- Email: [email protected]
Indentation effects on strain sensitivity of critical current for internal-tin Nb3Sn strand
Fusion Engineering and Design 137 (2018) 373–377
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Indentation effects on strain sensitivity of critical current for internal-tin Nb3Sn strand
T
⁎
Fang Liua, Fangyi Lia,b, Huajun Liua, , Chao Daia, Yu Wua, Yi Shia, Hongjun Maa,b, Zhehua Maoa,b, Yanyan Zhangc, Huan Jina, Jinggang Qina, Chao Zhoud a
The Institute of Plasma Physics of Chinese Academy of Sciences, P.O. Box 1123, Hefei, Anhui, 230031, PR China University of Science and Technology of China, Hefei, 230026, PR China c Zhejiang Jiuli Hi-Tech Metals Company Ltd, Huzhou, 313012, PR China d University of Twente, Netherlands b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nb3Sn Indentation Critical current Mechanical
The cable-in-conduit conductor (CICC) technology has been widely used in large scale superconducting magnet. The Short-Twist-Pitch (STP) design was aimed to avoid the conductor degradation during electromagnetic and thermal cycling. But manufacture of STP CICC deforms the cabled strands significantly. Indentations appear on superconducting strands after cabling and compaction. The conductor used for China Fusion Engineering Test Reactor (CFETR) magnet will be subjected to much higher Lorentz force than ITER. The STP CICC will be the first choice for the conductor design, since it showed no observed current sharing temperature Tcs degradation after electromagnetic cycling. In order to estimate the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, series measurements have been done on an internal-tin (IT) Nb3Sn strand, fabricated by Western Superconducting Technologies Co., Ltd (WST). The strand was artificially indented and cut to several sections for different kinds of measurements, which mainly included the critical current Ic versus axial strain measurements with Pacman device and stress-strain tests. The results show that the indentation has less impact on Ic, but more impact on mechanical performance. The sample preparation, test results and analysis have been described in details.
1. Introduction The CICC technology was widely used in large scale superconducting magnet system, i.e. ITER magnets, CFETR magnets, etc. Irreversible degradation has been found on the Nb3Sn CICCs for ITER magnets upon cyclic load and thermal cycles [1–3]. Then, the conductor structure was changed to STP structure, which exhibits high cable stiffness and fatigue behavior, limits strand movement thus to have high transport properties [4,5]. But the STP CICC causes severe strand deformation. Manufacture of CICC cable will have a risk to deform the strands because of the short twist pitch and tight cabling before the heat treatment [6,7]. Indentations on superconducting strands after cabling and compaction have been found on both NbTi strands and Nb3Sn strands from ITER conductors. In future, the conductor used for CFETR will endure much higher Lorentz force than ITER, e.g. 1200 kN/m [8]. In order to avoid the conductor performance degradation, the STP would be the first choice. But, the indentations on strands caused by conductor
⁎
manufacture cannot be avoided. To investigate the indented strand performance is essential for conductor design and performance analysis. Researches have been carried out on the performance of superconducting strands (including NbTi, Nb3Sn and Bi-2212 strands) with indentation. The results show that, the indentation has no effects on the critical current for NbTi; indentation exceeding deformation threshold has impact on the critical current and RRR for Nb3Sn strand; and the performance of Bi-2212 is most sensitive of indentation [9–11]. However, no research has been done on the dependence of Ic of an indented superconducting strand on the axial strain. The superconducting strands in CICC are subjected to large electromagnetic force and residual thermal stress during operation. Although the critical current or RRR are not degraded for the strand samples, the conductor performance might be degraded further due to the worse mechanical properties of the strands with indentation. Therefore, the influence of the indentation on mechanical properties and Ic characterization with axial strain for a Nb3Sn strand has been investigated
Corresponding author. E-mail address: [email protected] (H. Liu).
https://doi.org/10.1016/j.fusengdes.2018.10.017 Received 12 July 2018; Received in revised form 11 October 2018; Accepted 15 October 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Transverse cross-section of the IT Nb3Sn strand.
to determine the threshold of indentation depth, thus to provide essential information for cabling design. The strand samples, sample preparation, test procedure and test results are reported in the paper. Fig. 2. Indentation on the Nb3Sn strands: (a) definition and measurement for the indentation; (b) typical indentation on the strands of the cable after compaction and jacketing.
