Metallic materials — Fatigue testing — Axial plane bending method

This document specifies the conditions for conducting the plane bending fatigue test on an axial machine, constant-amplitude, force or displacement controlled, at room temperature (ideally between 10 °C and 35 °C) on metallic specimens, without deliberately introduced stress concentrations. This document does not include the reversed/partially loading test. The purpose of the test is to provide relevant results, such as the relation between applied stress and number of cycles to failure for a given material condition, expressed by hardness and microstructure, at various stress ratios. Although the shape, preparation and testing of specimens of rectangular and bevelled cross-section are specified, component testing and other specialized forms of testing are not included in this document. Fatigue tests on notched specimens are not covered by this document since the shape and size of notched test pieces have not been specified in any standard so far. Guidelines are given in Annex A. However, the fatigue-test procedures described in this document can be used for testing such notched specimens. It is possible for the results of a fatigue test to be affected by atmospheric conditions. Where controlled conditions are required, ISO 554:1976, 2.1 applies.

Matériaux métalliques — Essais de fatigue — Méthode par flexion plane axiale

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Published
Publication Date
02-Jun-2021
Current Stage
6060 - International Standard published
Start Date
03-Jun-2021
Completion Date
03-Jun-2021
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INTERNATIONAL ISO
STANDARD 22407
First edition
2021-06
Metallic materials — Fatigue testing —
Axial plane bending method
Matériaux métalliques — Essais de fatigue — Méthode par flexion
plane axiale
Reference number
ISO 22407:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 22407:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 22407:2021(E)
Contents Page

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

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

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

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

4 Symbols .......................................................................................................................................................................................................................... 4

4.1 Symbols related to specimen geometry ............................................................................................................................ 4

4.2 Symbols related to testing device ........................................................................................................................................... 4

4.3 Symbols related to fatigue test .................................................................................................................................................. 4

5 Principle of test ...................................................................................................................................................................................................... 5

6 Test plan ........................................................................................................................................................................................................................ 5

6.1 General outline ........................................................................................................................................................................................ 5

7 Specimen ....................................................................................................................................................................................................................... 5

7.1 Shape of specimens ............................................................................................................................................................................. 5

7.2 Size of specimen ..................................................................................................................................................................................... 6

7.3 Preparation of specimens .............................................................................................................................................................. 6

7.3.1 General...................................................................................................................................................................................... 6

7.3.2 Machining procedure ................................................................................................................................................... 7

7.3.3 Sampling and marking ................................................................................................................................................ 7

7.3.4 Dimensional checks ....................................................................................................................................................... 8

7.3.5 Storage and handling ................................................................................................................................................... 8

8 Apparatus ..................................................................................................................................................................................................................... 8

8.1 Testing machine ..................................................................................................................................................................................... 8

8.1.1 Introduction ......................................................................................................................................................................... 8

8.1.2 Force transducer .............................................................................................................................................................. 8

8.1.3 Displacement transducer ......................................................................................................................................... 9

8.1.4 Cycle counter ....................................................................................................................................................................... 9

8.1.5 Instrumentation for test monitoring .............................................................................................................. 9

8.1.6 Anti-rotation system ..................................................................................................................................................... 9

8.2 Testing device ........................................................................................................................................................................................... 9

9 Stress calculation ..............................................................................................................................................................................................10

9.1 Introduction ...........................................................................................................................................................................................10

9.2 Rectangular cross-section ..........................................................................................................................................................10

9.2.1 Angular corner ................................................................................................................................................................10

9.2.2 Rounded corner .............................................................................................................................................................10

9.3 Bevelled cross-section ...................................................................................................................................................................10

10 Stress homogeneity check .......................................................................................................................................................................11

10.1 Principle .....................................................................................................................................................................................................11

10.2 Measurement method ....................................................................................................................................................................11

10.3 Calculations .............................................................................................................................................................................................11

11 Test procedure .....................................................................................................................................................................................................11

11.1 Mounting of testing device ........................................................................................................................................................11

11.2 Mounting of specimen ...................................................................................................................................................................12

