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 supercond[.]
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.
Testing machine
A tensile machine control system that provides a constant cross-head speed shall be used
Grips must be designed with adequate structure and strength to securely hold the test specimen while ensuring a reliable connection to the tensile machine The gripping surfaces should be filed, knurled, or otherwise textured to prevent slippage during testing Gripping mechanisms can be screw-type or actuated through pneumatic or hydraulic means.
This document is copyrighted material licensed to BR Demo by Thomson Reuters (Scientific), Inc It was downloaded from subscriptions.techstreet.com on November 28, 2014, by James Madison Any reproduction or distribution of this material is prohibited, and it remains uncontrolled when printed.
Extensometer
The extensometer must weigh 30 g or less to avoid impacting the mechanical properties of the superconductive wire, and it is essential to prevent any bending moments from being applied to the test specimen.
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.
Length of specimen
The total length of the test specimen must include the inward distance between grips and the lengths of both grips Specifically, the inward distance between the grips should be at least 60 mm to accommodate the installation of the extensometer.
Removing insulation
The insulating coating on the test specimen surface must be removed using either a chemical or mechanical method, ensuring that the specimen surface remains undamaged (refer to Clause A.4).
Determination of cross-sectional area (S o )
To accurately measure the cross-sectional area of a specimen after removing the insulation coating, a micrometer or similar measuring device should be utilized For round wires, the cross-sectional area is calculated by taking the arithmetic mean of two orthogonal diameters In the case of rectangular wires, the area is determined by multiplying the thickness by the width Any necessary corrections for the corners of the cross-sectional area should be established through discussions among the involved parties.
Specimen gripping
The test specimen must be aligned with the tensile loading axis in a straight line when mounted on the grips of the tensile machine To prevent slipping and fracturing of the specimen, sandpaper can be used as a cushioning material on the gripped surfaces (refer to Clause A.6).
Pre-loading and setting of extensometer
To ensure accurate measurements, any slack in the specimen must be addressed by applying a force not exceeding one-tenth of the 0.2% proof strength of the composite before mounting the extensometer It is crucial to avoid deforming the test specimen during the extensometer installation, which should be positioned centrally between the grips, aligning the measurement direction with the specimen's axis Finally, the loading should be zeroed after installation.
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
Test
The tensile machine should be initiated once the cross-head speed is adjusted to the required level The extensometer and load cell signals must be plotted on the x-axis and y-axis, respectively, as illustrated in Figure 1 The process continues until the total strain approaches the specified threshold.
To reduce the force by approximately 10%, decrease it by 2% and consider omitting the removal of the extensometer if it is sufficiently robust to withstand the total strain and fracture shock during the test It is essential to ensure that no unnecessary force is applied to the test specimen during this process.
Increase the load to the previous level and continue testing until the test specimen fractures Measurements should be taken again if any slip or fracture occurs on the gripped surfaces of the specimen.
This article contains copyrighted material licensed to BR Demo by Thomson Reuters (Scientific), Inc It was downloaded on November 28, 2014, by James Madison, and any further reproduction or distribution is prohibited The content is considered uncontrolled when printed.
Line shifted by an offset of 0,2% parallel to the initial loading 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
The slope of the initial loading line is typically less steep than that of the unloading line To determine the 0.2% proof strength of the composite, two lines are drawn from the 0.2% offset point on the abscissa Point A is derived from the initial loading line, while Point B is obtained from the unloading line Additionally, Point C represents the second type of 0.2% proof strength of the composite, which occurs when the Nb-Ti component yields.