2. Samples and preparation The IT Nb3Sn strand with diameter of 0.82 mm, used for CFETR Central Solenoid Model Coil (CSMC) and produced by WST, was chosen for investigating the influence of indentation on comprehensive performance of the strand. Fig. 1 shows the cross-section image of the strand. Before the heat treatment, several samples were cut from the same strand piece for all the tests (including Ic, RRR, stress-strain and Ic as a function of strain test). All the samples were deformed with different indentation depth (including 0.15 mm, 0.25 mm and 0.35 mm) distribution before heat treatment. The indentation was marked with one anvil with a rounded end. The depth of indentation was investigated by micrometer and an auxiliary wire [8]. The depth of the indentation was defined by (d1+d2)−d, where d1 was the original diameter of the sample strand, d2 was the diameter of the auxiliary strand and d was the measured dimension, as shown in Fig. 2(a). An Nb3Sn cable with indentation on the strands after compaction and jacketing was shown in Fig. 2(b). For Ic test, the ITER standard barrel was employed [12]. The indented strand was wound onto the ITER barrel made of Ti-6Al-4 V. The indented part on the strand was centered on the barrel and the indentation was on the outside of the strand. The plated chromium in the two end part (which will be soldered on the Cu ring of the barrel) before Ic measurement was removed before winding [12,13].The distance between voltage taps was 250 mm. For RRR test, several straight strand sections with different indentation were heat treated together with the Ic samples. After heattreatment, samples were cut to around 5 cm with different indentation on the middle part for RRR test. For Ic(strain) characteristics measurements, the sample holders made of Ti-6Al-4 V were used to pre-shape and fix the Nb3Sn sample during heat-treatment in order to make the sample fit exactly on the test spring. The indentation with different depth was on the middle part of the sample and on the outside of the strand sample. After the heat treatment the sample was transferred to the Pacman spring and fixed with Sn-5 w. % Ag solder [14,15]. The schematic graph for sample instrumentation was shown in Fig. 3. Several reacted straight strand sections with length of around 15 cm
Fig. 3. Pacman sample instrumentation for Ic vs. strain characteristics with indentation on the outside.
have been used as the mechanical properties tests samples, which have different indentation depths on the middle part along the length. For the tests, a load cell with range of 2000 N was used to measure the tensile force of the sample wire, and a pair of extensometers was used to measure the elongation of the strand sample, as shown in Fig. 4. 3. Experimental To investigate the influence of indentation on comprehensive performance of the strand, Ic, RRR, Ic(strain) characteristics and mechanical properties were measured on reacted WST IT strand samples with 374
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Fig. 5. Ic and RRR versus indentation depth.
maximum compressive strain and then to tensile strain step-by-step with steps no larger than 0.1%. When the strain was around 0 or the Ic near to the maximum value, the applied strain step became smaller and less than 0.05%. The critical current as a function of intrinsic strain at 12 T and 4.2 K for all the samples were shown in Fig. 6. The intrinsic strain is equal to the applied strain minus the thermal pre-compressive strain [15]. Normalized Ic as a function of intrinsic strain were summarized in Fig. 7, which is defined as Ic / Ic (intrinsic strain = 0). It is obviously that when the indentation depth becomes larger, the critical current becomes more sensitive to the strain. For the strand with indentation depth of 0.35 mm, the irreversible strain limit, which was corresponding to the permanent reduction of the critical current [16,17], was around 0.25% while for all the other samples those were between 0.25% and 0.3%. It was difficult to get accurate irreversibility strain limit since the strain step was around 0.05%.
Fig. 4. Sample instrumentation for mechanical properties test with indention near the center part.
indentation of different depths. Ic measurements were performed at 12 T and 4.2 K. The critical criterion 10 μV/m was used. The RRR was obtained by the results of the resistance at 273 K divided by the resistance at 20 K. The resistances at 273 K were measured in ice-water mixture. And the measurement at 20 K was controlled by adjusting the height between the sample and the Liquid Helium surface. Ic(axial strain) characteristics for the Nb3Sn samples were tested by the Pacman device. As the same as Ic barrel measurements, the critical criterion 10 μV/m was used for Ic determination and the n value was determined from a power law fit in the range of 10 μV/m to 100 μV/m. All the measurements were carried out at 12 T. And the samples were immersed in Liquid Helium during the measurements. In order to compare the results for different samples, the sequences of strain applications were similar for all the samples. For mechanical properties tests, the reacted samples with different indentation were only measured at liquid helium temperature for comparison.