11.3 Rate of testing .......................................................................................................................................................................................12

11.4 Application of force or displacement ................................................................................................................................12

11.5 Recording of temperature and humidity .......................................................................................................................12

11.6 Criterion of failure and test termination .......................................................................................................................12

11.6.1 Criterion of failure .......................................................................................................................................................12

11.6.2 Test termination ............................................................................................................................................................13

11.7 Test validity .............................................................................................................................................................................................13

12 Presentation of fatigue results ............................................................................................................................................................13

© ISO 2021 – All rights reserved iii
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ISO 22407:2021(E)

13 Test report ................................................................................................................................................................................................................13

14 Measurement uncertainty .......................................................................................................................................................................14

Annex A (informative) Fatigue notched specimens ............................................................................................................................17

Bibliography .............................................................................................................................................................................................................................18

iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 22407:2021(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.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 22407:2021(E)
Metallic materials — Fatigue testing — Axial plane bending
method
1 Scope

This document specifies the conditions for conducting the plane bending fatigue test on an axial

machine, constant-amplitude, force or displacement controlled, at room temperature (ideally between

10 °C and 35 °C) on metallic specimens, without deliberately introduced stress concentrations. This

document does not include the reversed/partially loading test. The purpose of the test is to provide

relevant results, such as the relation between applied stress and number of cycles to failure for a given

material condition, expressed by hardness and microstructure, at various stress ratios.

Although the shape, preparation and testing of specimens of rectangular and bevelled cross-section are

specified, component testing and other specialized forms of testing are not included in this document.

Fatigue tests on notched specimens are not covered by this document since the shape and size of

notched test pieces have not been specified in any standard so far. Guidelines are given in Annex A.

However, the fatigue-test procedures described in this document can be used for testing such notched

specimens.

It is possible for the results of a fatigue test to be affected by atmospheric conditions. Where controlled

conditions are required, ISO 554:1976, 2.1 applies.
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

ASTM E2309/E2309M, Standard Practices for Verification of Displacement Measuring Systems and Devices

Used in Material Testing Machines
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
thickness of test section
thickness of reduced section of rectangular test specimen
Note 1 to entry: See Figure 1.
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ISO 22407:2021(E)
3.2
width of test section
width of reduced section of rectangular test specimen
Note 1 to entry: See Figure 1.
3.3
specimen length
overall length of test specimen
3.4
specimen cross-section
surface of the specimen cross-section
3.5
corner radius
radius of the corner of rectangular cross-section specimen
3.6
distance between inner loading points
distance between the axes of the two inner rollers
Note 1 to entry: See Figure 3.
3.7
distance between outer loading points
distance between the axes of the two outer rollers
Note 1 to entry: See Figure 3.
3.8
roller diameter
diameter of the four rollers
3.9
stress cycle
smallest segment of stress-time that is repeated identically
Note 1 to entry: See Figure 4.
3.10
maximum stress
max
greatest algebraic value of stress in a stress cycle
Note 1 to entry: See Figure 4.
3.11
mean stress

one-half the algebraic sum of the maximum stress and the minimum stress in a stress cycle

Note 1 to entry: See Figure 4.
2 © ISO 2021 – All rights reserved
---------------------- Page: 7 ----------------------
ISO 22407:2021(E)
3.12
minimum stress
min
least algebraic value of stress in a stress cycle
Note 1 to entry: See Figure 4.
3.13
stress amplitude

one-half the algebraic difference between the maximum stress and the minimum stress in a stress cycle

