SIST EN ISO 11782-2:2008

Corrosion of metals and alloys - Corrosion fatigue testing - Part 2: Crack propagation testing using precracked specimens (ISO 11782-2:1998)

Describes the fracture mechanics method of determining the crack growth rates of pre-existing cracks under cyclic loading in a controlled environment and the measurement of the treshold stress intensity factor range for crack growth below which the rate of crack advance falls below some defined limit agreed between parties. This part provides guidance and instruction on corrosion fatigue testing of metals and alloys in aqueous or gaseous environments.

Korrosion von Metallen und Legierungen - Prüfung der Schwingungskorrosion - Teil 2: Rissausbreitungsprüfung an angerissenen Proben (ISO 11782-2:1998)

1.1   Dieser Teil von ISO 11782 beschreibt ein Verfahren der Bruchmechanik zur Bestimmung der Wachs¬tumsgeschwindigkeiten bereits vorhandener Anrisse unter wechselnder Beanspruchung in einer kontrollierten Umgebung und Messung des für den Spannungsintensitätsfaktor beim Risswachstum kritischen Bereichs, den die Rissausbreitungsgeschwindigkeit nicht erreicht, wenn eine zwischen den Beteiligten vereinbarte Grenze unterschritten wird.
1.2   Der vorliegende Teil von ISO 11782 bietet einen Leitfaden und Anleitungen für die Schwingungs¬korrosionsprüfung von Metallen und Legierungen in wässrigen oder gasförmigen Korrosionsmedien

Corrosion des métaux et alliages - Essais de fatiguecorrosion - Partie 2: Essais d'amorce de rupture sur des éprouvettes préfissurées (ISO 11782-2:1998)

La présente partie de l'ISO 11782 décrit une méthode reposant sur la mécanique de la rupture, permettant de déterminer les vitesses de propagation de fissures préexistantes sous chargement cyclique en milieu contrôlé et de mesurer la plage de facteur d'intensité de contrainte limite pour la propagation de fissure au-dessous de laquelle la vitesse de progression de la fissure devient inférieure à une limite définie convenue entre les parties.  La présente partie de l'ISO 11782 donne des indications et des instructions sur les essais de fatigue-corrosion des métaux et des alliages en milieux aqueux ou gazeux.

Korozija kovin in zlitin - Ugotavljanje pokanja zaradi korozijske utrujenosti - 2. del: Preskus za ugotavljanje napredovanja razpok z vzorci z umetno razpoko (ISO 11782-2:1998)

General Information

• Status: Published
• Publication Date: 02-Jun-2008
• ICS: 77.060 - Corrosion of metals
• Technical Committee: IPKZ - Protection of metals against corrosion
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 07-May-2008
• Due Date: 12-Jul-2008
• Completion Date: 03-Jun-2008
• Ref Project: EN ISO 11782-2:2008 - Corrosion of metals and alloys - Corrosion fatigue testing - Part 2: Crack propagation testing using precracked specimens (ISO 11782-2:1998)

Relations

• Replaces: SIST ISO 11782-2:1999 - Corrosion of metals and alloys -- Corrosion fatigue testing -- Part 2: Crack propagation testing using precracked specimens
• Effective Date: 01-Jul-2008

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 11782-2:2008
01-julij-2008
1DGRPHãþD
SIST ISO 11782-2:1999
Korozija kovin in zlitin - Ugotavljanje pokanja zaradi korozijske utrujenosti - 2. del:
Preskus za ugotavljanje napredovanja razpok z vzorci z umetno razpoko (ISO
11782-2:1998)
Corrosion of metals and alloys - Corrosion fatigue testing - Part 2: Crack propagation
testing using precracked specimens (ISO 11782-2:1998)
Korrosion von Metallen und Legierungen - Prüfung der Schwingungskorrosion - Teil 2:
Rissausbreitungsprüfung an angerissenen Proben (ISO 11782-2:1998)
Corrosion des métaux et alliages - Essais de fatiguecorrosion - Partie 2: Essais d'amorce
de rupture sur des éprouvettes préfissurées (ISO 11782-2:1998)
Ta slovenski standard je istoveten z: EN ISO 11782-2:2008
ICS:
77.060 Korozija kovin Corrosion of metals
SIST EN ISO 11782-2:2008 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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EUROPEAN STANDARD
EN ISO 11782-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2008
ICS 77.060

