prEN IEC 61788-29:2026

• Status: Not Published
• Publication Date: 01-Feb-2028
• Technical Committee: CLC/SR 90 - Superconductivity
• Current Stage: 4020 - Enquiry circulated - Enquiry
• Start Date: 10-Jul-2026
• Due Date: 30-Oct-2026
• Completion Date: 10-Jul-2026
• Ref Project: oSIST prEN IEC 61788-29:2026 - Electromechanical properties measurement - Electromechanical test of practical rebco and bscco composite superconductors at liquid nitrogen temperature

Overview

prEN IEC 61788-29:2026 establishes an international standard for measuring the electromechanical properties of REBCO (Rare-Earth Barium Copper Oxide) and BSCCO (Bismuth Strontium Calcium Copper Oxide) composite superconductor tapes at liquid nitrogen temperature. Published by the International Electrotechnical Commission (IEC) Technical Committee 90 (Superconductivity) and adopted by CLC, this standard is a vital reference for both research and industrial applications involving high-temperature superconductors (HTS).

The test procedures focus on assessing changes in the critical current-a key parameter of superconducting performance-when tapes are subjected to uniaxial tensile stress under cryogenic conditions. By defining repeatable methods for specimen preparation, test execution, and data evaluation, IEC 61788-29 enables consistency and comparability in the characterization of advanced superconducting materials.

Keywords: Electromechanical properties measurement, REBCO, BSCCO, composite superconductors, liquid nitrogen temperature, critical current, tensile test, superconductivity standard.

Key Topics

• Electromechanical Testing under Cryogenic Conditions: The standard specifies tensile test methodologies for practical, coated conductor tapes, focusing on the effects of mechanical strain at liquid nitrogen temperature (77 K).

• Critical Current Measurement (Ic): Procedures for measuring the maximum direct current that can flow through a superconductor tape without significant resistance, as a function of applied tensile stress.

• Irreversible Stress Limit Determination: Clear criteria (e.g., 99% Ic recovery, 95% Ic retention) for identifying stress limits beyond which superconducting performance is compromised.

• Specimen Preparation and Handling: Guidelines to ensure consistent tape dimensions, proper mounting, and avoidance of pre-loading or damage during cooling and gripping.

• Testing Apparatus and Conditions:

  • Calibration requirements for tensile test machines and force-measuring systems (ISO 376, ISO 7500-1).
  • Cryogenic systems and safety considerations when working with liquid nitrogen.

• Data Recording and Reporting: Recommendations for reporting test conditions, stress-strain responses, and uncertainties following standardized terminology (IEC 60050-815 and related standards).

Applications

IEC 61788-29 delivers practical value for a range of users working with HTS materials:

• Superconducting Magnet and Power Device Manufacturing: Ensures that REBCO and BSCCO tapes meet mechanical and electrical requirements for reliability in demanding environments such as power cables, fault current limiters, and rotating machinery.

• Research and Development: Academic and industrial laboratories benchmarking new HTS materials benefit from standardized test protocols that underpin meaningful, reproducible results.

• Quality Assurance & Procurement: Provides objective benchmarks for evaluating supplier materials according to agreed international criteria, supporting procurement and regulatory compliance.

• Design and Engineering: Facilitates the selection of superconducting tapes with appropriate electromechanical performance, underpinning advanced design in cryogenic applications.

Related Standards

Users of IEC 61788-29 should also be aware of the following related standards, which provide complementary guidance:

• IEC 60050-815:2024 – International Electrotechnical Vocabulary: Superconductivity. Key terminology and definitions.
• IEC 61788-26 – Superconductivity: Critical current measurement for REBCO composite superconductors.
• IEC 61788-28 – Superconductivity: Mechanical properties measurement – Tensile test at cryogenic temperatures.
• ISO 376:2011 – Metallic materials: Calibration of force-proving instruments for uniaxial testing machines.
• ISO 7500-1:2018 – Calibration and verification of static uniaxial testing machines.

Conclusion

prEN IEC 61788-29:2026 is an essential standard for the consistent measurement of electromechanical properties in REBCO and BSCCO composite superconductors at cryogenic temperatures. By enabling accurate, comparable testing, it supports the safe, reliable advancement of superconducting technologies for the energy sector and scientific research.