4.3. Mechanical properties Samples with different indentation depth were tested using the stress-strain testing device. The results showed that the elastic Modulus (EM) for each sample with different indentation was close while the yield strength (YS) (σ0.2) was different, as shown in Fig. 8. Those might be caused by the strand cross-section changes after intended or Bauschinger Effect of the Copper. The strand cross section changes have been ignored and the original cross section was used for stress calculation to estimate the indentation effect directly.
4. Results and discussion 4.1. Ic and RRR The Ic for the barrel strand sample without indentation was 261 A at 12 T and 4.2 K. The indentation depth resulting to strand critical current with 95% of the Ic without indentation is defined as critical depth dc. The dc of the strand was around 0.35 mm according to the Ic results. As shown in Fig. 5, Ic was almost constant with the indentation that is less than dc, but Ic was degraded drastically with the indentation exceeding dc. The RRR was 182 for the strand sample without indentation. The degradation of RRR started from the indentation depth of 0.25 mm which is less than dc from the critical current. The RRR is more sensitive than the Ic to the indentation. From cross-sectional observation, the RRR degradation was caused by breakage of the barrier layer, which has the similar performance from the reference [11]. 4.2. Ic(strain) characteristics Samples with different indentation depths (including 0, 0.15 mm, 0.25 mm, 0.3 mm and 0.35 mm) were tested for Ic(strain) characterization by Pacman device. The applied strain sequence was from 0 to
Fig. 6. Ic as a function of strain for the samples with different indentation depth. 375
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Fig. 7. Normalized Ic as a function of strain for all the samples with different indentation depth.
Fig. 10. Ultimate stress as a function of indentation depth for the IT strand.
intrinsic strain of the stress-strain sample as about -0.2%, which was mainly caused by the thermal residual stress [18]. According to the Ic(strain) characteristics and mechanical properties of the strand samples with different indentation depths, the calculated normalized critical current as a function of axial stress is shown in Fig. 9. It was shown that after the sample yielded, the Ic decrease became much sensitive to the indentation depth. For example, when the applied stress was around 200 MPa, the Ic degraded to be around 98% with depth of 0.15 mm while that to be around 89% with depth of 0.25 mm and around 72% with depth of 0.35 mm, respectively. And the ultimate tensile strength (UTS) as a function of indentation depth was shown in Fig. 10. The UTS is much sensitive to the indentation depth, and reduces continuously along with the enlarging indentation depth. 5. Conclusion Aiming for investigating the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, systematic measurements have been done on a Nb3Sn IT strand. The strand was artificially indented for different measurements. It was found that RRR degradation was caused by the breakage of barrier layer with indentation. And Ic with ITER barrel decreased rapidly when the indentation depth larger than around 0.35 mm for the measured sample. The results showed that RRR is more sensitive to indentation depth compared to Ic. Furthermore, samples with different artificial indentation were measured by Pamcan device and stress-strain test setup to analyze the influence of indentation on mechanical properties and Ic (strain) characteristics for the strand. The results show that the critical current becomes more sensitive to the strain when the indentation depth gets larger. And from the mechanical tests results, the indentation depths mainly effect on the UTS and YS. As a result, the conductor manufacturing process should limit the deformation of Nb3Sn strands as less as possible for high CICC performance. The less indentation, the better the strand performance combined the mechanical properties and superconducting performance. The results provide solid basis for large CICC design for CFETR and DEMO magnet.
Fig. 8. Stress-strain curves for the samples with indentation depth of 0, 0.15 mm, 0.25 mm and 0.35 mm, respectively.
Fig. 9. The normalized critical current as a function of axial stress for the samples with indentation depth of 0, 0.15 mm, 0.25 mm and 0.35 mm, respectively.
