Corrosion of metals and alloys - Stress corrosion testing - Part 1: General guidance on testing procedures (ISO 7539-1:2012)

This part of ISO 7539 describes the general considerations that apply when designing and conducting tests to assess susceptibility of metals to stress corrosion. This part of ISO 7539 also gives some general guidance on the selection of test methods.

Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 1: Allgemeiner Leitfaden für Prüfverfahren (ISO 7539-1:2012)

1.1   Dieser Teil von ISO 7539 beschreibt die allgemeinen Betrachtungen, die für die Planung und Durchführung von Prüfungen zur Beurteilung der Anfälligkeit von Metallen gegenüber Spannungsrisskorrosion gelten.
1.2   Dieser Teil von ISO 7539 gibt zusätzlich einen allgemeinen Leitfaden für die Auswahl von Prüfverfahren.
ANMERKUNG 1   Die einzelnen Prüfverfahren sind im Detail nicht in diesem Teil von ISO 7539 behandelt. Diese sind Gegenstand der weiteren Teile von ISO 7539.
ANMERKUNG 2   Dieser Teil von ISO 7539 ist unter den Bedingungen des kathodischen Schutzes anwendbar.

Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 1: Lignes directrices générales relatives aux méthodes d'essai (ISO 7539-1:2012)

L'ISO 7539-1:2012 décrit les considérations générales qui s'appliquent à la mise au point et à la réalisation des essais servant à évaluer la sensibilité des métaux à la corrosion sous contrainte.
L'ISO 7539-1:2012 donne également des directives générales sur la sélection des méthodes d'essais.

Korozija kovin in zlitin - Ugotavljanje pokanja zaradi napetostne korozije - 1. del: Splošna navodila za postopke preskušanja (ISO 7539-1:2012)

Ta del standarda ISO 7539 opisuje splošne pogoje, ki veljajo pri projektiranju in izvajanju preskusov za ocenjevanje dovzetnosti kovin za korozijo. Ta del standarda ISO 7539 podaja tudi splošna navodila za izbiro metod preskušanja.

General Information

Status
Published
Publication Date
01-Jun-2014
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
14-May-2014
Due Date
19-Jul-2014
Completion Date
02-Jun-2014

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SLOVENSKI STANDARD
SIST EN ISO 7539-1:2014
01-julij-2014
1DGRPHãþD
SIST EN ISO 7539-1:1999

Korozija kovin in zlitin - Ugotavljanje pokanja zaradi napetostne korozije - 1. del:

Splošna navodila za postopke preskušanja (ISO 7539-1:2012)

Corrosion of metals and alloys - Stress corrosion testing - Part 1: General guidance on

testing procedures (ISO 7539-1:2012)

Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 1:

Allgemeiner Leitfaden für Prüfverfahren (ISO 7539-1:2012)

Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 1: Lignes

directrices générales relatives aux méthodes d'essai (ISO 7539-1:2012)
Ta slovenski standard je istoveten z: EN ISO 7539-1:2012
ICS:
77.060 Korozija kovin Corrosion of metals
SIST EN ISO 7539-1:2014 en

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

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SIST EN ISO 7539-1:2014
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SIST EN ISO 7539-1:2014
EUROPEAN STANDARD
EN ISO 7539-1
NORME EUROPÉENNE
EUROPÄISCHE NORM
December 2012
ICS 77.060 Supersedes EN ISO 7539-1:1995
English Version
Corrosion of metals and alloys - Stress corrosion testing - Part
1: General guidance on testing procedures (ISO 7539-1:2012)

Corrosion des métaux et alliages - Essais de corrosion Korrosion der Metalle und Legierungen - Prüfung der

sous contrainte - Partie 1: Lignes directrices générales Spannungsrisskorrosion - Teil 1: Allgemeiner Leitfaden für

relatives aux méthodes d'essai (ISO 7539-1:2012) Prüfverfahren (ISO/FDIS 7539-1:2012)

This European Standard was approved by CEN on 18 December 2012.

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, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United

Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2012 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7539-1:2012: E

worldwide for CEN national Members.
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SIST EN ISO 7539-1:2014
EN ISO 7539-1:2012 (E)
Contents Page

Foreword ..............................................................................................................................................................3

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SIST EN ISO 7539-1:2014
EN ISO 7539-1:2012 (E)
Foreword

This document (EN ISO 7539-1:2012) 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 June 2013, and conflicting national standards shall be withdrawn at

the latest by June 2013.

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 7539-1:1995.

According to the CEN/CENELEC Internal Regulations, the national standards organisations 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, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

Endorsement notice

The text of ISO 7539-1:2012 has been approved by CEN as a EN ISO 7539-1:2012 without any modification.