SLOVENSKI STANDARD
01-september-2026
Merjenje elektromehanskih lastnosti - Elektromehanski preskus praktičnih
kompozitnih superprevodnikov rebco in bscco pri temperaturi tekočega dušika
Electromechanical properties measurement - Electromechanical test of practical rebco
and bscco composite superconductors at liquid nitrogen temperature
Ta slovenski standard je istoveten z: prEN IEC 61788-29:2026
ICS:
17.220.20 Merjenje električnih in Measurement of electrical
magnetnih veličin and magnetic quantities
29.050 Superprevodnost in prevodni Superconductivity and
materiali conducting materials
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

90/563/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 61788-29 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2026-07-10 2026-10-02
SUPERSEDES DOCUMENTS:
90/551/CD, 90/557/CC
IEC TC 90 : SUPERCONDUCTIVITY
SECRETARIAT: SECRETARY:
Japan Mr Hideki II
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):

ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of CENELEC,
is drawn to the fact that this Committee Draft for Vote (CDV) is
submitted for parallel voting.
The CENELEC members are invited to vote through the CENELEC
online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are aware
and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some Countries” clau ses to be
included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for submitting ISC c lauses. (SEE
AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Superconductivity – Part 29: Electromechanical properties measurement – Electromechanical test under uniaxial
tensile load of REBCO and BSCCO composite superconductor tapes at liquid nitrogen temperature

