ISO 12108:2018

Metallic materials — Fatigue testing — Fatigue crack growth method

This document describes tests for determining the fatigue crack growth rate from the fatigue crack growth threshold stress-intensity factor range, ΔKth, to the onset of rapid, unstable fracture. This document is primarily intended for use in evaluating isotropic metallic materials under predominantly linear-elastic stress conditions and with force applied only perpendicular to the crack plane (mode I stress condition), and with a constant force ratio, R.

Matériaux métalliques — Essais de fatigue — Méthode d'essai de propagation de fissure en fatigue

General Information

• Status: Published
• Publication Date: 17-Jul-2018
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: ISO/TC 164/SC 4 - Fatigue, fracture and toughness testing
• Drafting Committee: ISO/TC 164/SC 4/WG 6 - Fatigue testing: elasto-plastic strain and crack growth conditions
• Current Stage: 9020 - International Standard under periodical review
• Start Date: 15-Jul-2023
• Completion Date: 15-Jul-2023

Relations

• Revises: ISO 12108:2012 - Metallic materials — Fatigue testing — Fatigue crack growth method
• Effective Date: 20-May-2017

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 12108
Third edition
2018-07
Metallic materials — Fatigue testing —
Fatigue crack growth method
Matériaux métalliques — Essais de fatigue — Méthode d'essai de
propagation de fissure en fatigue
Reference number
ISO 12108:2018(E)
©
ISO 2018

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ISO 12108:2018(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2018
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
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Published in Switzerland
ii © ISO 2018 – All rights reserved

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ISO 12108:2018(E)

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 4
4.1 Symbols . 4
4.2 Abbreviated terms for specimen identification . 5
5 Apparatus . 5
6 Specimens . 6
6.1 General . 6
6.2 Crack plane orientation . 6
6.3 Starter notch precracking details. 8
6.4 Stress-intensity factor .10
6.5 Specimen size .10
6.6 Specimen thickness .10
6.7 Residual stress . .10
7 Procedure.11
7.1 Fatigue precracking .11
7.2 Crack length measurement .11
−5
7.3 Constant-force-amplitude, Κ-increasing, test procedure for da/dN > 10 mm/cycle .12
−5
7.4 K-decreasing procedure for da/dN < 10 mm/cycle .13
8 Crack length measurement .15
8.1 Resolution .15
8.2 Interruption .15
8.3 Static force .15
8.4 Measurement interval .15
8.5 Symmetry.16
8.6 Out-of-plane cracking .16
8.7 Crack tip bifurcation .16
8.8 Non-visual crack length measurement .16
8.9 Visual crack length measurement .16
9 Calculations.16
9.1 Crack-front curvature .16
9.2 Determining the fatigue crack growth rate .17
9.2.1 General.17
9.2.2 Secant method .17
9.2.3 Incremental polynomial method .18
9.3 Determination of the fatigue crack growth threshold .18
10 Test report .18
10.1 General .18
10.2 Material .18
10.3 Test specimen .19
10.4 Precracking terminal values .19
10.5 Test conditions .19
10.6 Test analysis .20
10.7 Presentation of results .20
Annex A (normative) Compact tension (CT) specimen .27
Annex B (normative) Centre crack tension (CCT) specimen .32
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ISO 12108:2018(E)

Annex C (normative) Single edge notch tension (SENT) specimen .38
Annex D (normative) Single edge notch bend (SENB) specimens .41
Annex E (informative) Non-visual crack length measurement methodology — Electric
[9][12][36]
potential difference .48
Bibliography .51
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ISO 12108:2018(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 on 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 the following
URL: 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.
This third edition cancels and replaces the second edition (ISO 12108:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
— The document has been reorganized to move the formulae and drawings for each of the test
specimens from the main body of the document into a separate normative annex for each specimen.
— Guidance on the effects of residual stress on fatigue crack growth rate data has been expanded.
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ISO 12108:2018(E)