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SIST EN ISO 7539-1:2014
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SIST EN ISO 7539-1:2014
INTERNATIONAL ISO
STANDARD 7539-1
Second edition
2012-12-15
Corrosion of metals and alloys —
Stress corrosion testing —
Part 1:
General guidance on testing procedures
Corrosion des métaux et alliages — Essais de corrosion sous
contrainte —
Partie 1: Lignes directrices générales relatives aux méthodes d’essai
Reference number
ISO 7539-1:2012(E)
ISO 2012
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2012

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 copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – All rights reserved
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Terms and definitions ..................................................................................................................................................................................... 1

3 Background ................................................................................................................................................................................................................ 2

4 Selection of test method ............................................................................................................................................................................... 3

5 Stressing systems ................................................................................................................................................................................................. 4

5.1 General ........................................................................................................................................................................................................... 4

5.2 Constant total strain tests .............................................................................................................................................................. 4

5.3 Constant load tests .............................................................................................................................................................................. 5

5.4 Slow strain rate tests ......................................................................................................................................................................... 6

6 Environmental aspects ................................................................................................................................................................................... 7

6.1 General ........................................................................................................................................................................................................... 7

6.2 Temperature .............................................................................................................................................................................................. 7

6.3 Water chemistry .................................................................................................................................................................................... 7

6.4 Flow rate ....................................................................................................................................................................................................... 9

6.5 Electrochemical aspects .................................................................................................................................................................. 9

7 Time dependent issues ...............................................................................................................................................................................10

7.1 General ........................................................................................................................................................................................................10

7.2 Pitting and intergranular corrosion ...................................................................................................................................10

7.3 Hydrogen uptake ................................................................................................................................................................................10

8 Specimen design and manufacture ................................................................................................................................................11

8.1 General ........................................................................................................................................................................................................11

8.2 Surface condition ...............................................................................................................................................................................11

8.3 Area effects ..............................................................................................................................................................................................13

8.4 Pre-cracked test pieces .................................................................................................................................................................13

8.5 Welds ............................................................................................................................................................................................................14

9 Stress corrosion test cells .........................................................................................................................................................................14

10 Initiation of stress corrosion tests ..................................................................................................................................................15

11 Assessment and treatment of results ..........................................................................................................................................15

Annex A (informative) Guide to selection of mechanical test method ...........................................................................18

Bibliography .............................................................................................................................................................................................................................20

© ISO 2012 – All rights reserved iii
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(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 7539-1 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.

This second edition cancels and replaces the first edition (ISO 7539-1:1987), which has been

technically revised.

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 pre-cracked specimens for tests under constant load or constant displacement

— Part 7: Method for 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

— Part 10: Reverse U-bend method

— Part 11: Guidelines for testing the resistance of metals and alloys to hydrogen embrittlement and

hydrogen-assisted cracking
iv © ISO 2012 – All rights reserved
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SIST EN ISO 7539-1:2014
INTERNATIONAL STANDARD ISO 7539-1:2012(E)
Corrosion of metals and alloys — Stress corrosion testing —
Part 1:
General guidance on testing procedures
1 Scope

1.1 This part of ISO 7539 describes the general considerations that apply when designing and

conducting tests to assess susceptibility of metals to stress corrosion.

1.2 This part of ISO 7539 also gives some general guidance on the selection of test methods.

NOTE 1 Particular methods of test are not treated in detail in this part of ISO 7539. These are described in the

additional parts of ISO 7539.
NOTE 2 This part of ISO 7539 is applicable to cathodic protection conditions.
2 Terms and definitions
2.1
stress corrosion

process involving conjoint corrosion and straining of the metal due to applied or residual stress

2.2
threshold stress

〈stress corrosion〉 stress above which stress corrosion cracks initiate and grow, for the specified

test conditions
2.3
threshold stress intensity factor
ISCC

〈stress corrosion cracking〉 stress intensity factor above which stress corrosion crack propagation is

sustained

Note 1 to entry: The threshold stress intensity factor is a concept of linear elastic fracture mechanics (LEFM) and

is applicable when the plastic zone size is large compared with the microstructure and a high constraint to plastic

deformation prevails; i.e. under plain strain-predominant conditions. For growing stress corrosion cracks, LEFM

is not necessarily applicable in detail but is adopted as a pragmatic tool that is commonly used.

Note 2 to entry: Stress corrosion cracks may initiate at a surface or a surface defect and grow in the “short crack”

regime at stress levels below the apparent threshold stress intensity factor. However, LEFM is not applicable in

the short crack regime and sustained propagation of these cracks requires that the threshold stress intensity

factor be exceeded.
2.4
test environment

either a service environment, or an environment produced in the laboratory, to which the test specimen

is exposed and which is maintained constant or varied in an agreed manner

Note 1 to entry: In the case of stress corrosion, the environment is often quite specific (see Clause 6).

