prEN 3873

SLOVENSKI STANDARD
01-januar-2026
Aeronavtika - Preskusne metode za kovinske materiale - Ugotavljanje stopnje rasti
utrujenostnih razpok, z uporabo preskusnih kosov s kotno razpoko (CC)
Aerospace series - Test methods for metallic materials - Determination of fatigue crack
growth rates using Corner-Cracked (CC) test pieces
Luft- und Raumfahrt - Prüfverfahren für metallische Werkstoffe - Ermittlung der
Ermüdungsriss-Wachstumsraten an Probestücken mit Eckanriss (Corner-Crack)
Série aérospatiale - Méthodes d'essais applicables aux matériaux métalliques -
Détermination de la vitesse de propagation de fissure en fatigue à l'aide d'éprouvettes
avec fissure en coin.
Ta slovenski standard je istoveten z: prEN 3873
ICS:
49.025.05 Železove zlitine na splošno Ferrous alloys in general
49.025.15 Neželezove zlitine na Non-ferrous alloys in general
splošno
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
November 2025
ICS 49.025.05; 49.025.15
English Version
Aerospace series - Test methods for metallic materials -
Determination of fatigue crack growth rates using Corner-
Cracked (CC) test pieces
Série aérospatiale - Méthodes d'essais applicables aux Luft- und Raumfahrt - Prüfverfahren für metallische
matériaux métalliques - Détermination de la vitesse de Werkstoffe - Ermittlung der Ermüdungsriss-
propagation de fissure en fatigue à l'aide d'éprouvettes Wachstumsraten an Probestücken mit Eckanriss
avec fissure en coin. (Corner-Crack)
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee ASD-
STAN.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 3873:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Symbols and abbreviations . 6
5 Test method . 7
5.1 General. 7
5.2 Parameters influencing crack growth . 8
5.3 Applicability of results . 8
6 Test equipment . 9
6.1 Test machine . 9
6.1.1 General. 9
6.1.2 Load control . 9
6.1.3 Load alignment . 9
6.2 Calibration . 9
6.3 Temperature measurement and control . 9
6.4 Gripping of test pieces . 10
7 Test pieces . 10
7.1 Corner-crack (CC) test piece . 10
7.2 Stress-intensity Factor calculation . 10
7.3 Test piece size . 11
7.4 Crack plane orientation . 11
7.5 Residual stresses . 11
8 Procedures before testing . 11
8.1 Condition of test pieces . 11
8.2 Heating . 11
8.3 Number of tests . 11
9 Test procedure . 12
9.1 General. 12
9.2 Determination of crack depth . 12
9.2.1 General. 12
9.2.2 Notch preparation . 12
9.2.3 Pre-cracking . 12
9.3 Procedure . 13
9.4 Increasing-K-test . 14
9.5 Measurement intervals . 14
9.6 End of test . 14
10 Health and safety . 15
11 Evaluation of test results . 15
11.1 Test piece measurement . 15
11.1.1 General. 15
11.1.2 Polynomial method . 15
11.1.3 Secant method . 16
11.2 Crack closure correction − ΔK . 16
eff
12 Test report . 16
13 Optional information in a test report . 17
Annex A (normative) Information on measuring crack depths in corner-crack test pieces
with the direct-current Potential-drop method . 19
Annex B (informative) Stress-intensity function for corner-crack test pieces . 23
Annex C (normative) Guidelines on test piece handling and degreasing . 25
Annex D (informative) da/dN Test report form . 34
Bibliography . 35
European foreword
This document (prEN 3873:2025) has been prepared by ASD-STAN.
After enquiries and votes carried out in accordance with the rules of this Association, this document
has received the approval of the National Associations and the Official Services of the member
countries of ASD-STAN, prior to its presentation to CEN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 3873:2010.
This document includes the following significant technical changes with respect to EN 3873:2010:
— New structural update;
— Adding Figure C.9 “Wire placement”;
— New Annex D “da/dN Test report form”.
1 Scope
This document specifies the requirements for determining fatigue crack growth rates using corner-
crack (CC) test pieces. Crack development is measured using a potential-drop system, and the
calculated crack depths can be corrected via marker bands created on the fracture surface during the
test. Results are expressed in terms of the crack-tip stress-intensity range (ΔK), with crack depths and
test stress level noted.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
EN ISO 3785, Metallic materials — Designation of test specimen axes in relation to product texture(ISO
3785)
EN ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines —
Part 1: Tension/compression testing machines — Calibration and verification of the force-measuring
system(ISO 7500-1)
ASTM E1012, Standard practice for verification of test frame and specimen alignment under tensile and
compressive axial force application
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
crack depth
a
distance from the extrapolated original corner containing the notch to the centre of the crack front
(45° position)
Note 1 to entry: For the calculation of stress-intensity factor, the crack length shall be given in metres.
3.2
calculated (potential) crack depth
a
v
average crack depth due to the averaging nature of the potential determination method
Note 1 to entry: Calculation involving average lengths measured at several positions along the crack front
are best for correlation with the potential measurements (in millimetres).