PROPOSED STABILITY DATE: 2029
NOTE FROM TC/SC OFFICERS:
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IEC CDV 61788-29 © IEC 2026
1 CONTENTS
3 FOREWORD . 3
4 INTRODUCTION . 4
5 1 Scope . 6
6 2 Normative references . 6
7 3 Terms and definitions . 6
8 3.1 tensile stress R . 6
9 3.2 stress interval ΔR . 7
10 3.3 transition stress RT . 7
11 3.4   critical current I . 7
c
12 3.5 reversible stress limit for Ic degradation Rrev or Rret . 7
13 3.6 constant sweep rate method . 7
14 3.7 ramp-and-hold method . 7
15 4 Principles . 7
16 5 Apparatus . 8
17 5.1   General . 8
18 5.2 Testing machine . 8
19 5.3 Critical current measuring system . 9
20 6 Specimen preparation . 8
21 6.1   General . 9
22 6.2 Specimen length . 9
23 6.3 Determination of cross-sectional area (S0) . 9
24 7 Testing conditions . 9
25 7.1 Specimen gripping . 9
26 7.2 Cooling procedure. 10
27 7.3 Testing speed . 10
28 7.4 Critical current measurement . 10
29 7.5 Test . 10
30 8 Calculation of results . 11
31 8.1 Irreversible stress limit for I degradation R or R . 11
c rev ret
32 9 Uncertainty of measurand . 12
33 10 Test report . 13
34 10.1 Identification of specimen . 13
35 10.2 Reporting of test conditions . 13
36 10.3 Results . 13
37 Annex A . 14
38 A.1 Scope . 14
39 A.2 Gripping of the specimen . 14
40 A.3 Evaluation of the cross-sectional area of HTS tapes . 14
41 A.4 Electromechanical properties evaluation setup . 15
42 A.5 Effect of recovery condition in defining the electromechanical properties of HTS
43 tapes . 16
44 Annex B . 18
IEC CDV 61788-29 © IEC 2026
45 B.1 Relative standard uncertainty (RSU) . 18
46 B.2 Analysis of variance . 21
47 B.3 Type-B uncertainty analysis for electromechanical properties . 22
48 B.4 Correlation of uncertainties between I and electromechanical properties . 24
c
49 Bibliography . 26
51 Figure 1 – Schematic view of a specimen with voltage taps installed for critical current
52 measurement . 9
53 Figure 2 – Schematic variations of Ic/Ic0 and Icr/Ic0 during loading-unloading under uniaxial
54 tensile stress applied to HTS tape specimens at liquid nitrogen temperature, showing
55 determination of Rrev and Rret using 99 % Ic0 recovery criterion and 95 % Ic0 retention
56 criterion during loading . 12
57 Figure A.1 – (a) Sample holder mounted on the loading frame of the testing device. Cu
58 blocks serve as current terminals for I measurement and specimen gripping under tensile
c
59 loading at liquid nitrogen temperature. (b) Examples of alternative holders for Ic
60 measurement under tensile loading. 14
61 Figure A.2 – Schematic illustration of thickness and width measurements for the specimen . 15
62 Figure A.3 – Representative comparison data of irreversible stress limits determined using
63 different recovery conditions . 17
64 Figure B.1 – Distribution of initial critical current values measured for various HTS tapes
65 by participating laboratories . 20
66 Figure B.2 – Combined uncertainty of cross-sectional area measurements for each
67 specimen . 24
68 Figure B.3 – Combined uncertainty of (a) R , and (b) R for various HTS tapes . 24
ret rev
69 Figure B.4 – Correlation between combined uncertainty of (a) Ic and uRret and (b) Ic and
70 u for various HTS tapes . 26
Rrev
72 Table B.1 – Specifications of HTS tapes used in the international round-robin test . 18
73 Table B.2 – Electromechanical properties measurement conditions used by participating
74 laboratories . 19
75 Table B.3 – Statistical summary of initial Ic (Ic0) values measured at 77 K in self-field
76 conditions . 20
77 Table B.4 – Statistical summary of Rret values measured at 77 K in self-field conditions . 21
78 Table B.5 – Statistical summary of R values measured at 77 K in self-field conditions . 21
rev
79 Table B.6 – Summary of the F test results for the electromechanical properties of each
80 sample . 22
IEC CDV 61788-29 © IEC 2026
82 INTERNATIONAL ELECTROTECHNICAL COMMISSION
83 ____________
85 SUPERCONDUCTIVITY –
86 Part 29: Electromechanical properties measurement –
87 Electromechanical test under uniaxial tensile load of REBCO and BSCCO
88 composite superconductor tapes at liquid nitrogen temperature
90 FOREWORD
91 1) The International Electrotechnical Commission (IEC) is a worldwide organization for
92 standardization comprising all national electrotechnical committees (IEC National Committees).
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99 International, governmental, and non-governmental organizations liaising with the IEC also
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101 Standardization (ISO) in accordance with conditions determined by agreement between the two
102 organizations.
103 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible,
104 an international consensus of opinion on the relevant subjects since each technical committee
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122 upon, this IEC Publication or any other IEC Publications.
123 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced
