Metallic materials — Fatigue testing — Fatigue crack growth method

ISO 12108:2012 describes tests for determining the fatigue crack growth rate from the fatigue crack growth threshold stress-intensity factor range to the onset of rapid, unstable fracture. ISO 12108:2012 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 stress ratio.

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

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Publication Date
13-Aug-2012
Withdrawal Date
13-Aug-2012
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9599 - Withdrawal of International Standard
Start Date
18-Jul-2018
Completion Date
18-Jul-2018
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INTERNATIONAL ISO
STANDARD 12108
Second edition
2012-08-15
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 12108:2012(E)
ISO 2012
---------------------- Page: 1 ----------------------
ISO 12108:2012(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2012

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 2012 – All rights reserved
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ISO 12108:2012(E)
Contents Page

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

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

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

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

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

4 Symbols and abbreviated terms ..................................................................................................................... 3

4.1 Symbols ................................................................................................................................................................. 3

4.2 Abbreviated terms for specimen identification ........................................................................................... 4

5 Apparatus .............................................................................................................................................................. 5

5.1 Testing machine ................................................................................................................................................... 5

5.2 Cycle-counter ....................................................................................................................................................... 5

5.3 Grips and fixtures for CT specimens ............................................................................................................. 5

5.4 Grips and fixtures for CCT/SENT specimens .............................................................................................. 7

5.5 Grips and fixtures for the SENB specimens .............................................................................................. 11

5.6 Crack length measurement apparatus ........................................................................................................ 11

6 Specimens ...........................................................................................................................................................12

6.1 General .................................................................................................................................................................12

6.2 Crack plane orientation ...................................................................................................................................16

6.3 Starter notch precracking details .................................................................................................................18

6.4 Stress-intensity factor ......................................................................................................................................20

6.5 Specimen size ....................................................................................................................................................21

6.6 Specimen thickness .........................................................................................................................................22

6.7 Residual stresses ..............................................................................................................................................23

7 Procedure ............................................................................................................................................................23

7.1 Fatigue precracking ..........................................................................................................................................23

7.2 Crack length measurement .............................................................................................................................23

7.3 Constant-force-amplitude, Κ-increasing, test procedure for da/dN > 10 mm/cycle......................24

7.4 K-decreasing procedure for da/dN < 10 mm/cycle .................................................................................25

8 Crack length measurement .............................................................................................................................27

8.1 Resolution ...........................................................................................................................................................27

8.2 Interruption .........................................................................................................................................................27

8.3 Static force ..........................................................................................................................................................27

8.4 Measurement interval .......................................................................................................................................27

8.5 Symmetry .............................................................................................................................................................28

8.6 Out-of-plane cracking .......................................................................................................................................28

8.7 Crack tip bifurcation .........................................................................................................................................28

9 Calculations ........................................................................................................................................................28

9.1 Crack-front curvature .......................................................................................................................................28

9.2 Determining the fatigue crack growth rate .................................................................................................28

9.3 Determination of the fatigue crack growth threshold..............................................................................29

10 Test report ...........................................................................................................................................................29

10.1 General .................................................................................................................................................................29

10.2 Material .................................................................................................................................................................29

10.3 Test specimen ....................................................................................................................................................30

10.4 Precracking terminal values ...........................................................................................................................30

10.5 Test conditions ...................................................................................................................................................30

10.6 Test analysis .......................................................................................................................................................31

10.7 Presentation of results .....................................................................................................................................31

Annex A (informative) Non-visual crack length measurement methodology — Electric potential

[18] [24] [33]

difference ..........................................................................................................................................38

© ISO 2012 – All rights reserved iii
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ISO 12108:2012(E)

Bibliography .....................................................................................................................................................................41

iv © ISO 2012 – All rights reserved
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ISO 12108:2012(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 12108 was prepared by Technical Committee ISO/TC 164, Mechanical testing of metals, Subcommittee

SC 5, Fatigue testing.

This second edition cancels and replaces the first edition (ISO 12108:2002), which has been technically revised.

© ISO 2012 – All rights reserved v
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ISO 12108:2012(E)
Introduction

This International Standard 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-

[1]-[6]

intensity factor range, ΔK, as defined by the theory of linear elastic fracture mechanics . 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

[7]
specified in Clause 6. All values are given in SI units .

This International Standard 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 must remain predominantly in an elastic condition during testing and that buckling is precluded.

Specimen size may vary over a wide range. Proportional planar dimensions for six standard configurations

are presented. The choice of a particular specimen configuration may 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 may vary independent 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 International Standard may be used,

provided there exists an established stress-intensity factor calibration expression, i.e. stress-intensity factor

[9]-[11]
geometry function, g (a/W).
[12],[13] [14],[15]

Residual stresses , crack closure , specimen thickness, cyclic waveform, frequency and

environment, including temperature, may 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.