Acknowledgements This research is conducted under the support of national natural Science foundation of China (Grant No. 51477172 and 51677184) and National Magnetic Confinement Fusion Science Program (No.
In order to estimate the effects of mechanical properties on the critical current of the strand, it is reasonable to assume the initial 376
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2014GB105004).
2960–2966. [9] J.G. Qin, C. Dai, Q. Wang, et al., Impact of Indentation on the Critical Current of Bi2212 Round Wire, IEEE Trans. Appl. Supercond. 26 (4) (2016) 1–5. [10] S. Tomone, N. Yoshihiro, T. Yoshikazu, et al., Influence of indentation on the critical current of Nb3Sn strands, Phys. Procedia 67 (2015) 908–913. [11] J. Qin, W. Yu, J. Li, et al., New design of Cable-in-Conduit Conductor for application in future fusion reactors, Supercond. Sci. Technol. 30 (2017) 11. [12] F. Liu, L. Feng, B. Liu, et al., Implementation and analysis of ITER strand test of CNDA for world-wide benchmarking, Fusion Eng. Des. 88 (1) (2013) 17–22. [13] F. Liu, H. Liu, S. Liu, et al., Nb3Sn Strand Verification for ITER TF Conductors of CNDA, Fusion Sci. Technol. 66 (1) (2014) 208–213. [14] A. Godeke, M. Dhalle, A. Morelli, et al., A device to investigate the axial strain dependence of the critical current density in superconductors, Rev. Sci. Instrum. 75 (12) (2004) 5112–5118. [15] F. Liu, A. Nijhuis, H.J.G. Krooshoop, et al., Comparison of Critical Current Versus Axial Strain Measurements on Internal Tin Nb3Sn Strand at ASIPP and University of Twente, IEEE Trans. Appl. Supercond. 25 (3) (2015) 1–4. [16] L.F. Goodrich, N. Cheggour, X.F. Lu, et al., Method for determining the irreversible strain limit of Nb3Sn wires, Supercond. Sci. Technol. 24 (7) (2011) 2099–2117. [17] K. Ilin, K.A. Yagotintsev, C. Zhou, et al., Experiments and FE modeling of stressstrain state in ReBCO tape under tensile, torsional and transverse load, Supercond. Sci. Technol. 28 (5) (2015) 263–269. [18] N. Mitchell, Finite element simulations of elasto-plastic processes in Nb3Sn strands, Cryogenics 45 (7) (2005) 501–515.
References [1] A. Devred, I. Backbier, D. Bessette, et al., Challenges and status of ITER conductor production, Supercond. Sci. Technol. 27 (4) (2014) 044001. [2] D. Ciazynski, Review of Nb3Sn conductors for ITER, Fusion Eng. Des. 82 (5–14) (2007) 488–497. [3] P. Bruzzone, B. Stepanov, R. Wesche, et al., Results of a New Generation of ITER TF Conductor Samples in SULTAN, IEEE Trans. Appl. Supercond. 18 (2) (2008) 459–462. [4] N. Mitchell, A. Devred, D.C. Larbalestier, et al., Reversible and irreversible mechanical effects in real cable-in-conduit conductors, Supercond. Sci. Technol. 26 (11) (2013) 114004. [5] D. Bessette, Design of a Nb3Sn cable-in-conduit conductor to withstand the 60000 electromagnetic cycles of the ITER central solenoid, IEEE Trans. Appl. Supercond. 24 (3) (2013) 1–5. [6] Y. Takahashi, Y. Nabara, H. Ozeki, et al., Cabling technology of Nb3Sn, conductor for ITER central solenoid, IEEE Trans. Appl. Supercond. 24 (3) (2014) 1–4. [7] J. Qin, C. Dai, B. Liu, et al., Optimization of CFETR CSMC cabling based on numerical modeling and experiments, Supercond. Sci. Technol. 28 (12) (2015) 125008. [8] J. Zheng, X. Liu, Y. Song, et al., Concept design of CFETR superconducting magnet system based on different maintenance ports, Fusion Eng. Des. 88 (11) (2013)
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Indentation effects on strain sensitivity of critical current for internal-tin Nb3Sn strand
T
⁎
Fang Liua, Fangyi Lia,b, Huajun Liua, , Chao Daia, Yu Wua, Yi Shia, Hongjun Maa,b, Zhehua Maoa,b, Yanyan Zhangc, Huan Jina, Jinggang Qina, Chao Zhoud a
The Institute of Plasma Physics of Chinese Academy of Sciences, P.O. Box 1123, Hefei, Anhui, 230031, PR China University of Science and Technology of China, Hefei, 230026, PR China c Zhejiang Jiuli Hi-Tech Metals Company Ltd, Huzhou, 313012, PR China d University of Twente, Netherlands b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nb3Sn Indentation Critical current Mechanical
