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
Standards Content (Sample)
Metallic materials — Fatigue testing —
Fatigue crack growth method
Matériaux métalliques — Essais de fatigue — Méthode d'essai de
propagation de fissure en fatigue
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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
7.1 Fatigue precracking .11
7.2 Crack length measurement .11
7.3 Constant-force-amplitude, Κ-increasing, test procedure for da/dN > 10 mm/cycle .12
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
8.6 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
9.1 Crack-front curvature .16
9.2 Determining the fatigue crack growth rate .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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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
potential difference .48
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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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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-
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 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
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
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
expression, i.e. stress-intensity factor geometry function, g (a/W) .
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.
For crack growth rates above 10 mm/cycle, the typical scatter in test results generated in a single
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.
Service conditions can exist where varying ΔK under conditions of constant K or K control
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
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.
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.
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/
linear measure of a principal planar dimension of a crack from a reference plane to the crack tip
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.
fatigue crack growth rate
extension in crack length
force having the highest algebraic value in the cycle, a tensile force being positive and a compressive
force being negative
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force having the lowest algebraic value in the cycle, a tensile force being positive and a compressive
force being negative
algebraic difference between the maximum and minimum forces in a cycle
ΔF = F − F
algebraic ratio of the minimum force or stress to the maximum force or stress in a cycle
R = F /F
Note 1 to entry: R can also be calculated using the values of stress-intensity factors; R = K /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
maximum stress-intensity factor
highest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current
minimum stress-intensity factor
lowest algebraic value of the stress-intensity factor in a cycle, corresponding to F and current
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.
stress-intensity factor range
algebraic difference between the maximum and minimum stress-intensity factors in a cycle
ΔK = K − K
Note 1 to entry: The variables ΔK, R and K are related as follows: ΔK = (1 − R) K .
Note 2 to entry: For R ≤ 0 conditions, see 3.10 and 10.6.
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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 .
fatigue crack growth threshold stress-intensity factor range
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
10 mm/cycle. When reporting ΔK , the corresponding lowest decade of da/dN data used in its determination
should also be included.
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
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.
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.
stress-intensity factor geometry function
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
crack-front curvature correction length
difference between the average through-thickness crack length and the corresponding crack length at
the specimen faces during the test
fatigue crack length
length of the fatigue crack, as measured from the root of the machined notch
Note 1 to entry: See Figure 2.
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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linear measure of a principal planar dimension of a specimen from a reference plane to the specimen edge
4 Symbols and abbreviated terms
See Table 1.
Table 1 — Symbols and their designations
Symbol Designation Unit
C Normalized K-gradient mm
E Tensile modulus of elasticity MPa
F Force kN
F Maximum force kN
F Minimum force kN
ΔF Force range kN
K Stress-intensity factor MPa·m
K Maximum stress-intensity factor MPa·m
K Minimum stress-intensity factor MPa·m
ΔK Stress-intensity factor range MPa·m
ΔK Initial stress-intensity factor range MPa·m
ΔK Fatigue crack growth threshold stress-intensity factor range MPa·m
N Number of cycles cycle
R Force ratio kN/kN
R Ultimate tensile strength at the test temperature MPa
R 0,2 % proof strength at the test temperature MPa
a Crack length or size measured from the reference plane to the crack tip mm
a Crack-front curvature correction length mm
a Fatigue crack length measured from the notch root mm
a Machined notch length mm
a Precrack length mm
B Specimen thickness mm
Hole diameter for CT, SENT or CCT specimen, loading tup diameter for bend
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
da/dN Fatigue crack growth rate mm/cycle
Δa Change in crack length, crack extension mm
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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
5.1 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
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
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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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.
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
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