Introduction
This document is intended to provide specifications for generation of fatigue crack growth rate data.
Test results are expressed in terms of the fatigue crack growth rate as a function of crack-tip stress-
[15][16][17]
intensity factor range, ΔK, as defined by the theory of linear elastic fracture mechanics
[18][19][20]
. Expressed in these terms, the results characterize a material's resistance to subcritical
crack extension under cyclic force test conditions. This resistance is independent of specimen planar
geometry and thickness, within the limitations specified in Clause 6.
This document describes a method of subjecting a precracked notched specimen to a cyclic force. The
crack length, a, is measured as a function of the number of elapsed force cycles, N. From the collected
crack length and corresponding force cycles relationship, the fatigue crack growth rate, da/dN, is
determined and is expressed as a function of stress-intensity factor range, ΔK.
Materials that can be tested by this method are limited by size, thickness and strength only to the
extent that the material remains predominantly in an elastic condition during testing and that buckling
is precluded.
Specimen size can vary over a wide range. Proportional planar dimensions for six standard
configurations are presented. The choice of a particular specimen configuration can be dictated
by the actual component geometry, compression test conditions or suitability for a particular test
environment.
Specimen size is a variable that is subjective to the test material's 0,2 % proof strength and the
maximum stress-intensity factor applied during test. Specimen thickness can vary independently of the
planar size, within defined limits, so long as large-scale yielding is precluded and out-of-plane distortion
or buckling is not encountered. Any alternate specimen configuration other than those included in
this document can be used, provided there exists an established stress-intensity factor calibration
[21][22][23]
expression, i.e. stress-intensity factor geometry function, g (a/W) .
[24][25] [26][27]
Residual stresses , crack closure , specimen thickness, cyclic waveform, frequency and
environment, including temperature, can markedly affect the fatigue crack growth data but are in no
way reflected in the computation of ΔK, and so should be recognized in the interpretation of the test
results and be included as part of the test report. All other demarcations from this method should be
noted as exceptions to this practice in the final report.
−5
For crack growth rates above 10 mm/cycle, the typical scatter in test results generated in a single
[28] −5
laboratory for a given ΔK can be in the order of a factor of two . For crack growth rates below 10
mm/cycle, the scatter in the da/dN calculation can increase to a factor of 5 or more. To ensure the
correct description of the material's da/dN versus ΔK behaviour, a replicate test conducted with the
same test parameters is highly recommended.
[29]
Service conditions can exist where varying ΔK under conditions of constant K or K control
max mean
can be more representative than data generated under conditions of constant force ratio; however,
these alternate test procedures are beyond the scope of this document.
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INTERNATIONAL STANDARD ISO 12108:2018(E)
Metallic materials — Fatigue testing — Fatigue crack
growth method
WARNING — This document does not address safety or health concerns, should such issues
exist, that can be associated with its use or application. The user of this document has the sole
responsibility to establish any appropriate safety and health concerns.
1 Scope
This document describes tests for determining the fatigue crack growth rate from the fatigue crack
growth threshold stress-intensity factor range, ΔK , to the onset of rapid, unstable fracture.
th
This document is primarily intended for use in evaluating isotropic metallic materials under
predominantly linear-elastic stress conditions and with force applied only perpendicular to the crack
plane (mode I stress condition), and with a constant force ratio, R.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain 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
crack length
a
crack size
linear measure of a principal planar dimension of a crack from a reference plane to the crack tip
3.2
cycle
smallest segment of a force-time or stress-time function which is repeated periodically
Note 1 to entry: The terms “fatigue cycle”, “force cycle” and “stress cycle” are used interchangeably. The letter N
is used to represent the number of elapsed cycles.
3.3
fatigue crack growth rate
da/dN
extension in crack length
3.4
maximum force
F
max
force having the highest algebraic value in the cycle, a tensile force being positive and a compressive
force being negative
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ISO 12108:2018(E)

