SIST EN ISO 7539-6:2011

Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of precracked specimens for tests under constant load or constant displacement (ISO 7539-6:2011)

1.1 This part of ISO 7539 covers procedures for designing, preparing and using precracked specimens for
investigating susceptibility to stress corrosion. It gives recommendations for the design, preparation and use
of precracked specimens for investigating susceptibility to stress corrosion. Recommendations concerning
notched specimens are given in Annex A.
The term “metal” as used in this part of ISO 7539 includes alloys.
1.2 Because of the need to confine plasticity at the crack tip, precracked specimens 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.
1.3 Precracked specimens can be loaded with equipment for application of a constant load or can
incorporate a device to produce a constant displacement at the loading points. Tests conducted under
increasing displacement or increasing load are dealt with in ISO 7539-9.
1.4 A particular advantage of precracked specimens is that they allow data to be acquired from which
critical defect sizes, above which stress corrosion cracking can occur, can be estimated for components of
known geometry subjected to known stresses. They also enable rates of stress corrosion crack propagation to
be determined. The latter data can be taken into account when monitoring parts containing defects during
service.

Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 6: Vorbereitung und Anwendung von angerissenen Proben für die Prüfung unter konstanter Kraft oder konstanter Verformung (ISO 7539-6:2011)

Dieser Teil von ISO 7539 behandelt Verfahren für die Gestaltung, Vorbereitung und Anwendung von
angerissenen Proben zur Untersuchung der Beständigkeit von Metallen gegen Spannungsrisskorrosion.
Empfehlungen für gekerbte Proben werden im Anhang A angegeben.
In diesem Teil von ISO 7539 werden mit der Benennung „Metall“ auch Legierungen erfasst.
1.2 Für die Bewertung dünner Erzeugnisse, z. B. Feinblech oder Draht, sind angerissene Proben wegen
der Notwendigkeit, die Plastizität auf die Rissspitze zu beschränken, nicht geeignet; sie werden im
Allgemeinen für die Bewertung dickerer Erzeugnisse angewendet, zu denen Grobbleche, Stangen und
Schmiedestücke gehören. Sie können auch auf miteinander verschweißte Teile angewendet werden.
1.3 Zur Beanspruchung der angerissenen Proben können Prüfeinrichtungen angewendet werden, die eine
konstante Kraft aufbringen oder die eine Vorrichtung aufnehmen können, mit der eine konstante Verformung
an den Kraftangriffspunkten erzeugt wird. Prüfungen mit ansteigender Auslenkung oder ansteigender Last
sind nach ISO 7539-9 durchzuführen.
1.4 Ein spezieller Vorteil der angerissenen Proben besteht darin, dass mit ihrer Hilfe Daten erhalten werden
können, mit denen kritische Fehlergrößen, bei deren Überschreiten Spannungsrisskorrosionsanfälligkeit
auftreten kann, für Teile mit bekannter Geometrie, die bekannten Spannungen ausgesetzt sind, ermittelt
werden können. Mit Hilfe von angerissenen Proben ist es ferner möglich, die Ausbreitungsgeschwindigkeit
von Spannungskorrosionsrissen zu bestimmen. Diese Daten können für die Überwachung von Teilen
herangezogen werden, in denen beim Einsatz Fehler auftreten.

Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 6: Préparation et utilisation des éprouvettes préfissurées pour essais sous charge constante ou sous déplacement constant (ISO 7539-6:2011)

L'ISO 7539-6:2011 couvre les procédures de conception, de préparation et d'utilisation d'éprouvettes préfissurées servant à évaluer la sensibilité d'un métal à la corrosion sous contrainte. Elle donne des recommandations pour la conception, la préparation et l'utilisation d'éprouvettes préfissurées pour évaluer la sensibilité à la corrosion sous contrainte.
Pour les besoins de l'ISO 7539-6:2011, le terme «métal» inclut également les alliages.
Comme il est nécessaire de confiner la déformation plastique en fond de fissure, les éprouvettes préfissurées ne se prêtent pas à l'évaluation des produits minces tels que les tôles minces et les fils, et sont généralement utilisées pour des produits plus épais tels que les tôles fortes, les barres et les pièces forgées. Elles peuvent aussi être utilisées pour des pièces assemblées par soudage.
Les éprouvettes préfissurées peuvent être soumises à une contrainte à l'aide d'appareils exerçant une charge constante ou comprenant un dispositif qui engendre un déplacement constant des points d'application de la charge. Les essais sous déplacement croissant ou sous charge croissante sont traités dans l'ISO 7539‑9.
Les éprouvettes préfissurées présentent l'avantage de permettre l'acquisition de données dont on peut déduire les tailles critiques de défaut au-delà desquelles une fissuration par corrosion sous contrainte peut se produire au niveau de pièces de géométrie connue soumises à des efforts connus. Ces éprouvettes permettent également de déterminer la vitesse de propagation des fissures de corrosion sous contrainte. Ces dernières données peuvent être prises en compte dans le cadre de la surveillance, en service, de pièces comportant des défauts.

