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Nb3Sn superconducting wires at room temperature
Physica C 357±360 (2001) 1302±1305
www.elsevier.com/locate/physc
Tensile testing methods of Cu/Nb3Sn superconducting wires at room temperature K. Katagiri a,*, K. Kasaba a, M. Hojo b, K. Osamura b, M. Sugano b, A. Kimura c, T. Ogata d a
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan b Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan c The Furukawa Electric Co., Ltd., 500 Kiyotaki, Nikko 321-1493, Japan d National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan Received 16 October 2000; accepted 18 December 2000
Abstract Preliminary tests for determining the procedure of round robin test on tensile testing methods of Cu/Nb3 Sn composite wires at room temperature have been conducted. Two kinds of bronze processed wires were used. The following results were obtained. (1) The initial part of the stress±strain curve was liable to be varied. Beyond the stress level of about 80 MPa, stress± strain behaviors were basically identical regardless of the gripping method and the inter-grip distance although shifts in strain axis were inevitable. Pre-loading of about 40 MPa was fairly eective for obtaining a consistent stress±strain behavior for a wire. (2) A reliable modulus of elasticity could be determined using unloading curve regardless of the unloading stress level. (3) The stress±strain curves could be expressed as a simple exponential function, which ®tted the experimental data fairly well excepting initial part and was useful to determine the zero strain position of the curves. (4) The 0.2% proof stresses with a coecient of variance of less than 5% could be obtained using the zero strain position and the modulus of elasticity. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 06.20.F; 85.25.K Keywords: Cu/Nb3 Sn composite; Superconducting wire; Tensile test; Standardization
1. Introduction The committee draft for vote (CDV) on the standard room temperature tensile test method of Cu/Nb±Ti superconducting composite has been * Corresponding author. Tel.: +81-19- 621-6412; fax: +81-19624-3951. E-mail address: [email protected] (K. Katagiri).
approved at IEC/TC90 WG5, Boulder, in March 2000 [1]. At Montreal meeting of IEC/TC90 WG5, the room temperature tensile test method of Cu/ Nb3 Sn composite superconductors has been recommended as a candidate for the next work item of WG5. Diculties are anticipated because the Nb3 Sn ®laments are brittle and the stabilizer copper as well as bronze in the composite have been fully annealed and are in yielded conditions
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 4 8 2 - 8
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
due to thermal tensile stresses on cooling [2]. Therefore, the composite will behave plastically from the beginning of testing. The WG5 in New Material Center, NMC, has been devoted to prepare for the round robin test (RRT) through experiments to examine the items, which will cause diculties in the RRT. This paper describes results of the preliminary tests; the outline of it has been brie¯y reported at the Boulder meeting. 2. Experimental procedure Two kinds of bronze processed wire samples were used. The main parameters of the specimens are shown in Table 1. Three light extensometers inevitable for the accurate strain measurement were used according to the conclusions obtained from the RRT activities on the room temperature tensile tests of Cu/Nb±Ti superconducting composite [3,4]. Two of them have been developed for the RRT and are essentially the same to each other with a gauge length of 25 mm. Another one usable in the liquid He environment has a gauge length of 5 mm. Five laboratories have taken part in the experiment. Tensile testing machines used were servo-hydraulic machines and Instron type ones, the load capacity ranged from 2 to 100 kN. The stress±strain curve was drawn as a computer out put, of which the data were memorized via a data logger. An X±Y recorder out put was also used auxiliary.
fabrication/handling or o-axis setting to the machine. Beyond the stress level of about 80 MPa, however, stress±strain behaviors were basically identical regardless of the gripping method (soldering, with sand paper and with brass sleeve/glue) and the inter-grip distance form 40 to 100 mm, although relative shifts in strain axis were inevitable. Typical curves are shown in Fig. 1. Although pre-loading of about 40 MPa was fairly eective for sample H in obtaining a consistent stress±strain behavior (Fig. 2), the adequate preload level is thought to depend on the wire. Unlike the Cu/Nb±Ti composite wires, in which the initial stress±strain curve is almost linear accepting the initial portion, the present wire showed
Fig. 1. Stress±strain curves (Sample H; no pre-stressing).