Note 1 to entry: to entry:
σ = Δσ/2
Note 2 to entry: See Figure 4.
3.14
stress range
arithmetic difference between the maximum and minimum stress
Note 1 to entry: to entry:
Δσ = σ – σ
max min
Note 2 to entry: See Figure 4.
3.15
stress ratio
ratio of minimum to maximum stress during any single cycle of fatigue operation
Note 1 to entry: to entry:
R = σ /σ
σ min max
Note 2 to entry: See Figure 5.
3.16
load ratio
ratio of minimum to maximum load during any single cycle of fatigue operation
Note 1 to entry: to entry:
R = F /F
F min max
Note 2 to entry: See Figure 5.
3.17
number of cycles

number of smallest segments of the force-time, stress-time, strain-time, etc., function that is repeated

periodically
© ISO 2021 – All rights reserved 3
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ISO 22407:2021(E)
3.18
fatigue life
number of applied cycles to achieve a defined failure criterion
3.19
applied force
force applied during the test (for force-controlled test)
3.20
bending moment

constant moment between the inner rollers, calculated with the applied force and the distances

between the rollers (d and d )
1 2
Note 1 to entry: to entry:
M=−dd
4 Symbols
4.1 Symbols related to specimen geometry
Symbol Designation Unit
δ Thickness of test section mm
Reduced thickness of the bevelled
δ mm
specimen
w Width of test section mm
Reduced width of the bevelled
w mm
specimen
L Specimen length mm
I Second moment of area mm
Maximum distance from the neutral
d mm
nba
bending axis
S Specimen cross-section mm
r Corner radius mm
4.2 Symbols related to testing device
Symbol Designation Unit
Distance between inner loading
d mm
points
Distance between outer loading
d mm
points
D Roller diameter mm
4.3 Symbols related to fatigue test
Symbol Designation Unit
β Stress homogeneity for load i
σ Maximum stress MPa
max
4 © ISO 2021 – All rights reserved
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ISO 22407:2021(E)
Symbol Designation Unit
σ Mean stress MPa
σ Minimum stress MPa
min
σ Stress amplitude MPa
Δσ Stress range MPa
σ Test stress MPa
R Stress ratio
R Load ratio
N Number of cycles cycles
N Fatigue life cycles
F Applied force N
M Bending moment N-m
5 Principle of test

The principle of the test is to place a specimen between four rollers as shown in Figure 3. Then a constant

amplitude cyclic force is applied so that a constant amplitude tension stress is applied to the tested

surface of the specimen. The test is then continued until the specimen fails or until a predetermined

number of stress cycles is reached.

Nominally identical specimens are mounted on a fatigue testing machine and subjected to the loading

condition required to introduce cycles of plane bending stress. Any one of the types of cyclic stress

illustrated in Figure 5 may be used. The test waveform shall be constant-amplitude sinusoidal.

6 Test plan
6.1 General outline

Before commencing testing, the following shall be agreed by the parties concerned, unless specified

otherwise in the relevant product standard:
a) The form of specimen to be used (see 7.1);
b) The stress ratio(s) to be used;
c) The objective of the tests, i.e., which of the following is to be determined:
— the fatigue life at a specified stress amplitude;
— the fatigue strength at a specified number of cycles;
— a full Wöhler or S-N curve;
d) The number of specimens to be tested and the testing sequence;

e) The number of cycles at which a test on an unfailed specimen shall be terminated.

7 Specimen
7.1 Shape of specimens

The specimens are generally fully machined with a rectangular cross-section of uniform thickness over

the test section. In order to avoid crack initiation from corners, two solutions may be considered:

— machining of a radius on each corner (Figure 1);
© ISO 2021 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO 22407:2021(E)
[8]
— use of a bevelled cross-section specimen (Figure 2)

When it is desired to take account of the surface condition in which the metal will be used in actual

application, the surface of maximum stress should remain unmachined.
7.2 Size of specimen

In order to have a minimum influence of the shear stress on the bending stress (lower than 5 %,

expressed in Von Mises equivalent stress), the following ratio [Formula (1)] shall be respected:

<05, (1)
dd−

Due to the bending of the specimen, the distance between the contact points of the rollers on the

specimen changes when the load is applied. This change affects the stress level in the specimen

compared with the value calculated in Clause 9.

The error between the calculated stress, σ , and the real stress, σ , is proportional to the roller

cal real

diameter and can be approximated using Formula (2). This error shall be lower than 5 %.

