SIST EN ISO 7539-6:2018

SLOVENSKI STANDARD
01-december-2018
Nadomešča:
SIST EN ISO 7539-6:2011
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 7539-6:2018, popravljena različica 2018-11)
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)
Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 6:
Vorbereitung und Anwendung von angerissenen Proben für die Prüfung unter konstanter
Last oder konstanter Auslegung (ISO 7539-6:2018)
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 7539-6:2018, Version corrigée 2018-11)
Ta slovenski standard je istoveten z: EN ISO 7539-6:2018
ICS:
77.060 Korozija kovin Corrosion of metals
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 7539-6
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2018
EUROPÄISCHE NORM
ICS 77.060 Supersedes EN ISO 7539-6:2011
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
7539-6:2018, Corrected version 2018-11)
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
constante ou sous déplacement constant (ISO 7539- unter konstanter Last oder konstanter Auslegung (ISO
6:2018, Version corrigée 2018-11) 7539-6:2018)
This European Standard was approved by CEN on 27 August 2018.

This European Standard was corrected and reissued by the CEN-CENELEC Management Centre on 19 December 2018.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATIO N

EUROPÄISCHES KOMITEE FÜR NORMUN G

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7539-6:2018 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 7539-6:2018) 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, including for corrosion protection and corrosion testing of metals and alloys”
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 March 2019, and conflicting national standards shall
be withdrawn at the latest by March 2019.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 7539-6:2011.
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, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 7539-6:2018, Corrected version 2018-11 has been approved by CEN as EN ISO 7539-
6:2018 without any modification.

INTERNATIONAL ISO
STANDARD 7539-6
Fourth edition
2018-08
Corrected version
2018-11
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 7539-6:2018(E)
©
ISO 2018
ISO 7539-6:2018(E)
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
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Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
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Published in Switzerland
ii © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 4
5 Specimens . 5
5.1 General . 5
5.2 Specimen design . 7
5.3 Stress intensity factor considerations . .17
5.4 Specimen preparation .23
5.5 Specimen identification .25
6 Initiation and propagation of fatigue cracks .25
7 Procedure.27
7.1 General .27
7.2 Environmental considerations .27
7.3 Environmental chamber .28
7.4 Environmental control and monitoring .29
7.5 Determination of K by crack arrest .29
ISCC
7.6 Determination of K by crack initiation .32
ISCC
7.7 Measurement of crack velocity .34
8 Test report .35
Annex A (informative) Use of notched specimens for stress corrosion tests .36
Annex B (informative) Determination of crack growth velocity.39
Bibliography .40
ISO 7539-6:2018(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys, in
collaboration with the National Physical Laboratory (United Kingdom).
This fourth edition cancels and replaces the third edition (ISO 7539-6:2011), which has been technically
revised to revise Figure 14.
This corrected version of ISO 7539-6:2018 incorporates the following corrections:
— in Figure 2, the symbol “^” has been corrected to “≥” in two places.
A list of all parts in the ISO 7539 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2018 – All rights reserved

INTERNATIONAL STANDARD ISO 7539-6:2018(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
This document specifies 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 document includes alloys.
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.
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.
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 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.
ISO 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing
procedures
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7539-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
ISO 7539-6:2018(E)
3.1
crack length
a
distance 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
distance 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 distance between both sides of the specimen measured parallel to the direction of load (3.6)
application for compact tension, double cantilever beam and modified wedge-opening-loaded test pieces
3.6
load
P
force, 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 (3.6) to a
constant displacement specimen
3.8
deflection away from the loading line
V
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 (3.6) to a constant displacement specimen
3.9
modulus of elasticity
E
ratio of stress to strain without deviation in proportionality of the stress and strain (Hooke’s law)
2 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
3.10
stress intensity factor
K
I
function of applied load (3.6), crack length (3.1) 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 1 to entry: 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
document, 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
I
occurs 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 (3.10) 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 (3.10) in a cycle, corresponding to the maximum
load (3.6)
3.17
0,2 % proof stress
R
p0,2
stress which is applied to produce a plastic strain of 0,2 % during a tensile test
3.18
applied stress
σ
stress resulting from the application of load (3.6) to the specimen
ISO 7539-6:2018(E)
3.19
stress intensity factor coefficient
Y
factor derived from the stress analysis for a particular specimen geometry which relates the stress
intensity factor (3.10) for a given crack length (3.1) to the load (3.6) and specimen dimensions
3.20
load ratio in fatigue loading
R
algebraic ratio of minimum to maximum load (3.6) in a cycle:
P K
min min
R==
P K
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 (3.1) 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 1 to entry: Where X, Y and Z are defined as follows:
X is coincident with the direction of grain flow (longitudinal axis);
Z is coincident with the main working force used during manufacture of the material (short-
transverse 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 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
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.
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
Ic
thickness, B, shall not be less than:
 
