SIST EN 61788-6:2011

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Superprevodnost - 6. del: Meritve mehanskih lastnosti - Natezni preskus za superprevodnike iz kompozita Cu/Nb-Ti pri sobni temperaturiSupraleitfähigkeit - Teil 6: Messung der mechanischen Eigenschaften - Messung der Zugfestigkeit von Cu/NbTi-Verbundsupraleitern bei RaumtemperaturSupraconductivité - Partie 6: Mesures des propriétés mécaniques - Essai de traction à température ambiante des supraconducteurs composites de Cu/Nb-TiSuperconductivity - Part 6: Mechanical properties measurment - Room temperature tensile test of Cu/Nb-Ti composite superconductors77.040.10Mehansko preskušanje kovinMechanical testing of metals29.050Superprevodnost in prevodni materialiSuperconductivity and conducting materialsICS:Ta slovenski standard je istoveten z:EN 61788-6:2011SIST EN 61788-6:2011en01-oktober-2011SIST EN 61788-6:2011SLOVENSKI
STANDARD
EUROPEAN STANDARD EN 61788-6 NORME EUROPÉENNE
EUROPÄISCHE NORM August 2011
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Management Centre: Avenue Marnix 17, B - 1000 Brussels
© 2011 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61788-6:2011 E
ICS 29.050; 77.040.10 Supersedes EN 61788-6:2008
English version
Superconductivity -
Part 6: Mechanical properties measurement -
Room temperature tensile test of Cu/Nb-Ti composite superconductors (IEC 61788-6:2011)
Supraconductivité -
Partie 6: Mesure des propriétés mécaniques -
Essai de traction à température ambiante des supraconducteurs composites de Cu/Nb-Ti (CEI 61788-6:2011)
Supraleitfähigkeit -
Teil 6: Messung der mechanischen Eigenschaften -
Messung der Zugfestigkeit von Cu/Nb-Ti-Verbundsupraleitern bei Raumtemperatur(IEC 61788-6:2011)
This European Standard was approved by CENELEC on 2011-08-15. CENELEC 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 Central Secretariat or to any CENELEC 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 CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.
Foreword The text of document 90/267/FDIS, future edition 3 of IEC 61788-6, prepared by IEC TC 90, Superconductivity was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN 61788-6:2011. The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2012-05-15 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2014-08-15
This document supersedes EN 61788-6:2008. EN 61788-6:2011 includes the following significant technical changes with respect to EN 61788-6:2008:
– specific example of uncertainty estimation related to mechanical tests was supplemented as Annex C.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.
Endorsement notice The text of the International Standard IEC 61788-6:2011 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following notes have to be added for the standards indicated: IEC 61788-5 NOTE
Harmonized as EN 61788-5. ISO 3611:2010 NOTE
Harmonized as EN ISO 3611:2010 (not modified).