σσ− 33Dd + d
calreal R 21
= Δr (2)
cal ()dd− ()dd+2
21 21

where the maximum displacement of the grips during the test, Δr, can be determined with Formula (3):

Δr=−dd dd+2 (3)
()()
21 21
12EI
where
E is the Young modulus of the tested specimen material.
7.3 Preparation of specimens
7.3.1 General

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

important to observe the following recommendations 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, these deviations shall be noted in the test report.

The surface conditions of the specimens have an effect on the test results. This effect is generally

associated with one or more of the following factors:
— the specimen surface roughness;
— the presence of residual stresses;
— alteration in the microstructure of the material;
— introduction of contaminants.

The recommendations given in 7.3.2, 7.3.3 and 7.3.4 allow for the influence of these factors to be reduced

to a minimum.
6 © ISO 2021 – All rights reserved
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ISO 22407:2021(E)
7.3.2 Machining procedure
7.3.2.1 General

The machining procedure selected can produce residual stresses on the specimen surface that are

likely to affect the test results. These stresses can be induced by heat gradients at the machining stage,

stresses associated with deformation of the material, or microstructural alterations. Their influence

is less when tested at high stress levels because they can be partially or totally relaxed. However, they

shall be reduced by using an appropriate final machining procedure, especially prior to a final polishing

stage.

It is recommended to check for such surface residual stresses using X-ray diffraction techniques, for

example.

The surface roughness is commonly quantified by the mean roughness value or equivalent (e.g. ten-

[1]

point roughness or maximum height of irregularities) . An arithmetic mean deviation of less than

0,2 µm (or equivalent) shall be specified.

Because the fatigue life is more dependent on local roughness, maximum height of profile value gives

additional information on the surface roughness and thus shall be measured and reported.

Another important parameter not covered by mean roughness is the presence of localized machining

scratches. Final grinding followed by longitudinal mechanical polishing is particularly recommended. A

low magnification check (approximately ×20) shall not show any transverse scratches within the gauge

length.

Longitudinal grinding rather than tool operation (milling) followed by polishing may be preferred.

If heat treatment is to be carried out after rough finishing of the specimens, it is preferable to carry out

the final polishing after the heat treatment. If this is not possible, the heat treatment should be carried

out in a vacuum or in inert gas to prevent oxidation of the specimen. This treatment shall not alter the

microstructural characteristics of the material under study. The specifics of the heat treatment and

machining procedure shall be reported with the test results.

In order to avoid premature crack initiation for raw corners, it is recommended to round angles by

polishing or even better to machine a given radius.
7.3.2.2 Alteration of the microstructure of the material

This phenomenon can be caused by an increase in temperature and/or by strain-hardening induced

by machining or grinding. It can be a matter of a change in phase or, more frequently, of surface

recrystallization. The immediate effect of this is to make a test invalid, as the material tested is no

longer the initial material. Every precaution should therefore be taken to avoid this risk.

If there is any doubt, a microstructure of the surface of the specimen shall be completed.

7.3.2.3 Introduction of contaminants

The mechanical properties of certain materials deteriorate when in the presence of certain elements or

compounds. An example of this is the effect of chlorine on steels and titanium alloys. These elements

shall therefore be avoided in the products used (cutting fluids, etc.). Rinsing and degreasing of

specimens prior to storage is also recommended.
7.3.3 Sampling and marking

The sampling of test materials from a semi-finished product or a component can have a major influence

on the results obtained during the test. It is therefore necessary for this sampling to be carried out

© ISO 2021 – All rights reserved 7
---------------------- Page: 12 ----------------------
ISO 22407:2021(E)

with full knowledge of the situation. If necessary, a sampling drawing, attached to the test report, shall

indicate clearly:
— the position of each specimen;

— the characteristic directions in which the semi-finished product has been worked (direction of

rolling, extrusion, etc., as appropriate);
— the marking/identifying of each of the specimens.