124 publications is indispensable for the correct application of this publication.
125 9) IEC draws attention to the possibility that the implementation of this document may involve the
126 use of (a) patent(s). IEC takes no position concerning the evidence, validity or applicability of
127 any claimed patent rights in respect thereof. As of the date of publication of this document, IEC
128 had not received notice of (a) patent(s), which may be required to implement this document.
IEC CDV 61788-29 © IEC 2026
129 However, implementers are cautioned that this may not represent the latest information, which
130 may be obtained from the patent database available at https://patents.iec.ch [and/or]
131 www.iso.org/patents. IEC shall not be held responsible for identifying any or all such patent
132 rights.
133 IEC 61788-29 has been prepared by IEC technical committee 90: SUPERCONDUCTIVITY. It is an
134 International Standard.
136 The text of this International Standard is based on the following documents:
Draft Report on voting
90/XX/CD 90/537/RVN
139 Full information on the voting for its approval can be found in the report on voting indicated in the
140 above table.
141 The language used for the development of this International Standard is English.
142 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in accordance
143 with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available at www.iec.ch/
144 members_experts/refdocs. The main document types developed by IEC are described in greater detail at
145 www.iec.ch/publications.
146 The committee has decided that the contents of this document will remain unchanged until the stability date
147 indicated on the IEC website under webstore.iec.ch in the data related to the specific document. At this
148 date, the document will be
149 • reconfirmed,
150 • withdrawn
151 • replaced by a revised edition, or
152 • amended.
162 INTRODUCTION
163 Several types of composite superconductors have now been commercialized. The rare-earth-based
164 oxide superconductor (SC) with the chemical formula RE1Ba2Cu3O7-δ is used in practical SC tapes, where
IEC CDV 61788-29 © IEC 2026
165 the rare-earth element RE is Y, Dy, Gd, Eu, Nd, Ho, and Sm or a mixture of them. This type of practical SC
166 tape is usually called a REBCO coated conductor. A typical architecture consists of a substrate of Hastelloy,
167 Ni-W alloy, or stainless steel, a buffer layer consisting of plural oxides, an SC layer, protection layers of Ag,
168 and electroplated copper layers. The substrate and buffer layers act as templates to facilitate the well-
169 oriented crystal grains of the SC layer. To resist the strong electromagnetic force, the tapes are in some
170 cases externally reinforced by laminating thin foils of stainless steel, copper, or a copper alloy.
171 Two types of bismuth–based oxide superconductors with the chemical formula (Bi, Pb)2Sr2Can-1Cun
172 O where n = 2 or 3, have been developed as practical SC tapes/wires. In either case, superconducting
2n+4
173 filaments are embedded in a pure silver matrix. This type of practical SC tape/wire is usually called BSCCO
174 tape/wire. In the Bi-2223 tapes/wires, a silver alloy outer sheath surrounds the matrix to increase mechanical
175 strength. They are made exclusively in a flat tape-shaped form. Tapes are commercially available in which
176 thin metallic sheets of copper alloy, stainless steel, or nickel alloy are soldered on both sides of the SC tapes
177 to improve the stress dependence of critical current and the mechanical properties.
178 Practical composite superconductors have a high current density and a small cross-sectional area.
179 The major application of composite superconductors is to build electrical power devices and
180 superconducting magnets. While the magnet is being manufactured, complex stresses/strains are applied
181 to its windings, and when it is energized, a large electromagnetic force is applied to the superconducting
182 tapes due to their high current density. It is therefore indispensable to determine the electromechanical
183 properties of the practical SC tapes at cryogenic temperatures.
IEC CDV 61788-29 © IEC 2026
185 SUPERCONDUCTIVITY –
186 Part 29: Electromechanical properties measurement –
187 Electromechanical test under uniaxial tensile load of REBCO and BSCCO
188 composite superconductor tapes at liquid nitrogen temperature
189 1 Scope
190 This part of IEC 61788 specifies a stress-based tensile test method for determining the electromechanical
191 properties of practical REBCO and BSCCO composite superconductor tapes at liquid nitrogen temperature
192 by measuring the critical current (Ic) under uniaxial tensile loading without the use of extensometers. This
193 stress-based test method is intended to determine the irreversible stress limits for I degradation under
c
194 tensile loading.
195 When REBCO tapes are tested using this procedure, they are a rectangular cross-section with an area of