© ISO 2012 – All rights reserved 1
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)
2.5
start of test

time when the stress is applied or when the specimen is exposed to the test environment, whichever

occurs later
2.6
crack initiation time

period from the start of a test to the time when a crack is detectable by the means employed

2.7
time to failure

period elapsing between the start of a test and the occurrence of failure, the criterion of failure being the

first appearance of cracking or the total separation of the test piece, or some agreed intermediate condition

2.8
slow strain rate test

test for evaluating the susceptibility of a metal to stress corrosion cracking that most commonly involves

pulling a tensile specimen to failure in a representative environment at a constant displacement rate, the

−5 −1 −8 −1

displacement rate being chosen to generate nominal strain rates usually in the range 10 s to 10 s

Note 1 to entry: Slow strain rate testing may also be applied to specimens in bend.

2.9
strain to failure

strain at which failure occurs in a slow strain rate test expressed usually as the plastic strain to failure

See ISO 7539-7.
2.10
average crack velocity
maximum depth of crack(s) due to stress corrosion, divided by the test time
2.11
orientation

direction of applied tensile stress of a test specimen with respect to some specified direction in the

product from which it was prepared, e.g. the rolling direction in the plate
3 Background

3.1 Although it is generally agreed that cracking is the usual result of stress corrosion, other

manifestations such as intergranular corrosion or elongated fissures, which are enhanced by the presence

of stress, have also to be recognized.

As far as this part of ISO 7539 is concerned, all phenomena involving metal dissolution or the action of

hydrogen introduced into the metal as the result of simultaneous effects of a corrosive environment and

a tensile stress are included, except for embrittlement by liquid metal and exfoliation corrosion.

3.2 There exists a wide diversity of methods used for assessing the stress corrosion properties of

metals. Each has its own particular advantage in certain situations.

3.3 Stress corrosion cracking depends on both the exposure conditions and the mechanical and

microstructural characteristics of the material and susceptibility or resistance to stress corrosion can only be

defined in that context. Thus, for example, there is no intrinsic threshold stress intensity factor for a material.

3.4 Ideally, in order to establish the risk of stress corrosion in a given application, it is necessary to

carry out simulation testing under all likely service exposure conditions. In practice, this is difficult, if

not impossible, and rarely achieved, but a number of “standard tests” have been found as a result of

experience to provide reasonable guidance on likely service behaviour for given specific applications.

However, these laboratory “standard tests” are only appropriate to service conditions where experience

2 © ISO 2012 – All rights reserved
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)

has shown an appropriate relationship, however empirical, to exist. The fact that a given alloy passes or

does not pass a test previously found useful in relation to another alloy may or may not be significant and

a test that discriminates correctly between alloys used for a given application will not necessarily provide

safe guidance if the exposure conditions are different. The use of a standard test beyond the point for

which there is experience therefore requires validation.

3.5 In the following clauses, attention is drawn particularly to the fact that the stress corrosion process

can be extremely sensitive to small changes in exposure or test conditions. The user of materials is

responsible for selecting the conditions under which stress corrosion tests are performed and the fact that

some tests are described in this part of ISO 7539 does not imply that these tests are the most appropriate

ones for any given situation. The justification for describing these tests in a standard is that they are in

widespread use and have been proven as valid for specific or common equipment-environment systems.

However, the responsibility for interpretation of the test results remains with the user of materials and it

is in no way diminished by the existence of this standard.

3.6 In addition to specific parts of ISO 7539 to cover the most widely used methods, it is considered

that this more general document, concerned with the selection of test details and the interpretation of

results, is required.
4 Selection of test method

4.1 Before embarking on a programme of stress corrosion testing, a decision has to be made regarding

which type of test is appropriate. Such a decision depends largely upon the purpose of the test and the

information required. While some tests attempt to reproduce service conditions as closely as possible

and are of value to the plant engineer, others may be designed to study a mechanistic aspect of failure. In

the former, for example, restrictions of material, space, time, etc., may mean the use of a relatively simple

test procedure whereas in other circumstances more sophisticated testing techniques may be essential.

Thus, studies of crack propagation rates may involve the use of pre-cracked specimens, although these

may be inappropriate when considering, for example, the effects of surface finish. Although a number of

sophisticated techniques are available, the adoption of a simple test may prove of great value in some

circumstances when more elaborate techniques cannot be used.

4.2 When selecting a test method of the pass/fail type, it is important to realize that this should not

be so severe that it leads to the condemnation of a material that would prove adequate for a particular

service condition, nor should it be so trifling as to encourage the use of a material in circumstances where

rapid failure would ensue.

