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)

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.

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)

Le présent document spécifie les procédures de conception, de préparation et d'utilisation d'éprouvettes préfissurées servant à évaluer la sensibilité à la corrosion sous contrainte. Il donne des recommandations pour la conception, la préparation et l'utilisation d'éprouvettes préfissurées pour évaluer la sensibilité à la corrosion sous contrainte. Des recommandations relatives aux éprouvettes entaillées sont données dans l'Annexe A.
Pour les besoins du présent document, 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 en 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 7539-6:2018)

Ta dokument določa postopke za načrtovanje, pripravo in uporabo preskušancev z umetno razpoko za ugotavljanje dovzetnosti za napetostno korozijo. Podaja priporočila za načrtovanje, pripravo in uporabo preskušancev z umetno razpoko za ugotavljanje dovzetnosti za napetostno korozijo. Priporočila za zarezane vzorce so podana v dodatku A.
Izraz »kovina«, kot se uporablja v tem dokumentu, vključuje zlitine.
Zaradi omejitve plastičnosti pri vrhu razpoke preskušanci z umetno razpoko niso primerni za ocenjevanje tankih izdelkov, kot je pločevina ali žice, in se v splošnem uporabljajo za debelejše izdelke, vključno s palicami plošč in kovanimi izdelki. Uporabljajo se lahko tudi za dele, spojene z varjenjem.
Preskušanci z umetno razpoko so lahko naloženi z opremo za uporabo stalne obremenitve oziroma lahko vključujejo napravo, ki zagotavlja stalen premik na točkah obremenitve. Preskusi, izvedeni ob povečanem premiku ali povečani obremenitvi, so obravnavani v standardu ISO 7539-9.
Posebna prednost preskušancev z umetno razpoko je v tem, da omogočajo pridobivanje podatkov, na podlagi katerih je mogoče oceniti ključne velikosti okvar, nad katerimi lahko pride do pokanja zaradi napetostne korozije, za sestavne dele z znano geometrijo, ki so podvrženi znanim obremenitvam. Omogočajo tudi ugotavljanje hitrosti širjenja razpok zaradi napetostne korozije. Te podatke je mogoče upoštevati pri spremljanju delov, ki vsebujejo okvare, med obratovanjem.

General Information

Status
Published
Public Enquiry End Date
19-Dec-2017
Publication Date
14-Nov-2018
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
19-Oct-2018
Due Date
24-Dec-2018
Completion Date
15-Nov-2018

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SLOVENSKI STANDARD
SIST EN ISO 7539-6:2018
01-december-2018
1DGRPHãþD
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)

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)

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)
Ta slovenski standard je istoveten z: EN ISO 7539-6:2018
ICS:
77.060 Korozija kovin Corrosion of metals
SIST EN ISO 7539-6:2018 de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST EN ISO 7539-6:2018
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SIST EN ISO 7539-6:2018
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)

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) 7539-6:2018)
This European Standard was approved by CEN on 27 August 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 NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
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.
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SIST EN ISO 7539-6:2018
EN ISO 7539-6:2018 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 7539-6:2018
EN ISO 7539-6:2018 (E)
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 has been approved by CEN as EN ISO 7539-6:2018 without any

modification.
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SIST EN ISO 7539-6:2018
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SIST EN ISO 7539-6:2018
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
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SIST EN ISO 7539-6:2018
ISO 7539-6:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© 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.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 7539-6:2018
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 2018 – All rights reserved iii
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SIST EN ISO 7539-6:2018
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
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SIST EN ISO 7539-6:2018
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 2018 – All rights reserved 1
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SIST EN ISO 7539-6:2018
ISO 7539-6:2018(E)
3.1
crack length

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

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
side-to-side dimension of the specimen being tested
3.4
reduced thickness at side grooves
minimum side-to-side dimension between the notches in side-grooved specimens
3.5
specimen half-height

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

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

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

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

ratio of stress to strain without deviation in proportionality of the stress and strain (Hooke’s law)

2 © ISO 2018 – All rights reserved
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SIST EN ISO 7539-6:2018
ISO 7539-6:2018(E)
3.10
stress intensity factor

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
stress intensity applied at the commencement of the stress corrosion test
3.12
plane strain fracture toughness

critical value of K at which the first significant environmentally independent extension of the crack

occurs under the influence of rising stress intensity under conditions of high resistance to plastic

deformation
3.13
provisional value of K
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

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
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
p0,2

stress which is applied to produce a plastic strain of 0,2 % during a tensile test

3.18
applied stress
stress resulting from the application of load (3.6) to the specimen
© ISO 2018 – All rights reserved 3
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SIST EN ISO 7539-6:2018
ISO 7539-6:2018(E)
3.19
stress intensity factor coefficient

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
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
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SIST EN ISO 7539-6:2018
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

thickness, B, shall not be less than:
 
25,  
 
p02,
 
and that, where possible, larger specimens where both a and B are at least:
 
4 
 
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:
 
25,  
 
p02,
 
and preferably should be not less than:
 
 
 
p,02
 
where K is the stress intensity to be applied during testing.

The threshold stress intensity value eventually determined should be substituted for K in the first of

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 2018 – All rights reserved 5
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SIST EN ISO 7539-6:2018
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 mea
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