Preparation of RRT for Tensile Test on REBCO Tapes at Cryogenic Temperature

Transcription

1 Preparation of RRT for Tensile Test on REBCO Tapes at Cryogenic Temperature Nadezda Bagrets 1, Klaus-Peter Weiss 1, Kozo Osamura 2, Hyung-Seop Shin 3 INSTITUTE FOR THECHNICAL PHYSICS (ITEP) 1 Institute for Technical Physics, KIT, Germany 2 Research Institute for Applied Sciences, Sakyo-ku, Kyoto, Japan 3 Department of Mechanical Design Engineering Andong National University, Andong, Korea KIT Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft

2 Outline! Motivation! Room temperature RRT! Low temperature stress-strain investigations on REBCO wires! Preliminary guideline for low temperature RRT! Conclusions and outlook

3 Motivation! a round robin test is an inter-laboratory test (measurement, analysis, or experiment) performed independently several times! identical samples are tested using identical or similar test procedure! results analysis from different participants allows to conclude about the reproducibility of a test method or process, and about proficiency of participating labs

4 Motivation! For participating labs: Ø self check Ø joint publication Ø international ISO/IEC Standard for cryogenic temperature tensile test for REBCO wires! For tapes producers: Ø test results Ø international ISO/IEC Standard for cryogenic temperature tensile test for REBCO wires! For everybody: Networking

5 Room temperature RRT K. Osamura at al, Supercond. Sci. Technol. (2014) ! Rectangular cross-section with an area of 0.3 mm 2 to 4 mm 2! Stress-strain curves were measured, from that moduli of elasticity E 0 and E U (unloading line after % of strain), and R p0.2-0 and R p0.2-u 0.2% proof strengths were determined! After each test the results are qualified using equation 1-δ< E 0 /E U <1+ δ (δ = 0.3)! The results obtained from all participants are evaluated with statistic tools. Analysis of variance is done for inter- and intralaboratory data sets

6 Low temperature studies in many applications (e.g. high field magnets) a large hoop stress acting as tensile stress applied along of a wire is an issue mostly focused on the effect of strain (stress) on critical current I c the strain(stress) limit below which there is no or negligible (typically less than 5%) degradation of I c is practically important irreversible strain (stress) is intrinsically determined by permanent damages in the conductor, and strongly affected by the mechanical properties of the substrate and stabilizer materials Y. Zhang at al, IEEE Trans. Appl. Supercond. VOL. 26, NO. 4, JUNE

7 Stress-strain studies at 77K! Increase in the thickness of the plated copper stabilizer or with addition of a laminated stabilizer, the elastic moduli of the conductors decreased S. Ochiai at al, IEEE Trans. Appl. Supercond. (2012) R. P. Walsh at al, IEEE Trans. Appl. Supercond. (2012) R. Nisay at al, Progress in Supercond. And Cryogenics (2013) 29-33! For two-component wire a strain can be calculated by the equation: σ =F/A=(F1 + F2)/A=(σ1*A1 + σ2*a2)/(a1 + A2) F1 and F2 are the tensile load on component 1 and 2, A1 and A2 are the cross sectional area of component 1 and 2 σ1 and σ2 are the stress in component 1 and 2. But mechanical properties of a stabilizer made by different techniques are different and different from that of a bulk metal and there is lack of data on bare (no stabilizer) REBCO tapes 2016 Y. Zhang at al, IEEE Trans. Appl. Supercond. VOL. 26, NO. 4, JUNE

8 Preparation of RRT at cryogenic temperatures To be used as guideline, analogous to room temperature test! Scope: This test is used to measure the modulus of elasticity and 0.2 % proof strength. Commercially available REBCO wires with rectangular cross-section with an area of 0.3 mm 2 to 4 mm 2 should be tested! Terms and definitions: tensile stress R, strain A, extensometer gauge length L G, distance between grips L o, modulus of elasticity E, 0.2 % proof strength R p0,2, fracture strength R f, tensile stress R el and strain A el at elastic limit are defined.! Principle: The test consists of straining a test piece within a cryostat system by a tensile force, generally to fracture, for the purpose of determining the mechanical properties at cryogenic temperatures. With single extensometer system, the determination of E U and R p0,2-u is recommended, whereas with double extensometer system all quantities can be defined

9 Preparation of RRT at cryogenic temperatures! Apparatus: The testing machine and the extensometer shall conform to ISO and ISO 9513, respectively. The calibration shall obey ISO 376.! Testing machine: should meet cryogenic requirements (materials, safety).! Extensometers: should be 30 g or less. In order to prevent the influence of heating of the strain gauges on strain signal due to bubbles on the gauge grid, the bridge voltage of the strain gauge system should be lowered to approximately 1 V. As the extensometer calibration factor might vary with temperature the extensometer has to be calibrated at test temperature

10 Preparation of RRT at cryogenic temperatures! Specimen preparation: no bending or pre-loading when the specimen is handled manually. Length of specimen L =2 L g +L GL +2 L x, L x =0.7 L GL! Determination of cross-sectional area (S o ): SCS thickness is not uniform R. P. Walsh at al, IEEE Trans. Appl. Supercond. (2012) Ø For each specimen, the thickness is systematically measured across the width and down the length, and approximately 20 measurements are averaged Ø Image of cross-sectional area is made using microscope and crosssectional area can be determined with a software

