................................................................................19

9.1 Aim of the study ..................................................................................................................................19

9.2 Material .................................................................................................................................................19

9.3 Specimen..............................................................................................................................................19

9.4 Test equipment details........................................................................................................................19

9.5 Description of test methodology .......................................................................................................19

9.6 Test termination technique including definition of failure..............................................................19

9.7 Deviations from specified test tolerances or recommended procedures .....................................19

9.8 Test conditions ....................................................................................................................................20

9.9 Presentation of results........................................................................................................................20

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ISO 12111:2011(E)

Annex A (informative) Representative diagrams ...........................................................................................21

Annex B (informative) Modulus of elasticity determination..........................................................................24

Bibliography ......................................................................................................................................................25

iv © ISO 2011 – All rights reserved
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ISO 12111:2011(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards

adopted by the technical committees are circulated to the member bodies for voting. Publication as an

International Standard requires approval by at least 75 % of the member bodies casting a vote.

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.

ISO 12111 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 5, Fatigue testing.
© ISO 2011 – All rights reserved v
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ISO 12111:2011(E)
Introduction

The fatigue lives of structural components subjected to simultaneously occurring thermal and mechanical

loadings are often of critical interest and concern to design engineers. A common approach to investigating

the behaviours of materials subjected to combined thermal and mechanical loadings is to idealize the

conditions of a critical material element on a uniaxial laboratory test specimen. The test condition is one where

cyclic, theoretically uniform, within the test section, temperature and strain fields are externally imposed,

simultaneously varied and controlled. Such a test is designated as “thermomechanical fatigue”, commonly

abbreviated as TMF.

In order to ensure reliability and consistency of results from different laboratories, it is necessary to generate

and collect all data using test methodologies that comply with an established standard.

This International Standard addresses both the generation and presentation of TMF data.

vi © ISO 2011 – All rights reserved
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INTERNATIONAL STANDARD ISO 12111:2011(E)
Metallic materials — Fatigue testing — Strain-controlled
thermomechanical fatigue testing method
1 Scope

This International Standard is applicable to the TMF testing of uniaxially loaded metallic specimens under

strain control. Specifications allow for any constant cyclic amplitude of mechanical strain and temperature with

any constant cyclic mechanical strain ratio and any constant cyclic temperature-mechanical strain phasing.

NOTE A list and sketch of the most common cyclic types is shown in Annex A.

The range of cycles considered corresponds to that which is generally considered as the low-cycle fatigue

domain, that is, N ≤ 10 .
2 Normative references

The following referenced documents are indispensable for the application 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 7500-1:2004, Metallic materials — Verification of static uniaxial testing machines — Part 1:

Tension/compression testing machines — Verification and calibration of the force-measuring system

ISO 9513, Metallic materials — Calibration of extensometer systems used in uniaxial testing

ISO 12106, Metallic materials — Fatigue testing — Axial-strain-controlled method
ISO 23718, Metallic materials — Mechanical testing — Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 23718 and ISO 12106 and the

following apply.
3.1
stress

F /A , where F is the instantaneous force and A is the original cross-sectional area at room temperature

i o i o
3.2
original gauge length

length on the specimen between extensometer measurement points at room temperature and zero strain

NOTE This definition avoids the complexity of a continually varying gauge length due to thermal expansion and

contraction.
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ISO 12111:2011(E)
3.3
gauge length
instantaneous length on the specimen between extensometer measurement points
3.4
strain

∆L/L , where ∆L is the change in length and L is the gauge length measured at room temperature

o o
3.5
total strain
tot
algebraic sum of the mechanical and thermal strains:
ε = ε + ε
tot m th
3.6
thermal strain
strain corresponding to the free expansion induced by a change in temperature
3.7
mechanical strain

strain that is independent of temperature and is associated with the applied force on the specimen

3.8
elastic strain

strain component resulting when the stress is divided by the temperature-dependent Young's modulus

3.9
inelastic strain

strain component resulting when the elastic strain is subtracted from the mechanical strain

3.10
cycle

smallest segment of the strain-temperature-time pattern that is repeated periodically