- 3 - EN 61788-6:2011 Annex ZA (normative)
Normative references to international publications with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
NOTE
When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-815 - International Electrotechnical Vocabulary -
Part 815: Superconductivity - -
ISO 376 - Metallic materials - Calibration of force-proving instruments used for the verification of uniaxial testing machines EN ISO 376 -
ISO 6892-1 - Metallic materials - Tensile testing -
Part 1: Method of test at room temperature
EN ISO 6892-1 -
ISO 7500-1 - Metallic materials - Verification of static uniaxial testing machines -
Part 1: Tension/compression testing
machines - Verification and calibration of the force-measuring system EN ISO 7500-1 -
ISO 9513 - Metallic materials - Calibration of extensometers used in uniaxial testing EN ISO 9513 -
IEC 61788-6 Edition 3.0 2011-07 INTERNATIONAL STANDARD NORME INTERNATIONALE Superconductivity –
Part 6: Mechanical properties measurement – Room temperature tensile test
of Cu/Nb-Ti composite superconductors
Supraconductivité –
Partie 6: Mesure des propriétés mécaniques – Essai de traction à température ambiante des supraconducteurs composites de Cu/Nb-Ti
INTERNATIONAL ELECTROTECHNICAL COMMISSION COMMISSION ELECTROTECHNIQUE INTERNATIONALE V ICS 29.050; 77.040.10 PRICE CODE CODE PRIX ISBN 978-2-88912-580-7
– 2 – 61788-6  IEC:2011 CONTENTS FOREWORD . 4 INTRODUCTION . 6 1 Scope . 7 2 Normative references . 7 3 Terms and definitions . 7 4 Principle . 8 5 Apparatus . 8 5.1 Conformity . 8 5.2 Testing machine . 8 5.3 Extensometer . 9 6 Specimen preparation. 9 6.1 Straightening the specimen . 9 6.2 Length of specimen . 9 6.3 Removing insulation . 9 6.4 Determination of cross-sectional area (So) . 9 7 Testing conditions . 9 7.1 Specimen gripping . 9 7.2 Pre-loading and setting of extensometer . 9 7.3 Testing speed. 9 7.4 Test . 10 8 Calculation of results . 12 8.1 Tensile strength (Rm) . 12 8.2 0,2 % proof strength (Rp0,2A and
Rp0,2B) . 12 8.3 Modulus of elasticity (Eo and Ea) . 12 9 Uncertainty . 12 10 Test report. 13 10.1 Specimen . 13 10.2 Results . 13 10.3 Test conditions . 13 Annex A (informative)
Additional information relating to Clauses 1 to 10 . 14 Annex B (informative)
Uncertainty considerations . 19 Annex C (informative)
Specific examples related to mechanical tests . 23 Bibliography . 32
Figure 1 – Stress-strain curve and definition
of modulus of elasticity and 0,2 % proof strengths . 11 Figure A.1 – An example of the light extensometer, where R1 and R3 indicate the corner radius . 15 Figure A.2 – An example of the extensometer provided with balance weight and vertical specimen axis . 16 Figure C.1 – Measured stress versus strain curve of the rectangular cross section NbTi wire and the initial part of the curve . 23 Figure C.2 – 0,2 % offset shifted regression line, the raw stress versus
strain curve and the original raw data of stress versus strain . 29
61788-6  IEC:2011 – 3 – Table B.1 – Output signals from two nominally identical extensometers . 20 Table B.2 – Mean values of two output signals . 20 Table B.3 – Experimental standard deviations of two output signals. 20 Table B.4 – Standard uncertainties of two output signals . 21 Table B.5 – Coefficient of variations of two output signals. 21 Table C.1 – Load cell specifications according to manufacturer’s data sheet . 26 Table C.2 – Uncertainties of displacement measurement . 26 Table C.3 – Uncertainties of wire width measurement . 27 Table C.4 – Uncertainties of wire thickness measurement . 27 Table C.5 – Uncertainties of gauge length measurement . 27 Table C.6 – Calculation of stress at 0 % and at 0,1 % strain using the zero offset regression line as determined in Figure C.1b). . 28 Table C.7 – Linear regression equations computed for the three shifted lines and for the stress versus strain curve in the region where the lines intersect . 29 Table C.8 – Calculation of strain and stress at the intersections of the three shifted lines with the stress strain curve . 30 Table C.9 – Measured stress versus strain data and the computed
stress based on a linear fit to the data in the region of interest . 31
– 4 – 61788-6  IEC:2011 INTERNATIONAL ELECTROTECHNICAL COMMISSION _____________
SUPERCONDUCTIVITY –
Part 6: Mechanical properties measurement –
Room temperature tensile test of Cu/Nb-Ti
composite superconductors
FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 61788-6 has been prepared by IEC technical committee 90: Superconductivity. This third edition cancels and replaces the second edition published in 2008. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: – specific example of uncertainty estimation related to mechanical tests was supplemented as Annex C. SIST EN 61788-6:2011

61788-6  IEC:2011 – 5 – The text of this standard is based on the following documents: FDIS Report on voting 90/267/FDIS 90/278/RVD
Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. A list of all parts of the IEC 61788 series, published under the general title Superconductivity, can be found on the IEC website. The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to the specific publication. At this date, the publication will be
• reconfirmed, • withdrawn, • replaced by a revised edition, or • amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it contains colours which are considered to be useful for the correct understanding of its contents. Users should therefore print this document using a colour printer.