The specimens shall carry a mark/identification during each different stage of their preparation. Such a

mark/identification may be applied using any reliable method in an area not likely to disappear during

machining or likely to adversely affect the quality of the test.
Before testing, identification shall be applied to each end of the specimen.
7.3.4 Dimensional checks

The dimensions should be measured on completion of the final machining stage using a method of

metrology that does not alter the fatigue test result.
7.3.5 Storage and handling

After preparation, the specimens should be stored so as to prevent any risk of damage (scratching by

contact, oxidation, etc.). The use of individual container is recommended. In certain cases, storage in a

vacuum or in a desiccator filled with silica gel is necessary.

Handling should be reduced to the minimum necessary. Particular attention shall be given to avoid any

mark or scratch.
8 Apparatus
8.1 Testing machine
8.1.1 Introduction

The tests shall be carried out on a tension-compression machine, designed for a smooth start-up. For a

negative load ratio, no backlash when passing through zero is required. The machine shall have lateral

rigidity and accurate alignment.

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.

8.1.2 Force transducer

The force transducer shall have axial and lateral rigidity. Its capacity shall be suitable for the forces

applied during the test. It shall be fatigue rated and suitable for the forces applied during the test.

The indicated force as recorded at the output from the computer in an automated s

...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 22407
ISO/TC 164/SC 4
Metallic materials — Fatigue testing —
Secretariat: ANSI
Axial plane bending method
Voting begins on:
2021­03­04
Matériaux métalliques — Essais de fatigue — Méthode par flexion
plane axiale
Voting terminates on:
2021­04­29
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 22407:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2021
---------------------- Page: 1 ----------------------
ISO/FDIS 22407:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH­1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/FDIS 22407:2021(E)
Contents Page

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

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

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

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

4 Symbols .......................................................................................................................................................................................................................... 4

4.1 Symbols related to specimen geometry ............................................................................................................................ 4

4.2 Symbols related to testing device ........................................................................................................................................... 4

4.3 Symbols related to fatigue test .................................................................................................................................................. 4

5 Principle of test ...................................................................................................................................................................................................... 5

6 Test plan ........................................................................................................................................................................................................................ 5

6.1 General outline ........................................................................................................................................................................................ 5

7 Specimen ....................................................................................................................................................................................................................... 5

7.1 Shape of specimens ............................................................................................................................................................................. 5

7.2 Size of specimen ..................................................................................................................................................................................... 6

7.3 Preparation of specimens .............................................................................................................................................................. 6

7.3.1 General...................................................................................................................................................................................... 6

7.3.2 Machining procedure ................................................................................................................................................... 7

7.3.3 Sampling and marking ................................................................................................................................................ 7

7.3.4 Dimensional checks ....................................................................................................................................................... 8

7.3.5 Storage and handling ................................................................................................................................................... 8

8 Apparatus ..................................................................................................................................................................................................................... 8

8.1 Testing machine ..................................................................................................................................................................................... 8

8.1.1 Introduction ......................................................................................................................................................................... 8

8.1.2 Force transducer .............................................................................................................................................................. 8

8.1.3 Displacement transducer ......................................................................................................................................... 9

8.1.4 Cycle counter ....................................................................................................................................................................... 9

8.1.5 Instrumentation for test monitoring .............................................................................................................. 9

8.1.6 Anti-rotation system ..................................................................................................................................................... 9

8.2 Testing device ........................................................................................................................................................................................... 9

9 Stress calculation ..............................................................................................................................................................................................10

9.1 Introduction ...........................................................................................................................................................................................10

9.2 Rectangular cross­section ..........................................................................................................................................................10

9.2.1 Angular corner ................................................................................................................................................................10

9.2.2 Rounded corner .............................................................................................................................................................10

9.3 Bevelled cross­section ...................................................................................................................................................................10

10 Stress homogeneity check .......................................................................................................................................................................10

10.1 Principle .....................................................................................................................................................................................................10

10.2 Measurement method ....................................................................................................................................................................11

10.3 Calculations .............................................................................................................................................................................................11

11 Test procedure .....................................................................................................................................................................................................11

11.1 Mounting of testing device ........................................................................................................................................................11

11.2 Mounting of specimen ...................................................................................................................................................................11

11.3 Rate of testing .......................................................................................................................................................................................12

11.4 Application of force or displacement ................................................................................................................................12