Korozija kovin in zlitin - Preskušanje napetostne korozije - 6. del: Priprava in uporaba preskušancev z umetno razpoko za preskuse pri konstantni obremenitvi ali konstantni deformaciji (ISO 7539-6:2011)

1.1 Ta del ISO 7539 zajema postopke za načrtovanje, pripravo in uporabo preskušancev z umetno razpoko za preiskavo dovzetnosti za napetostno korozijo. Navaja priporočila za načrtovanje, pripravo in uporabo preskušancev z umetno razpoko za preiskavo dovzetnosti za napetostno korozijo. Priporočila v zvezi s preskušanci z zarezo so navedena v dodatku A.
Izraz »kovina«, kot se uporablja v tem delu ISO 7539, vključuje tudi zlitine.
1.2 Zaradi potrebe po omejitvi plastičnosti na odprtju razpoke preskušanci z umetno razpoko niso primerni za vrednotenje tankih proizvodov, kot so listi ali žice, in se v splošnem uporabljajo za debelejše proizvode, vključno s ploščami, palicami in odkovki. Uporabljajo se lahko tudi za dele, združene z varjenjem.
1.3 Preskušanci z umetno razpoko se lahko obremenijo z opremo za vnos konstantne obremenitve, lahko pa vključujejo pripomoček, ki izvaja konstantno deformacijo na točkah obremenitve. Preskuse, opravljene z naraščajočo deformacijo ali naraščajočo obremenitvijo, obravnava ISO 7539-9.
1.4 Konkretna prednost preskušancev z umetno razpoko je, da omogočajo pridobivanje podatkov, iz katerih se lahko ocenijo kritične velikosti okvare, nad katerimi lahko pride do pokanja zaradi napetostne korozije, komponent znane geometrije, na katere deluje znana napetost. Omogočajo tudi določevanje stopnje širjenja razpoke zaradi napetostne korozije. Zadnji podatki se lahko upoštevajo pri spremljanju delov, ki vsebujejo okvaro, med delovanjem.

General Information

• Status: Withdrawn
• Publication Date: 25-Oct-2011
• Withdrawal Date: 11-Nov-2018
• ICS: 77.060 - Corrosion of metals
• Technical Committee: IPKZ - Protection of metals against corrosion
• Current Stage: 9900 - Withdrawal (Adopted Project)
• Start Date: 12-Nov-2018
• Due Date: 05-Dec-2018
• Completion Date: 12-Nov-2018
• Ref Project: EN ISO 7539-6:2011 - Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of precracked specimens for tests under constant load or constant displacement (ISO 7539-6:2011)

Relations

• Replaced By: SIST EN ISO 7539-6:2018 - Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of precracked specimens for tests under constant load or constant displacement (ISO 7539-6:2018, Corrected version 2018-11)
• Effective Date: 01-Dec-2018
• Replaces: SIST EN ISO 7539-6:2003 - Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement (ISO 7539-6:2003)
• Effective Date: 01-Dec-2011