3. Results and discussion 3.1. Stress±strain curves The initial part of each stress±strain curves varied due presumably to the pre-bending during Table 1 Speci®cation of bronze processed Cu/Nb3 Sn wires Wire
H
K
Diameter No. ®lament Fil. diameter Cu ratio Twist pitch Stabilizer
0.81 mm 4579 4 lm 1.5 16 mm External
1.27 mm 14 322 4:6 lm 0.21 100 mm Internal
1303
Fig. 2. Eects of pre-stressing (Sample H).
1304
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
curve from the beginning to the end of the test. It is presumed to be due to premature yielding of copper and bronze on cooling from the reaction heat treatment temperature. Because the true zero strain in a stress±strain curve is unknown, some procedure to estimate it is needed. Fortunately, the stress±strain curves upto the strain of 0.5% could be expressed as a simple n exponential function, r a e b , which ®tted the experimental data fairly well excepting the
Table 2 Modulus of elasticity (Sample H and K) Unload level (MPa)
E (GPa) Unload 1
Load 1
HI01
112 167 208
109 108 109
107 103 104
HI02
88 136 183 211
114 109 106 115
113 109 110 107
126 259
127 121
134 120
KI02
strain region in the beginning (Fig. 1). As an increase of the starting value of strain for ®tting range upto 0.5%, the parameters appeared to tend to converge to certain values. The parameter b is used to determine the zero strain position of the curves. 3.2. Modulus of elasticity As in the case of Cu/Nb±Ti wires, the modulus of elasticity from initial loading curve varied signi®cantly. A reliable modulus could be determined using unloading curve regardless of the unloading stress level. Especially, the initial part of unloading line is preferable for the modulus because it represents the modulus without reverse yielding of the copper within the wire. Some of the values are shown in Table 2. The coecient of variation (COV; standard deviation divided by the average) was less than 5%. 3.3. 0.2% proof stress Table 3 shows typical values of the 0.2% proof stress and the ®tting parameters a, b and n for
Table 3 0.2% proof stress and ®tting parameters a, b and n Sample number
Fitting parameters a
(a) Sample H HH01 HH03 HH05 HH07 HH09 HH10 HI01
250 254 253 252 252 252 255
r0:2 (MPa) b
n 0:072 0:092 0:085 0:068 0:100 0:110 0:065
0.88 0.83 0.84 0.84 0.83 0.87 0.83
80 88 87 86 88 82 89 COV: 0.040
(b) Sample K KO00 KO40 KO80 KO120 KF02 KF03
420 415 415 423 394 389
0:015 0:028 0:070 0:110 0:040 0:052
0.86 0.86 0.83 0.82 0.77 0.80
161 158 167 174 170 159 COV: 0.039
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
sample H and K. The 0.2% proof stress was obtained using the zero strain position and the modulus of elasticity mentioned above. The parameters vary each other to some extent depending on the laboratories. Not withstanding the variation in the parameters, COV's of less than 5% are acceptable. Present tensile testing method appears promising for a standard.
1305
Acknowledgements This work was performed as the activity of WG5, JNC of IEC/TC90, being conducted by NMC, Osaka Science and Technology Center and supported by Agency of Industrial Science and Technology, MITI, Japan. The authors wish to thank group members for useful discussion. Helpful experimental works by Ms. T. Matsuoka, Kyoto University and Messrs. Y. Shoji and M. Kamesawa, Iwate University are acknowledged.
4. Summary Describing stress±strain curves in a simple exponential function using parameter-®tting method solves the main diculty of deciding the zero strain point. Reasonable COV's were obtained for both the modulus of elasticity and 0.2% proof stress. Based on the results of the preliminary tests and extensive discussion on the scope of the testing method including internal Sn diusion wires, the RRT for the room temperature tensile test method of Cu/Nb3 Sn composite superconductors is about to start in the NMC.
References [1] K. Osamura, A. Nylas, M. Shimada, H. Moriai, M. Hojo, T. Fuse, M. Sugano, in: N. Koshizuka, S. Tajima (Eds.), Advances in Superconductivity XI/2, Springer, 1999, pp. 1515±1518. [2] S. Ochiai, K. Osamura, Acta metallurgica 37 (9) (1989) 2539. [3] S. Sakai, K. Osamaura, M. Hojo, T. Ogata, M. Shimada, T. Haruyama, et al. (Eds.), Proc. 16th CEC/ICMC, Elsevier, 1996, pp. 1791±1794. [4] M. Shimada, M. Hojo, H. Moriai, K. Osamura, Cryogenic Engng. 33 (1998) 665, in Japanese.