– 6 – 61788-6  IEC:2011 INTRODUCTION The Cu/Nb-Ti superconductive composite wires currently in use are multifilamentary composite material with a matrix that functions as a stabilizer and supporter, in which ultrafine superconductor filaments are embedded. A Nb-40~55 mass % Ti alloy is used as the superconductive material, while oxygen-free copper and aluminium of high purity are employed as the matrix material. Commercial composite superconductors have a high current density and a small cross-sectional area. The major application of the composite superconductors is to build superconducting magnets. While the magnet is being manufactured, complicated stresses are applied to its windings and, while it is being energized, a large electromagnetic force is applied to the superconducting wires because of its high current density. It is therefore indispensable to determine the mechanical properties of the superconductive wires, of which the windings are made. SIST EN 61788-6:2011

61788-6  IEC:2011 – 7 – SUPERCONDUCTIVITY –
Part 6: Mechanical properties measurement –
Room temperature tensile test of Cu/Nb-Ti
composite superconductors
1 Scope This part of IEC 61788 covers a test method detailing the tensile test procedures to be carried out on Cu/Nb-Ti superconductive composite wires at room temperature. This test is used to measure modulus of elasticity, 0,2 % proof strength of the composite due to yielding of the copper component, and tensile strength. The value for percentage elongation after fracture and the second type of 0,2 % proof strength due to yielding of the Nb-Ti component serves only as a reference (see Clauses A.1 and A.2).
The sample covered by this test procedure has a round or rectangular cross-section with an area of 0,15 mm2 to 2 mm2 and a copper to superconductor volume ratio of 1,0 to 8,0 and without the insulating coating. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60050-815, International Electrotechnical Vocabulary – Part 815: Superconductivity ISO 376, Metallic materials – Calibration of force-proving instruments used for the verification of uniaxial testing machines
ISO 6892-1, Metallic materials – Tensile testing – Part 1: Method of test at room temperature ISO 7500-1, Metallic materials – Verification of static uniaxial testing machines – Part 1: Tension/compression testing machines – Verification and calibration of the force-measuring system ISO 9513, Metallic materials – Calibration of extensometers used in uniaxial testing 3 Terms and definitions For the purposes of this document, the definitions given in IEC 60050-815 and ISO 6892-1, as well as the following, apply. 3.1
tensile stress
tensile force divided by the original cross-sectional area at any moment during the test SIST EN 61788-6:2011

– 8 – 61788-6  IEC:2011 3.2
tensile strength
Rm tensile stress corresponding to the maximum testing force NOTE The symbol σUTS is commonly used instead of Rm. 3.3
extensometer gauge length length of the parallel portion of the test piece used for the measurement of elongation by means of an extensometer 3.4
distance between grips
Lg length between grips that hold a test specimen in position before the test is started 3.5
0,2 % proof strength
Rp0,2 (see Figure 1) stress value where the copper component yields by 0,2 %
NOTE 1 The designated stress, Rp0,2A or Rp0,2B corresponds to point A or B in Figure 1, respectively. This strength is regarded as a representative 0,2 % proof strength of the composite. The second type of 0,2 % proof strength is defined as a 0,2 % proof strength of the composite where the Nb-Ti component yields by 0,2 %, the value of which corresponds to the point C in Figure 1 as described complementarily in Annex A (see Clause A.2). NOTE 2 The symbol σ0,2 is commonly used instead of Rp0,2. 3.6