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 7539-6:2011
01-december-2011
1DGRPHãþD
SIST EN ISO 7539-6:2003
Korozija kovin in zlitin - Preskušanje napetostne korozije - 6. del: Priprava in
uporaba preskušancev z umetno razpoko za preskuse pri konstantni obremenitvi
ali konstantni deformaciji (ISO 7539-6:2011)
Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of
precracked specimens for tests under constant load or constant displacement (ISO 7539
-6:2011)
Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 6:
Vorbereitung und Anwendung von angerissenen Proben für die Prüfung unter konstanter
Kraft oder konstanter Verformung (ISO 7539-6:2011)
Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 6:
Préparation et utilisation des éprouvettes préfissurées pour essais sous charge
constante ou sous déplacement constant (ISO 7539-6:2011)
Ta slovenski standard je istoveten z: EN ISO 7539-6:2011
ICS:
77.060 Korozija kovin Corrosion of metals
SIST EN ISO 7539-6:2011 en,fr
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 7539-6:2011

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SIST EN ISO 7539-6:2011


EUROPEAN STANDARD
EN ISO 7539-6

NORME EUROPÉENNE

EUROPÄISCHE NORM
October 2011
ICS 77.060 Supersedes EN ISO 7539-6:2003
English Version
Corrosion of metals and alloys - Stress corrosion testing - Part
6: Preparation and use of precracked specimens for tests under
constant load or constant displacement (ISO 7539-6:2011)
Corrosion des métaux et alliages - Essais de corrosion Korrosion der Metalle und Legierungen - Prüfung der
sous contrainte - Partie 6: Préparation et utilisation des Spannungsrisskorrosion - Teil 6: Vorbereitung und
éprouvettes préfissurées pour essais sous charge Anwendung von angerissenen Proben für die Prüfung unter
constante ou sous déplacement constant (ISO 7539- konstanter Kraft oder konstanter Verformung (ISO 7539-
6:2011) 6:2011)
This European Standard was approved by CEN on 14 October 2011.

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-CENELEC 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-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, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.





EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2011 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7539-6:2011: E
worldwide for CEN national Members.

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SIST EN ISO 7539-6:2011
EN ISO 7539-6:2011 (E)
Contents Page
Foreword .3
2

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SIST EN ISO 7539-6:2011
EN ISO 7539-6:2011 (E)
Foreword
This document (EN ISO 7539-6:2011) has been prepared by Technical Committee ISO/TC 156 "Corrosion of
metals and alloys" in collaboration with 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 April 2012, and conflicting national standards shall be withdrawn at the
latest by April 2012.
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.
This document supersedes EN ISO 7539-6:2003.
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, Croatia, 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 7539-6:2011 has been approved by CEN as a EN ISO 7539-6:2011 without any modification.

3

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SIST EN ISO 7539-6:2011

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SIST EN ISO 7539-6:2011

INTERNATIONAL ISO
STANDARD 7539-6
Third edition
2011-10-15

Corrosion of metals and alloys — Stress
corrosion testing —
Part 6:
Preparation and use of precracked
specimens for tests under constant load
or constant displacement
Corrosion des métaux et alliages — Essais de corrosion sous
contrainte —
Partie 6: Préparation et utilisation des éprouvettes préfissurées pour
essais sous charge constante ou sous déplacement constant




Reference number
ISO 7539-6:2011(E)
©
ISO 2011

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)

COPYRIGHT PROTECTED DOCUMENT


©  ISO 2011
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]
Web www.iso.org
Published in Switzerland

ii © ISO 2011 – All rights reserved

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
Contents Page
Foreword . iv
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 2
4  Principle . 4
5  Specimens . 5
5.1  General . 5
5.2  Specimen design . 6
5.3  Stress intensity factor considerations . 17
5.4  Specimen preparation . 22
5.5  Specimen identification . 23
6  Initiation and propagation of fatigue cracks . 23
7  Procedure . 25
7.1  General . 25
7.2  Environmental considerations . 25
7.3  Environmental chamber. 26
7.4  Environmental control and monitoring . 27
7.5  Determination of K by crack arrest . 28

ISCC
7.6  Determination of K by crack initiation . 31
ISCC
7.7 Measurement of crack velocity . 32
8  Test report . 33
Annex A (normative) Use of notched specimens for stress corrosion tests . 35
Annex B (normative) Determination of crack growth velocity . 38