www.elsevier.com/locate/physc
Tensile testing methods of Cu/Nb3Sn superconducting wires at room temperature K. Katagiri a,*, K. Kasaba a, M. Hojo b, K. Osamura b, M. Sugano b, A. Kimura c, T. Ogata d a
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan b Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan c The Furukawa Electric Co., Ltd., 500 Kiyotaki, Nikko 321-1493, Japan d National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan Received 16 October 2000; accepted 18 December 2000
Abstract Preliminary tests for determining the procedure of round robin test on tensile testing methods of Cu/Nb3 Sn composite wires at room temperature have been conducted. Two kinds of bronze processed wires were used. The following results were obtained. (1) The initial part of the stress±strain curve was liable to be varied. Beyond the stress level of about 80 MPa, stress± strain behaviors were basically identical regardless of the gripping method and the inter-grip distance although shifts in strain axis were inevitable. Pre-loading of about 40 MPa was fairly eective for obtaining a consistent stress±strain behavior for a wire. (2) A reliable modulus of elasticity could be determined using unloading curve regardless of the unloading stress level. (3) The stress±strain curves could be expressed as a simple exponential function, which ®tted the experimental data fairly well excepting initial part and was useful to determine the zero strain position of the curves. (4) The 0.2% proof stresses with a coecient of variance of less than 5% could be obtained using the zero strain position and the modulus of elasticity. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 06.20.F; 85.25.K Keywords: Cu/Nb3 Sn composite; Superconducting wire; Tensile test; Standardization
1. Introduction The committee draft for vote (CDV) on the standard room temperature tensile test method of Cu/Nb±Ti superconducting composite has been * Corresponding author. Tel.: +81-19- 621-6412; fax: +81-19624-3951. E-mail address: [email protected] (K. Katagiri).
approved at IEC/TC90 WG5, Boulder, in March 2000 [1]. At Montreal meeting of IEC/TC90 WG5, the room temperature tensile test method of Cu/ Nb3 Sn composite superconductors has been recommended as a candidate for the next work item of WG5. Diculties are anticipated because the Nb3 Sn ®laments are brittle and the stabilizer copper as well as bronze in the composite have been fully annealed and are in yielded conditions
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 4 8 2 - 8
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
due to thermal tensile stresses on cooling [2]. Therefore, the composite will behave plastically from the beginning of testing. The WG5 in New Material Center, NMC, has been devoted to prepare for the round robin test (RRT) through experiments to examine the items, which will cause diculties in the RRT. This paper describes results of the preliminary tests; the outline of it has been brie¯y reported at the Boulder meeting. 2. Experimental procedure Two kinds of bronze processed wire samples were used. The main parameters of the specimens are shown in Table 1. Three light extensometers inevitable for the accurate strain measurement were used according to the conclusions obtained from the RRT activities on the room temperature tensile tests of Cu/Nb±Ti superconducting composite [3,4]. Two of them have been developed for the RRT and are essentially the same to each other with a gauge length of 25 mm. Another one usable in the liquid He environment has a gauge length of 5 mm. Five laboratories have taken part in the experiment. Tensile testing machines used were servo-hydraulic machines and Instron type ones, the load capacity ranged from 2 to 100 kN. The stress±strain curve was drawn as a computer out put, of which the data were memorized via a data logger. An X±Y recorder out put was also used auxiliary.
fabrication/handling or o-axis setting to the machine. Beyond the stress level of about 80 MPa, however, stress±strain behaviors were basically identical regardless of the gripping method (soldering, with sand paper and with brass sleeve/glue) and the inter-grip distance form 40 to 100 mm, although relative shifts in strain axis were inevitable. Typical curves are shown in Fig. 1. Although pre-loading of about 40 MPa was fairly eective for sample H in obtaining a consistent stress±strain behavior (Fig. 2), the adequate preload level is thought to depend on the wire. Unlike the Cu/Nb±Ti composite wires, in which the initial stress±strain curve is almost linear accepting the initial portion, the present wire showed
Fig. 1. Stress±strain curves (Sample H; no pre-stressing).
3. Results and discussion 3.1. Stress±strain curves The initial part of each stress±strain curves varied due presumably to the pre-bending during Table 1 Speci®cation of bronze processed Cu/Nb3 Sn wires Wire
H
K
Diameter No. ®lament Fil. diameter Cu ratio Twist pitch Stabilizer
0.81 mm 4579 4 lm 1.5 16 mm External
1.27 mm 14 322 4:6 lm 0.21 100 mm Internal
1303
Fig. 2. Eects of pre-stressing (Sample H).