modulus of elasticity
E gradient of the straight portion of the stress-strain curve in the elastic deformation region
4 Principle The test consists of straining a test piece by tensile force, generally to fracture, for the purpose of determining the mechanical properties defined in Clause 3. 5 Apparatus 5.1 Conformity The test machine and the extensometer shall conform to ISO 7500-1 and ISO 9513, respectively. The calibration shall obey ISO 376. The special requirements of this standard are presented here. 5.2 Testing machine A tensile machine control system that provides a constant cross-head speed shall be used. Grips shall have a structure and strength appropriate for the test specimen and shall be constructed to provide an effective connection with the tensile machine. The faces of the grips shall be filed or knurled, or otherwise roughened, so that the test specimen will not slip on them during testing. Gripping may be a screw type, or pneumatically or hydraulically actuated. SIST EN 61788-6:2011

61788-6  IEC:2011 – 9 – 5.3 Extensometer The weight of the extensometer shall be 30 g or less, so as not to affect the mechanical properties of the superconductive wire. Care shall also be taken to prevent bending moments from being applied to the test specimen (see Clause A.3). 6 Specimen preparation 6.1 Straightening the specimen When a test specimen sampled from a bobbin needs to be straightened, a method shall be used that affects the material as little as possible. 6.2 Length of specimen The total length of the test specimen shall be the inward distance between grips plus both grip lengths. The inward distance between the grips shall be 60 mm or more, as requested for the installation of the extensometer. 6.3 Removing insulation If the test specimen surface is coated with an insulating material, that coating shall be removed. Either a chemical or mechanical method shall be used, with care taken not to damage the specimen surface (see Clause A.4).
6.4 Determination of cross-sectional area (So) A micrometer or other dimension-measuring apparatus shall be used to obtain the cross-sectional area of the specimen after the insulation coating has been removed. The cross-sectional area of a round wire shall be calculated using the arithmetic mean of the two orthogonal diameters. The cross-sectional area of a rectangular wire shall be obtained from the product of its thickness and width. Corrections to be made for the corners of the cross-sectional area shall be determined through consultation among the parties concerned (see Clause A.5).
7 Testing conditions 7.1 Specimen gripping The test specimen shall be mounted on the grips of the tensile machine. At this time, the test specimen and tensile loading axis must be on a single straight line. Sand paper may be inserted as a cushioning material to prevent the gripped surfaces of the specimen from slipping and fracturing (see Clause A.6).
7.2 Pre-loading and setting of extensometer If there is any slack in the specimen when it is mounted, a force not greater than one-tenth of the 0,2 % proof strength of the composite shall be applied to take up the slack before the extensometer is mounted. When mounting the extensometer, care shall be taken to prevent the test specimen from being deformed. The extensometer shall be mounted at the centre between the grips, aligning the measurement direction with the specimen axis direction. After installation, loading shall be zeroed.