3.5
minimum force
F
min
force having the lowest algebraic value in the cycle, a tensile force being positive and a compressive
force being negative
3.6
force range
ΔF
algebraic difference between the maximum and minimum forces in a cycle
ΔF = F − F
max min
3.7
force ratio
R
stress ratio
algebraic ratio of the minimum force or stress to the maximum force or stress in a cycle
R = F /F
min max
Note 1 to entry: R can also be calculated using the values of stress-intensity factors; R = K /K .
min max
3.8
stress-intensity factor
K
magnitude of the ideal crack-tip stress field for the opening mode force application to a crack in a
homogeneous, linear-elastically stressed body, where the opening mode of a crack corresponds to the
force being applied to the body perpendicular to the crack faces only (mode I)
Note 1 to entry: The stress-intensity factor is a function of applied force, crack length, specimen size and
geometry.
3.9
maximum stress-intensity factor
K
max
highest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current
max
crack length
3.10
minimum stress-intensity factor
K
min
lowest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current
min
crack length
Note 1 to entry: This definition remains the same, regardless of the minimum force being tensile or compressive.
For a negative force ratio (R < 0), there is an alternate, commonly used definition for the minimum stress-
intensity factor, K = 0. See 3.11.
min
3.11
stress-intensity factor range
ΔK
algebraic difference between the maximum and minimum stress-intensity factors in a cycle
ΔK = K − K
max min
Note 1 to entry: The variables ΔK, R and K are related as follows: ΔK = (1 − R) K .
max max
Note 2 to entry: For R ≤ 0 conditions, see 3.10 and 10.6.
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ISO 12108:2018(E)

Note 3 to entry: When comparing data developed under R ≤ 0 conditions with data developed under R > 0
conditions, it can be beneficial to plot the da/dN data versus K .
max
3.12
fatigue crack growth threshold stress-intensity factor range
ΔK
th
asymptotic value of ΔK for which da/dN approaches zero
Note 1 to entry: For most materials, the threshold is defined as the stress-intensity factor range corresponding to
−7
10 mm/cycle. When reporting ΔK , the corresponding lowest decade of da/dN data used in its determination
th
should also be included.
3.13
normalized K-gradient
C = (1/K) dK/da
fractional rate of change of K with increased crack length, a
C = 1/K (dK/da) = 1/K (dK /da) = 1/K (dK /da) = 1/ΔK (dΔK/da)
max max min min
3.14
K-decreasing test
test in which the value of the normalized K-gradient, C, is negative
Note 1 to entry: A K-decreasing test is conducted by reducing the stress-intensity factor either by continuously
shedding or by a series of steps, as the crack grows.
3.15
K-increasing test
test in which the value of C is positive
Note 1 to entry: For standard specimens, a constant force amplitude results in a K-increasing test where the
value of C is positive and increasing.
3.16
stress-intensity factor geometry function
g (a/W)
mathematical expression, based on experimental, numerical or analytical results, that relates the
stress-intensity factor to force and crack length for a specific specimen configuration
3.17
crack-front curvature correction length
a
cor
difference between the average through-thickness crack length and the corresponding crack length at
the specimen faces during the test
3.18
fatigue crack length
a
fat
length of the fatigue crack, as measured from the root of the machined notch
Note 1 to entry: See Figure 2.
3.19
notch length
a
n
length of the machined notch, as measured from the load line to the notch root
Note 1 to entry: See Figure 2.
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ISO 12108:2018(E)

3.20
specimen width
W
linear measure of a principal planar dimension of a specimen from a reference plane to the specimen edge
4 Symbols and abbreviated terms
4.1 Symbols
See Table 1.
Table 1 — Symbols and their designations
Symbol Designation Unit
Loading
−1
C Normalized K-gradient mm
E Tensile modulus of elasticity MPa
F Force kN
F Maximum force kN
max
F Minimum force kN
min
ΔF Force range kN
1/2
K Stress-intensity factor MPa·m
1/2
K Maximum stress-intensity factor MPa·m
max
1/2
K Minimum stress-intensity factor MPa·m
min
1/2
ΔK Stress-intensity factor range MPa·m
1/2
ΔK Initial stress-intensity factor range MPa·m
i
1/2
ΔK Fatigue crack growth threshold stress-intensity factor range MPa·m
th
N Number of cycles cycle
R Force ratio kN/kN
R Ultimate tensile strength at the test temperature MPa
m
R 0,2 % proof strength at the test temperature MPa
p0,2
Geometry
a Crack length or size measured from the reference plane to the crack tip mm
a Crack-front curvature correction length mm
cor
a Fatigue crack length measured from the notch root mm
fat
a Machined notch length mm
n
a Precrack length mm
p
B Specimen thickness mm
Hole diameter for CT, SENT or CCT specimen, loading tup diameter for bend
D mm
specimens
g(a/W) Stress-intensity factor geometry function unitless
h Notch height mm
W Specimen width measured from the reference plane to the specimen edge mm
(W − a) Uncracked ligament mm
Crack growth
da/dN Fatigue crack growth rate mm/cycle
Δa Change in crack length, crack extension mm
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ISO 12108:2018(E)