© ISO 2011 – All rights reserved iii

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 7539-6 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys, in
collaboration with the National Physical Laboratory (United Kingdom).
This third edition cancels and replaces the second edition (ISO 7539-6:2003), of which it constitutes a minor
revision.
ISO 7539 consists of the following parts, under the general title Corrosion of metals and alloys — Stress
corrosion testing:
 Part 1: General guidance on testing procedures
 Part 2: Preparation and use of bent-beam specimens
 Part 3: Preparation and use of U-bend specimens
 Part 4: Preparation and use of uniaxially loaded tension specimens
 Part 5: Preparation and use of C-ring specimens
 Part 6: Preparation and use of precracked specimens for tests under constant load or constant
displacement
 Part 7: Method fo slow strain rate testing
 Part 8: Preparation and use of specimens to evaluate weldments
 Part 9: Preparation and use of pre-cracked specimens for tests under rising load or rising displacement
The following parts are under preparation:
 Part 10: Testing of alloys using reverse U-bend test method
 Part 11: Guidelines for testing the resistance of metals and alloys to hydrogen embrittlement and
hydrogen assisted cracking

iv © ISO 2011 – All rights reserved

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SIST EN ISO 7539-6:2011
INTERNATIONAL STANDARD ISO 7539-6:2011(E)

Corrosion of metals and alloys — Stress corrosion testing —
Part 6:
Preparation and use of precracked specimens for tests under
constant load or constant displacement
1 Scope
1.1 This part of ISO 7539 covers procedures for designing, preparing and using precracked specimens for
investigating susceptibility to stress corrosion. It gives recommendations for the design, preparation and use
of precracked specimens for investigating susceptibility to stress corrosion. Recommendations concerning
notched specimens are given in Annex A.
The term “metal” as used in this part of ISO 7539 includes alloys.
1.2 Because of the need to confine plasticity at the crack tip, precracked specimens 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.
1.3 Precracked specimens can be loaded with equipment for application of a constant load or can
incorporate a device to produce a constant displacement at the loading points. Tests conducted under
increasing displacement or increasing load are dealt with in ISO 7539-9.
1.4 A particular advantage of precracked specimens is that they allow data to be acquired from which
critical defect sizes, above which stress corrosion cracking can occur, can be estimated for components of
known geometry subjected to known stresses. They also enable rates of stress corrosion crack propagation to
be determined. The latter data can be taken into account when monitoring parts containing defects during
service.
2 Normative references
The following 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 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing
procedures
ISO 11782-2:1998, Corrosion of metals and alloys — Corrosion fatigue testing — Part 2: Crack propagation
testing using precracked specimens
© ISO 2011 – All rights reserved 1

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7539-1 and the following apply.
3.1
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
3.2
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
3.3
specimen thickness
B
side-to-side dimension of the specimen being tested
3.4
reduced thickness at side grooves
B
n
minimum side-to-side dimension between the notches in side-grooved specimens
3.5
specimen half-height
H
50 % of the specimen height measured parallel to the direction of load application for compact tension, double
cantilever beam and modified wedge-opening-loaded test pieces
3.6
load
P
load which, when applied to the specimen, is considered positive if its direction is such as to cause the crack
faces to move apart
3.7
deflection at loading point axis
V
LL
crack opening displacement produced at the loading line during the application of load to a constant
displacement specimen
3.8
deflection away from the loading line
V
0
crack opening displacement produced at a location remote from the loading plane, e.g. at knife edges located
at the notch mouth, during the application of load to a constant displacement specimen
3.9
modulus of elasticity
E
elastic modulus (i.e. stress/strain) in tension
2 © ISO 2011 – All rights reserved

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
3.10
stress intensity factor
K
I
function of applied load, crack length and specimen geometry having dimensions of stress  length which
uniquely define 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 with 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 part of ISO 7539, mode I is assumed and
the subscript I is implied everywhere.
3.11
initial stress intensity factor
K
Ii
stress intensity applied at the commencement of the stress corrosion test
3.12
plane strain fracture toughness
K
Ic
critical value of K at which the first significant environmentally independent extension of the crack occurs

