IEC 61788-6:2011
(Main)Superconductivity - Part 6: Mechanical properties measurement - Room temperature tensile test of Cu/Nb-Ti composite superconductors
Superconductivity - Part 6: Mechanical properties measurement - Room temperature tensile test of Cu/Nb-Ti composite superconductors
IEC 61788-6:2011 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. 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. This third edition cancels and replaces the second edition published in 2008. It constitutes a technical revision which adds a new Annex C including specific examples of uncertainty estimation related to mechanical tests.
Supraconductivité - Partie 6: Mesure des propriétés mécaniques - Essai de traction à température ambiante des supraconducteurs composites de Cu/Nb-Ti
La CEI 61788-6:2011 spécifie une méthode d'essai détaillant les modes opératoires d'essai de traction à exécuter sur des fils composites supraconducteurs de Cu/Nb-Ti à température ambiante. Cet essai est utilisé pour mesurer le module d'élasticité, la charge d'épreuve à 0,2 % du composite due à la déformation du composant de cuivre et à la résistance à la traction. La valeur du pourcentage d'allongement après fracture et le second type de charge d'épreuve à 0,2 % due à la déformation du composant en Nb-Ti ne servent que de référence. L'échantillon spécifié par le présent mode opératoire d'essai a une section transversale circulaire ou rectangulaire avec une surface de 0,15 mm2 à 2 mm2 et un rapport volumique entre le cuivre et le supraconducteur de 1,0 à 8,0 et sans revêtement isolant. Cette troisième édition annule et remplace la deuxième édition parue en 2008. Elle constitue une révision technique avec l'ajout d'une Annexe C décrivant un exemple spécifique d'estimation d'incertitude concernant les essais mécaniques.
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Standards Content (Sample)
IEC 61788-6 ®
Edition 3.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
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
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IEC 61788-6 ®
Edition 3.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
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
PRICE CODE
INTERNATIONALE
CODE PRIX V
ICS 29.050; 77.040.10 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 (S ) . 9
o
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 (R ) . 12
m
8.2 0,2 % proof strength (R and R ) . 12
p0,2A p0,2B
8.3 Modulus of elasticity (E and E ) . 12
o a
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
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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.
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.
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
2 2
area of 0,15 mm to 2 mm 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
– 8 – 61788-6 IEC:2011
3.2
tensile strength
R
m
tensile stress corresponding to the maximum testing force
NOTE The symbol σUTS is commonly used instead of R .
m
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
L
g
length between grips that hold a test specimen in position before the test is started
3.5
0,2 % proof strength
R (see Figure 1)
p0,2
stress value where the copper component yields by 0,2 %
NOTE 1 The designated stress, R or R corresponds to point A or B in Figure 1, respectively. This
p0,2A p0,2B
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 σ is commonly used instead of R .
0,2 p0,2
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.
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 (S )
o
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
–4 –3
The strain rate shall be 10 /s to 10 /s during the test using the extensometer. After
–3
removing the extensometer, the strain rate may be increased to a maximum of 10 /s.
– 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.
61788-6 IEC:2011 – 11 –
E
C
B
A
ε
a
D
0 0,2 0,5 1,0 1,5 2,0
Strain (%)
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
Stress (MPa)
– 12 – 61788-6 IEC:2011
8 Calculation of results
8.1 Tensile strength (R )
m
Tensile strength R shall be the maximum force divided by the original cross-sectional area of
m
the wire before loading.
8.2 0,2 % proof strength (R and R )
p0,2A p0,2B
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 R shall be determined as follows: the
p0,2A
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 R shall be determined as follows: the linear
p0,2B
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:
R = F / S (1)
p0,2i i o
where
R is the 0,2 % proof strength (MPa) at each point;
p0,2i
F is the force (N) at each point;
i
S is the original cross-sectional area (in square millimetres) of the test specimen;
o
Further, i = A and B.
8.3 Modulus of elasticity (E and E )
o a
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 + ε )/(S ∆ε) (2)
a o
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;
ε is the strain just after unloading as shown in Figure 1.
a
E is designated as E when using the initial loading curve (ε = 0), and as E when using the
o a a
unloading curve (ε ≠ 0).
a
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, E , and the 0,2 % proof strength,
o
R , are expected to be 2,0 % (N=1) and 0,78 % (N=1), respectively, where N indicates
p0,2A
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 (R )
m
b) 0,2 % proof strengths (R and R )
p0,2A p0,2B
and E with ε )
c) Modulus of elasticity (E
o a a
The following information shall be reported as necessary.
d) Second type of 0,2 % proof strength (R )
p0,2C
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
– 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 (L − L ) / L (A.1)
u c c
where
A is the percentage elongation after fracture;
L is the initial distance between cross-heads;
c
L is the distance between cross-heads after fracture.
u
A.3 Second type of 0,2 % proof strength (R )
p0,2C
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 R corresponding to
p0,2C
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 = (R − R ) /R (A.2)
m p0,2A p0,2A
The R is one of the important parameters describing the mechanical property of the
p0,2C
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
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
R1
∅2,2
R3
3,5
IEC 2365/07
Figure A.1 – An example of the light extensometer,
where R1 and R3 indicate the corner radius
0,3
3,3
26,7
– 16 – 61788-6 IEC:2011
Dimensions in millimetres
a) Top view
Bar spring
b) Side view
Stopper
Specimen
Strain gauge
Balance weight
Frame
22 35 Cross spring plate
Frame
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
G.L. 25
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 E determined under the loading curve was found to be
o
always smaller than the modulus E under unloading. The reason is attributed to the following
a
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 E < E .
o a
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.
is the quantity determined with the lowest
The second type of 0,2 % proof strength R
p0,2C
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
– 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] .
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
. The appropriate coverage factor was used when converting the original number to the
1/ 3
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 T
...
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