English Version
Corrosion of metals and alloys - Corrosion fatigue testing - Part
2: Crack propagation testing using precracked specimens (ISO
11782-2:1998)
Corrosion des métaux et alliages - Essais de fatigue- Korrosion von Metallen und Legierungen - Prüfung der
corrosion - Partie 2: Essais d'amorce de rupture sur des Schwingungskorrosion - Teil 2: Rissausbreitungsprüfung an
éprouvettes préfissurées (ISO 11782-2:1998) angerissenen Proben (ISO 11782-2:1998)
This European Standard was approved by CEN on 21 March 2008.
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. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2008 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 11782-2:2008: E
worldwide for CEN national Members.

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EN ISO 11782-2:2008 (E)
Contents Page
Foreword.3

2

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EN ISO 11782-2:2008 (E)
Foreword
The text of ISO 11782-2:1998 has been prepared by Technical Committee ISO/TC 156 “Corrosion of metals
and alloys” of the International Organization for Standardization (ISO) and has been taken over as EN ISO
11782-2:2008 by Technical Committee CEN/TC 262 “Metallic and other inorganic coatings” the secretariat of
which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by October 2008, and conflicting national standards shall be withdrawn at
the latest by October 2008.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 11782-2:1998 has been approved by CEN as a EN ISO 11782-2:2008 without any
modification.

3

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INTERNATIONAL ISO
STANDARD 11782-2
First edition
1998-07-15
Corrosion of metals and alloys — Corrosion
fatigue testing —
Part 2:
Crack propagation testing using precracked
specimens
Corrosion des métaux et alliages — Essais de fatigue-corrosion —
Partie 2: Essais d'amorce de rupture sur des éprouvettes préfissurées
A
Reference number
ISO 11782-2:1998(E)

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ISO 11782-2:1998(E)
Contents Page
1 Scope . 1
2 Normative reference . 1
3 Definitions . 1
4 Test. 3
Principle of corrosion fatigue crack propagation testing .
4.1 3
4.2 Specimens for corrosion fatigue crack propagation testing 4
5 Apparatus . 5
6 Fatigue precracking . 6
7 Test conditions . 7
Test procedure .
8 9
9 Test report . 11
Annex A (informative) Information on methods for measuring
crack lengths . 13
©  ISO 1998
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 the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet [e-mail]
Printed in Switzerland
ii

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©
ISO ISO 11782-2:1998(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.
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.
International Standard ISO 11782-2 was prepared by Technical Committee
ISO/TC 158, Corrosion of metals and alloys.
ISO 11782 consists of the following parts, under the general title Corrosion
of metals and alloys — Corrosion fatigue testing:
— Part 1: Cycles to failure testing
— Part 2: Crack-propagation testing using precracked specimens
Annex A of this part of ISO 11782 is for information only.
iii

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ISO 11782-2:1998(E) ISO
Introduction
Crack propagation testing employs precracked specimens to provide
information on the threshold conditions and on rates of corrosion fatigue
crack growth. These data can be used in the design and evaluation of
engineering structures where corrosion fatigue crack growth can dominate
component life.
Because of the need to maintain elastically constrained conditions at the
crack tip, the precracked specimens used for crack propagation tests are
not suitable for the evaluation of thin products such as sheet or wire and
are generally used for thicker products including plate, bar and forgings.
They can also be used for parts joined by welding.
The results of corrosion fatigue testing are suitable for direct application
only when the service conditions exactly parallel the test conditions
especially with regard to material, environmental and stressing
considerations.
iv