7.3 Testing speed
The strain rate shall be 10–4/s to 10–3/s during the test using the extensometer. After removing the extensometer, the strain rate may be increased to a maximum of 10–3/s. SIST EN 61788-6:2011

– 10 – 61788-6  IEC:2011 7.4 Test The tensile machine shall be started after the cross-head speed has been set to the specified level. The signals from the extensometer and load cell shall be plotted on the abscissa and ordinate, respectively, as shown in Figure 1. When the total strain has reached approximately 2 %, reduce the force by approximately 10 % and then remove the extensometer. The step of removing the extensometer can be omitted in the case where the extensometer is robust enough not to be damaged by the total strain and the fracture shock of this test. At this time, care shall be taken to prevent unnecessary force from being applied to the test specimen. Then, increase loading again to the previous level and continue testing until the test specimen fractures. Measurement shall be made again if a slip or fracture occurs on the gripped surfaces of the test specimen. SIST EN 61788-6:2011

61788-6  IEC:2011 – 11 –
0 100 200 300 400 500 600 700 0 0,5 1,0 1,5 2,0 Stress
(MPa) Strain
(%) 0,2 A B C D E 1 2 3 4 5 6 εa IEC
1597/11
Key  Initial loading line  Line shifted by an offset of 0,2% parallel to the initial loading line  Unloading line  Line shifted by an offset of 0,2% parallel to the unloading line  Second linear part of loading line  Line shifted by an offset of 0,2% parallel to the second linear loading line NOTE 1 When the total strain has reached ~2 % (point E), the load is reduced by 10 % and the extensometer is removed, if necessary. Then, the load is increased again. NOTE 2 The slope of the initial loading line is usually smaller than that of the unloading line. Then, two lines can be drawn from the 0,2 % offset point on the abscissa to obtain 0,2 % proof strength of the composite due to yielding of the copper component. Point A is obtained from the initial loading line, and Point B is obtained from the unloading line. Point C is the second type of 0,2 % proof strength of the composite where the Nb-Ti component yields. Figure 1 – Stress-strain curve and definition
of modulus of elasticity and 0,2 % proof strengths SIST EN 61788-6:2011

– 12 – 61788-6  IEC:2011 8 Calculation of results 8.1 Tensile strength (Rm) Tensile strength Rm shall be the maximum force divided by the original cross-sectional area of the wire before loading. 8.2 0,2 % proof strength (Rp0,2A and
Rp0,2B) The 0,2 % proof strength of the composite due to yielding of the copper component is determined in two ways from the loading and unloading stress-strain curves as shown in Figure 1. The 0,2 % proof strength under loading Rp0,2A shall be determined as follows: the initial linear portion under loading of the stress-strain curve is moved 0,2 % in the strain axis (0,2 % offset line under loading) and the point A at which this linear line intersects the stress-strain curve shall be defined as the 0,2 % proof strength under loading. The 0,2 % proof strength of the composite under unloading Rp0,2B shall be determined as follows: the linear portion under unloading is to be moved parallel to the 0,2 % offset strain point. The intersection of this line with the stress-strain curve determines the point B that shall be defined as the 0,2 % proof strength. This measurement shall be discarded if the 0,2 % proof strength of the composite is less than three times the pre-load specified in 7.2. Each 0,2 % proof strength shall be calculated using formula (1) given below:
Rp0,2i = Fi / So (1) where
Rp0,2i
is the 0,2 % proof strength (MPa) at each point;
Fi
is the force (N) at each point;
So
is the original cross-sectional area (in square millimetres) of the test specimen; Further, i = A and B. 8.3 Modulus of elasticity (Eo and Ea)
Modulus of elasticity shall be calculated using the following formula and the straight portion, either of the initial loading curve or of the unloading one.
E = ∆F (1 + εa)/(So ∆ε) (2) where
E
is the modulus of elasticity (MPa);
∆F
is the increments (N) of the corresponding force;
∆ε
is the increment of strain corresponding to ∆F;
εa
is the strain just after unloading as shown in Figure 1.
E is designated as Eo when using the initial loading curve (εa = 0), and as
Ea when using the unloading curve (εa ≠ 0). 9 Uncertainty Unless otherwise specified, measurements shall be carried in a temperature range between 280 K and 310 K. A force measuring cell with a combined standard uncertainty not greater than 0,5 % shall be used. An extensometer with a combined standard uncertainty not greater than 0,5 % shall be used. The dimension-measuring apparatus shall have a combined standard uncertainty not greater than 0,1 %. The target combined standard uncertainties are defined by root square sum (RSS) procedure, which is given in Annex B.