11.5 Recording of temperature and humidity .......................................................................................................................12

11.6 Criterion of failure and test termination .......................................................................................................................12

11.6.1 Criterion of failure .......................................................................................................................................................12

11.6.2 Test termination ............................................................................................................................................................12

11.7 Test validity .............................................................................................................................................................................................12

12 Presentation of fatigue results ............................................................................................................................................................13

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/FDIS 22407:2021(E)

13 Test report ................................................................................................................................................................................................................13

14 Measurement uncertainty .......................................................................................................................................................................14

Annex A (informative) Fatigue notched specimens ............................................................................................................................17

Bibliography .............................................................................................................................................................................................................................18

iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/FDIS 22407:2021(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 non­governmental, 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.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 22407:2021(E)
Metallic materials — Fatigue testing — Axial plane
bending method
1 Scope

This document specifies the conditions for conducting the plane bending fatigue test on an axial

machine, constant-amplitude, force or displacement controlled, at room temperature (ideally between

10 °C and 35 °C) on metallic specimens, without deliberately introduced stress concentrations. This

document does not include the reversed/partially loading test. The purpose of the test is to provide

relevant results, such as the relation between applied stress and number of cycles to failure for a given

material condition, expressed by hardness and microstructure, at various stress ratios.

Although the shape, preparation and testing of specimens of rectangular and bevelled cross­section are

specified, component testing and other specialized forms of testing are not included in this document.

Fatigue tests on notched specimens are not covered by this document since the shape and size of

notched test pieces have not been specified in any standard so far. Guidelines are given in Annex A.

However, the fatigue­test procedures described in this document can be used for testing such notched

specimens.

It is possible for the results of a fatigue test to be affected by atmospheric conditions. Where controlled

conditions are required, ISO 554:1976, 2.1 applies.
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 7500­1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:

Tension/compression testing machines — Calibration and verification of the force-measuring system

ASTM E2309/E2309M, Standard Practices for Verification of Displacement Measuring Systems and Devices

Used in Material Testing Machines
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
thickness of test section
thickness of reduced section of rectangular test specimen
Note 1 to entry: See Figure 1.
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3.2
width of test section
width of reduced section of rectangular test specimen
Note 1 to entry: See Figure 1.
3.3
specimen length
overall length of test specimen
3.4
specimen cross-section
surface of the specimen cross­section
3.5
corner radius
radius of the corner of rectangular cross­section specimen
3.6
distance between inner loading points
distance between the axes of the two inner rollers
Note 1 to entry: See Figure 3.
3.7
distance between outer loading points
distance between the axes of the two outer rollers
Note 1 to entry: See Figure 3.
3.8
roller diameter
diameter of the four rollers
3.9
stress cycle
smallest segment of stress-time that is repeated identically
Note 1 to entry: See Figure 4.
3.10
maximum stress
max
greatest algebraic value of stress in a stress cycle
Note 1 to entry: See Figure 4.
3.11
mean stress

one-half the algebraic sum of the maximum stress and the minimum stress in a stress cycle

Note 1 to entry: See Figure 4.
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3.12
minimum stress
min
least algebraic value of stress in a stress cycle
Note 1 to entry: See Figure 4.
3.13
stress amplitude

one-half the algebraic difference between the maximum stress and the minimum stress in a stress cycle

Note 1 to entry: to entry:
σ = Δσ/2
Note 2 to entry: See Figure 4.
3.14
stress range
arithmetic difference between the maximum and minimum stress
Note 1 to entry: to entry:
Δσ = σ – σ
max min
Note 2 to entry: See Figure 4.
3.15
stress ratio
ratio of minimum to maximum stress during any single cycle of fatigue operation
Note 1 to entry: to entry:
R = σ /σ
σ min max
Note 2 to entry: See Figure 5.
3.16
load ratio
ratio of minimum to maximum load during any single cycle of fatigue operation
Note 1 to entry: to entry:
R = F /F
F min max
Note 2 to entry: See Figure 5.
3.17
number of cycles

number of smallest segments of the force­time, stress­time, strain­time, etc., function that is repeated

