1、BSI Standards PublicationBS ISO 18488:2015Polyethylene (PE) materialsfor piping systems Determination of StrainHardening Modulus in relationto slow crack growth TestmethodBS ISO 18488:2015 BRITISH STANDARDNational forewordThis British Standard is the UK implementation of ISO 18488:2015.The UK partic
2、ipation in its preparation was entrusted to TechnicalCommittee PRI/88/4, Test Methods - Plastic Piping.A list of organizations represented on this committee can beobtained on request to its secretary.This publication does not purport to include all the necessaryprovisions of a contract. Users are re
3、sponsible for its correctapplication. The British Standards Institution 2015. Published by BSI StandardsLimited 2015ISBN 978 0 580 84898 8ICS 23.040.20; 23.040.45Compliance with a British Standard cannot confer immunity fromlegal obligations.This British Standard was published under the authority of
4、 theStandards Policy and Strategy Committee on 31 August 2015.Amendments issued since publicationDate Text affectedBS ISO 18488:2015 ISO 2015Polyethylene (PE) materials for piping systems Determination of Strain Hardening Modulus in relation to slow crack growth Test methodMatriaux polythylne (PE) p
5、our systmes de canalisations Dtermination du module dcrouissage en relation avec la propagation lente de fissures Mthode dessaiINTERNATIONAL STANDARDISO18488First edition2015-09-01Reference numberISO 18488:2015(E)BS ISO 18488:2015ISO 18488:2015(E)ii ISO 2015 All rights reservedCOPYRIGHT PROTECTED DO
6、CUMENT ISO 2015, Published in SwitzerlandAll rights reserved. Unless otherwise specified, 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 writte
7、n permission. Permission can be requested from either ISO at the address below or ISOs member body in the country of the requester.ISO copyright officeCh. de Blandonnet 8 CP 401CH-1214 Vernier, Geneva, SwitzerlandTel. +41 22 749 01 11Fax +41 22 749 09 47copyrightiso.orgwww.iso.orgBS ISO 18488:2015IS
8、O 18488:2015(E)Foreword ivIntroduction v1 Scope . 12 Normative references 13 Terms and definitions . 14 Principle 35 Apparatus . 36 Test specimens 46.1 Test specimen geometry and dimensions . 46.2 Test specimen preparation . 47 Test procedure 58 Data analysis . 69 Test report . 6Annex A (informative
9、) Neo-Hookean constitutive model . 8Bibliography .10 ISO 2015 All rights reserved iiiContents PageBS ISO 18488:2015ISO 18488:2015(E)ForewordISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing Inter
10、national 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 wit
11、h 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,
12、 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 t
13、he 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
14、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 on the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISOs
15、 adherence to the WTO principles in the Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary informationThe committee responsible for this document is ISO/TC 138, Plastics pipes, fittings and valves for the transport of fluids, Subcommittee SC 5, General properties of pi
16、pes, fittings and valves of plastic materials and their accessories Test methods and basic specifications .iv ISO 2015 All rights reservedBS ISO 18488:2015ISO 18488:2015(E)IntroductionResistance to slow crack growth is related in general to the lifetime of polyethylene and thus, the lifetime of poly
17、ethylene products, e.g. pipes and fittings. The slow crack growth behaviour can be regarded as a combination of yield stress and the capability of disentanglement of tie molecules as reported by Kramer and Brown.3,6,7The disentanglement capability of a polymer will determine its resistance against s
18、low crack growth.The strain hardening modulus of a polymer is a measure of the disentanglement capability of the tie molecules of this polymer and is an intrinsic property. The strain hardening modulus of polyethylene is obtained from a stress-strain curve above the natural draw ratio. The stress-st
19、rain curve of a compression moulded sample is relatively easily obtained using a tensile test apparatus equipped with an optical extensometer. The test time of the strain hardening modulus is a consequence of the speed of tensile testing and is therefore constant for all measurements and independent
20、 of the slow crack growth property of the tested material itself.The strain hardening modulus value allows discrimination between materials. It has been demonstrated that the strain hardening modulus corresponds very well with several environmental stress cracking test methods for high density polye