61788-6  IEC:2011 – 13 – There are no reliable experimental data with respect to uncertainties on moduli of elasticity and 0,2 % proof strengths as mentioned in Clause A.7. As described in Annex C, on the other hand, their uncertainties could be evaluated from the experimental conditions, of which parts are indicated above like uncertainty of force measuring cell. Consequently the relative expanded uncertainties (k=2) for the modulus of elasticity, Eo , and the 0,2 % proof strength, Rp0,2A,
are expected to be 2,0 % (N=1) and 0,78 % (N=1), respectively, where N indicates the time of repeated tests. NOTE Uncertainties reported in the present text, if used for the purpose of practical assessment, have to be taken under the specific considerations with detailed caution as indicated in Annex B.
10 Test report
10.1 Specimen a) Name of the manufacturer of the specimen b) Classification and/or symbol c) Lot number The following information shall be reported as necessary. d) Raw materials and their chemical composition e) Cross-sectional shape and dimension of the wire f) Filament diameter g) Number of filaments h) Twist pitch of filaments i) Copper to superconductor ratio 10.2 Results a) Tensile strength (Rm) b) 0,2 % proof strengths (Rp0,2A and
Rp0,2B) c) Modulus of elasticity (Eo and Ea with εa) The following information shall be reported as necessary. d) Second type of 0,2 % proof strength (Rp0,2C) e) Percentage elongation after fracture (A) 10.3 Test conditions a) Cross-head speed b) Distance between grips c) Temperature The following information shall be reported as necessary. d) Manufacturer and model of testing machine e) Manufacturer and model of extensometer f) Gripping method SIST EN 61788-6:2011

– 14 – 61788-6  IEC:2011 Annex A
(informative)
Additional information relating to Clauses 1 to 10
A.1 General This annex gives reference information on the variable factors that can seriously affect the tensile test methods, together with some precautions to be observed when using the standard. A.2 Percentage elongation after fracture (A) In Cu/NbTi superconductive wires there is a difference in strength between the copper and NbTi, and the wire is often deformed in waves by the shock of fracture. In such a case, it is difficult to find the elongation accurately after fracture using the butt method. Hence, the measurement of elongation after fracture should serve only as a reference. The movement of the cross-head may be used to find the approximate value for elongation after fracture, instead of using the butt method, as shown below. To use this method, the cross-head position at fracture must be recorded. Use the following formula to obtain the elongation after fracture, given in percentage.
A = 100 (Lu − Lc) / Lc (A.1) where
A
is the percentage elongation after fracture;
Lc
is the initial distance between cross-heads;
Lu
is the distance between cross-heads after fracture. A.3 Second type of 0,2 % proof strength (Rp0,2C) The second type of 0,2 % proof strength, at which the Nb-Ti component yields, is defined reasonably on the basis of the rule-of-mixture for the bimetallic composite including continuous filaments. As indicated in Figure 1, it should be the stress Rp0,2C corresponding to point C, at which the straight portion of the loading curve after the point A is moved by 0,2 % along the strain axis intersects the stress-strain curve. The relevant straight portion is usually observed for the commercial Cu/Nb-Ti superconductive wires, because the copper component deforms plastically in a linear behaviour. Often the stress-strain curve does not show any straight line, but is rounded off for some wires, when they have high copper/non-copper ratio and are highly cold worked. It has been empirically made clear that the rounded-off appearance is observed when the following k-factor is less than 0,4:
k = (Rm − Rp0,2A) /Rp0,2A (A.2)
The Rp0,2C is one of the important parameters describing the mechanical property of the composite material in the scientific viewpoint, but its use is not always demanded in the engineering sense. A.4 Extensometer When using a special type of extensometer, which is attached with an unremovable spacer for determining the gauge length, it may introduce a problem during
the unloading of the wire to zero force. To avoid a compressive force on the spacer, the actual gauge length must be SIST EN 61788-6:2011

61788-6  IEC:2011 – 15 – adjusted during installation with sufficient clearance. If the clearance after unloading is not negligible, it must be included in calculating the strain values. If the test specimen is thin and the extensometer is relatively heavy, any bending moment caused by the weight of the extensometer can stress the specimen, eventually resulting in the specimen yielding. To avoid this, a light extensometer with a balance weight is to be carefully attached. Alternatively, a sufficiently light extensometer without a balance weight is also acceptable to use. Figure A.1 shows an extensometer made with a Ti alloy, with a total mass of about 3 g. It is so light that even a single use without a balance weight could provide enough uncertainty according to the procedure of the present standard. Figure A.2 shows one of the lightest extensometers commercially available, with a total mass of 31 g together with a balance weight. Using it, a round robin test (RRT) was conducted in Japan and good results were obtained. The results were used to establish the present international standard.