periodically
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3.18
fatigue life
number of applied cycles to achieve a defined failure criterion
3.19
applied force
force applied during the test (for force­controlled test)
3.20
bending moment

constant moment between the inner rollers, calculated with the applied force and the distances

between the rollers (d and d )
1 2
Note 1 to entry: to entry:
M=−dd
4 Symbols
4.1 Symbols related to specimen geometry
Symbol Designation Unit
δ Thickness of test section mm
Reduced thickness of the bevelled
δ mm
specimen
w Width of test section mm
Reduced width of the bevelled
w mm
specimen
L Specimen length mm
I Second moment of area mm
Maximum distance from the neutral
d mm
nba
bending axis
S Specimen cross­section mm
r Corner radius mm
4.2 Symbols related to testing device
Symbol Designation Unit
Distance between inner loading
d mm
points
Distance between outer loading
d mm
points
D Roller diameter mm
4.3 Symbols related to fatigue test
Symbol Designation Unit
σ Maximum stress MPa
max
σ Mean stress MPa
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Symbol Designation Unit
σ Minimum stress MPa
min
σ Stress amplitude MPa
Δσ Stress range MPa
σ Test stress MPa
R Stress ratio
R Load ratio
N Number of cycles cycles
N Fatigue life cycles
F Applied force N
M Bending moment N­m
5 Principle of test

The principle of the test is to place a specimen between four rollers as shown in Figure 3. Then a constant

amplitude cyclic force is applied so that a constant amplitude tension stress is applied to the tested

surface of the specimen. The test is then continued until the specimen fails or until a predetermined

number of stress cycles is reached.

Nominally identical specimens are mounted on a fatigue testing machine and subjected to the loading

condition required to introduce cycles of plane bending stress. Any one of the types of cyclic stress

illustrated in Figure 5 may be used. The test waveform shall be constant-amplitude sinusoidal.

6 Test plan
6.1 General outline

Before commencing testing, the following shall be agreed by the parties concerned, unless specified

otherwise in the relevant product standard:
a) The form of specimen to be used (see 7.1);
b) The stress ratio(s) to be used;
c) The objective of the tests, i.e., which of the following is to be determined:
— the fatigue life at a specified stress amplitude;
— the fatigue strength at a specified number of cycles;
— a full Wöhler or S­N curve;
d) The number of specimens to be tested and the testing sequence;

e) The number of cycles at which a test on an unfailed specimen shall be terminated.

7 Specimen
7.1 Shape of specimens

The specimens are generally fully machined with a rectangular cross-section of uniform thickness over

the test section. In order to avoid crack initiation from corners, two solutions may be considered:

— machining of a radius on each corner (Figure 1);
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[8]
— use of a bevelled cross­section specimen (Figure 2)

When it is desired to take account of the surface condition in which the metal will be used in actual

application, the surface of maximum stress should remain unmachined.
7.2 Size of specimen

In order to have a minimum influence of the shear stress on the bending stress (lower than 5 %,

expressed in Von Mises equivalent stress), the following ratio [Formula (1)] shall be respected:

<05, (1)
dd−

Due to the bending of the specimen, the distance between the contact points of the rollers on the

specimen changes when the load is applied. This change affects the stress level in the specimen

compared with the value calculated in Clause 9.

The error between the calculated stress, σ , and the real stress, σ , is proportional to the roller

cal real

diameter and can be approximated using Formula (2). This error shall be lower than 5 %.

σσ− 33Dd + d
calreal R 21
= Δr (2)
cal ()dd− ()dd+2
21 21

where the maximum displacement of the grips during the test, Δr, can be determined with Formula (3):

Δr=−dd dd+2 (3)
()()
21 21
12EI
where
E is the Young modulus of the tested specimen material.
7.3 Preparation of specimens
7.3.1 General

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

important to observe the following recommendations 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, these deviations shall be noted in the test report.

The surface conditions of the specimens have an effect on the test results. This effect is generally

associated with one or more of the following factors:
— the specimen surface roughness;
— the presence of residual stresses;
— alteration in the microstructure of the material;
— introduction of contaminants.