21、thylene.4,5,8 ISO 2015 All rights reserved vBS ISO 18488:2015BS ISO 18488:2015Polyethylene (PE) materials for piping systems Determination of Strain Hardening Modulus in relation to slow crack growth Test method1 ScopeThis International Standard specifies a method for the determination of the strain
22、 hardening modulus which is used as a measure for the resistance to slow crack growth of polyethylene.The strain hardening modulus is obtained from stress-strain curves on compression moulded samples. This International Standard describes how such measurement is performed and how the strain hardenin
23、g modulus shall be determined from such a curve. Details of the required equipment, precision, and sample preparation for the generation of meaningful data are given.This International Standard provides a method that is valid for all types of polyethylene, independent from the manufacturing technolo
24、gy, comonomer, catalyst type, that are used for pipes and fittings applications.NOTE This method could be developed for materials for other applications.2 Normative referencesThe following documents, in whole or in part, are normatively referenced in this document and are indispensable for its appli
25、cation. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.ISO 527-1, Plastics Determination of tensile properties Part 1: General principlesISO 7500-1, Metallic materials Verification of stat
26、ic uniaxial testing machines Part 1: Tension/compression testing machines Verification and calibration of the force-measuring systemISO 9513, Metallic materials Calibration of extensometer systems used in uniaxial testing3 Terms and definitionsFor the purposes of this document, the following terms a
27、nd definitions apply.NOTE The symbols and their terms and definitions, as given below, are in line with ISO 527-1 and/or ISO 16241.3.1gauge lengthl0initial distance between the gauge marks on the central part of the test specimenNote 1 to entry: It is expressed in millimetres (mm).3.2thicknesshsmall
28、er initial dimension of the rectangular cross-section in the central part of a test specimenNote 1 to entry: It is expressed in millimetres (mm).INTERNATIONAL STANDARD ISO 18488:2015(E) ISO 2015 All rights reserved 1BS ISO 18488:2015ISO 18488:2015(E)3.3widthblarger initial dimension of the rectangul
29、ar cross-section in the central part of a test specimenNote 1 to entry: It is expressed in millimetres (mm).3.4test speedvrate of separation of the gripping jawsNote 1 to entry: It is expressed in millimetres per minute (mm/min).3.5lengthldistance between the gauge marks on the central part of the t
30、est specimen at any given momentNote 1 to entry: It is expressed in millimetres (mm).3.6stressnormal force per unit area of the original cross-section within the gauge length (3.1)Note 1 to entry: It is expressed in megapascals (MPa).3.7stress at yieldystress at the strain at yield (3.10)Note 1 to e
31、ntry: It is expressed in megapascals (MPa).3.8true stresstruedraw ratio (3.11) multiplied with the normal force per unit area of the original cross-section within the gauge length (3.1)Note 1 to entry: It is expressed in megapascals (MPa).3.9strainincrease in length (3.5) per unit original length of
32、 the gaugeNote 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).3.10strain at yieldyield strainyfirst occurrence in a tensile test of strain increase without a stress increaseNote 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).2 ISO 2015 All
33、rights reservedBS ISO 18488:2015ISO 18488:2015(E)3.11draw ratiolength (3.5) per unit original length of the gaugeNote 1 to entry: It is expressed as a dimensionless ratio, or as a percentage (%).3.12strain hardening modulusslope of the Neo-Hookean constitutive model between a true strain (3.9) of 8
34、and up to the point of maximum stress (3.6), but not above 12Note 1 to entry: It is expressed in megapascals (MPa).4 PrincipleTest specimens cut from compression moulded sheet are subjected to a tensile test at 80 C. The stress-strain curve is obtained sufficiently beyond the natural draw ratio. The
35、 strain hardening modulus is determined from the slope of this curve in the area after the natural draw ratio.5 Apparatus5.1 Tensile-testing machine, complying with ISO 527-1 and capable of maintaining a test speed of (20 2) mm/min.5.1.1 Load cell, which shall comply with Class 1, as defined in ISO