2 2
196 0,06 mm to 5,0 mm (corresponding to tapes 2,0 mm to 12,5 mm in width and 0,03 mm to 0,4 mm in
197 thickness). When BSCCO tapes are tested, they are a rectangular cross-section with an area of 0,3 mm to
198 2,5 mm (corresponding to tape-shaped ones 2,0 mm to 5,0 mm in width and 0,15 mm to 0,5 mm in
199 thickness).
200 2 Normative references
201 The following documents are referred to in the text in such a way that some or all of their content
202 constitutes requirements of this document. For dated references, only the edition cited applies. For
203 undated references, the latest edition of the referenced document (including any amendments) applies.
204 IEC 60050-815:2024, International Electrotechnical Vocabulary (IEV) – Part 815: Superconductivity
205 IEC 61788-26, Ed. 1: Superconductivity - Part 26: Critical current measurement – DC critical current of
206 RE-Ba-Cu-O composite superconductors
207 IEC 61788-28, CDV: Superconductivity – Part 28: Mechanical properties measurement – Tensile test of
208 practical REBCO and BSCCO composite superconductors at cryogenic temperatures
209 ISO 376:2011, Metallic materials – Calibration of force-proving instruments used for the verification of
210 uniaxial testing machines
211 ISO 7500-1:2018, Metallic materials – Calibration and verification of static uniaxial testing machines – Part
212 1: Tension/compression testing machines – Calibration and verification of the force-measuring system
213 3 Terms and definitions
214 For the purposes of this document, the terms and definitions given in IEC 60050-815 and ISO 6892, as
215 well as the following, apply.
216 ISO and IEC maintain terminology databases for use in standardization at the following addresses:
217 • IEC Electropedia: available at https://www.electropedia.org/
218 • ISO Online browsing platform: available at https://www.iso.org/obp
219 3.1
220 tensile stress
221 R
222 Tensile force divided by the original cross-sectional area (So) of the test piece at any moment during the
223 test.
IEC CDV 61788-29 © IEC 2026
224 3.2
225 stress interval
226 ΔR
227 Stress range set to measure the behavior of the critical current (I ) under the tensile stress applied to the
c
228 superconducting tape specimen during a stress-based tensile test.
229 3.3
230 transition stress
231 R
T
232 Stress at the point where ΔR changes from a broad to a narrow range while measuring the Ic under tensile
233 stress (see Figure 2).
234 Note: R is used to establish the irreversible stress limit for Ic degradation accurately. It typically corresponds to the stress level at
T
235 which I decreases to approximately 95 % of its initial critical current (I ). Alternatively, if applicable, the transition stress may be set to
c c0
236 80% of the specimen's yield stress [1]. This alternative may be used as guidance for the preliminary test described in Clause 7.5.
237 3.4
238 critical current
239 I
c
240 Maximum direct current that can be regarded as flowing through an HTS tape specimen practically without
241 resistance, defined by a specified electric field criterion (E ) at a specified temperature.
c
242 Note: I measured at the state before the load or deformation is applied is regarded as the initial I , I . I in the unloaded state is
c c c0 c
243 referred to as the retained or residual I (I ).
c cr
244 3.5
245 irreversible stress limit for I degradation
c
246 R or R
rev ret
247 Irreversible stress limit for Ic degradation may be identified using two criteria: Rrev is based on the 99 % Ic0
248 recovery criterion (see Figure 2) and corresponds to the stress level just before irreversible I degradation
c
249 begins during loading-unloading under tensile stress R. At this point, after unloading, the measured critical
250 current in the unloaded state (I ), which is also known as the recovered or retained I , does not recover to
cr c
251 a level exceeding 99 % of Ic0 in subsequent loading-unloading cycles.
252 R is based on the 95 % I retention criterion (see Figure 2) during loading without engaging unloading and
ret c0
253 corresponds to the stress at which Ic drops to 95 % of Ic0. This provides an alternative practical criterion to
254 assess the electromechanical response of HTS tapes under tensile loading.
255 3.6
256 constant sweep rate method
257 Voltage-current (U-I) data acquisition method where a current is swept at a constant rate from zero to a
258 current above I , and where the U-I data are acquired continuously or frequently.
c
259 3.7
260 ramp-and-hold method
261 U-I data acquisition method where a current is swept in stages from zero to a current above Ic, where the
262 current is held for an appropriate period at each stage, during which the U-I data are acquired
263 continuously or frequently.
264 4 Principles
265 This stress-based tensile test method combines the Ic measurement method with a uniaxial tensile test on
266 HTS tapes at liquid nitrogen temperature to determine electromechanical properties, such as the irreversible
267 stress limit for Ic degradation. The specimen shall be mounted on the loading frame of a testing machine
268 equipped with a critical current measuring system (see Figure A.1).
IEC CDV 61788-29 © IEC 2026
269 The electromechanical properties shall be evaluated by measuring Ic under controlled tensile stress
270 conditions during loading and, where applicable, unloading. The irreversible stress limits for I degradation
c
271 may be determined using either: a) the 99 % Ic0 recovery criterion (R ev); or b) the 95 % Ic0 retention criterion
r
272 during loading (R ).
ret