Published by American Society for Testing and Materials (ASTM International), available at:
https://www.astm.org/.
3.3
stress-intensity factor (general)
K
load parameter which characterises the stress field at the crack tip
Note 1 to entry: It is a function of load, crack depth and test piece geometry (in MPa √m).
3.4
fatigue crack growth threshold
ΔK
th
asymptotic value of ΔK for which da/dN approaches zero
Note 1 to entry: When reporting ΔK , the corresponding lowest decade of near threshold data used in its
th
determination shall be given.
4 Symbols and abbreviations
a crack depth (in meter)
a final crack depth (in millimetre)
e
initial crack depth (in millimetre)
ai
a measured crack depth (optical, post-test fracture surface micrography or with SEM)
m
a calculated (potential) crack depth (in millimetre)
v
Δa crack growth increment (in millimetre)
a/W normalized crack depth (in millimetre/millimetre)
C Normalized K-gradient C = (1/K × dK/da). For load-shedding to attain a desired initial ΔK,
C defines the fractional rate of change of K with increasing crack depth a.
C = 1/K × dK/da = 1/K × dK /da = 1/K × dK /da = 1/ΔK × dΔK /da, (in
max. max. min. min.
−1
mm )
da/dN fatigue crack growth rate (FCGR) (in metre per cycle)
E Young's modulus (in gigapascal)
f frequency (in Hertz)
F mean force (expressed in kilonewton)
m
F maximum tensile force applied to the test piece during a cycle (expressed in kilonewton)
max.
F minimum tensile force (in kilonewton)
min.
ΔF force range (in kilonewton), ΔF = F − F
max. min.
K stress-intensity factor (general)
K maximum value of K during a loading cycle, corresponding to the maximum tensile force
max.
applied (in MPa √m)
Kmin. minimum value of K during a loading cycle (in MPa √m)
ΔK range of K during a loading cycle = K − K = (1 − R)*K (in MPa √m)
max. min. max.
ΔK effective range of K, due to crack closure-induced reduction applied ΔK (in MPa √m)
eff
ΔK fatigue crack growth threshold
th
N Number of loading cycles, Stress cycle (fatigue cycle, load cycle) is the smallest segment of
the loading waveform spectrum which is repeated periodically
N' number of stress cycles between two marker cycles
N number of stress cycles in a marker cycle
m
ΔN stress cycle difference
q resolution of crack depth measuring system (expressed in millimetres)
r notch radius (expressed in millimetre)
R force ratio (= F /F = K /K )
min. max. min. max.
R*m ratio Fmin./Fmax. during a marker cycle
R 0,2 % offset yield strength (Proof Stress R ) at test temperature (expressed in megapascal)
p p0,2
R tensile strength at test temperature (expressed in megapascal)
m
σ flow stress – here defined as the arithmetic mean of R and R (expressed in megapascal)
f p m
W test piece width (in millimetre)
Z axial distance from crack plane to each wire used for potential measurement (expressed in
millimetre) − (2Z = wire separation distance)
5 Test method
5.1 General
The corner-crack (CC) test piece is useful in determining da/dN for components where the cracks
usually appear at a corner, such as in holes in turbine disks. The determination involves the use of an
axially-loaded test piece of square or rectangular cross-section. It may be loaded in tension and
compression for positive and negative stress ratio testing if suitable end designs permit backlash-free
loading.
A carefully defined and produced notch or a small arc strike enables cracking to be initiated at the
centre of the reduced section. A fatigue crack is induced at the root of the notch by cyclic loading, and
its growth is monitored by a suitable method, e.g. by potential-drop techniques. As the crack grows,
the force range applied to the test piece is maintained or reduced in a controlled manner until the
cracks are of sufficient depth for the influence of the notch and the crack initiation method to be
negligible, and the ΔK has reached the lowest level of interest. The test is then carried out. The force
range is maintained constant and the crack depth recorded as a function of elapsed cycles. These data
are then subjected to numerical analysis, enabling da/dN to be determined as a function of ΔK.
The majority of metallic materials can be tested using the method described here, provided that the
force applied is such as to ensure that the plastic zone in front of the crack tip is small in relation to the
remaining cross section (linear-elastic criterion).
The test piece used here is a corner-crack (CC) test piece. See Figure C.1.
In the standard crack-growth test the load amplitude is assumed to be constant throughout the test,
after the required ΔK level and R-ratio is reached. Another load range can be added if certain transient
effects are to be investigated.
The range of the stress-intensity factor ΔK is given by Formula (1):
ΔK = K − K (1)
max min.
.
where the ratio R
K
max.
R= (2)
K
min.
applies.
From Formula (1) and Formula (2) it follows that, for:
R > 0,
ΔK = (1 − R) K (3)
max.
The reference point for measuring the crack depth with corner-cracked test pieces is the original
corner of the test piece, determined by the projections of the sides of the test piece on the fracture
surface adjacent to the notch. Possible rounding of the corner during test piece manufacture will result
in this reference point being no longer on the fracture surface. This rounding shall be determined to
obtain a “Zero-point offset” between the reference point and the rounded corner where the
measurement wire is welded, which is used in the calibration of the potential-drop measurements.
The purpose of the crack propagation measurements is to allocate the relevant load cycles N to the