I
under the influence of rising stress intensity under conditions of high resistance to plastic deformation
3.13
provisional value of K
Ic
K
Q
K = K when the validity criteria for plane strain predominance are satisfied
Q Ic
3.14
threshold stress intensity factor for susceptibility to stress corrosion cracking
K
ISCC
stress intensity factor above which stress corrosion cracking will initiate and grow for the specified test
conditions under conditions of high resistance to plastic deformation, i.e. under plane strain predominant
conditions
3.15
provisional value of K
ISCC
K
QSCC
K = K when the validity criteria for plane strain predominance are satisfied
QSCC ISCC
3.16
maximum stress intensity factor
K in fatigue

max
highest algebraic value of the stress intensity factor in a cycle, corresponding to the maximum load
3.17
0,2 % proof stress
R
p0,2
stress which must be applied to produce a plastic strain of 0,2 % during a tensile test
3.18
applied stress

stress resulting from the application of load to the specimen
3.19
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
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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
3.20
load ratio in fatigue loading
R
algebraic ratio of minimum to maximum load in a cycle:
PK
min min
R
PK
max max
3.21
crack velocity
instantaneous rate of stress corrosion crack propagation measured by a continuous crack monitoring
technique
3.22
average crack velocity
average rate of crack propagation calculated by dividing the change in crack length due to stress corrosion by
the test duration
3.23
specimen orientation
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 identified by the letters X, Y and Z
NOTE
Z is coincident with the main working force used during manufacture of the material (short-transverse axis);
X is coincident with the direction of grain flow (longitudinal axis);
Y is normal to the X and Z axes.
4 Principle
4.1 The use of precracked specimens acknowledges the difficulty of ensuring that crack-like defects
introduced during either manufacture or subsequent service are totally absent from structures. Furthermore,
the presence of such defects can cause a susceptibility to stress corrosion cracking which in some materials
(e.g. titanium) may not be evident from tests under constant load on smooth specimens. The principles of
linear elastic fracture mechanics can be used to quantify the stress situation existing at the crack tip in a
precracked specimen or structure in terms of the plane strain-stress intensity.
4.2 The test involves subjecting a specimen in which a crack has been developed by fatigue from a
machined notch to either a constant load or displacement at the loading points during exposure to a
chemically aggressive environment. The objective is to quantify the conditions under which environmentally
assisted crack extension can occur in terms of the threshold stress intensity for stress corrosion cracking,
K , and the kinetics of crack propagation.
ISCC
4.3 The empirical data can be used for design or life prediction purposes, in order to ensure either that the
stresses within large structures are insufficient to promote the initiation of environmentally assisted cracking,
whatever pre-existing defects may be present, or that the amount of crack growth which would occur within
the design life or inspection periods can be tolerated without the risk of unstable failure.
4.4 Stress corrosion cracking is influenced by both mechanical and electrochemical driving forces. The
latter can vary with crack depth, opening or shape because of variations in crack-tip chemistry and electrode
potential and may not be uniquely described by the fracture-mechanics stress intensity factor.
4.5 The mechanical driving force includes both applied and residual stresses. The possible influence of the
latter shall be considered in both laboratory testing and the application to more complex geometries.
Gradients in residual stress in a specimen may result in non-uniform crack growth along the crack front.
4 © ISO 2011 – All rights reserved

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
5 Specimens
5.1 General
5.1.1 A wide range of standard specimen geometries of the type used in fracture toughness tests may be
applied. The particular type of specimen used will be dependent upon the form, the strength and the
susceptibility to stress corrosion cracking of the material to be tested and also on the objective of the test.
5.1.2 A basic requirement is that the dimensions be sufficient to maintain predominantly triaxial (plane
strain) conditions in which plastic deformation is limited to the vicinity of the crack tip. Experience with fracture
toughness testing has shown that, for a valid K measurement, both the crack length, a, and the thickness, B,
Ic
shall not be less than
2

K
Ic

2,5

R
p0,2

and that, where possible, larger specimens where both a and B are at least
2

K
Ic
4

R
p0,2

shall be used to ensure adequate constraint.
From the point of view of fracture mechanics, a minimum thickness from which an invariant value of K is
ISCC
obtained cannot be specified at this time. The presence of an aggressive environment during stress corrosion
may reduce the extent of plasticity associated with fracture and hence the specimen dimensions needed to
limit plastic deformation. However, in order to minimize the risk of inadequate constraint, it is recommended
that similar criteria to those used during fracture toughness testing also be used regarding specimen
dimensions, i.e. both a and B shall be not less than
2