The cable-in-conduit conductor (CICC) technology has been widely used in large scale superconducting magnet. The Short-Twist-Pitch (STP) design was aimed to avoid the conductor degradation during electromagnetic and thermal cycling. But manufacture of STP CICC deforms the cabled strands significantly. Indentations appear on superconducting strands after cabling and compaction. The conductor used for China Fusion Engineering Test Reactor (CFETR) magnet will be subjected to much higher Lorentz force than ITER. The STP CICC will be the first choice for the conductor design, since it showed no observed current sharing temperature Tcs degradation after electromagnetic cycling. In order to estimate the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, series measurements have been done on an internal-tin (IT) Nb3Sn strand, fabricated by Western Superconducting Technologies Co., Ltd (WST). The strand was artificially indented and cut to several sections for different kinds of measurements, which mainly included the critical current Ic versus axial strain measurements with Pacman device and stress-strain tests. The results show that the indentation has less impact on Ic, but more impact on mechanical performance. The sample preparation, test results and analysis have been described in details.
1. Introduction The CICC technology was widely used in large scale superconducting magnet system, i.e. ITER magnets, CFETR magnets, etc. Irreversible degradation has been found on the Nb3Sn CICCs for ITER magnets upon cyclic load and thermal cycles [1–3]. Then, the conductor structure was changed to STP structure, which exhibits high cable stiffness and fatigue behavior, limits strand movement thus to have high transport properties [4,5]. But the STP CICC causes severe strand deformation. Manufacture of CICC cable will have a risk to deform the strands because of the short twist pitch and tight cabling before the heat treatment [6,7]. Indentations on superconducting strands after cabling and compaction have been found on both NbTi strands and Nb3Sn strands from ITER conductors. In future, the conductor used for CFETR will endure much higher Lorentz force than ITER, e.g. 1200 kN/m [8]. In order to avoid the conductor performance degradation, the STP would be the first choice. But, the indentations on strands caused by conductor
⁎
manufacture cannot be avoided. To investigate the indented strand performance is essential for conductor design and performance analysis. Researches have been carried out on the performance of superconducting strands (including NbTi, Nb3Sn and Bi-2212 strands) with indentation. The results show that, the indentation has no effects on the critical current for NbTi; indentation exceeding deformation threshold has impact on the critical current and RRR for Nb3Sn strand; and the performance of Bi-2212 is most sensitive of indentation [9–11]. However, no research has been done on the dependence of Ic of an indented superconducting strand on the axial strain. The superconducting strands in CICC are subjected to large electromagnetic force and residual thermal stress during operation. Although the critical current or RRR are not degraded for the strand samples, the conductor performance might be degraded further due to the worse mechanical properties of the strands with indentation. Therefore, the influence of the indentation on mechanical properties and Ic characterization with axial strain for a Nb3Sn strand has been investigated
Corresponding author. E-mail address: [email protected] (H. Liu).
https://doi.org/10.1016/j.fusengdes.2018.10.017 Received 12 July 2018; Received in revised form 11 October 2018; Accepted 15 October 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Transverse cross-section of the IT Nb3Sn strand.
to determine the threshold of indentation depth, thus to provide essential information for cabling design. The strand samples, sample preparation, test procedure and test results are reported in the paper. Fig. 2. Indentation on the Nb3Sn strands: (a) definition and measurement for the indentation; (b) typical indentation on the strands of the cable after compaction and jacketing.