4.2 Abbreviated terms for specimen identification
CT Compact tension
CCT Centre cracked tension
SENT Single edge notch tension
SEN B3 Three-point single edge notch bend
SEN B4 Four-point single edge notch bend
SEN B8 Eight-point single edge notch bend
5 Apparatus
5.1 Testing machine.
5.1.1 The testing machine shall have smooth start-up and a backlash-free force train if passing through
zero force (tension – compression). Cycle to cycle variation of the peak force during precracking shall be
less than ±5 % and shall be held to within ±2 % of the desired peak force during the test. ΔF shall also
be maintained to within ±2 % of the desired range during test. A practical overview of test machines and
[36][37]
instrumentation is available .
5.1.2 If a dynamic force calibration is appropriate or required (e.g. by the purchaser), it should be
conducted according to ISO 4965-1. Dynamic force calibration is appropriate when inertial forces act
on the force transducer or any dynamic errors occur in the electronics of the force indicating system,
as described in ISO 4965-1. Test frequency and amplitude as well as grip mass can affect the inertial
forces acting on the force transducer. Examples for which dynamic force calibration can be appropriate
are configurations with the load cell on the moving piston or the part.
5.1.3 In terms of testing machine alignment, asymmetry of the crack front is an indication of
misalignment. For tension-compression testing, the length of the force train should be as short and stiff
as practical. Non-rotating joints should be used to minimize off-axis motion. It is important that adequate
attention be given to alignment of the testing machine and during machining and installation of the grips
in the testing machine. Regarding the relevance of alignment, a distinction shall be made between:
— Crack growth tests with rigid gripping and rigid load train which can also undergo compressive
forces and stresses (e.g. corner crack test pieces): a sufficient alignment of the load train can be
important for these test pieces to obtain correct and reproducible crack growth data, and
— Crack growth tests only with tensile load and fixed with bolts and using cardanic joints (e.g. CT-
specimens): due to the use of cardanic or similar joints in the load train alignment checks are not
necessary.
If an alignment check is appropriate (e.g. when using a rigid load train and grips) and required (e.g. by
the purchaser), it should be conducted according to ISO 23788 and using an alignment class 5 according
to ISO 23788. If alignment check is conducted, the results shall be reported.
5.1.4 Accuracy of the force measuring system shall be verified periodically in the testing machine.
The calibration for the force transducer shall be traceable to a national organization of metrology. The
force measuring system shall be designed for tension and compression fatigue testing and possess
great axial and lateral rigidity. The indicated force, as recorded as the output from the computer in an
automated system or from the final output recording device in a non-computer system, shall be within
the permissible variation from the actual force. The force transducer's capacity shall be sufficient to cover
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ISO 12108:2018(E)

the range of force measured during a test. Errors greater than 1 % of the difference between minimum
and maximum measured test force are not acceptable.
The force measuring system shall be temperature compensated, not have zero drift greater than
0,002 % of full scale, nor have a sensitivity variation greater than 0,002 % of full scale over a 1 °C
change. During elevated and cryogenic temperature testing, suitable thermal shielding/compensation
shall be provided to the force measuring system so it is maintained within its compensation range.
5.2 Cycle-counter.
An accurate digital device is required to count elapsed force cycles. A timer is to be used only as a
verification check on the accuracy of the counter. It is preferred that individual force cycles be counted.
−5
However, when the crack velocity is below 10 mm/cycle, counting in increments of 10 cycles is
acceptable.
5.3 Crack length measurement apparatus.
Accurate measurement of crack length during the test is very important. There are a number of
visual and non-visual apparati that can be use
...