For crack growth rates above 10 mm/cycle, the typical scatter in test results generated in a single laboratory

[16] −5

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 may 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.

[17]

Service conditions may exist where varying ΔK under conditions of constant K or K control may be

max mean

more representative than data generated under conditions of constant force ratio; however, these alternate test

procedures are beyond the scope of this International Standard.
vi © ISO 2012 – All rights reserved
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INTERNATIONAL STANDARD ISO 12108:2012(E)
Metallic materials — Fatigue testing — Fatigue crack growth
method
1 Scope

This International Standard 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.

This International Standard 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 stress ratio, R.
2 Normative references

The following normative 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 4965-1, Metallic materials — Dynamic force calibration for uniaxial fatigue testing — Part 1: Testing systems

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
crack length

linear measure of a principal planar dimension of a crack from a reference plane to the crack tip

NOTE This is also called crack size.
3.2
cycle

smallest segment of a force-time or stress-time function which is repeated periodically

NOTE The terms “fatigue cycle”, “force cycle” and “stress cycle” are used interchangeably. The letter N is used to

represent the number of elapsed force cycles.
3.3
fatigue crack growth rate
da/dN
extension in crack length
3.4
maximum force
max

force having the highest algebraic value in the cycle; a tensile force being positive and a compressive force

being negative
3.5
minimum force
min

force having the lowest algebraic value in the cycle; a tensile force being positive and a compressive force

being negative
© ISO 2012 – All rights reserved 1
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ISO 12108:2012(E)
3.6
force range
the algebraic difference between the maximum and minimum forces in a cycle
ΔF = F − F
max min
3.7
force ratio
algebraic ratio of the minimum force to maximum force in a cycle
R = F /F
min max
NOTE 1 R is also called the stress ratio.

NOTE 2 R may also be calculated using the values of stress-intensity factors; R = K /K .

min max
3.8
stress-intensity factor

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 The stress-intensity factor is a function of applied force, crack length, specimen size and geometry.

3.9
maximum stress-intensity factor
max

highest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current crack length

max
3.10
minimum stress-intensity factor
min

lowest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current crack length

min

NOTE 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

algebraic difference between the maximum and minimum stress-intensity factors in a cycle

ΔK = K − K
max min
NOTE 1 The force variables ΔK, R and K are related as follows: ΔK = (1 − R) K .
max max
NOTE 2 For R ≤ 0 conditions, see 3.10 and 10.6.

NOTE 3 When comparing data developed under R ≤ 0 conditions with data developed under R > 0 conditions, it may be

beneficial to plot the da/dN data versus K .
max
3.12
fatigue crack growth threshold stress-intensity factor range
asymptotic value of ΔK for which da/dN approaches zero

NOTE For most materials, the threshold is defined as the stress-intensity factor range corresponding to 10 mm/cycle.

When reporting ΔK , the corresponding lowest decade of da/dN data used in its determination should also be included.

2 © ISO 2012 – All rights reserved
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ISO 12108:2012(E)
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 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 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
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
fat
length of the fatigue crack, as measured from the root of the machined notch
NOTE See Figure 12.
3.19
notch length
length of the machined notch, as measured from the load line to the notch root
NOTE See Figure 12.
4 Symbols and abbreviated terms
4.1 Symbols
See Table 1.
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ISO 12108:2012(E)
Table 1 — Symbols and their designations
Symbol Designation Unit
Loading
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
1/2
ΔK Fatigue crack growth threshold stress-intensity factor range MPa·m
N Number of cycles 1
R Force ratio or stress ratio 1
R Ultimate tensile strength at the test temperature MPa
R 0,2 % proof strength at the test temperature MPa
p0,2
Geometry
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
a Precrack length mm
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 1
h Notch height mm
W Specimen width, distance from reference plane to edge of specimen mm
(W − a) Minimum uncracked ligament mm
Crack growth
da/dN Fatigue crack growth rate mm/cycle
Δa Change in crack length, crack extension mm
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
4 © ISO 2012 – All rights reserved
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ISO 12108:2012(E)
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 General

The testing machine shall have smooth start-up and a backlash-free force train if passing through zero force.

See ISO 4965-1. 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

[33], [34]

the desired range during test. A practical overview of test machines and instrumentation is available .

5.1.2 Testing machine alignment

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.

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.

Asymmetry of the crack front is an indication of misalignment; a strain gauged specimen similar to the test article

under investigation can be used in aligning the force train and to minimize nonsymmetrical stress distribution

and/or bending strain to less than 5 %.
5.1.3 Force measuring system

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 noncomputer system, shall be within the permissible variation from the actual force. The

force transducer’s capacity shall be sufficient to cover 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. However, when the

crack velocity is below 10 mm/cycle, counting in increments of 10 cycles is acceptable.