36、7500-1. The load cell shall be able to accurately measure forces in the range of 40 N for 0,30 mm thick samples and 120 N for 1,0 mm thick samples.5.1.2 Extensometer, which shall comply with Class 1, as defined in ISO 9513. The traverse displacement shall not be used as a measure of strain. For a 0,
37、30 mm thick specimen, a non-contact extensometer is preferred.5.1.3 Temperature chamber, to control the temperature at (80 1) C.5.1.4 Clamps, remote operation to enable closing and opening without the need to open the temperature chamber is recommended.5.2 Devices for measuring the thickness and wid
38、th of the test specimens.5.2.1 Thickness shall be measured with a device with an accuracy of 0,005 mm and a device with a contact dimension of less than the width of the parallel specimen section (4,0 mm).5.2.2 Width shall be measured with a device with an accuracy of 0,01 mm. Care shall be taken no
39、t to change the width of the test specimen by deformation. Therefore, it is recommended to use a microscope to measure width to avoid deforming the test specimen.5.3 Punch knife, in accordance with 6.1 shall be used. ISO 2015 All rights reserved 3BS ISO 18488:2015ISO 18488:2015(E)6 Test specimens6.1
40、 Test specimen geometry and dimensionsThe test specimen geometry as shown in Figure 1 shall be used. A large clamping area of the specimen to avoid slippage in the clamps is required to accurately measure the strain hardening regime.Figure 1 Test specimenTable 1 Dimensions of test specimensDimension
41、 Size mmL Start length between clamps 30,0 0,5l0Gauge length 12,5 0,1l1Length of narrow parallel sided portion16,0 1,0l2Length between the parallel portions of the clamp area46 1,0l3Minimum overall lengtha70R1Radius 10,0 0,5R2Radius 8,0 0,5b1Width at narrow parallel sided portion4,0 0,1b2Width at en
42、ds 20,0 1,0h Thickness 0,30 + 0,05/-0,03 or 1,0 0,1aA greater overall length may be necessary to ensure that only the wide end tabs come into contact with the machine grips, thus avoiding “shoulder breaks”.6.2 Test specimen preparationThe PE granules shall be compression moulded into sheets of 0,30
43、mm or 1,0 mm thickness according to the moulding conditions given in Table 2, but in case of dispute, the 0,30 mm thickness shall be used.4 ISO 2015 All rights reservedBS ISO 18488:2015ISO 18488:2015(E)Table 2 Conditions for compression moulding of sheetThicknessMoulding temperatureAverage cooling r
44、ateaPreheating timebFull pressureFull-pressure timemm C C/min min MPa min0,30 or 1,0 180 15 2 5 to 15 5 5 1aDemoulding temperature (MPa) for 8 +GCp2012(3)whereC is a mathematical parameter of the constitutive model describing the yield stress extrapolated to = 0.Accuracy of fit of data (R2) greater
45、than 0,9 shall be achieved.9 Test reportThe test report shall include the following information:a) a reference to this International Standard, i.e. ISO 18488:2015;b) complete identification of the material tested including type, source, manufacturers designation, and test piece (manufacturer, method
46、 of manufacture, production date);c) minimum thickness of each test specimen;d) mean width of each test specimen;e) number of the test specimen tested;f) the stress at yield for each test specimen;g) the mean value and the standard deviation of the stress at yield for the total number of test specim
47、ens tested;h) calculated strain hardening modulus for each test specimen;i) the mean value and the variation coefficient of the strain hardening modulus for the total number of test specimens tested;6 ISO 2015 All rights reservedBS ISO 18488:2015ISO 18488:2015(E)j) test temperature and any recorded
48、variation;k) statement as to whether any test specimens have been rejected and replaced, and, if so, the reasons and reasons for testing non-conforming specimens;l) any factors which may have affected the results such as any incidents or any operating details not specified in this International Stan
49、dard;m) date of measurement. ISO 2015 All rights reserved 7BS ISO 18488:2015ISO 18488:2015(E)Annex A (informative) Neo-Hookean constitutive modelA.1 GeneralThe important role of the underlying molecular entanglement network has been described by Kramer and Brown3,6,7and is reflected by the strain hardening behaviour. It is proposed4,8that the amount of strain hardening in a tensile test is a measure of fibril deformation resistance. The str
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