ISO 12108:2012

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

General Information

• Status: Withdrawn
• Publication Date: 13-Aug-2012
• Withdrawal Date: 13-Aug-2012
• 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 - Fatigue, fracture and toughness testing
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 18-Jul-2018
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 12108:2018 - Metallic materials - Fatigue testing - Fatigue crack growth method
• Effective Date: 20-May-2017
• Revises: ISO 12108:2002 - Metallic materials - Fatigue testing - Fatigue crack growth method
• Effective Date: 15-Apr-2008

Standards Content (Sample)

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 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
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Published in Switzerland
ii © ISO 2012 – All rights reserved

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
−5
7.3 Constant-force-amplitude, Κ-increasing, test procedure for da/dN > 10 mm/cycle.24
−5
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
Bibliography .41
iv © ISO 2012 – All rights reserved

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.
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.
−5
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

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.
th
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
a
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
N
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
F
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
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
the algebraic difference between the maximum and minimum forces in a cycle
ΔF = F − F
max min
3.7
force ratio
R
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
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 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 crack length
max
3.10
minimum stress-intensity factor
K
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
ΔK
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
ΔK
th
asymptotic value of ΔK for which da/dN approaches zero
−8
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.
th
2 © ISO 2012 – All rights reserved

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
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 See Figure 12.
3.19
notch length
a
n
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.
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 1
R Force ratio or stress ratio 1
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 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

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
−5
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,
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.
a
Loading rod thread.
b
Through diameter.
c
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

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
e
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.
a
D = W / 3.
b
See Figure 12 for notch detail.
c
Reference plane.
d
Recommended thickness: B ≤ 0,5W.
e
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
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 tension-tension testing of a specimen with a width greater than 75 mm, distributing the force across the
specimen width with multiple pin holes is recommended. A serrated grip surface at the specimen-grip interface
increases the force that can be transferred. With this force application arrangement, the gauge length between
the innermost rows of pin holes must be at least 3W.
Surface roughness values in micrometres
NOTE 1 The machined notch is centred to within ± 0,002W.
NOTE 2 The faces are parallel to ± 0,05 mm/mm.
NOTE 3 The two faces are not out-of-plane more than 0,05 mm.
NOTE 4 The crack length is measured from the reference plane of the longitudinal centreline.
NOTE 5 The clevis and pin loading system is not suitable for a force ratio R < 0.
NOTE 6 Special gripping systems may be used for a force ratio R < 0 such as shown in Figure 4.
a
See Figure 12 for notch detail.
b
D = 2W /3.
c
Reference plane.
Figure 3 — Standard pinned end centre cracked tension, CCT, specimen for 2W ≤ 75 mm
5.4.3 Tension-compression testing of a CCT specimen
A backlash-free gripping arrangement shall be used for tension-compression testing of the CCT specimen.
Various commercially available pneumatic and hydraulic wedge grips that provide adequate clamping force
may be used. The minimum gauge length for a clamped CCT specimen is 2,4W.
For tension-compression testing of a CCT specimen, Figure 4 presents a design that affords a simple
backlash free grip that provides improved force transfer through multiple pins plus frictional force transfer via
specimen clamp-up with the serrated gripping surfaces. The compressive condition between the pins and the
specimen’s end surfaces, induced by drawing the wedges together, affords large reverse force excursions
8 © ISO 2012 – All rights reserved

while circumventing elongation of the pin holes. The minimum gauge length for this specimen is 2,4W between
the grip end surfaces and 3W between the inner rows of pins, as stated above.
Dimensions in millimetres
Key
1 Serrated sideplate surface
2 Countersunk cap screw
3 Lock nut
NOTE 1 Made of hardened steel, e.g. ≥ 40 HRC.
NOTE 2 Serrated side plates vary in thickness to accommodate approximately 2 mm to 3 mm, range in thickness B.
a
Body drilled.
Figure 4 — Example of backlash free grip for a CCT specimen
5.4.4 Alignment of CCT specimen grips
The CCT specimen is sensitive to misalignment and nonsymmetrical force application, especially in tension-
compression testing where gimbaled connections are not used, which can readily lead to violation of the
through thickness crack curvature and/or symmetry validity criteria. It is recommended that bending strain
be checked periodically with a panel specimen similar to the one being tested and instrumented with strain
[22]
gauges, as shown in Figure 5 . This technique can be used to minimize the bending strain. See 5.1.2.
1 to 4 locations indicating faces on the specimen
5 to 8 locations indicating strain gauges applied to the specimen.
a
Plane A.
[22]
Figure 5 — Strain gauge arrangement for an instrumented panel alignment specimen
[22]
5.4.5 Bending strain calculation for the arrangement shown in Figure 5 :
The average axial strain, ε , for the flat panel calibration specimen is calculated using:
a
εε++εε+
()
56 78
ε =
a
where ε , ε , ε and ε are the measured strains.
5 6 7 8
The equivalent strain at the centre of the four faces 1 to 4 is calculated using:
  