crack depth a. The measurements (a-N points, see Figure C.2) are normally evaluated in the form of a
da/dN versus ΔK curve (see Figure C.3). It is not always the case that the crack propagation can be
described by the range of stress-intensity factor ΔK. If it cannot be so described, other laws can be
applied, e.g. crack growth rate as a function of K .
max.
5.2 Parameters influencing crack growth
The crack growth behaviour depends on a number of parameters. The framework within which the
test is to be carried out needs to be precisely defined in the written test procedure to avoid undesired
effects on the results.
The most important factors affecting the results are:
a) temperature and environment;
b) load spectrum;
The test parameters R, dwell time and loading frequency shall be defined and recorded before testing
commences. The results can also be affected by the loading history, including interruption times, e.g.
stop and restart of cycling to check surface crack length or other parameters, work stoppage at
weekends.
c) residual stresses
Residual stresses are usually ignored, as they are difficult to determine, and a duplication of the
residual stresses in a component is very difficult to obtain in a test piece. Their presence in a
component will affect the life of the component and should be regarded in the use of the crack growth
data. The presence of unexpected residual stresses in the test piece may be witnessed in an asymmetry
of the crack front.
5.3 Applicability of results
The crack-growth determination is generally used:
a) to investigate the influence of fatigue-crack growth on the predicted life of a component, or for
evaluating the crack-growth resistance of a material or heat-treat condition;
b) to define the requirements of non-destructive testing; and
c) to determine quantitatively various factors (e.g. load, microstructure, manufacture).
6 Test equipment
6.1 Test machine
6.1.1 General
Tests shall be performed with a feed-back load-controlled servohydraulic or electromechanical test
system designed for smooth loading from first load cycle without exceeding the desired F . The
max
system shall be capable of halting the cycling at desired intervals of cycles or crack depth, at a desired
potential level, or at will, to enable measurements of the optical crack depth, potential or thermal
potential, without stopping the test or causing overloads during the following restart.
6.1.2 Load control
The test system shall satisfy the following requirements in accordance with EN ISO 7500-1:
a) accuracy of electronic force measurement: ± 0,5 % and ± 0,25 % of nominal range respectively;
b) accuracy of control throughout testing: better than 0,5 % of specified value of ΔF;
c) recording instrument voltage requirements (upper and lower stress range, cycles): digital
recorder is recommended;
d) recording accuracy throughout testing: better than 0,25 %.
6.1.3 Load alignment
Good alignment in the load train is essential for ensuring loading symmetry. An alignment test shall be
carried out. The loading train shall be rigid, to avoid loading eccentricity as the crack grows, which
would influence the applied stress-intensity factor at the crack tip. Alignment shall be carried out in
accordance with ASTM E1012.
6.2 Calibration
All instruments shall be calibrated at least once a year, as well as after every incident that may have
affected the calibration accuracy. Multiplication factors (e.g. × 10 or × 100) shall not be used when
× −2 - ×
counting the cycles, unless the factor is less than 10 , where 10 mm/cycle is to be measured.
Thermocouples shall be calibrated every six months in accordance with EN ISO 3785.
6.3 Temperature measurement and control
Temperature of the test piece shall be measured by a calibrated Platinum/ Rh (Type R) or Chromel-
Alumel (Type K) thermocouple in adequate thermal contact with the test piece, at the centreline of one
face adjacent to the notch, 2 mm to 4 mm above or below the crack plane. Shielding of the junction
from radiation is not necessary if the difference in indicated temperature from an unshielded bead and
a bead inserted in a hole in the test piece has been shown to be less than one-half the permitted
variation shown below.
Throughout the test, the temperature shall not deviate from the specified values by more than the
following:
For elevated temperature tests up to (1 000 ± 3) °C: (1 000 ± 4) °C to (1 100 ± 4) °C.
Temperatures shall be recorded and monitored, irrespective of the accuracy of temperature control,
by each change of 1 °C.
The recording accuracy shall be better than 0,25 % of the specified value.
Room temperature variations; i.e. at night, over weekends, shall be known and limited to ± 5°C.
For tests at elevated temperatures, a three-zone furnace featuring electronic PID control shall be used.
The grips shall also be heated.
6.4 Gripping of test pieces
To compensate for the weight of the grips and fixtures, the load cell shall be adjusted to ensure that the
test piece is not under stress at zero indicated force.
7 Test pieces
7.1 Corner-crack (CC) test piece
The corner-crack (CC) test piece is illustrated in Figure C.1. CC test pieces may be used with positive or
negative R-ratio loading, assuming the gripping system can transmit loads without backlash.
7.2 Stress-intensity Factor calculation
The stress-intensity factor for the CC test piece shall be calculated according to Formulae (5) to (7):
First, the geometry factor for the 45° position is calculated:
2 3
Y = 0,574 + 1,199 (1 − a/W) − 1,324 (1 − a/W) + 0,4845 (1 − a/W) (4)
°
Then, for crack depths a ≤ 0,2 W:
Y = 1,143 (1 + 0,6 a/W) (1 + 0,7 a/W) (2/π) Y (5)
45°
and for crack depths a > 0,2 W:
2 2