Figure 1 – Stress-strain curve and definition of modulus of elasticity and 0,2 % proof strengths
Tensile strength (R m )
Tensile strength R m shall be the maximum force divided by the original cross-sectional area of the wire before loading
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 p0,2A shall be determined as follows: the initial linear portion under loading of the stress-strain curve is moved 0,2 % in the strain axis
The 0.2% proof strength under loading is defined at point A, where a 0.2% offset line intersects the stress-strain curve To determine the 0.2% proof strength under unloading, denoted as R_{p0.2B}, the linear portion of the unloading curve is shifted parallel to the 0.2% offset strain point The intersection of this adjusted line with the stress-strain curve identifies point B, which represents the 0.2% proof strength However, this measurement is invalidated if the 0.2% proof strength of the composite is less than three times the pre-load specified in section 7.2.
Each 0,2 % proof strength shall be calculated using formula (1) given below:
R p0,2i is the 0,2 % proof strength (MPa) at each point;
F i is the force (N) at each point;
S o is the original cross-sectional area (in square millimetres) of the test specimen;
Modulus of elasticity (E o and E 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 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 a is the strain just after unloading as shown in Figure 1
E is designated as E o when using the initial loading curve (ε a = 0), and as E a when using the unloading curve (ε a ≠ 0)
Unless otherwise specified, measurements shall be carried in a temperature range between
For accurate measurements, a force measuring cell and an extensometer must have a combined standard uncertainty of no greater than 0.5% Additionally, the dimension-measuring apparatus should maintain a combined standard uncertainty of no more than 0.1% These target combined standard uncertainties are determined using the root square sum (RSS) procedure outlined in Annex B.
This article contains copyrighted material licensed to BR Demo by Thomson Reuters (Scientific), Inc It was downloaded on November 28, 2014, by James Madison, and any further reproduction or distribution is prohibited The content is uncontrolled when printed.
There is a lack of reliable experimental data regarding the uncertainties associated with the moduli of elasticity and 0.2% proof strengths as outlined in Clause A.7 However, as detailed in Annex C, these uncertainties can be assessed based on the experimental conditions, including factors such as the uncertainty of the force measuring cell Therefore, the relative expanded uncertainties (k=2) for the modulus of elasticity, \(E_o\), and the 0.2% proof strength can be evaluated.
R p0,2A , are expected to be 2,0 % (N=1) and 0,78 % (N=1), respectively, where N indicates the time of repeated tests
It is essential to approach the uncertainties presented in this document with careful consideration, as outlined in Annex B, especially when applying them for practical assessments.
Specimen
a) Name of the manufacturer of the specimen b) Classification and/or symbol c) Lot number
The necessary information to be reported includes the raw materials and their chemical composition, the cross-sectional shape and dimensions of the wire, the filament diameter, the number of filaments, the twist pitch of the filaments, and the copper to superconductor ratio.
Results
a) Tensile strength (R m ) b) 0,2 % proof strengths (R p0,2A and R p0,2B ) c) Modulus of elasticity (E o and E a with ε 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)
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
Additional information relating to Clauses 1 to 10
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
In Cu/NbTi superconductive wires there is a difference in strength between the copper and
NbTi wire often experiences wave-like deformation due to fracture shock, making accurate elongation measurement challenging when using the butt method Therefore, elongation measurements post-fracture should be considered as reference values only An alternative approach involves using the cross-head movement to estimate elongation after fracture, requiring the recording of the cross-head position at the moment of fracture The elongation after fracture can then be calculated using a specific formula expressed in percentage.
A is the percentage elongation after fracture;
L c is the initial distance between cross-heads;
L u is the distance between cross-heads after fracture
A.3 Second type of 0,2 % proof strength ( R p0,2C )
The 0.2% proof strength of the Nb-Ti component is determined using the rule-of-mixture for bimetallic composites with continuous filaments As shown in Figure 1, this strength corresponds to the stress \( R_{p0.2C} \) at point C, where the loading curve's straight portion after point A intersects the stress-strain curve after a 0.2% strain shift This linear behavior is typically observed in commercial Cu/Nb-Ti superconductive wires, as the copper component exhibits plastic deformation However, for wires with a high copper/non-copper ratio and significant cold work, the stress-strain curve may appear rounded rather than linear This rounded appearance is empirically linked to a k-factor of less than 0.4, defined as \( k = \frac{(R_m - R_{p0.2A})}{R_{p0.2A}} \).