K
Ic
25,  
 
R
p02,
 
and that, where possible, larger specimens where both a and B are at least:
 
K
Ic
4 
 
R
p,02
 
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 obtained cannot be specified at this time. The presence of an aggressive environment during
ISCC
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:
 
K
I
25,  
 
R
p02,
 
and preferably should be not less than:
 
K
I
 
 
R
p,02
 
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
I
these 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
ISCC
be based on an estimate of the K of the material (in the first instance, it is better to over-estimate
ISCC
the K value and therefore use a larger specimen than may eventually be found necessary). Where
ISCC
ISO 7539-6:2018(E)
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
QSCC
determining stress corrosion 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.
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
ISCC
stress corrosion 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
ISCC
crack from a pre-existing fatigue crack 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.
6 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
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.
5.2.2 Constant load specimens can be of two distinct types:
a) those in which the stress intensity increases with increasing crack length;
b) those in which the stress intensity is effectively independent of crack length.
Type a) is suitable for K determinations and studies of crack propagation rates as a function of K ,
ISCC I
while type b) is useful for fundamental studies of stress corrosion mechanisms.
5.2.3 Increasing-K constant load specimens can be subjected to either tension or bend loading.
Depending on the design, tension-loaded specimens can experience stresses at the crack tip which are
predominantly tensile (as in remotely-loaded tension types such as the centre-cracked plate) or contain
a significant bend component (as in crackline-loaded types such as compact tension specimens). The
presence of significant bending stress at the crack tip can adversely affect the crack path stability during
stress corrosion testing and can facilitate crack branching in certain materials. Bend specimens can be
loaded in 3-point, 4-point or cantilever bend fixtures.
5.2.4 Constant-K constant load specimens can be subjected to either torsion loading as in the case of
the double-torsion single edge cracked plate specimen, or tension loading as in the case of contoured
double-cantilever-beam specimens. Although loaded in tension, the design of the latter specimens
produces crackline bending with an associated tendency for crack growth out of plane, which can be
curbed by the use of side grooves.
5.2.5 Constant displacement specimens are usually self-loaded by means of a loading bolt in one arm
which impinges on either an anvil or a second loading bolt in the opposite arm. Two types are available:
a) those which are (W−a) dominated, such as the modified wedge-opening-loaded (modified WOL)
specimen in which the proximity of the back face to the crack tip influences the crack tip stress field;
b) those which are (W−a) indifferent, such as the double-cantilever-beam (DCB) specimen in which
the back face is sufficiently distant from the crack tip to ensure that its position has a negligible
effect on the crack tip stress field.
5.2.6 A number of the specimen geometries described above have specific advantages which have
caused them to be frequently used for stress corrosion testing. These include the following:
a) cantilever bend specimens, which are easy to machine and inexpensive to test under constant load;
b) compact tension (CTS) specimens, which minimize the material requirement for constant load
testing;
c) self-loaded double-cantilever-beam (DCB) specimens, which are easy to test under constant
displacement in service situations;
ISO 7539-6:2018(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
8 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
d) modified wedge-opening-loaded (modified WOL) specimens, which are also self-loaded and
minimize the material requirement for constant displacement testing;
e) C-shaped specimens, which can be machined from thick-walled cylinders in order to study the
radial propagation of longitudinally oriented cracks under constant load.
Details of standard specimen designs for each of these types of specimen are given in Figures 2 to 6.
5.2.7 If required, for example if fatigue crack initiation and/or propagation is difficult to control
satisfactorily, a chevron notch configuration as shown in Figure 7 may be used. If required, its included
angle may be increased from 90° to 120°.
5.2.8 Where it is necessary to measure crack opening displacements, as during the application of
deflection to constant displacement specimens, knife edges for the location of displacement gauges can
be machined into the mouth of the notch, as shown in Figure 8 a). Alternatively, separate knife edges can
either be screwed or glued onto the specimen at opposite sides of the notch, as shown in Figure 8 b).
Details of a suitable tapered beam displacement gauge are given in Figure 9.
Dimensions in millimetres, surface roughness values in micrometres
Key
W width
B thickness = 0,5W
N notch width = 0,065W maximum (if W > 25 mm) or 1,5 mm maximum (if W ≤ 25 mm)
l effective notch length = 0,25W to 0,45W
a effective crack length = 0,45W to 0,55W
Figure 2 — Proportional dimensions and tolerances for cantilever bend test pieces