2. Samples and preparation The IT Nb3Sn strand with diameter of 0.82 mm, used for CFETR Central Solenoid Model Coil (CSMC) and produced by WST, was chosen for investigating the influence of indentation on comprehensive performance of the strand. Fig. 1 shows the cross-section image of the strand. Before the heat treatment, several samples were cut from the same strand piece for all the tests (including Ic, RRR, stress-strain and Ic as a function of strain test). All the samples were deformed with different indentation depth (including 0.15 mm, 0.25 mm and 0.35 mm) distribution before heat treatment. The indentation was marked with one anvil with a rounded end. The depth of indentation was investigated by micrometer and an auxiliary wire [8]. The depth of the indentation was defined by (d1+d2)−d, where d1 was the original diameter of the sample strand, d2 was the diameter of the auxiliary strand and d was the measured dimension, as shown in Fig. 2(a). An Nb3Sn cable with indentation on the strands after compaction and jacketing was shown in Fig. 2(b). For Ic test, the ITER standard barrel was employed [12]. The indented strand was wound onto the ITER barrel made of Ti-6Al-4 V. The indented part on the strand was centered on the barrel and the indentation was on the outside of the strand. The plated chromium in the two end part (which will be soldered on the Cu ring of the barrel) before Ic measurement was removed before winding [12,13].The distance between voltage taps was 250 mm. For RRR test, several straight strand sections with different indentation were heat treated together with the Ic samples. After heattreatment, samples were cut to around 5 cm with different indentation on the middle part for RRR test. For Ic(strain) characteristics measurements, the sample holders made of Ti-6Al-4 V were used to pre-shape and fix the Nb3Sn sample during heat-treatment in order to make the sample fit exactly on the test spring. The indentation with different depth was on the middle part of the sample and on the outside of the strand sample. After the heat treatment the sample was transferred to the Pacman spring and fixed with Sn-5 w. % Ag solder [14,15]. The schematic graph for sample instrumentation was shown in Fig. 3. Several reacted straight strand sections with length of around 15 cm
Fig. 3. Pacman sample instrumentation for Ic vs. strain characteristics with indentation on the outside.
have been used as the mechanical properties tests samples, which have different indentation depths on the middle part along the length. For the tests, a load cell with range of 2000 N was used to measure the tensile force of the sample wire, and a pair of extensometers was used to measure the elongation of the strand sample, as shown in Fig. 4. 3. Experimental To investigate the influence of indentation on comprehensive performance of the strand, Ic, RRR, Ic(strain) characteristics and mechanical properties were measured on reacted WST IT strand samples with 374
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Fig. 5. Ic and RRR versus indentation depth.
maximum compressive strain and then to tensile strain step-by-step with steps no larger than 0.1%. When the strain was around 0 or the Ic near to the maximum value, the applied strain step became smaller and less than 0.05%. The critical current as a function of intrinsic strain at 12 T and 4.2 K for all the samples were shown in Fig. 6. The intrinsic strain is equal to the applied strain minus the thermal pre-compressive strain [15]. Normalized Ic as a function of intrinsic strain were summarized in Fig. 7, which is defined as Ic / Ic (intrinsic strain = 0). It is obviously that when the indentation depth becomes larger, the critical current becomes more sensitive to the strain. For the strand with indentation depth of 0.35 mm, the irreversible strain limit, which was corresponding to the permanent reduction of the critical current [16,17], was around 0.25% while for all the other samples those were between 0.25% and 0.3%. It was difficult to get accurate irreversibility strain limit since the strain step was around 0.05%.
Fig. 4. Sample instrumentation for mechanical properties test with indention near the center part.
indentation of different depths. Ic measurements were performed at 12 T and 4.2 K. The critical criterion 10 μV/m was used. The RRR was obtained by the results of the resistance at 273 K divided by the resistance at 20 K. The resistances at 273 K were measured in ice-water mixture. And the measurement at 20 K was controlled by adjusting the height between the sample and the Liquid Helium surface. Ic(axial strain) characteristics for the Nb3Sn samples were tested by the Pacman device. As the same as Ic barrel measurements, the critical criterion 10 μV/m was used for Ic determination and the n value was determined from a power law fit in the range of 10 μV/m to 100 μV/m. All the measurements were carried out at 12 T. And the samples were immersed in Liquid Helium during the measurements. In order to compare the results for different samples, the sequences of strain applications were similar for all the samples. For mechanical properties tests, the reacted samples with different indentation were only measured at liquid helium temperature for comparison.