273 The U-I characteristic shall be measured in accordance with IEC 61788-26.
274 It is the responsibility of the user of this document to establish appropriate safety and health practices and
275 to determine the applicability of regulatory limitations prior to use. Specific precautionary statements are
276 given below.
277 5 Apparatus
278 5.1   General
279 The testing apparatus shall conform to the specifications outlined in ISO 7500-1. Calibration procedures
280 shall comply with ISO 376. Furthermore, this standard imposes specific additional requirements, detailed
281 as follows:
282 5.2 Testing machine
283 The testing machine shall include a tensile machine control system capable of maintaining a constant
284 crosshead speed and a cryostat system capable of maintaining a constant, homogeneous specimen
285 temperature during testing.
286 The grips shall have sufficient structure and strength appropriate for the test specimen and shall provide a
287 firm connection to the tensile machine. The grip faces may be filed or knurled, or otherwise roughened to
288 prevent specimen slippage during testing. Gripping may be a screw type, depending on the cryogenic test
289 system.
290 Structural materials used for grips and load-transferring components shall maintain sufficient strength and
291 toughness at liquid nitrogen temperature. Materials with low thermal conductivity shall be used to reduce
292 heat flow into the cryostat system. Care shall be taken to ensure that the specimen is not allowed to be
293 slack and that compressive stresses or shocks do not occur. Abrupt removal of the load can cause specimen
294 buckling, which could invalidate the test.
295 The load cell capacity shall be selected according to the expected tensile load range of the specimen.
296 Hazards exist in measurements at cryogenic conditions. Commonly, cryogenic liquids (e.g., liquid nitrogen
297 or liquid helium) are used to cool the specimen directly by bath cooling or gas exchange. Other cryogenic
298 systems may also be applied. Direct contact of the skin with cold liquid or gas transfer lines, storage Dewars,
299 or other components can cause immediate skin damage. Safety precautions for handling cryogenic devices
300 shall be observed.
301 5.3 Critical current measuring system
302 The apparatus used for measuring the U-I characteristic shall consist of a specimen mounted on the upper
303 and lower grip supports using Cu blocks for the current terminals, a Dewar with an open bath, and a U-I
304 measuring system.
305 The U-I measuring system shall include a DC source, a data acquisition system, preamplifiers, filters,
306 voltmeters, or a combination thereof. A computer-assisted data acquisition system shall be used.
307 6 Specimen preparation
308 6.1   General
309 Care shall be taken to prevent bending or pre-loading during specimen handling. The specimen shall be
310 used in its original shape, and care shall be taken when cutting specimens to avoid delamination, particularly
311 for REBCO tapes.
IEC CDV 61788-29 © IEC 2026
312 6.2 Specimen length
313 The length (L) of the test specimen shall be determined as the combined measurement of the distance
314 between the inward points of the grips and the respective lengths of the grips themselves. This definition
315 is specified as follows.
316 L = L1 + 2 × L2 + 2 × L3 + L4     (1)
318 Figure 1 – Schematic view of a specimen with voltage taps installed for critical current
319 measurement
321 L1, L2, L3 >> W             (2)
322 The individual components of the L calculation are defined as follows:
323 • L is the distance between voltage taps;
324 • L2 is the length of the current contact;
325 • L3 is the shortest distance from a current contact to a voltage tap;
326 • L is the length of a voltage tap;
327 • W is the width of the specimen to be measured.
329 The schematic of the specimen length is shown in Figure 1.
330 The dimension L2 may be increased as the specimen's current-carrying capacity increases. This
331 increase becomes more significant when dealing with specimens that have stainless steel substrates
332 or other high-resistivity materials serving as backings or reinforcements. When higher voltage
333 sensitivity is required, L1 may be increased.
334 6.3 Determination of cross-sectional area (S )
335 A micrometer or other dimension-measuring apparatus shall be used to determine the cross-sectional area
336 (S0) of the specimen after removal of the insulation coating. Multiple measurements shall be performed to
337 account for dimensional non-uniformity.
338 The thickness shall be measured using a suitable micrometer equipped with a blade or spherical anvil. This
339 choice of equipment helps reduce errors.
340 7 Testing conditions
341 7.1 Specimen gripping
342 The specimen mounting procedure shall follow IEC 61788-28.
343 The specimen shall be mounted on the upper and lower grip supports at room temperature, then cooled to
344 liquid nitrogen temperature. The specimen shall be firmly clamped using Cu blocks serving as current
345 terminals. Care shall be taken to avoid damage to the superconducting layer of the specimen (see Clause
IEC CDV 61788-29 © IEC 2026
346 A.2).
347 When the specimen is mounted on the grips, the specimen and tensile loading axis shall be aligned along
348 a straight line. Sandpaper may be inserted as a cushioning material between the specimen and the grip
349 support to prevent the gripped surfaces of the specimen from slipping or fracturing. In this case, the
350 specimen and each grip support shall be electrically insulated using suitable insulating materials such as