Y = [0,1 (a/W) + 0,29 (a/W) + 1,081] × [0,75 (a/W) − 0,185 (a/W) + 1,019] × [(0,9
(a/W) − 0,21 (a/W) + 1,02)] 2/π × Y (6)
°
and
FY××π a
K = × (7)
W 2 10
where
a is the crack depth (expressed in centimetres);
W is the test piece width (expressed in centimetres);
F is the force (expressed in kilonewtons);
K is the stress-intensity (expressed in MPa√m).
A check for this calculation may be made with the following input and results:
a 1 mm (0,1 cm);
W 0,8 cm;
F 32 kN.
here
Y 0,691;
K 19,365.
7.3 Test piece size
For the results to be valid, the test pieces shall be subjected to a stress within the elasticity range of the
material for all values of the applied load.
7.4 Crack plane orientation
The crack plane orientation, as related to the characteristic direction of the product is identified with a
hyphenated letter code as in Figure C.4.
The letter(s) preceding the hyphen represent the loading direction normal to the crack plane.
The letter(s) following the hyphen represent the expected direction of crack extension.
For wrought metals the letter L always denotes the direction of principal processing deformation,
T denotes the direction of least deformation and the letter S is the third orthogonal direction.
C denotes the circumferential direction and R the radial direction in a disk, while L denotes the
direction along the longitudinal axis of the disk.
7.5 Residual stresses
In test pieces where stress relief has not been applied, or where forging may have introduced residual
stresses which cannot be adequately relieved, the crack growth rate and/or crack symmetry may be
affected, particularly at lower ΔK levels.
8 Procedures before testing
8.1 Condition of test pieces
The test pieces shall be cleaned and measured before testing commences. Test pieces shall be
degreased and cleaned in accordance with the guidelines in Annex C. The notch depth and
measurement wire spacing shall be measured before the test, since the wire spacing cannot be
determined after the test, and the notch depth is necessary to determine the initial stress-intensity
levels.
8.2 Heating
The maximum heating rate shall be 1 K/s. After the specimen temperature reaches that specified, at
least 30 min shall be allowed for stabilization.
8.3 Number of tests
The number of tests depends on the use to which the data are to be put. In any case, at least two tests
shall be carried out for each set of tested parameters. As far as possible, the tests shall be identical so
that the scatter can be attributed to material effects. If this is not possible, all tests shall be carried out
within the same ΔK range, with similar number of measurement points and similar measurement
intervals.
9 Test procedure
9.1 General
The test should be conducted at constant load amplitude (ΔF). However, crack growth measurements
under variable load amplitude may be desired, especially when obtaining specific information from a
limited number of test pieces. In this case the procedure shall be such as to exclude undesired
transient effects.
9.2 Determination of crack depth
9.2.1 General
Crack-depth determinations shall be made using the potential-drop method:
The DC (direct current) potential-drop method determines the crack depth through the increase in
potential (V), measured across the mouth of the crack, from an initial reference potential V measured
o
at a known or estimated initial crack depth at test temperature, induced by a constant current (~ 10 A)
passing through the plane of the crack. For a constant current flow, the electric potential or voltage
drop across the crack plane will increase with increasing crack size due to modification of the
electrical field and associated perturbation of the current streamlines. The relationship between
potential and crack depth depends on the arrangement of the current- and measurement leads on the
test piece. Annex A gives information on the use of this method. An AC (alternating current) potential-
drop method may instead be used, and also requires calibration.
9.2.2 Notch preparation
To facilitate crack initiation at low stress ratios, the notch root radius should be on the order
of < 0,05 mm. The notch depth used depends on the initial K required for initiation. For nickel-based
alloys, either the arc-strike technique with an effective notch depth of 0,1 mm or a diamond-sawed
notch of 0,25 mm depth have proven effective. An EDM notch of 0,1 mm width or smaller and 0,1 mm
depth can be effectively used for the initiation of the fatigue crack with titanium alloys.
9.2.3 Pre-cracking
The condition of the test piece (e.g. heat treatment) when initiating the pre-crack shall be the same as
that with which testing is carried out. No intermediate heat treatments between pre-cracking and
testing are allowed. The purpose of pre-cracking is to provide sharp fatigue cracks of sufficient depth
so that the K-calibration expression is no longer influenced by the starter notches and that the
subsequent fatigue crack growth rate is not influenced by the pre-cracking force history. Frequently a
ΔK is required to initiate the crack that is larger than the ΔK desired as the starting point for the test. In
this case, the forces shall be stepped down to meet the desired starting criteria (see Figure C.9).
If F and a are the maximum load and crack depth in one step j, and F  and a are the
max., j j max., j + 1 j + 1
corresponding values in the next step j + 1, the conditions of Formulae (8) and (9) shall be met:
F
max., j
F ≥ (8)
max., j+1
12,
(a 10 % change is a good initial recommendation; later steps of only 5 % may be necessary to avoid
excessive delays before crack growth resumes after each step).