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©
INTERNATIONAL STANDARD  ISO ISO 11782-2:1998(E)
Corrosion of metals and alloys — Corrosion fatigue testing —
Part 2:
Crack propagation testing using precracked specimens
1  Scope
1.1  This part of ISO 11782 describes the fracture mechanics method of determining the crack growth rates of pre-
existing cracks under cyclic loading in a controlled environment and the measurement of the threshold stress
intensity factor range for crack growth below which the rate of crack advance falls below some defined limit agreed
between parties.
1.2  This part of ISO 11782 provides guidance and instruction on corrosion fatigue testing of metals and alloys in
aqueous or gaseous environments.
2  Normative reference
The following standard contains provisions which, through reference in this text, constitute provisions of this part of
ISO 11782. At the time of publication, the edition indicated was valid. All standards are subject to revision, and
parties to agreements based on this part of ISO 11782 are encouraged to investigate the possibility of applying the
most recent edition of the standard indicated below. Members of IEC and ISO maintain registers of currently valid
International Standards.
ISO 7539-1:1987, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing
procedures.
3  Definitions
For the purposes of this part of ISO 11782, the following definitions apply.
3.1  corrosion fatigue: Process involving conjoint corrosion and alternating straining of the metal, often leading to
cracking.
NOTE —  Corrosion fatigue may occur when a metal is subjected to cyclic straining in a corrosive environment.
3.2  force, P: Force applied to the specimen considered positive when its direction is such as to cause the crack
faces to move apart.
3.3  maximum force, P : Algebraic maximum value of force during a loading cycle.
max
3.4  minimum force, P : Algebraic minimum value of force during a loading cycle.
min
3.5  Difference between the algebraic maximum and minimum values of the force.
force range, DP:
1