351 GFRP (G10) spacers. During specimen mounting, bending or deformation shall be prevented (see Clause
352 A.2).
353 7.2 Cooling procedure
354 The system shall be cooled at a controlled rate to prevent damage from thermal stresses. The cool-down
355 procedure shall be documented in the test report (setup, cool-down rate).
356 Pre-loading caused by thermal contraction of the specimen or gripping device shall be avoided. Feedback-
357 controlled fixtures or systems may be used to minimize unintended loading during cool-down. In the case
358 of pre-loading during cool-down, the exerted stress shall be reported.
359 Testing shall commence after the system has reached thermal equilibrium at liquid nitrogen temperature
360 (active boiling around the parts immersed in liquid nitrogen has ceased). The temperature within the cryostat
361 shall be monitored during testing. Add liquid nitrogen if the level drops to near the upper grip of the specimen.
362 7.3 Testing speed
363 After the specimen has been cooled to liquid nitrogen temperature, tensile loading shall be applied in
364 displacement control mode. The tensile load corresponding to the desired stress level shall be maintained
365 constant during I measurement using a stop-and-hold method.
c
366 A crosshead speed between 0,05 mm/min and 2 mm/min is recommended. Different loading rates (e.g., 2
367 and 0,05 mm/min for I measurements below and above the R ) may be used to improve measurement
c T
368 efficiency and stability.
369 7.4 Critical current measurement
370 The Ic measurement procedure shall, in principle, be performed in accordance with IEC 61788-26.
371 I shall be measured after the specimen and loading frame have reached liquid nitrogen temperature.
c
372 When using the constant-sweep-rate method, the sweep rate shall be selected to avoid influencing the
373 voltage measurement.
374 When using the ramp-and-hold method, the current sweep rate between stages shall be lower than the
375 equivalent of ramping from zero current to I in 3 seconds. Data acquisition at each stage shall start as soon
c
376 as the flow or creep voltage generated by the current ramp can be considered negligible. The current drift
377 during each current setpoint shall be less than 1 % of I .
c
378 The U-I characteristic shall be recorded with increasing current at each stress level.
379 After completion of the measurements, the specimen shall be warmed up to room temperature.
380 7.5 Test
381 Following these procedures, the tensile machine shall be started after the testing speed is set to the
382 specified level. The stress applied to the specimen shall be calculated from the output signal of the load
383 cell.
384 The test shall consist of: a) a preliminary test for determining RT; and b) a regular test for determining Rrev
385 or R .
ret
386 Prior to tensile loading, the initial critical current Ic0 of the specimen shall be measured at liquid nitrogen
387 temperature in self-field conditions without mechanical deformation.
IEC CDV 61788-29 © IEC 2026
388 During the preliminary test step, Ic shall be roughly measured while increasing R using a broad stress interval
389 (ΔR), typically between 50 MPa and 100 MPa, depending on the tape structure. The normalized critical
390 current Ic/Ic0 shall be plotted as a function of the applied R. The transition stress (RT) shall be defined as the
391 stress level at which Ic decreases to approximately 95 % to 97% of Ic0, depending on the tape structure.
392 During the regular test, Ic under each R shall be measured using a new specimen taken from the same
393 batch. Below R , I shall be measured using a broad interval ΔR. When the applied R reaches R , I shall
T c T c
394 be measured using a narrow ΔR, typically between 10 MPa and 20 MPa, or 1/5 of the broad ΔR, until
395 electrical failure occurs or irreversible degradation is observed.
396 For the determination of Rrev, unloading shall be performed to approximately 5-10 N after each loading step,
397 and the retained critical current I shall be measured. Following that, as R increases, I measurements shall
cr c
398 be repeated using the loading-unloading scheme until Ic degradation becomes significant and irreversible.
399 If it is necessary to obtain I data over the entire stress range, I may be measured through unloading from
cr cr
400 the beginning of the test.
401 For the determination of R , I shall be measured continuously during loading without unloading procedures.
ret c
402 8 Calculation of results
403 8.1 Irreversible stress limit for I degradation R and R
c rev ret
404 Figure 2 shows representative variations of Ic/Ic0 and Icr/Ic0 during loading-unloading under tensile stresses
405 (R) applied to the specimen at liquid nitrogen temperature. The figure also shows the determination of R
rev
406 and Rret using the 99 % Ic0 recovery criterion and the 95 % Ic0 retention criterion during loading.
407 One curve (marked with ■) is formed by plotting I /I during loading. When I decreases to 97 % I , ΔR
c c0 c c0
408 changes from a broad interval to a narrow interval at RT, and unloading begins. The other curve (marked
409 with □) is formed by plotting I /I measured in the unloaded state against the R just before unloading.
cr c0
410 When measuring Ic during the loading-unloading scheme, if Icr after unloading remains above 99 % Ic0, the
411 I degradation is considered reversibly recovered. If the applied stress is further increased by the narrow
c
412 ΔR (10 MPa - 20 MPa), then Ic is measured again after repeating the loading-unloading scheme.