εε=− εε−+()ε //22WW()22− d ;
15aa 8
  
  
εε=− εε−+ε //22WW22− d ;
() ()
36aa 7
  
εε=+ε /2 ; εε=+ε /2 .
() ()
25 6 47 8
The local bending strains at the centre of each of the four faces are calculated using:
b =−εε ; b =−εε ; b =−εε ; b =−εε .
11 a 22 a 33 a 44 a
The maximum bending strain percentage in plane A can then be calculated as follows:
 
βε%/=−bb 22+−bb //100 ≤5%
() ()
13 24 a
 
10 © ISO 2012 – All rights reserved

5.5 Grips and fixtures for the SENB specimens
5.5.1 Tension-compression grips for the SEN B8 specimen
The eight-point bend specimen is also suited for tension-compression testing. In gripping the eight-point bend
specimen, the top and bottom tups are rigidly tied together with a line-to-line fit to the specimen’s surfaces.
Precautions shall be taken to eliminate backlash and secondary moments.
5.5.2 Tension-tension testing of SENB specimens
The general principles of the bend test fixture suitable for tension-tension testing of the SENB specimen are
illustrated in Figure 6. The fixture is designed to minimize frictional effects by allowing the support rollers to
rotate and move apart slightly as force is applied to the specimen, hence permitting rolling contact. Thus, the
support rollers are allowed limited motion along plane surfaces parallel to the notched side of the specimen,
but are initially positively positioned against stops that set the span length and are held in place by low-tension
[23]
springs (such as rubber bands). Fixtures and rollers shall be made of high hardness (>40 HRC) steel .
5.6 Crack length measurement apparatus
5.6.1 General
Accurate measurement of crack length during the test is very important. There are a number of visual and
non-visual apparati that can be used to determine the crack length. A brief description of a variety of crack
length measurement methods is included in Reference [26]. The required crack length measurements are the
average of the through-the thickness crack lengths, as covered in 9.1.
5.6.2 Non-visual crack length measurement
There are a number of non-visual measurement techniques. Most lend themselves to automated data
acquisition and determine the average crack length, reflecting the crack-front curvature, if it exists. Crack-
[36]-[38] [39]-[41]
opening-displacement compliance , AC and DC electric potential difference (EPD) , back face
[36], [42] [43]- [45]
strain , and side face foil crack gauges are all acceptable techniques, provided the resolution
requirements covered in 8.1 be met. (Information on the methodology of crack length determination through
the use of EPD is provided in Annex A.)
5.6.3 Visual crack length measurement
In the past, the most common visual crack length measurement technique used a micrometer thread travelling
microscope with low magnification (×20 to ×50). This technique measures the surface crack length during the test
and may need to be corrected to the actual through-thickness crack size upon test completion, as covered in 9.1.
Dimensions in millimetres
Surface roughness values in micrometres
Key
1 test specimen
2 loading rod
3 test fixture
4 support rollers
NOTE Support rollers and specimen contact surface of loading rod should be parallel to each other to ± 0,002W (TIR).
a
Bosses for springs or rubber bands.
b
0,6x support roller diameter.
c
1,1x support roller diameter.
Figure 6 — Fixture for tension-tension forcing of a SEN B3 specimen
6 Specimens
6.1 General
Proportional dimensions of six standard specimens: a compact tension (CT); a centre cracked tension (CCT)
and three-, four- and eight-point single edge notch bends [(SEN B3), (SEN B4) and (SEN B8)]; and single
edge notch tension (SENT) are presented in Figures 7, 3, 8, 9, 10 and 2, respectively. A variety of specimen
configurations is presented to accommodate the component geometry available and test environment and/or
force application conditions during a test. Machining tolerances and surface finishes are also given in Figures 7
to 10. The CT, SEN B3 and SEN B4 specimens are recommended for tension-tension test conditions only.
12 © ISO 2012 – All rights reserved

The specimen shall have the same metallurgical structure as the material for which the crack growth rate is
being determined. The test specimen shall be in the fully machined condition and in the final heat-treated state
that the material will see in service.
Surface roughness values in micrometres
NOTE 1 The machined notch is centred to within ± 0,005W.
NOTE 2 The surfaces are perpendicular and parallel to within ± 0,002W (TIR).
NOTE 3 The crack length is measured from the reference plane of the loading pin holes centerline.
NOTE 4 This specimen is recommended for notch root tension at a force ratio R > 0 only.
a
Reference plane.
b
See Figure 12 for notch detail.
c
Recommended thickness: W / 20 ≤ B ≤ W / 2.
d
The suggested minimum dimensions are W = 25 mm and a = 0,2W.
p
Figure 7 — Standard compact tension, CT, specimen for fatigue crack growth rate testing
Surface roughness values in micrometres
NOTE 1 The machined notch is centre 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.
a
See Figure 12 for notch detail.
b
Reference plane.
c
Recommended thickness: 0,2W ≤ B ≤ W.
d
D ≥ W/8.
Figure 8 — Standard three-point single edge notch bend, SEN B3, specimen
14 © ISO 2012 – All rights reserved