K
I
2,5

R
p0,2

and preferably should be not less than
2

K
I
4

R
p0,2

where K is the stress intensity to be applied during testing.
I
The threshold stress intensity value eventually determined should be substituted for K in the first of these
I
expressions as a test for its validity.
5.1.3 If the specimens are to be used for the determination of K , the initial specimen size should be
ISCC
based on an estimate of the K of the material (in the first instance, it is better to over-estimate the K

ISCC ISCC
value and therefore use a larger specimen than may eventually be found necessary). Where the service
application involves the use of material of insufficient thickness to satisfy the conditions for validity, it is
permissible to test specimens of similar thickness, provided that it is clearly stated that the threshold intensity
value obtained, K , is of relevance only to that specific application. Where determining stress corrosion
QSCC
crack growth behaviour as a function of stress intensity is required, the specimen size shall be based on an
estimate of the highest stress intensity at which crack growth rates are to be measured.
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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
5.1.4 Two basic types of specimen can be used:
a) those intended for testing under constant displacement, which are invariably self-loaded by means of
built-in loading bolts;
b) those intended for testing under constant load, for which an external means of load application is required.
5.1.5 Constant displacement specimens, being self-loaded, have the advantage of economy in use since no
external stressing equipment is required. Their compact dimensions also facilitate exposure to operating
service environments. They can be used for the determination of K by the initiation of stress corrosion
ISCC
cracks from the fatigue precrack, in which case a series of specimens must be used to pinpoint the threshold
value, or by the arrest of a propagating crack since, under constant displacement testing conditions, the stress
intensity decreases progressively as crack propagation occurs. In this case, a single specimen will suffice in
principle, but, in practice, the use of several specimens (not less than three) is often recommended, taking into
account the disadvantages described in 5.1.6.
5.1.6 The disadvantages of constant displacement specimens are as follows:
a) applied loads can only be measured indirectly by displacement changes;
b) oxide formation or corrosion products can either wedge open the crack surfaces, thus changing the
applied displacement and load, or can block the crack mouth, thus preventing the ingress of corrodent
and impairing the accuracy of crack length measurements by electrical resistance methods;
c) crack branching, blunting or growth out of plane can invalidate crack arrest data;
d) crack arrest must be defined by crack growth below some arbitrary rate which can be difficult to measure
accurately;
e) elastic relaxation of the loading system during crack growth can cause increased displacement and
higher loads than expected;
f) plastic relaxation due to time-dependent processes within the specimen can cause lower loads than
expected;
g) it is sometimes impossible to introduce the test environment prior to application of the load, which can
retard crack initiation during subsequent testing.
5.1.7 Constant load specimens have the advantage that stress parameters can be quantified with
confidence. Since crack growth results in increasing crack opening, there is less likelihood that oxide films will
either block the crack or wedge it open. Crack length measurements can be readily made via a number of
continuous monitoring methods. A wide choice of constant load specimen geometries is available to suit the
form of the test material, the experimental facilities available and the objectives of the test. This means that
crack growth can be studied under either bend or tension loading conditions. The specimens can be used for
either the determination of K by the initiation of a stress corrosion crack from a pre-existing fatigue crack
ISCC
using a series of specimens, or for measurements of crack growth rates. Constant load specimens can be
loaded during exposure to the test environment in order to avoid the risk of unnecessary incubation periods.
5.1.8 The principal disadvantage of constant load specimens is the expense and bulk associated with the
need for an external loading system. Bend specimens can be tested in relatively simple cantilever beam
equipment, but specimens subjected to tension loading require constant load creep rupture or similar testing
machines. In this case, the expense can be minimized by testing chains of specimens connected by loading
links which are designed to prevent unloading on the failure of specimens. The size of these loading systems
means that it is difficult to test constant load specimens under operating conditions, but they can be tested in
environments bled off from operating systems.
5.2 Specimen design
5.2.1 Figure 1 shows some of the precracked specimen geometries which are used for stress corrosion
testing.
6 © ISO 2011 – All rights reserved

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)

NOTE Stress intensity factor coefficients for the specimens shown above are available in the published literature.
Figure 1 — Precracked specimen geometries for stress corrosion testing
© ISO 2011 – All rights reserved 7

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SIST EN ISO 7539-6:2011
ISO 7539-6:2011(E)
5.2.2 Constant load specimens can be of two distinct types:
a) those in which the stress intensity increas
...