ISO 7539-6:2018(E)
Dimensions in millimetres, surface roughness values in micrometres
Key
W net width
C total width = 1,25W minimum
B thickness = 0,5W
H half-height = 0,6W
D hole diameter = 0,25W
F half-distance between hole outer edges = 1,6D
N notch width = 0,065W maximum
l effective notch length = 0,25W to 0,40W
a effective crack length = 0,45W to 0,55W
Figure 3 — Proportional dimensions and tolerances for compact tension test pieces
10 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
Dimensions in millimetres, surface roughness values in micrometres
Key
1 screw tip radius 12,5 to 50
H half-height
B thickness = 2H
W net width = 10H minimum
C total width = W + d
d screw diameter = 0,75H minimum
N notch width = 0,14H maximum
l effective notch length = 2H
NOTE 1 “A” surfaces should be perpendicular and parallel, as applicable, to within 0,002H TIR.
NOTE 2 At each side point, “B” should be equidistant from the top and bottom surface to within 0,001H.
NOTE 3 The bolt centreline should be normal to the specimen centreline to within 1°.
NOTE 4 The bolt material should be similar to that of the specimen, fine-threaded with a square or Allen-
screw head.
Figure 4 — Proportional dimensions and tolerances for double-cantilever-beam test pieces
ISO 7539-6:2018(E)
Dimensions in millimetres, surface roughness values in micrometres
Key
B thickness
W net width = 2,55B
C total width = 3,20B
H half-height = 1,24B
D hole diameter = 0,718B ± 0,003B
l effective notch length = 0,77B
N notch width = 0,06B
T thread diameter = 0,625B
F distance from hole centreline to notch centreline = 0,239B
a
All over.
NOTE 1 The surface should be perpendicular and parallel, as applicable, to within 0,002H TIR.
NOTE 2 The bolt centreline should be normal to the specimen centreline to within 1°.
NOTE 3 The bolt material should be similar to that of the specimen, fine-threaded with a square or Allen-
screw head.
Figure 5 — Proportional dimensions and tolerances for modified wedge-opening-loaded
test pieces
12 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
Dimensions in millimetres, surface roughness values in micrometres
Key
W net width
B thickness = 0,50W ± 0,01W
X distance from the hole axis to a tangent with the inner surface = 0,50W ± 0,005W
N notch width = 1,5 mm minimum (0,1W maximum)
l notch length = 0,3W
Z distance from the hole axis to face of specimen = 0,25W ± 0,01W
T distance from the hole axis to outer surface = 0,25W ± 0,01W
D diameter of holes = 0,25W ± 0,005W
NOTE All surfaces should be perpendicular and parallel, as applicable, to within 0,002W TIR and “E” surfaces
should be perpendicular to “Y” surfaces to within 0,02W TIR.
Figure 6 — Proportional dimensions and tolerances for C-shaped test pieces
ISO 7539-6:2018(E)
Dimensions in millimetres
a
Mill with a 60° cutter; notch root radius 0,3 mm maximum for all test piece sizes.
Figure 7 — Chevron notch
14 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
a) Integral type
b) Screw-on type
NOTE Provided adequate strength can be ensured, the above knife edges may be fixed using adhesive.
Figure 8 — Knife edges for location of displacement gauges
ISO 7539-6:2018(E)
Dimensions in millimetres
a) Displacement gauge mounted on a test piece
b) Dimensions of beams
16 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
c) Bridge measurement circuit
Key
A, B terminals
V voltage
a
This dimension should be 3,8 × the minimum initial gauge length.
b
Beam thickness taper: 0,5 to 0,8.
Strain gauges and materials should be selected to suit the test environment.
Figure 9 — Details of tapered beam displacement gauge
5.3 Stress intensity factor considerations
5.3.1 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=×σ× a
I
where
Q is the geometrical constant;
σ is the applied stress;
a is the crack length.
5.3.2 The solutions for K for specimens of a particular geometry and loading method can be established
I
by means of finite element stress analysis or by either experimental or theoretical determinations of
specimen compliance.
ISO 7539-6:2018(E)
5.3.3 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, or a/H for (W−a) indifferent specimens, where W is
the width and H is the half-height of the specimen, through a stress intensity function of the form:
YP
=
K
I
BW
for compact tension or C-shaped specimens or:
YP
=
K
I
Ba
for T-type wedge-opening-loaded specimens or:
YP
=
K
I
BH
for double-cantilever-beam specimens.
5.3.4 Where it is necessary to use side-grooved specimens in order to curb crack branching tendencies,
etc., shallow side grooves (usually 5 % of the specimen thickness on both sides) can be used. Either semi-
circular or 60° V-grooves can be used, but it should be noted that, even with semi-circular side grooves of
up to 50 % of the specimen thickness, it is not always possible to maintain the crack in the desired plane
of extension. Where side grooves are used, the effect of the reduced thickness, B , due to the grooves on
n
the stress intensity, can be taken into account by replacing B by:
BB×
n
in the above expressions; however, the influence of side grooving on the stress intensity factor is
far from established and correction factors should be treated with caution, particularly if deep side
grooves are used.
5.3.5 Solutions for Y for specimens with geometries which are often used for stress corrosion testing
are given in Figures 10 to 14.
18 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
Dimensions in millimetres
EV×+HH30(,a 6HH) +
yLL
K =
I
 