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ISO
ISO 11782-2:1998(E)
3.6  stress intensity factor, K : Function of applied load, crack length and specimen geometry having dimensions
I
1/2
of stress (length) which uniquely defines the elastic stress field intensification at the tip of a crack subjected to
opening mode displacements (mode I).
NOTE —  It has been found that stress intensity factors, calculated assuming that specimens respond purely elastically,
correlate the behaviour of real cracked bodies provided that the size of the zone of plasticity at the crack tip is small compared
to the crack length and the length of the uncracked ligament. In this standard, mode I is assumed and the subscript I is implied
everywhere.
3.7  maximum stress intensity factor, K , in fatigue: Highest algebraic value of the stress intensity factor in a
max
cycle corresponding to the maximum load.
3.8  minimum stress intensity factor, K , in fatigue: Lowest algebraic value of the stress intensity factor in a
min
cycle.
NOTE —  This value corresponds to the minimum load when the stress ratio, R, is greater than zero and is set equal to zero
when R is less than or equal to zero.
3.9  range of stress intensity factor, DK, in fatigue: Algebraic difference between the maximum and minimum
stress intensity factors in a cycle:
DKK=−K
max min
3.10  threshold stress intensity factor range, DK , in fatigue: Value of the stress intensity factor range below
th
which the rate of crack advance becomes insignificant for the application.
3.11  stress ratio, R, in fatigue loading: Algebraic ratio of the minimum and maximum force in a cycle
P K
min min
R==
P K
max max
3.12  cycle: Smallest segment of the load- or stress-time function which is repeated periodically. The terms fatigue
cycle, load cycle and stress cycle are also commonly used.
3.13  fatigue crack growth rate, da/dN: Rate of crack extension caused by fatigue loading and expressed in
terms of crack extension per cycle.
3.14  stress intensity factor coefficient, Y: Factor derived from the stress analysis for a particular specimen
geometry which relates the stress intensity factor for a given crack length to the load and specimen dimensions.
3.15  plane strain fracture toughness, K : The critical value of K at which the first significant environmentally
lc
independent extension of the crack occurs under the influence of rising stress intensity under conditions of high
constraint to plastic deformation.
3.16  specimen orientation: The fracture plane of the specimen identified in terms of firstly the direction of
stressing and secondly the direction of crack growth expressed with respect to three reference axes. These are
identified by the letters X, Y and Z.
where
Z  is coincident with the main working force employed during manufacture of the material (short-transverse
axis);
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ISO
ISO 11782-2:1998(E)
X  is coincident with the direction of grain flow (longitudinal axis);
Y  is normal to the X and Z axes (see figure 1).
3.17  crack length, a: Effective crack length measured from the crack tip to either the mouth of the notch or the
loading point axis depending on the specimen geometry.
3.18  specimen width, W: Effective width of the specimen measured from the back face to either the face
containing the notch or the loading plane depending on the specimen geometry.
3.19  waveform: Shape of the peak-to-peak variation of load as a function of time.
3.20  cyclic frequency: Number of cycles per unit time, usually expressed in terms of cycles per second (Hz).
4  Test
4.1  Principle of corrosion fatigue crack propagation testing
A fatigue pre-crack is induced in a notched specimen by cyclic loading. As the crack grows the loading conditions
are adjusted until the values of DK and R are appropriate for the subsequent determination of DK or crack growth
th
rates and the crack is of sufficient length for the influence of the notch to be negligible.
Corrosion fatigue crack propagation tests are then conducted using cyclic loading under environmental and
stressing conditions relevant to the particular application. During the test, crack length is monitored as a function of
elapsed cycles. These data are subjected to numerical analysis so that the rate of crack growth, da/dN, can be
expressed as a function of the stress intensity factor range, DK.
Crack growth rates presented in terms of DK are generally independent of the geometry of the specimen used. The
principle of similitude allows the comparison of data obtained from a variety of specimen types and allows da/dN
versus DK data to be used in the design and evaluation of engineering structures provided that appropriate
mechanical, chemical and electrochemical test conditions are employed. An important deviation from the principle of
similitude can occur in relation to short cracks because of crack-tip chemistry differences, microstructurally sensitive
growth and crack tip shielding considerations.
The threshold stress intensity factor range for corrosion fatigue, DK may be higher or lower than the threshold in
th
air depending on the particular metal/environment conditions. It may be determined by a controlled reduction in load
range (see 6.3) until the rate of growth becomes insignificant for the specific application. Practically, from a
measurement perspective it is necessary to assign a value to this (see 8.5).
NOTE —  Both crack growth rate measurements and threshold stress intensity factor range determinations can be markedly
affected by residual stresses. Thermal stress relief should, therefore, be considered prior to testing, but if this is not acceptable,
the possibility of an effect should be recognized in the interpretation of the results. In particular, the presence of residual
stresses can lead to an apparent dependence of DK on specimen thickness. Thickness effects can also arise in principle in
th
relation to hydrogen charging and also where through-thickness transport of fluid occurs in flowing aqueous solutions. In the
latter case it should be recognized that solution transport via the crack sides in the through-thickness direction is an artifact of
the fracture mechanics specimen and may not be representative of cracking in service.
Results of corrosion fatigue crack growth rate tests for many metals have shown that the relationship between
da/dN and DK can differ significantly from the three-stage relationship usually observed for tests in air, as shown in
figure 2. The shape of the curve depends on the material/environment system and for some cases time-dependent
(as distinct from cycle-dependent) cracking modes can ensue which can enhance crack growth producing
frequency-dependent growth rate plateaux as shown in figure 2.
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ISO
ISO 11782-2:1998(E)
4.2  Specimens for corrosion fatigue crack propagation testing
4.2.1  General
A wide range of standard specimen geometries of the type used in fracture toughness testing may be used. The
particular type of specimen selected will be dependent upon the form of the material to be tested and the conditions
of test.
Pin-loaded specimens such as compact tension (CT) specimens are not suitable for tests with R values of zero or
less than zero because of backlash effects. For such purposes four-point single edge notch bend (SENB4) or centre
cracked tension (CCT) specimens loaded by friction grips are suitable.
A basic requirement is that the dimensions of the specimens be sufficient to maintain predominantly triaxial (plane
strain) conditions in which plastic deformation is limited in the vicinity of the crack tip. Experience with fracture
toughness testing has shown that for a valid K measurement a, B and (W-a) should not be less than
lc
2
 