Surface roughness values in micrometres
NOTE 1 The machined notch is centred to within ± 0,005W.
NOTE 2 The surfaces are parallel and perpendicular to within ± 0,002W (TIR).
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.
a
See Figure 12 for notch detail.
b
Reference plane.
c
Recommended thickness: 0,2W ≤ B ≤ W.
d
D ≥ W/8.
Figure 9 — Standard four-point single-edge-notch bend, SEN B4, specimen
Surface roughness values in micrometres
NOTE 1 The machined notch is centred to within ± 0,005W.
NOTE 2 The surfaces are parallel and perpendicular to within ± 0,002W (TIR).
NOTE 3 The crack length is measured from the reference loading plane containing the starter V-notch.
NOTE 4 Specimen suitable for R ≤ 0, provided backlash and secondary moment loading by grips be avoided.
a
See Figure 12 for notch detail.
b
Reference plane.
c
Recommended thickness: 0,2W ≤ B ≤ W.
d
D ≥ W/8.
Figure 10 — Standard eight-point single edge notch bend, SEN B8, specimen
6.2 Crack plane orientation
The crack plane orientation, as related to the characteristic direction of the product, is identified in Figure 11.
The letter(s) preceding the hyphen represent(s) the force direction normal to the crack plane; the letter(s)
following the hyphen represent the expected direction of crack extension. For wrought metals, the letter X
always denotes the direction of principal processing deformation, Y denotes the direction of least deformation
and the letter Z is the third orthogonal direction. If the specimen orientation does not coincide with the product’s
characteristic direction, then two letters are used before and/or after the hyphen to identify the normal to the
crack plane and/or expected direction of crack extension.
NOTE For rectangular sections of wrought metals, a commonly used alternative designation system uses the letter
L to denote the direction of principal processing deformation (maximum grain flow), T to denote the direction of least
deformation, and S for the third orthogonal direction.
16 © ISO 2012 – All rights reserved

a) Basic identification
b) Non-basic identification
c) Radial grain flow, axial working direction
d) Axial grain flow, radial working direction
a
Grain flow.
Figure 11 — Fracture plane orientation identification
6.3 Starter notch precracking details
The envelope and various acceptable machined notch configurations and precracking details for the specimens
are presented in Figure 12.
The machined notches in the SENB and CCT specimens are determined by practical machining limitations; the
K-calibration does not have a notch size limitation. However, a CCT specimen’s minimum notch length, 2a , of
n
at least 0,2W is required when using the compliance method for crack length determination to ensure accurate
crack length measurements.
The starter notch for the standard specimens may be made via electrical discharge machining (EDM), milling,
broaching or saw cutting. To facilitate precracking, the notch root radius should be as small as practical,
typically less than 0,2 mm. For aluminium, saw cutting the final 0,5 mm starter notch depth with a jeweler’s saw
is acceptable.
18 © ISO 2012 – All rights reserved

NOTE 1 Crack length is measured from reference plane.
NOTE 2 Notch height, h, should be minimized.
NOTE 3 A hole of radius r < 0,05W is allowed for ease of machining the notch in a CCT specimen.
a
Reference plane.
b
Root radius.
Maximum notch height
Specimen type Notch length Minimum precrack length
a h a
n p
CT 0,1W ≤ a ≤ 0,15W ≤ 1 mm for W ≤ 25 a ≥ a + h, or
n p n
CCT W/16 for W > 25 a ≥ a + 1 mm, or
p n
a ≥ a + 0,1B, whichever is
p n
greater.
a ≥ 0,2W for CT only
p
Figure 12 — Notch detail and minimum fatigue precracking requirements
6.4 Stress-intensity factor
6.4.1 General
The stress-intensity factor for all standard specimen configurations is calculated using the following relationship:
F a
K = g() (1)
12/
W
BW
The stress-intensity factor geometry function,ga(/W) , for each standard specimen configuration is calculated
using the following expressions.
6.4.2 Compact tension, CT, specimen
23 4
a ()20++αα(,886 46,,41−+3321αα47,,25− 6α )
g()= (2)
32/
W
()1−α
where α =aW/ ; the expression is valid for 02,/≤≤aW 10, . See Figure 7.
6.4.3 Centre cracked tension, CCT, specimen
[24]-[26]
For the centre cracked tension specimen, CCT, the stress-intensity factor geometry function is given by :
a θ
12/ 24
g()=−( )(0,,707 10 007 20θθ+ ,)007 0 (3)
W cosθ
where θ =πaW/ 2 radians; the expression is valid for 02<=α aW/.21< ,00 Here, it is recommended that
the crack length, a, be the average of the four measurements from the centreline refe
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