3.11
maximum
greatest algebraic value of a variable within one cycle
3.12
minimum
least algebraic value of a variable within one cycle
3.13
mean
one-half of the algebraic sum of the maximum and minimum values of a variable
3.14
range
algebraic difference between the maximum and minimum values of a variable
3.15
amplitude
half the range of a variable
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ISO 12111:2011(E)
3.16
fatigue life
number of applied cycles, N , to achieve a defined failure criterion
EXAMPLE An example of this is found in 7.11.
3.17
hysteresis loop
closed curve of the stress-mechanical strain response during one cycle
3.18
mechanical strain ratio
minimum mechanical strain divided by the maximum mechanical strain
3.19
phase angle

angle between temperature and mechanical strain, defined with respect to the temperature as reference

variable

NOTE The phase angle is expressed in degrees. A positive phase angle (0 < Φ < 180) means that the maximum of

the mechanical strain lags behind the maximum temperature.
4 Symbols
D diameter of grip end of specimen, mm
d diameter of cylindrical gauge section, mm
E modulus of elasticity, Young's modulus
L original gauge length, mm
N cycles to failure, cycles
n cycle number
R mechanical strain ratio = ε /ε
ε min max
T temperature, °C
∆ range of a parameter
ε strain, unit in % or dimensionless
σ stress, MPa
Φ phase angle, degrees
Subscripts (if used):
m mechanical
max maximum
min minimum
th thermal
tot total
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ISO 12111:2011(E)
5 Apparatus
5.1 Testing machine
5.1.1 General

The tests shall be conducted on a tension-compression machine designed for smooth start-up with no

backlash when passing through zero force. The machine shall be capable of controlling strain and measuring

force when applying the recommended waveform. It may be hydraulic or electromechanical.

The load frame shall have high lateral stiffness when the crosshead is in the operating position and accurate

alignment (both parallelism and concentricity) between the load train support references.

The complete load train (including force transducer, pullrods/grips, and specimen) shall also have high lateral

stiffness to minimize specimen bending.
5.1.2 Force measuring system

The force measuring system, comprising force transducer, conditioner and readout. This system shall meet

the requirements of ISO 7500-1, Class 1 over the range of forces expected during the test series.

NOTE Class 1 requires that force indication errors should not exceed ± 1 % of reading over the verified range.

The force transducer shall be suitable for the forces applied during the test.

The force transducer shall be temperature compensated and not have a zero drift or sensitivity variation

greater than 0,002 % of full scale per one degree Celsius. During the test, it shall be maintained within its

temperature compensated range.
5.1.3 Specimen gripping device

The gripping device shall transmit the cyclic forces to the specimen without backlash for the duration of the

test. The geometric qualities of the device shall ensure correct alignment in order to meet the requirements

specified in 5.1.4.

NOTE It is good design practice to reduce the number of mechanical interfaces to a minimum.

The gripping device shall ensure that the alignment is reproducible over successive specimens.

The gripping device materials shall be selected so as to ensure correct functioning across the range of test

conditions.
5.1.4 Load train alignment

Load frame, including grips, shall be aligned using a specimen, with a geometry as similar as possible to that

of the test specimen, instrumented with strain gauges. The permitted maximum bending strain due to the

machine shall be no more than 50 microstrain at zero force or 5 % of the applied axial mechanical strain,

whichever is the greater. This shall be carried out at 12-month intervals and in the following events:

a) as part of the commissioning procedure of a newly acquired testing machine;

b) after an accidental buckling of a specimen, unless it can be demonstrated that the alignment has not

changed; and
c) if any adjustment has been made to the load train.
[13]
NOTE A relevant procedure is given in VAMAS Report No. 42 .
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ISO 12111:2011(E)
5.2 Strain measuring system

The strain measuring system (optical, mechanical, etc.) including the extensometer and its associated

electronics shall conform to ISO 9513, Class 1.
For gauge lengths less than 15 mm, a Class 0.5 system is recommended.
The strain shall be measured on the specimen using an axial extensometer.

The extensometer shall be suitable for measuring dynamic strain over long periods during which there should

be minimal drift, slippage or instrument hysteresis. It shall measure axial extension directly over the gauge

length of the specimen.