Dimensions in millimetres
3,5 R3 0,3 3,3 27 1 26 ∅2,2 R1 26,7 30 5 7 IEC
2365/07
Figure A.1 – An example of the light extensometer, where R1 and R3 indicate the corner radius SIST EN 61788-6:2011

– 16 – 61788-6  IEC:2011 Dimensions in millimetres
G.L. 25 13 37
Bar spring a) Top view b) Side view Strain gauge Frame Stopper Specimen Balance weight 22 35 Frame Cross spring plate Gauge length setting hole IEC
1598/11
Figure A.2 – An example of the extensometer provided with balance weight and vertical specimen axis NOTE Further information about extensometers is obtainable from the Japanese National Committee of
IEC/TC90, ISTEC, 10-13, Shinonome 1-chome Koto-ku, Tokyo 135-0062, Japan, Tel 81-3-3536-7214, Fax 81-3-3536-7318, e-mail Koki TSUNODA Since the superconductive composite wire is covered with a soft copper, a scratch in the surface of the specimen made as it is mounted can be a starting point of fracture. Care should therefore be taken when handling the specimen. A.5 Insulating coating
The coating on the surface of the test specimen should be removed using an appropriate organic solvent that would not damage the specimen. If the coating material is not dissolved by the organic solvent, a mechanical method should be used with care to prevent the copper from being damaged. If the coating is not removed, it affects the strength to only a small extent. For example, tensile strength decreases by less than 3 % for a low-strength wire which has a high copper ratio of 7. The coating is not designed as a structural component. An SIST EN 61788-6:2011

61788-6  IEC:2011 – 17 – analysis of measurement as a three-component composite, i.e. copper, Nb-Ti and insulating coating, is too complicated to conduct. Therefore this test method covers a bare wire in order to maintain the level of uncertainty.
A.6 Cross-sectional area
Where even lower uncertainty is required, the cross-sectional area may be obtained by correcting the radius of the corner of the rectangular wire finished by dies, using the value given on the manufacturing specifications. For rolling or Turk's-head finish, the radius of the corner is not controlled and a correction is made using a microphotograph of the cross-section.