K
1 max., j

aa−≥ (9)
jj+1
3πR
p

The best initial K shall be determined for each material. But if this is not known, a value of
max.
0,000 08*E √m may be initially used until experience is gained. A net section stress of 500 MPa to
600 MPa is recommended for high-strength nickel alloys.
9.3 Procedure
a) For the DC method, apply ~ 0,1·W amperes direct current to produce an initial potential of
~ 1 mV.
b) A fatigue pre-crack of 0,03 mm shall be produced using a stress intensity range of ~ 10 MPa√m to
15 MPa√m until a potential change of 0,005 mV to 0,01 mV is noted. This change represents
approx. 0,01 mm to 0,04 mm crack extension, and usually indicates the crack is growing steadily.
For some materials, small incremental increases in ΔK will be necessary to initiate a crack. Here it
is useful to drop the minimum load used into the compressive range, as well as increasing the
tensile maximum load, to avoid net section stresses too near to yield.
c) The loading shall then be adjusted for the desired minimum ΔK for the test, while the pre-crack
extends to a = ~ 0,3 mm. Figure C.6 shows a schematic of the load-shedding process typically
recommended (ASTM E 647); the shedding process shall be accelerated for CC test pieces due to
the small test piece dimensions.
The temperature during pre-cracking should be the same as during testing to avoid transient effects
which tend to retard the initial crack growth after pre-cracking. Pre-cracking may be performed at
room temperature, to enable monitoring the pre-crack length on the specimen sides. To save time
during load shedding, higher frequencies than during testing may be used initially (e.g. 5 Hz to 20 Hz),
but the final 0,05 mm of pre-crack growth should be performed using a waveform having similar
loading rates as the waveform used during testing.
If hold times at maximum load are used during CGR testing at elevated temperatures, the initial
0,05 mm to 0,1 mm of growth data may be influenced by such transient effects, and should be
considered as suspect, especially if a gradual transition to higher rates is evident after switching to a
hold time.
If reduction of the frequency from pre-cracking to test conditions allows F to increase, due to test
max.
machine control characteristics, then all frequency changes, including stops and restarts, shall be
immediately preceded by a precautionary 10 % reduction in F and F to avoid F overshoot, and
max. min. max.
later increased.
d) The potential at this reference crack depth and F shall be recorded and, if pre-cracking was
min.
performed at RT the test piece is heated, still at F to the test temperature, if elevated
min.,
temperature tests are required. If so, the potential at the test temperature shall be recorded and
used to obtain the temperature correction coefficient.
V ()RT
Y =
(10)
TC
V ()Test Temp.
This coefficient may be used to correct the potentials measured, for use in the crack depth formula
(see Annex A).
Optimally, the test piece shall be pre-cracked at the test temperature. The reference potential shall be
taken at RT in the notched condition, or after heating, to obtain Y . A different potential-drop
TC
calibration will be necessary, however, due to differences between potentials for notches and cracks of
the same depths.
If the desired R-ratio is higher than 0,1, further cycling shall now be performed to reach the desired R-
ratio, reducing the loading range by 10 % after each 0,1 mm of crack extension by increasing the
minimum load. For materials with R ~ 1 000 MPa, and for low R-ratios, reductions may be made after
p
each 0,05 mm of extension, permitting the use of higher initial pre-cracking stress-intensities, or
enabling the possibility to reach lower stress-intensities for the initial testing conditions without
excessive pre-crack depths.
This can also be achieved by initially cycling at the R-ratio desired for the test, with a ΔK of ~ 16 MPa
√m, but the resulting higher K may require larger crack extension increments to avoid retardation,
max.
due to the larger crack tip plastic zone at elevated temperature.
The final transition to testing parameters involves changing to the test frequency, which may involve
hold times. The choice of sinus or trapezoidal waveform depends partly on the ability to accurately
measure the potential at the maximum load each cycle. If the test frequency is less than 2 Hz, the
transient effects may require up to 0,2 mm of crack growth before constant growth rates are obtained.
This frequency, and the possible transient zone sizes, depends on the material, test temperature and
hold time. Switching the control mode to constant ΔK, with continuous decreasing control of F and
max.