4.3. Mechanical properties Samples with different indentation depth were tested using the stress-strain testing device. The results showed that the elastic Modulus (EM) for each sample with different indentation was close while the yield strength (YS) (σ0.2) was different, as shown in Fig. 8. Those might be caused by the strand cross-section changes after intended or Bauschinger Effect of the Copper. The strand cross section changes have been ignored and the original cross section was used for stress calculation to estimate the indentation effect directly.
4. Results and discussion 4.1. Ic and RRR The Ic for the barrel strand sample without indentation was 261 A at 12 T and 4.2 K. The indentation depth resulting to strand critical current with 95% of the Ic without indentation is defined as critical depth dc. The dc of the strand was around 0.35 mm according to the Ic results. As shown in Fig. 5, Ic was almost constant with the indentation that is less than dc, but Ic was degraded drastically with the indentation exceeding dc. The RRR was 182 for the strand sample without indentation. The degradation of RRR started from the indentation depth of 0.25 mm which is less than dc from the critical current. The RRR is more sensitive than the Ic to the indentation. From cross-sectional observation, the RRR degradation was caused by breakage of the barrier layer, which has the similar performance from the reference [11]. 4.2. Ic(strain) characteristics Samples with different indentation depths (including 0, 0.15 mm, 0.25 mm, 0.3 mm and 0.35 mm) were tested for Ic(strain) characterization by Pacman device. The applied strain sequence was from 0 to
Fig. 6. Ic as a function of strain for the samples with different indentation depth. 375
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Fig. 7. Normalized Ic as a function of strain for all the samples with different indentation depth.
Fig. 10. Ultimate stress as a function of indentation depth for the IT strand.
intrinsic strain of the stress-strain sample as about -0.2%, which was mainly caused by the thermal residual stress [18]. According to the Ic(strain) characteristics and mechanical properties of the strand samples with different indentation depths, the calculated normalized critical current as a function of axial stress is shown in Fig. 9. It was shown that after the sample yielded, the Ic decrease became much sensitive to the indentation depth. For example, when the applied stress was around 200 MPa, the Ic degraded to be around 98% with depth of 0.15 mm while that to be around 89% with depth of 0.25 mm and around 72% with depth of 0.35 mm, respectively. And the ultimate tensile strength (UTS) as a function of indentation depth was shown in Fig. 10. The UTS is much sensitive to the indentation depth, and reduces continuously along with the enlarging indentation depth. 5. Conclusion Aiming for investigating the effects of indentation on the superconducting and mechanical properties of Nb3Sn strands, systematic measurements have been done on a Nb3Sn IT strand. The strand was artificially indented for different measurements. It was found that RRR degradation was caused by the breakage of barrier layer with indentation. And Ic with ITER barrel decreased rapidly when the indentation depth larger than around 0.35 mm for the measured sample. The results showed that RRR is more sensitive to indentation depth compared to Ic. Furthermore, samples with different artificial indentation were measured by Pamcan device and stress-strain test setup to analyze the influence of indentation on mechanical properties and Ic (strain) characteristics for the strand. The results show that the critical current becomes more sensitive to the strain when the indentation depth gets larger. And from the mechanical tests results, the indentation depths mainly effect on the UTS and YS. As a result, the conductor manufacturing process should limit the deformation of Nb3Sn strands as less as possible for high CICC performance. The less indentation, the better the strand performance combined the mechanical properties and superconducting performance. The results provide solid basis for large CICC design for CFETR and DEMO magnet.
Fig. 8. Stress-strain curves for the samples with indentation depth of 0, 0.15 mm, 0.25 mm and 0.35 mm, respectively.
Fig. 9. The normalized critical current as a function of axial stress for the samples with indentation depth of 0, 0.15 mm, 0.25 mm and 0.35 mm, respectively.
Acknowledgements This research is conducted under the support of national natural Science foundation of China (Grant No. 51477172 and 51677184) and National Magnetic Confinement Fusion Science Program (No.
In order to estimate the effects of mechanical properties on the critical current of the strand, it is reasonable to assume the initial 376
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F. Liu et al.
2014GB105004).
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