5.3 Grips and fixtures for CT specimens

Force is applied to a CT specimen through pinned joints. The choice of this specimen and gripping arrangement

necessitates tension-tension test conditions only. Figure 1 shows the clevis and mating pin assembly used at

both the top and bottom of a CT specimen to apply the force perpendicular to the machined starter notch and

crack plane. Suggested dimensions are expressed as a proportion of specimen width, W, or thickness, B, since

these dimensions can vary independently within the limits specified in Clause 6. The pin holes have a generous

clearance over the pin diameter, 0,2W minimum, to minimize resistance to specimen and pin in-plane rotation

[35]

which has been shown to cause nonlinearity in the force versus displacement response . A surface finish,

© ISO 2012 – All rights reserved 5
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ISO 12108:2012(E)

Ra, range of 0,8 µm to 1,6 µm is suggested for grip surfaces. With this grip-and-pin arrangement, materials

with low proof strength may sustain plastic deformation at the specimen pin hole; similarly, when testing high

strength materials and/or when the clevis displacement exceeds 1,05B, a stiffer force pin, i.e. a diameter

greater than 0,225W, may be required. As an alternative approach to circumvent plastic deformation, a flat

bottom clevis hole may be used along with a pin diameter equaling 0,24W. Any heat treatable steel thermally

processed to a 0,2 % proof strength of 1 000 MPa used in fabricating the clevises will usually provide adequate

strength and resistance to fretting, galling and fatigue.

In addition to the generous pin hole clearance, the mating surfaces shall be prepared to minimize friction

which could invalidate the provided K-calibration expression. The use of high viscosity lubricants and greases

has been shown to cause hysteresis in the force versus displacement response and is not recommended if

compliance measurements are required.
Key
1 clevis
2 pin

NOTE For high strength materials or large pin displacements, the pin may be stiffened by increasing the diameter to

0,24W along with using D-shaped flat bottom holes.
Loading rod thread.
Through diameter.
These surfaces are perpendicular and parallel as applicable to within 0,05W.
Figure 1 — Clevis and pin assembly for gripping a CT specimen
6 © ISO 2012 – All rights reserved
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ISO 12108:2012(E)
5.4 Grips and fixtures for CCT/SENT specimens
5.4.1 General

Force can be applied to CCT and SENT specimens through pinned joints and/or through frictional clamping

grips. Gripping for the CCT and SENT specimens depends on specimen width and whether the test condition is

to be tension-tension or tension-compression. The minimum CCT specimen gauge length varies with gripping

arrangement and shall provide a uniform stress distribution in the gauge length during the test.

Under certain conditions, the CCT specimen can be prone to general and localized buckling. The use of

[49]
buckling constraints is recommended.

Formula (6) is applicable only for a single pinned end SENT specimen, as shown in Figure 2. The SENT pinned

end specimen (Figure 2) is appropriate for tension-tension test conditions only.

Formula (7) is applicable for a SENT specimen with clamped ends and is appropriate for both tension and

compression force conditions. For the clamped-end SENT specimen, the grips must be sufficiently stiff to

circumvent any rotation of the specimen ends or any lateral movement of the crack plane; the presence of

[29]

either condition introduces errors into the stress-intensity factor calculation .

Surface roughness values in micrometres
NOTE 1 The machined notch is centred to within ± 0,005W (TIR ).
NOTE 2 The surfaces are parallel and perpendicular to within ± 0,002W.

NOTE 3 The crack length is measured from the reference loading plane containing the starter V-notch.

NOTE 4 This specimen is recommended for notch root tension at a force ratio R > 0 only.

D = W / 3.
See Figure 12 for notch detail.
Reference plane.
Recommended thickness: B ≤ 0,5W.
Total indicated reference value.
Figure 2 — Standard single edge notch tension, SENT, specimen
5.4.2 Tension-tension testing of a CCT specimen

For tension-tension testing of a specimen with a width 2W, less than 75 mm, as shown in Figure 3, a clevis with

single force pin is acceptable for gripping provided the specimen gauge length, defined here as the distance

between the pin hole centrelines, be at least 6W. Shims may be helpful in circumventing fretting fatigue at the

specimen’s pin hole. Another step that can be taken to prevent crack initiation at the pin holes is the welding

or adhesive bonding of reinforcement plates or tabs to the gripping area, especially when testing very thin

materials. Cutting the test section down in width to form a “dog bone” shaped specimen design is another

© ISO 2012 – All rights reserved 7
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ISO 12108:2012(E)

measure that can be adopted to circumvent failure at the pin holes; here the gauge length is defined as the

uniform width section and it shall be at least 3,4W in length.
For tens
...

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