F to maintain ΔK constant over the next 0,1 mm to 0,2 mm of growth, will show the size of this
min.
transient, as the crack growth rises to a stable rate. During data analysis, any transient data shall be
deleted if this transient was not the object of the test.
9.4 Increasing-K-test
At this point the control mode shall be switched to constant F and F and ΔK is allowed to
max. min.,
increase with crack extension. The final crack depth, as determined from the potential drop (PD), shall
be limited to a/W ≤ 0,5. Test interruptions shall be kept to a minimum. If the test is interrupted, a
change in growth rate may occur after resumption of cycling.
Measurements of the actual initial and final crack depths, taken from the actual post-test fracture
surface, shall be used to correct the calculated crack depths. For tests where the final crack depth is
too large (e.g. > 0,5 W), the relationship between the crack depth and potential measured may no
longer be linear, skewing the analysis. Inclusion of periodic “Marker loads” of a few thousand cycles,
cycling at the same K as during testing, but with a significantly higher or lower R-ratio, can
max.
produce visible “beach marks” at known cycle counts and known potentials, which can be used to
calibrate the crack depth calculations based on potential, using the measured crack depths at the
beach marks (see Figure C.7). Thus, any deviations from linearity can be determined and shall be used
to limit the range of data used. At slow crack growth rates, the stress-intensity factor range of the
marker cycles may be below the threshold, in which case no propagation during the marker cycles can
be expected, and the transition to and from the marker cycling may not be visible on the fracture
surface. At moderate temperatures, the markers may also be difficult to observe. Dark-field
microscopy or post-test oxidation of the fracture surface may be of advantage.
9.5 Measurement intervals
The interval should be large in relation to the accuracy of the measurement method, but small
compared to the K-gradient of the test piece.
For potential-drop methods, crack-depth potential should be recorded at least every 15 min. For low
growth rates, longer intervals may be used, and the stability of the measurements over time should be
ensured. The interval chosen should permit adequate sampling of the crack depth at short crack
depths, but should not be less than ten times the measurement accuracy. Crack depth shall be
recorded every ~ 0,1 mm. An interval of 1/20th of the initial potential is recommended (0,05 mV
when V = 1 mV). The resulting da/dN data shall present at least ten points in each growth rate decade.
o
9.6 End of test
Breaking the test piece during the fatigue test shall be avoided. The test shall be stopped at or before
the normalized crack depth a/W = 0,5, and before the net section stress reaches the yield level at the
test temperature. The potential shall be measured and recorded immediately before the stop (one
cycle to three cycles) to enable accurate post-test correction of the calculated crack depths. The test
piece shall be heat-tinted as necessary to enable a clearly-defined final crack depth to be measured on
the fracture surface. For elevated temperature tests, it is recommended that the test piece shall be
cooled to at least room temperature and that additional cycling at the previous stress levels be
performed to extend the fatigue crack beyond the test end, before loading in tension to failure. The
fracture surfaces of both halves shall be protected from further damage, or contact with each other.
10 Health and safety
Exposure limits as given in EC regulation 93/72/EWG for cleaning solvents and for artificial mineral
fibres used for sealing furnace gaps shall be observed.
11 Evaluation of test results
11.1 Test piece measurement
11.1.1 General
If the crack front is visually recognizable, the depth of the pre-cracks and the final depths of the fatigue
cracks shall be measured at the end of the test. The actual crack depths shall then used to correct the
individual crack depth measurements by a linear adjustment method.
The crack growth rate da/dN shall be calculated as follows:
11.1.2 Polynomial method
In this method, a polynomial of the second degree is fitted to (2 n + 1) consecutive points in the
– N diagram.
aaverage
The following applies:
a = b + b (N ) + b (N ) (11)
i 0 1 eff 2 eff
where
NC−
i 1
(12)
N =
eff
C
and
NN+
in−+i n
C = (13)
N + N
i+n in−
C = (14)
The coefficients b , b and b shall be determined by the least-squares method over the range a
0 1 2 i –
≤ a ≤ a . The recommended value for n is 5.
n i i + n
The crack growth rate at point N shall be determined with the aid of the following relation:
i
 