The recommendations given in 7.3.2, 7.3.3 and 7.3.4 allow for the influence of these factors to be reduced

to a minimum.
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7.3.2 Machining procedure
7.3.2.1 General

The machining procedure selected can produce residual stresses on the specimen surface that are

likely to affect the test results. These stresses can be induced by heat gradients at the machining stage,

stresses associated with deformation of the material, or microstructural alterations. Their influence is

less when tested at high stress levels because they can be partially or totally relaxed. However, they shall

be reduced by using an appropriate final machining procedure, especially prior to a final polishing stage.

It is recommended to check for such surface residual stresses using X-ray diffraction techniques, for

example.

The surface roughness is commonly quantified by the mean roughness value or equivalent (e.g. ten-

[1]

point roughness or maximum height of irregularities) . An arithmetic mean deviation of less than

0,2 µm (or equivalent) shall be specified.

Because the fatigue life is more dependent on local roughness, maximum height of profile value gives

additional information on the surface roughness and thus shall be measured and reported.

Another important parameter not covered by mean roughness is the presence of localized machining

scratches. Final grinding followed by longitudinal mechanical polishing is particularly recommended.

A low magnification check (approximately ×20) shall not show any transverse scratches within the

gauge length.

Longitudinal grinding rather than tool operation (milling) followed by polishing may be preferred.

If heat treatment is to be carried out after rough finishing of the specimens, it is preferable to carry out

the final polishing after the heat treatment. If this is not possible, the heat treatment should be carried

out in a vacuum or in inert gas to prevent oxidation of the specimen. This treatment shall not alter the

microstructural characteristics of the material under study. The specifics of the heat treatment and

machining procedure shall be reported with the test results.

In order to avoid premature crack initiation for raw corners, it is recommended to round angles by

polishing or even better to machine a given radius.
7.3.2.2 Alteration of the microstructure of the material

This phenomenon can be caused by an increase in temperature and/or by strain-hardening induced

by machining or grinding. It can be a matter of a change in phase or, more frequently, of surface

recrystallization. The immediate effect of this is to make a test invalid, as the material tested is no

longer the initial material. Every precaution should therefore be taken to avoid this risk.

If there is any doubt, a microstructure of the surface of the specimen shall be completed.

7.3.2.3 Introduction of contaminants

The mechanical properties of certain materials deteriorate when in the presence of certain elements or

compounds. An example of this is the effect of chlorine on steels and titanium alloys. These elements

shall therefore be avoided in the products used (cutting fluids, etc.). Rinsing and degreasing of

specimens prior to storage is also recommended.
7.3.3 Sampling and marking

The sampling of test materials from a semi-finished product or a component can have a major influence

on the results obtained during the test. It is therefore necessary for this sampling to be carried out

with full knowledge of the situation. If necessary, a sampling drawing, attached to the test report, shall

indicate clearly:
— the position of each specimen;
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— the characteristic directions in which the semi-finished product has been worked (direction of

rolling, extrusion, etc., as appropriate);
— the marking/identifying of each of the specimens.

The specimens shall carry a mark/identification during each different stage of their preparation. Such a

mark/identification may be applied using any reliable method in an area not likely to disappear during

machining or likely to adversely affect the quality of the test.
Before testing, identification shall be applied to each end of the specimen.
7.3.4 Dimensional checks

The dimensions should be measured on completion of the final machining stage using a method of

metrology that does not alter the fatigue test result.
7.3.5 Storage and handling

After preparation, the specimens should be stored so as to prevent any risk of damage (scratching by

contact, oxidation, etc.). The use of individual container is recommended. In certain cases, storage in a

vacuum or in a desiccator filled with silica gel is necessary.

Handling should be reduced to the minimum necessary. Particular attention shall be given to avoid any

mark or scratch.
8 Apparatus
8.1 Testing machine
8.1.1 Introduction

The tests shall be carried out on a tension­compression machine, designed for a smooth start­up. For a

negative load ratio,
...

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