A.7 Gripping force A weak gripping force results in slippage and a strong gripping force can break the gripped surface. Care should therefore be used when adjusting the gripping force. A.8 Uncertainty The Japanese National Committee of IEC TC90 fulfilled the domestic RRT in 1996 by contributions of eight research groups [1] in order to evaluate only the coefficient of variation of experimental data on moduli of elasticity and 0,2 % proof strengths [2], but not their uncertainties. It is, however, not possible to deduce their uncertainties at the present time, because their original data have been insufficient to evaluate uncertainties. Only the way to know the uncertainty is to evaluate it by using the numerical computation based on type B statistics as the procedure is given in Annex C and its results are described in Clause 9 of the main text. Empirical facts with respect to the scattering source of measured values are described in the following. The modulus of elasticity Eo determined under the loading curve was found to be always smaller than the modulus Ea under unloading. The reason is attributed to the following handling issues: the bending of the wire specimen, the misalignment of sample gripping with respect to the load axis and a weak grip, and so on. Also, it is pointed out that the copper component is in a plastic state at room temperature before the test, depending on a degree of thermal contraction during cooling from the heat treating temperature. As a whole, the initial loading curve with non-linearity causes the result of Eo < Ea. The German National Committee of IEC TC90 reported that the modulus of elasticity can be determined with small uncertainty when adopting an initial linear loading at zero-offset. This low uncertainty was achieved by using two light extensometers (Figure A.1) which enabled the cancelling of the possible initial bending effects and ensured a high degree of linearity for the zero-offset loading line. Care must be taken while handling specimens in order not to induce strain to the copper component. Otherwise, the 0,2 % proof strength of the composite due to yielding of the copper component would increase due to work hardening. Allowable pre-loading limit should be taken into consideration in this fact. The second type of 0,2 % proof strength Rp0,2C is the quantity determined with the lowest uncertainty, that should serve only as reference. Care must, however, be taken to ensure an existence of a straight portion in the stress-strain curve after the point A in Figure 1 SIST EN 61788-6:2011

– 18 – 61788-6  IEC:2011 A.9 Reference documents of Annex A [1] SHIMADA, M., HOJO, M., MORIAI, H. and OSAMURA. K. Jpn. Cryogenic Eng, 1998, 33, p. 665. [2] OSAMURA, K., NYILAS, A., SHIMADA, M., MORIAI, H., HOJO, M., FUSE T. and SUGANO, M. Adv. Superconductivity, 1999, XI, p.1515.
61788-6  IEC:2011 – 19 – Annex B
(informative)
Uncertainty considerations
B.1 Overview In 1995, a number of international standards organizations, including IEC, decided to unify the use of statistical terms in their standards. It was decided to use the word “uncertainty” for all quantitative (associated with a number) statistical expressions and eliminate the quantitative use of “precision” and “accuracy.” The words “accuracy” and “precision” could still be used qualitatively.
The terminology and methods of uncertainty evaluation are standardized in the Guide to the Expression of Uncertainty in Measurement (GUM) [1] 1. It was left to each TC to decide if they were going to change existing and future standards to be consistent with the new unified approach. Such change is not easy and creates additional confusion, especially for those who are not familiar with statistics and the term uncertainty. At the June 2006 TC 90 meeting in Kyoto, it was decided to implement these changes in future standards.
Converting “accuracy” and “precision” numbers to the equivalent “uncertainty” numbers requires knowledge about the origins of the numbers. The coverage factor of the original number may have been 1, 2, 3, or some other number. A manufacturer’s specification that can sometimes be described by a rectangular distribution will lead to a conversion number of 3/1. The appropriate coverage factor was used when converting the original number to the equivalent standard uncertainty. The conversion process is not something that the user of the standard needs to address for compliance to TC 90 standards, it is only explained here to inform the user about how the numbers were changed in this process. The process of converting to uncertainty terminology does not alter the user’s need to evaluate their measurement uncertainty to determine if the criteria of the standard are met.
The procedures outlined in TC 90 measurement standards were designed to limit the uncertainty of any quantity that could influence the measurement, based on the Convener’s engineering judgment and propagation of error analysis. Where possible, the standards have simple limits for the influence of some quantities so that the user is not required to evaluate the uncertainty of such quantities. The overall uncertainty of a standard was then confirmed by an interlaboratory comparison.
B.2 Definitions Statistical definitions can be found in three sources: the GUM, the International Vocabulary of Basic and General Terms in Metrology (VIM)[2], and the NIST Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results (NIST)[3]. Not all statistical terms used in this standard are explicitly defined in the GUM. For example, the terms “relative standard uncertainty” and “relative combined standard uncertainty” are used in the GUM (5.1.6, Annex J), but they are not formally defined in the GUM (see [3]). B.3 Consideration of the uncertainty concept Statistical evaluations in the past frequently used the coefficient of variation (COV) which is the ratio of the standard deviation and the mean (N.B. the COV is often called the relative standard deviation). Such evaluations have been used to assess the precision of the ————————— 1
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