The R p0,2C is a crucial parameter for characterizing the mechanical properties of composite materials from a scientific perspective, although its application is not always necessary in engineering contexts.
Using a specialized extensometer with a fixed spacer for gauge length measurement can lead to issues when unloading the wire to zero force To prevent compressive force on the spacer, it is essential to accurately determine the actual gauge length.
When installing equipment, it is crucial to ensure that there is adequate clearance If the clearance observed after unloading is significant, it should be factored into the calculations for strain values.
To prevent specimen yielding due to bending moments from a heavy extensometer, it is essential to use a lightweight extensometer with a balance weight or a sufficiently light one without a balance weight For instance, an extensometer made from a titanium alloy weighing approximately 3 g can be used without a balance weight while still adhering to standard procedures Additionally, one of the lightest commercially available extensometers, weighing 31 g with a balance weight, was utilized in a round robin test (RRT) in Japan, yielding positive results that contributed to the establishment of the current international standard.
Figure A.1 – An example of the light extensometer, where R1 and R3 indicate the corner radius
Bar spring a) Top view b) Side view
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
The superconductive composite wire, which is encased in a soft copper layer, is susceptible to fractures from surface scratches Consequently, it is essential to handle the specimen with care to prevent damage.
To ensure the integrity of the test specimen, it is essential to remove the coating using a suitable organic solvent that does not harm the specimen If the solvent fails to dissolve the coating, a careful mechanical method should be employed to avoid damaging the copper While the presence of the coating may slightly reduce strength, such as a less than 3% decrease in tensile strength for a low-strength wire with a high copper ratio of 7, it is important to note that the coating is not intended to serve as a structural component.
The analysis of measurement as a three-component composite, including copper, Nb-Ti, and insulating coating, is overly complex To reduce uncertainty, this test method focuses on a bare wire.
To achieve lower uncertainty in measuring the cross-sectional area, it is essential to correct the radius of the corner of rectangular wire as specified in the manufacturing guidelines In cases of rolling or Turk's-head finishes, where the corner radius is not regulated, corrections are made by analyzing a microphotograph of the cross-section.
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
In 1996, the Japanese National Committee of IEC TC90 completed the domestic RRT with contributions from eight research groups to assess the coefficient of variation of experimental data on moduli of elasticity and 0.2% proof strengths However, the original data was insufficient to determine uncertainties, making it impossible to deduce them at this time The only method to evaluate uncertainty is through numerical computation based on type B statistics, as outlined in Annex C and detailed in Clause 9 of the main text.
The study reveals that the modulus of elasticity during loading (Eo) is consistently lower than that during unloading (Ea) This discrepancy is attributed to several handling issues, including wire specimen bending, misalignment of sample gripping relative to the load axis, and inadequate grip strength Additionally, it is noted that the copper component may be in a plastic state at room temperature prior to testing, influenced by the degree of thermal contraction during cooling from heat treatment Overall, the initial non-linear loading curve contributes to the observed result of Eo being less than Ea.
The German National Committee of IEC TC90 found that the modulus of elasticity can be accurately determined with minimal uncertainty by applying an initial linear loading at zero-offset This low uncertainty was attained through the use of two light extensometers, which effectively eliminated potential initial bending effects and maintained a high level of linearity in the zero-offset loading line.
When handling specimens, it is crucial to avoid inducing strain on the copper component, as this can lead to an increase in the 0.2% proof strength of the composite due to work hardening Therefore, it is important to consider the allowable pre-loading limit in this context.
The 0.2% proof strength R p0.2C is a reference value characterized by the lowest uncertainty It is essential to verify the presence of a straight segment in the stress-strain curve beyond point A, as illustrated in Figure 1.
[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