11 Preparation of RRT at cryogenic temperatures! Testing procedure Ø Specimen gripping: the specimen and tensile loading axis shall be aligned to be a straight line. Slipping and fracturing of specimen should be avoided, bending or deformation should be prevented (since fracture on gripping is possible, testing to fracture is not mandatory) Ø Setting of extensometer: test specimen should be not deformed due to extensometer s sharp edges, pre-loading of the specimen should be avoided

12 Preparation of RRT at cryogenic temperatures! Cooling procedure! cryogenic liquid can enter freely to surround the gauge s active elements to avoid gas bubbles and associated clip gauge noise slow cool down - avoiding damage due to thermal stresses How fast? e.g. 10 min from RT to 77K H-S. Shin at al, Supercond. Sci. Technol. (2012) ! pre-loading of the specimen during cool down due to thermal contraction of the specimen or the gripping device: 1. thermal contraction of specimen due to cool down can be evaluated separately and then taken into account 2. unloading fixture can be used 3. a force free mode can be activated in the tensile machine for stress free cool down! testing may begin after the system has reached thermal equilibrium at cryogenic temperature. The temperature within the cryostat shall be monitored during the test Nadezda Bagrets

13 Preparation of RRT at cryogenic temperatures! Testing speed The initial strain rate shall be 10 5 /s to 5x10 4 /s during the test using the extensometer. These values can be attained well with a constant machine stroke of mm/min.! Test E 0 : the tensile machine shall be started after the testing speed has been set to the specified level. The strain and stress calculated from the output signals of extensometer and load cell, respectively shall be plotted on the abscissa and ordinate of the diagram. E U : When the total strain has reached a value of approx %, the tensile force shall be reduced by approximately 30 % to 40 %. Then, the load shall be increased again to the previous level and the test shall be continued to the fracture Nadezda Bagrets

14 Preparation of RRT at cryogenic temperatures! Results evaluation Ø Modulus of elasticity (E): both modulus of elasticity E 0 and E U are to be determined. Ø 0.2 % proof strength (R p0,2-0 and R p0,2-u ): are determined from the initial loading part and the unloading/ reloading part of the stress-strain curve

15 Conclusions and outlook! RRT for tensile test on REBCO wires at room temperature is used as baseline for cryogenic temperature RRT on REBCO wires! Preliminary guideline for RRT for tensile test on REBCO wires at cryogenic temperatures was briefly presented! So far 7 Labs (Research Institute for Applied Sciences; Andong National University ; University of Twente; SuperPower Inc.; Siemens AG; KIT CryoMaK, NHMFL), and 2 tape provider (Theva and SuperOX) agreed to participate in RRT! Amount of tests and time schedule to be defined after feedback from all participants

16 Preparation of RRT at cryogenic temperatures! Uncertainty requirements: Ø The temperature shall be reported to an uncertainty of ±0.1 K, measured by means of a pressure sensor or an appropriate temperature sensor. The difference between the specimen temperature and the bath/gas temperature shall be minimized Ø Force measuring cell with the relative standard uncertainty less than 0.1%, valid between zero and the maximum force value shall be used Ø The extensometers should have the relative standard uncertainty of strain less than 0.5 % Ø The displacement measuring transducer used for the calibration should have the relative standard uncertainty less than 0.1 % Ø The relative standard uncertainty values for E and R 0,2p have to be included after summarizing the results of the international RRT

17 Preparation of RRT at cryogenic temperatures! Test reporting Ø Specimen Ø Test conditions a) Name of the manufacturer of the specimen b) Classification and/or symbol c) Lot number Optionally: d) Raw materials and their chemical composition e) Cross-sectional shape and dimension of the wire f) Non-superconductor to superconductor ratio a) Initial strain rate b) Distance between grips c) Manufacturer and model of testing machine d) Manufacturer and model of extensometer e) Gripping method Ø Results a) Test temperature (T) b) Modulus of elasticity E 0 and E U c) 0.2 % proof strengths (R p0,2-0 and R p0,2-u ) Optionally: d) Percentage elongation to fracture (A) derived from the stress-strain curves and the location of the fracture (i. e. within the extensometer or at the grips) e) Tensile strength (R m )

18 Preparation of RRT at cryogenic temperatures! Evaluation of experimental results: Ø After performing the test the total average of N measurands is calculated using q =( 1/N ) j=1 j k=1 k qjk K. Osamura at al, Supercond. Sci. Technol. (2014) Ø F test to be used (as it was done in room temperature RRT) as a tool to investigate the scattering in test results. It allows to conclude if the major scattering source is within laboratory or among laboratories for each characteristic of tapes reported by labs. Symmetric data set is required from each lab for F test

19 Substrate effects on critical current M. Sugano at al, Supercond. Sci. Technol. (2005) S344-S350 Quenching in tapes under tension ( %) is attributed to the discontinuous yielding of the Hastelloy C-276 substrate

20 Stabilizer effects on critical current! With increase of electroplated Cu volume fraction the stress tolerance decreased (due to softer nature of copper) while strain tolerance increased (due to compressive stress to superconducting layer during cool down to 77K) S. Ochiai at al, IEEE Trans. Appl. Supercond. (2012) ! Additional brass stabilizer improves the reversible strain (due to additional compressive stress to superconducting layer during cool down to 77K, and higher fracture toughness of brass making deformation more uniform) H-S. Shin at al, Supercond. Sci. Technol. (2012)