1304
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
curve from the beginning to the end of the test. It is presumed to be due to premature yielding of copper and bronze on cooling from the reaction heat treatment temperature. Because the true zero strain in a stress±strain curve is unknown, some procedure to estimate it is needed. Fortunately, the stress±strain curves upto the strain of 0.5% could be expressed as a simple n exponential function, r a e b , which ®tted the experimental data fairly well excepting the
Table 2 Modulus of elasticity (Sample H and K) Unload level (MPa)
E (GPa) Unload 1
Load 1
HI01
112 167 208
109 108 109
107 103 104
HI02
88 136 183 211
114 109 106 115
113 109 110 107
126 259
127 121
134 120
KI02
strain region in the beginning (Fig. 1). As an increase of the starting value of strain for ®tting range upto 0.5%, the parameters appeared to tend to converge to certain values. The parameter b is used to determine the zero strain position of the curves. 3.2. Modulus of elasticity As in the case of Cu/Nb±Ti wires, the modulus of elasticity from initial loading curve varied signi®cantly. A reliable modulus could be determined using unloading curve regardless of the unloading stress level. Especially, the initial part of unloading line is preferable for the modulus because it represents the modulus without reverse yielding of the copper within the wire. Some of the values are shown in Table 2. The coecient of variation (COV; standard deviation divided by the average) was less than 5%. 3.3. 0.2% proof stress Table 3 shows typical values of the 0.2% proof stress and the ®tting parameters a, b and n for
Table 3 0.2% proof stress and ®tting parameters a, b and n Sample number
Fitting parameters a
(a) Sample H HH01 HH03 HH05 HH07 HH09 HH10 HI01
250 254 253 252 252 252 255
r0:2 (MPa) b
n 0:072 0:092 0:085 0:068 0:100 0:110 0:065
0.88 0.83 0.84 0.84 0.83 0.87 0.83
80 88 87 86 88 82 89 COV: 0.040
(b) Sample K KO00 KO40 KO80 KO120 KF02 KF03
420 415 415 423 394 389
0:015 0:028 0:070 0:110 0:040 0:052
0.86 0.86 0.83 0.82 0.77 0.80
161 158 167 174 170 159 COV: 0.039
K. Katagiri et al. / Physica C 357±360 (2001) 1302±1305
sample H and K. The 0.2% proof stress was obtained using the zero strain position and the modulus of elasticity mentioned above. The parameters vary each other to some extent depending on the laboratories. Not withstanding the variation in the parameters, COV's of less than 5% are acceptable. Present tensile testing method appears promising for a standard.
1305
Acknowledgements This work was performed as the activity of WG5, JNC of IEC/TC90, being conducted by NMC, Osaka Science and Technology Center and supported by Agency of Industrial Science and Technology, MITI, Japan. The authors wish to thank group members for useful discussion. Helpful experimental works by Ms. T. Matsuoka, Kyoto University and Messrs. Y. Shoji and M. Kamesawa, Iwate University are acknowledged.
4. Summary Describing stress±strain curves in a simple exponential function using parameter-®tting method solves the main diculty of deciding the zero strain point. Reasonable COV's were obtained for both the modulus of elasticity and 0.2% proof stress. Based on the results of the preliminary tests and extensive discussion on the scope of the testing method including internal Sn diusion wires, the RRT for the room temperature tensile test method of Cu/Nb3 Sn composite superconductors is about to start in the NMC.
References [1] K. Osamura, A. Nylas, M. Shimada, H. Moriai, M. Hojo, T. Fuse, M. Sugano, in: N. Koshizuka, S. Tajima (Eds.), Advances in Superconductivity XI/2, Springer, 1999, pp. 1515±1518. [2] S. Ochiai, K. Osamura, Acta metallurgica 37 (9) (1989) 2539. [3] S. Sakai, K. Osamaura, M. Hojo, T. Ogata, M. Shimada, T. Haruyama, et al. (Eds.), Proc. 16th CEC/ICMC, Elsevier, 1996, pp. 1791±1794. [4] M. Shimada, M. Hojo, H. Moriai, K. Osamura, Cryogenic Engng. 33 (1998) 665, in Japanese.