The transducer section of the extensometer should be protected from thermal fluctuations that give rise to drift.

Given the transient nature of the temperature in a TMF test, it is recommended that the extensometer is

actively cooled, so that the transducer section of the extensometer remains isothermal during the course of

the test.

The kinematic design of contacting extensometry should be such that lateral or angular motions of the

specimen contact zone do not cause the extensometer contact points or knife edges to slip.

The contact pressure and operating force of the extensometer should be low enough to avoid damaging the

specimen surface and giving rise to crack initiation at the extensometer contact points or knife edges.

5.3 Heating system

The heating system shall be capable of applying the maximum heating and cooling rates required by the TMF

test series.

To minimize radial temperature gradients with a direct induction heating system, it is advisable to select a

generator with a sufficiently low frequency (typically in the several hundred kHz range and lower). This will

help to minimize “skin effects” during heating.

During a test, the specimen temperature shall be measured using thermocouples, pyrometers, RTDs, or other

such temperature-measurement devices.

For thermocouples, direct contact between the thermocouple and the specimen shall be achieved without

causing incipient failure at the point of contact.

NOTE Commonly used methods of attachment are: resistance spot welding (outside the gauge length) and fixing by

binding or by pressing a sheathed thermocouple against the specimen surface.

If the temperature within the gauge section is measured with an optical pyrometer, steps shall be taken during

calibration to address possible variation in the specimen's thermal emissivity over the duration of the test.

Potential solutions may include two-colour pyrometers and pre-oxidizing the specimen surface.

5.4 Instrumentation for test monitoring

A computerized system capable of carrying out the task of collecting and processing force, extension,

temperature, and cycle count data digitally is recommended. Sampling frequency of data points shall be

sufficient to ensure correct definition of the hysteresis loop especially in the regions of reversals. Different data

collection strategies will affect the number of data points per loop needed, however, typically 200 points per

loop are required.
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ISO 12111:2011(E)

Alternatively, an analog system capable of measuring the same data may be used and would include:

⎯ an X-Y-Y recorder used to record force, extension, and temperature hysteresis loops;

⎯ a strip-chart recorder for several time-dependent parameters: force, extension and temperature;

⎯ a peak detector per signal;
⎯ a cycle counter.

The recorders may be replaced with storage devices capable of reproducing the recorded signals either in

photographic or analog form. These devices are necessary when the rate of recorded signals is greater than

the maximum slew-rate of the recorder. They allow permanent records to be reproduced subsequently at a

lower rate.
5.5 Checking and verification of the apparatus

The testing machine and its control and measurement systems should be checked regularly.

Specifically, each transducer and associated electronics shall always be checked as a unit.

⎯ The force measuring system(s) shall be verified according to ISO 7500-1.
⎯ The strain measuring system(s) shall be verified according to ISO 9513.

⎯ The temperature measuring system(s) shall be traceable to the relevant national standard.

It is good practice before each series of tests to check the base length of the extensometer, the force cell and

extensometer calibrations using a shunt resistor or other suitable method and the thermocouple or pyrometer

calibrations.
6 Specimens
6.1 Geometry
6.1.1 General

The total specimen bending is comprised of bending from load frame misalignment and specimen bending

from test specimen asymmetry. To minimize the bending contribution due to the test specimen, it is important

to carefully control deviations from the intended test specimen geometry.
6.1.2 Solid round specimens

6.1.2.1 The gauge portion of the specimen in a TMF test 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.

This geometry should fulfil the following conditions:
⎯ provide a uniform cylindrical gauge portion;

⎯ minimize the risk of buckling in compression to avoid failure initiation at the transition radius;

⎯ provide a uniform strain distribution over its whole gauge portion;
⎯ allow the extensometer to measure the strain without interference or slippage.
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ISO 12111:2011(E)

The parallel length of the specimen shall be longer than the extensometer gauge length. However, it shall not

exceed L + (d/2) to reduce risk of failure outside the extensometer gauge length.