40(,aa++6HH)
 
NOTE This expression was derived from elastic compliance theory and, although its inaccuracy and validity
a
limits are not well-defined, it has been used over the range 25≤≤ . For greater confidence, it is recommended
H
that an empirical compliance be used.
Figure 10 — Stress intensity factor solution for double-cantilever-beam specimen [(W−a)
indifferent]
ISO 7539-6:2018(E)
a
V, the crack-opening displacement (COD) for a rigid bolt, is a constant (g − g ).
i
YP
K =
I
Ba
20 © ISO 2018 – All rights reserved

ISO 7539-6:2018(E)
2 3 4 5
a a a a a
         
where Y =30,,96 −+−195 8 730,,6 1186 3 +754,6
         
W W W W W
         
NOTE This expression was derived from elastic compliance theory and, although its inaccuracy and validity
a
limits are not well-defined, it has been used over the range 03,,≤≤08 . For greater confidence, it is
W
recommended that an empirical compliance be used.
Figure 11 — Stress intensity factor solution for modified wedge-opening-loaded specimen

YP
K =
I
BW
1 a
 
where Y =62, 1 −−1 in the case where S = 1,5 W
 
W
 
a
 
1−
 
W
 
NOTE This expression was originally derived from the combined techniques of stress analysis and
compliance and, although its inaccuracy and validity limits are not well-defined, it has been used over the range
a
02,,≤≤06 . For greater confidence, it is recommended that an empirical compliance be used.
W
Figure 12 — Stress intensity factor solution for cantilever bend specimens
SIST EN ISO 7539-
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