b NC−

da
11i
 
+ 2b (15)

 2 
dN c

C
 
The stress-intensity factor K shall be calculated for the value a corresponding to N .
i i
=
In generating the best fit for the a − N data, the polynomial regression shall use the minimum residual
of the x- and y-residuals for the best fit. At the start of the test, the flat progression of the a − N curve
lends itself better to a regression based on a least-squares fit of the y-residuals, while at the end of the
test the steep curve progression lends itself better to a regression based on the x-residuals.
−5
A modification of this method is useful at slow growth rates (<10 mm/cycle). The data points used
are not individually incremented, but rather a, N points within a selectable Δa interval, usually five
times to ten times the crack depth measurement resolution.
11.1.3 Secant method
In this method, the slope of the line between two adjacent points in the a − N diagram shall be
calculated (see Figure C.2).
The following applies:

aa−

da
1+ii
= (16)


dN N − N

ii+1
This method results in large data scatter if the Δa interval is small.
The geometric mean value
a aa• (17)
i ii+1
shall be used for determining the corresponding stress intensity factor ΔK .
i
11.2 Crack closure correction − ΔKeff
At low or negative R-ratios, the flanks of the crack surface usually come into contact before the
minimum load is reached during unloading. Thus, the crack tip does not experience the full ΔK
applied
and the CGR is lower than it would be for the same ΔK at higher R-ratios. If it is necessary to
distinguish ΔK from the effective ΔK (ΔK ), compliance measurements may be used to determine
applied eff
the minimum force required to fully open the crack, F , and the corre
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