413 If I after unloading falls below 99 % I , I is no longer reversibly recovered, and the specimen exhibits
cr c0 c
414 irreversible degradation due to damage or cracks induced in the superconducting layer. The tensile stress
415 just before
IEC CDV 61788-29 © IEC 2026
1.10
1.05
R
rev
99 % I recovery
c0
1.00
0.95
95 % I retention
c0
R
T
0.90 R
ret
0.85
0.80
REBCO Tape
I /I
0.75 c c0
I /I
cr c0
0.70
0 200 400 600 800 1000
Uniaxial tensile stress (MPa)
417 Figure 2 – Schematic variations of I /I and I /I during loading-unloading under uniaxial
c c0 cr c0
418 tensile stress applied to HTS tape specimens at liquid nitrogen temperature, showing
419 determination of R and R using 99 % I recovery criterion and 95 % I retention
rev ret c0 c0
420 criterion during loading
422 unloading is defined as the irreversible stress limit for Ic degradation (Rrev) using the 99 % Ic0 recovery
423 criterion, as shown in Figure 2. As for the validity or practical applicability of the 99 % I recovery criterion,
c0
424 its effect on the measured quantity Rrev has been examined in the international round-robin test and is
425 described in Clause A.5.
427 Instead of measuring the reversible recovery behavior of Ic in superconductors using a repeated loading-
428 unloading process, an alternative practical procedure may be applied, even after changing to a narrow ΔR.
429 In this case, I is continuously measured while increasing the applied stress without unloading, and the
c
430 stress at which the measured Ic reaches 95 % Ic0 shall be defined as the irreversible stress limit for Ic
431 degradation (Rret), using the 95 % Ic0 retention criterion during loading, as shown in Figure 2. The 95 % Ic0
432 retention criterion refers to the stress at which Ic degrades 5% from its Ic0 due to either tensile or compressive
433 loading.
435 9 Uncertainty of measurand
436 The specimen shall be immersed in the cooling liquid during testing. The temperature difference between
437 the specimen and the cooling medium shall be minimized. Unless otherwise specified, measurements
438 shall be carried out in a liquid nitrogen bath whose temperature is maintained between 76,8 K and 77,7 K.
439 A force measuring cell with a relative standard uncertainty less than 0,1 %, valid between zero and the
440 maximum force value, shall be used.
441 According to the IEC 61788-26, the relative standard uncertainty of I measurement shall be less than 3 %.
c
442 The relative standard uncertainty (RSU) of this method may be estimated from the coefficient of variation
443 divided by the square root of the number of repeated measurements. The RSU corresponding to the number
I /I , I /I
c c0 cr c0
IEC CDV 61788-29 © IEC 2026
444 of specimens tested for the measured irreversible stress limits, Rrev and Rret, shall be calculated using
445 formula (3):
446 𝑋 (𝑁) = 𝑋 ⁄ 𝑁    (3)
√
RSU COV
447 where
448 X (N) is the value of relative standard uncertainty;
RSU
449 N is the number of specimens tested;
450 X is the value of the averaged coefficient of variation for all data tested.
COV
452 According to the international round-robin test, the relative standard uncertainty of R and R is less than
rev ret
453 3 % (see Annex B).
454 10 Test report
455 10.1  Identification of specimen
456 a) name of the manufacturer of the specimen
457 b) classification and/or symbol
458 c) lot number
459 d) cross-sectional area
460 e) cross-sectional shape and dimension of the specimen (thickness and width)
461 10.2 Reporting of test conditions
462 The following values shall be reported.
463 a)  Ic0 values with corresponding electric field criteria
464 b)  specimen temperature and/or ambient pressure
465 c)  cool-down procedure (setup, cool-down rate)
466 Reporting of the following is optional.
467 d) initial length, width, and thickness of the specimen (L, W, t)
468 e)  distance between the voltage taps (voltage tap separation, L1)
469 f)  length of the current contacts (gripped part, L )
470 g)  shortest distance from a current contact to a voltage tap (L3)
471 h)  distance between grips
472 10.3 Results
473 Results of the following experimental data shall be reported.
474 a)  irreversible stress limits: Rrev and/or Rret
475 b) I values measured at each applied tensile stress
c
476 Note: For a routine test, the retained I /or the ratio (I /I ) under one specific stress may be reported. According to the agreement  b
c cr c0
477 etween the parties, the report in a) may be omitted, and only the I value at a specific stress shall be reported.
c
478 Reporting of the following is optional.
479 c)  n-values at each applied stress
480 d)  stress intervals used (ΔR): broad and narrow
481 e)  transition stress (RT)
482 f)  number of repeated loading-unloading cycles until R was reached
rev
IEC CDV 61788-29 © IEC 2026
484 Annex A
485 (Informative)
486 Additional information relating to measurement, apparatus, and calculation
487 A.1 Scope
488 Certain factors can influence the measured quantities. This annex provides additional information regarding
489 such factors. For more detailed guidance and information, users are encouraged to refer to Annex B, which
490 provides additional examples and uncertainty evaluation results for these influential factors.
491 A.2 Gripping of the specimen
492 A weak gripping force results in slippage, but a strong gripping force can break the gripped surface. Care is
493 therefore required when adjusting the gripping force. A cushioning material such as sandpaper may be
494 inserted between the specimen and the GFRP (G10) insulation spacer on each grip support, no
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