K
lc
25,  
s
 y 
 
where s is the yield strength.
y
It is recommended that a similar criterion be used to ensure adequate constraint during corrosion fatigue crack
growth testing where K is substituted for K in the above expression.
max lc
4.2.2  Specimen design
Specimen geometries which are frequently used for corrosion fatigue crack growth rate testing include the following:
a) three-point single edge notch bend (SENB3);
b) four-point single edge notch bend (SENB4);
c) compact tension (CT);
d) centre-cracked tension (CCT).
Details of standard specimen designs for each of these types of specimen are given in figures 3 to 6 and permitted
notch geometries are given in figure 7. Suitable machining tolerances are given in table 1.
4.2.3  Stress intensity factor considerations
It can be shown, using elastic theory, that the stress intensity factor acting at the tip of a crack in specimens or
structures of various geometries can be expressed by relationships of the form:
KQ= s a
I
where
Q is the geometrical constant;
s is the applied stress;
a is the crack length.
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ISO
ISO 11782-2:1998(E)
Stress intensity factors can be calculated by means of a dimensionless stress intensity coefficient, Y, related to
crack length expressed in terms of a/W (where W is the width of the specimen) through a stress intensity factor
function of the form:
YP
K =
I
12/
BW
NOTE —  Where P < 0, K = 0. Nevertheless, it should not be assumed that negative loading will have no influence on the rate
of crack growth.
The values of Y appropriate to the four specimen geometries discussed above are given in tables 2 to 5.
4.2.4  Specimen preparation
Specimens of the required orientation (see figure 1) shall, where possible, be machined in the fully heat-treated
condition, i.e. in the material condition of interest. For specimens in material that cannot easily be completely
machined in the fully heat-treated condition, the final heat-treatment may be given prior to the notching and finishing
operations provided that at least 0,5 mm per face is removed from the thickness at the finish machining stage.
However, heat treatments may be carried out on fully machined specimens in cases where heat treatment will not
result in detrimental surface conditions, residual stresses, quench cracking or distortion.
After machining, the specimens shall be fully degreased in order to ensure that no contamination of the crack tip
occurs during subsequent fatigue precracking or corrosion fatigue crack propagation testing. In cases where it is
necessary to attach electrodes to the specimens by soldering or brazing for crack length monitoring purposes, the
specimens should be degreased following this operation prior to precracking in order to remove traces of remnant
flux.
4.2.5  Specimen identification
Specimen identification marks may be stamped or scribed on either the face of the specimen bearing the notch or
the end faces parallel to the notch.
5  Apparatus
5.1  Environmental chamber
The environmental chamber shall completely enclose the test section of the specimen. Wherever possible, the
gripped portions shall be excluded from contact with the solution environment to prevent galvanic effects and
crevice corrosion. If this is not possible, appropriate measures shall be taken through, for example, the use of
similar metals, electrical insulation or coatings. An adequate volume of solution to metal area ratio is required
(dependent on reaction rates and exposure time) and a circulation system is usually necessary. For conditions of
applied potential or applied current a separate compartment for the counter electrode may be necessary to limit any
effects caused by reaction products from this electrode. Non-metallic materials are recommended for the
environmental chamber and circulation system where this is practicable. These materials shall be inert. Note that
glass and certain plastics are not inert at elevated temperatures. Where metallic chambers are necessary these
shall be electrically insulated from the specimen to prevent galvanic interaction.
For tests in gaseous environment an all-metal-chamber is preferred.
5.2  Crack length measurement
The most commonly used techniques for the measurement of crack length are described in annex A. Optical
methods of measurement are often precluded by the environment and test chamber and, in any case, provide
guidance only to the surface length of a crack. Enhancement of crack visibility by removal of corrosion products
may perturb the local electrochemistry and is not recommended. Methods that measure the average crack length
5