6.1.2.2 Taking into account these requirements, the experience gained by a large number of laboratories

and the results of calculations taken from different types of specimens (see References [1], [2], [3], [10] and

[11]), the following geometric dimensions [see Figures 1 to 4 in which (L /d) > 2] are specified:

⎯ diameter of cylindrical gauge length: d ≥ 5 mm;
⎯ gauge length: L ≥ d;
⎯ transition radius (from parallel section to grip end): r ≥ 2d;
⎯ diameter of grip end: D ≥ d;

⎯ length of reduced section or distance between grips for a constant cross section specimen: L < 8d.

Other geometric cross sections and gauge lengths may be used for specimens provided that the uniform

distribution of stresses, strains and temperatures in the gauge length are ensured.

6.1.2.3 It is important that general tolerances of the specimens respect the three following properties:

⎯ parallelism = 0,005D or better;
⎯ concentricity = 0,005D or better;
⎯ perpendicularity = 0,005D or better.
(These values are expressed in relation to the axis or reference plane.)

6.1.2.4 The dimensions of end connections shall be defined as a function of the testing machine.

The recommended end connections are as follows:
⎯ smooth cylindrical connection (hydraulic collets);
⎯ button-end connection.

The gripping device shall locate the specimen and provide axial alignment. It should not permit backlash. The

design of the gripping device will depend on the specimen end details. A number of examples are given in

Figures 1 to 4.

In general, designs in which specimen alignment depends on screw threads are not recommended.

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ISO 12111:2011(E)
Dimensions in millimetres

Figure 1 — Cylindrical smooth shank TMF tube (see Reference [1] in the Bibliography)

Dimensions in millimetres

Figure 2 — Cylindrical threaded shank TMF tube (see Reference [2] in the Bibliography)

Dimensions in millimetres

Figure 3 — Cylindrical smooth shank TMF tube (see Reference [3] in the Bibliography)

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ISO 12111:2011(E)
Dimensions in millimetres
Figure 4 — Rectangular solid sample (see Reference [10] in the Bibliography)
6.1.3 Round tubular specimens

In general, the considerations discussed in 6.1.2 also apply to tests on tube specimens. In addition, tolerances

should be maintained such that wall thickness variation is maintained within 1 % of the nominal wall thickness

around the circumference.

Tubular specimens have the advantage over solid specimens of minimizing the radial temperature gradient.

The specimen wall should be sufficiently thick in order to be representative of the material microstructure.

As a general rule, the ratio of mean diameter to wall thickness should be in the range 5 to 30, in order to

satisfy thin-wall specimen criteria. Buckling tendencies at high axial strain ranges will tend to push specimen

design to the lower end of this range.
6.1.4 Solid rectangular specimens

In general, the considerations discussed in 6.1.2 also apply to tests on rectangular specimens. However,

these tests require specific geometries and fixtures in order to avoid problems of buckling.

The gripping system may necessitate the use of flat mechanical or hydraulic jaws. However, flat parallel jaw

faces require additional measures to ensure alignment in the two unconstrained degrees of freedom.

In general, 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 end thickness as well as to avoid

failure in the grips.
6.2 Preparation of specimens

In any TMF test programme designed to characterize the intrinsic properties of a material, it is important to

observe the recommendations given in 6.3 to 6.7 in the preparation of specimens. A deviation from these

recommendations is possible if the test programme aims to determine the influence of a specific factor

(surface treatment, oxidation, etc.) that is incompatible with these recommendations. In all cases, this

deviation shall be noted in the test report.
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ISO 12111:2011(E)
6.3 Machining procedure
6.3.1 General

The machining procedure selected may produce residual stresses on the specimen surface likely to affect the

test results. These stresses may be induced by heat gradients at the machining stage, stresses associated

with deformation of the material or microstructural alterations. Their influence is less marked in tests involving

relatively high temperatures because they are partially or totally relaxed upon preliminary thermal cycling.

However, they are to be reduced by using an appropriate final machining procedure, especially prior to a final

polishing stage. For harder materials, grinding rather than tool operation (turning or milling) may be preferred.

⎯ Grinding: from 0,1 mm of the final dimension at a rate of no more than
...

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