4.3 The aim of stress corrosion testing is usually to provide information more quickly than can be

obtained from service experience, but at the same time predictive of service behaviour. Among the most

common approaches employed to achieve this are the use of higher stress, slow continuous straining,

pre-cracked specimens, higher concentration of species in test environment than in service environment,

increased temperature, and electrochemical stimulation. It is important however, that these methods be

controlled in such a way that the details of the failure mechanism are not changed.

4.4 If it is too difficult to reproduce the service conditions exactly, it may be useful to analyse the stress

corrosion process in order to determine as far as possible the main factors operating at different stages.

The stress corrosion test then selected may involve only one step of the corrosion mechanism.

4.5 A brief guide to the selection of test methods is included in Annex A.
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)
5 Stressing systems
5.1 General

Methods of loading test pieces, whether initially plain (i.e. nominally free from notches or pre-cracks),

notched or pre-cracked, can be conveniently grouped according to whether they involve

a) a constant total strain (see 5.2);
b) a constant load (see 5.3);
c) an applied slow strain rate (see 5.4).

In the case of pre-cracked specimens, threshold conditions are defined in terms of a stress intensity

value K and tests may also be conducted under constant stress intensity conditions. Knowledge of

ISCC

the limitations of the various methods is at least as important as the choice of method of stressing.

5.2 Constant total strain tests

5.2.1 These form by far the most popular type of test as a group, since bend tests in a variety of forms

come into this category. Furthermore, they simulate the fabrication stresses that are frequently associated

with service failures.

5.2.2 Material in sheet form is frequently tested by bending; plate material is tested under tension or as

C-rings, with the latter also used for testing tubular products and other semi-finished products of round

cross-section.

5.2.3 Bend tests have the attraction of employing simple, and therefore frequently cheap, specimens and

restraining jigs The tests may involve deforming the specimen plastically into a U-shape or adopting 2-point,

3-point, or 4-point bend configurations with a nominal applied stress at or below yield. For materials with

a discrete yield point, elastic theory can be used to calculate the stress when testing at applied stresses

up to that stress level. More commonly, and especially for corrosion resistant alloys, a discrete yield point

is not observed and it is necessary to attach strain gauges to the specimen and deflect the specimen to

achieve the desired level of total strain (usually up to a maximum of the 0,2 % plastic strain)

5.2.4 Tubular material may be tested in the form of C-rings or O-rings, the former being stressed by

partial opening or closing of the gap and the latter by forced insertion of a plug that is appropriately

oversized for the bore. The C-ring has also been found to be particularly useful for testing thick product

forms, e.g. aluminium alloys in the short transverse direction.

5.2.5 Constant total strain tensile tests are sometimes preferred to bend tests because the initial stress is

more readily characterized and through–thickness gradients of stress have to be considered in bend specimens.

5.2.6 The restraining frame used for either bend or tensile tests should be sufficiently stiff that constant

displacement is maintained throughout the test.

NOTE The stiffness of the stressing frame employed may also influence the time to failure of a specimen

because of stress relaxation, quite apart from any effect that it may have upon the initial stress level.

5.2.7 The use of restraining frames may be avoided by employing internally stressed specimens

containing residual stresses as the result of inhomogeneous deformation. The latter may be introduced

by plastic bending, e.g. by producing a bulge in sheet or plate material, or by welding. However, such

tests involve problems in systematic variation of the initial stress, which usually achieves maximum

values in the region of the yield stress. Moreover, elastic spring-back, in introducing residual stresses by

bulging plate or partially flattening tube, may cause problems. Where welding is involved the structural

modifications may raise difficulties, unless the test is simulative of a practical situation.

4 © ISO 2012 – All rights reserved
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SIST EN ISO 7539-1:2014
ISO 7539-1:2012(E)

5.2.8 Constant total strain specimens are sometimes loaded by being placed initially into conventional

testing machines or similar devices and then, while being maintained in the strained condition, having a

restraining frame attached. When the load applied by the testing machine is removed, the specimen remains

stressed by virtue of the restraint imposed by the frame, the assumption being made that the strain in the

specimen remains constant as the restraint is transferred from the testing machine to the frame. Strain

gauging can be used to confirm that there is no stress relaxation in the specimen. When testing at elevated

temperature, consideration should be given to the change of material properties with temperature.

5.2.9 Stress relaxation may occur because of creep of the material, specimen thinning, or because some

of the displacement is taken up by opening of the crack/s formed.

NOTE 1 Creep relaxation is most significant at elevated temperatures but can be important at ambient

temperature in some cases (e.g. duplex stainless steels). The extent of relaxation should be assessed before testing

and consideration of the value of constant total strain testing made, recognizing also that dynamic plastic strain

is an inherent feature during any transient creep process.

NOTE 2 Specimen thinning is best assessed at the end of the test and the increased effective stress evaluated,

accounting for any significant non-uniformity of thinning.

NOTE 3 The extent to which crack opening relaxes the stress will be dependent on the numb

...

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