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ISO
ISO 11782-2:1998(E)
across the thickness of the specimen are generally preferred. These include electrical resistance methods. AC and
DC potential drop measurements are suitable but should be checked to ensure that they exert no detectable
influence on the rate of corrosion fatigue crack propagation and appropriate methods should be used to eliminate
galvanic effects. Compliance methods based on measurement of displacement across the notch or of strain in the
back face of the specimen opposite the notch can also be used.
6  Fatigue precracking
6.1  General
The machine used for fatigue cracking should have a method of loading such that the stress distribution is
symmetrical about the notch and the applied force should be known to an accuracy within – 2,5 %.
In corrosion fatigue studies in the laboratory an artificial precracking procedure is introduced to provide a sharpened
fatigue crack of adequate size and straightness. In principle, this procedure can affect subsequent crack growth
depending on the frequency used, the manner in which the loading parameters are adjusted and whether
precracking is conducted in air or in the test environment.
In some materials, the introduction of the corrosion fatigue test environment during the precracking operation will
promote a change from the normal ductile transgranular mode of fatigue cracking to a less ductile corrosion fatigue
mode. This may facilitate the subsequent initiation of cracking during corrosion fatigue testing. However, unless
corrosion fatigue testing is conducted immediately following the precracking operation, corrodent remaining at the
crack tip may promote blunting due to corrosive attack. For this reason, it is recommended that, unless agreed
otherwise between the parties, fatigue precracking should be conducted in the normal laboratory air environment. In
this case, precracking can be expedited by the use of high cyclic frequency.
6.2  Precracking procedure
Conduct fatigue precracking with the specimen fully heat-treated to the condition in which it is to be tested until the
crack extends beyond the notch at the side surfaces by at least 0,025W or 1,25 mm, whichever is greater.
The final K during precracking shall not exceed the initial K for which test data are to be obtained. Ideally,
max max
-8
precracking should be conducted without reduction in the value of K . This is feasible for da/dN . 10 m/cycle
max
but impractical for lower growth rates (see 6.3).
-
8
NOTE —  The DK values to give growth rates of about 10 m/cycle are:
1/2
— steels, nickel, titanium and copper alloys: DK = 13 MPa�m
1/2
— aluminium alloys: DK = 6 MPa�m
The K value can be evaluated from the R value of interest. The value of K shall not exceed 0,7K .
max max lc
The K value can be as important as K during precracking; K will dictate crack wake effects. For example,
min max min
high versus low crack wake effects can dramatically affect corrosion fatigue testing results. Transient d /d
R R a N
(crack closure influenced) behaviour can result.
At the end of precracking check that the surface crack lengths do not differ by more than 0,1a. If the fatigue crack
departs more than – 5° from the plane of symmetry the specimen is not suitable for further testing.
The precracked specimen may be stored in a dessicated vessel until required. Long storage periods should be
avoided because of possible crack tip blunting or contamination effects.
DK
6.3  Precracking for low crack growth rates or determination
th
-8
For da/dN , 10 m/cycle and for determination of the threshold DK (see 8.5) the precracking procedure described
in 6.2 should be followed initially. A load-shedding procedure is then adopted until the lowest DK or crack growth
rate of interest is achieved.
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ISO 11782-2:1998(E)
Cyclically load the specimen, smoothly varying K with crack length according to:
max
KK=−expCaa
()
[]
max s k s
where
a is the crack length after the preliminary precracking stage (see 6.2);
s
K is the corresponding value of K ;
s max
- 1
C is a load shedding factor; (C = -100 m is generally satisfactory when a and a are expressed in
k k s
metres).
Continue load shedding, varying P so that the stress ratio R remains constant and equal to R , the value after the
min s
preliminary precracking.
NOTE —  Continuous load shedding by computer control is recommended. If step shedding of load is employed the reduction
in P shall not exceed 10 % of the previous value, and adjustments should not be made until the crack has grown by at least the
2
 
prior plane strain plastic zone size RK=01,/s,m.
pm[]ax y
 
An alternative method of precracking for low crack growth rates or threshold DK determination can be used for high
R values simply by increasing K while maintaining K constant until the relevant DK value is obtained.
min max
Assuming the notch to behave as a crack of the equivalent length, cyclically load the specimen such that K
max
equals the value of interest and K is derived from the target value of R.
min
When a reaches a , cyclically load the specimen, smoothly varying K with crack length according to
s min
KK=−11−R expCa−a
() ( )}
ss { s
min[]k
-1
where C is a load shedding factor (C = -280 m is generally satisfactory when a and a are expressed in metres).
k k s
Vary P so that K remains constant and equal to K . Continue until the appropriate DK value is obtained.
max max s
NOTE —  K (1 - R ) which equals DK at the beginning of the determination can conceivably be less than DK at this value of
s s th
R = R and this test method would clearly be inappropriate.
s
7  Test conditions
7.1  Environmental considerations
Because of the specificity of metal-environment interactions, it is essential that corrosion fatigue crack propagation
tests are conducted under environmental conditions which are closely controlled (see paragraphs 3 and 4 below).
The environmental testing conditions depend upon the intent of the test but, ideally, should be the same as those
prevailing for the intended use of the alloy or comparable to the anticipated service
...