- Email: [email protected]
VAMAS round robin test on bending strain effect measurement of Bi-2223 tapes
Physica C 382 (2002) 7–11 www.elsevier.com/locate/physc
VAMAS round robin test on bending strain effect measurement of Bi-2223 tapes K. Itoh *, T. Kuroda, H. Wada National Institute for Materials Science, Sakura 3-13, Tsukuba, 305-0003 Ibaraki, Japan
Abstract In the framework of Versailles project on advanced materials and standards (VAMAS) a round robin test (RRT) has been implemented to determine a standard test method for the bending strain effect measurement by using three kinds of Ag alloy sheathed Bi-2223 multifilamentary tapes and with the participation of 12 laboratories worldwide. In the RRT, specially designed bending devices having curvatures corresponding to 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% bending strains were prepared and delivered to the participants. Bending of a specimen was done at room temperature and the critical current of the specimen was measured at 77 K and zero magnetic field. The bending and subsequent critical current measurement were started from 0% strain, and repeated on a single specimen up to 1.0%. The critical current vs. bending strain curves obtained were compared among laboratories whereby the critical current was normalized to that for the 0% strain state. The coefficient of variation (COV) ( ¼ the average divided by the standard deviation) was very small for the bending strain of 0.2%, increased at 0.4% strain, and remained at the same level up to 1.0% strain. The COV was, however, kept within 6.2% for all strains. Results were further compared in terms of bending strain limit and the COV in bending strain limit among participants was found to be 13.9%, rather large. Possible error sources are discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.72.Hs; 74.80.Bj; 81.70.Bt Keywords: Bi-2223/Ag tape; Critical current; Bending strain; Round robin test
1. Introduction Due to many successful efforts to reinforce the mechanical property of Bi-base high temperature superconductors by multifilamentarization and application of hard sheath material such as Ag– Mg alloy, the mechanical strain tolerance of the conductors has been improved from 0.1% to 0.2% for the early stage conductors to more than 0.5%. *
Corresponding author. Tel.: +81-298-59-5081; fax: +81298-59-5023. E-mail address: [email protected] (K. Itoh).
To know the effects of stress/strain on the superconducting properties and to be able to estimate the strain tolerance is very important practically for engineers developing superconducting devices such as coils, transformers, motors and power transmission lines, since superconductors in such applications are subjected to high mechanical loads that can significantly degrade the superconducting properties. There are three test methods routinely applied: uniaxial tensile test, transverse load test and bending test. We can estimate the strain tolerance of a conductor loaded with hoop stresses by the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 1 8 7 - 5
8
K. Itoh et al. / Physica C 382 (2002) 7–11
uniaxial tensile test, where the critical current, Ic is measured as a function of tensile strain at cryogenic temperatures. In the transverse load test where Ic is measured under compressive transverse strain, we can estimate the transverse strain limit in designing superconducting coils. The bending test is effective to estimate the strain tolerance when transferring a conductor from drum to drum and when winding it on a coil former. Versailles project on advanced materials and standards (VAMAS)/technical working area 16 (TWA16, superconducting materials) decided in 1998 to start a project of the prestandardization work on the bending test via a round robin test (RRT). The reason for the selection of the bending test was that the bending test is rather simple compared to other tests and many people can participate in the project. A working plan for the RRT was proposed in 1999 based on preliminary studies and a survey of the literature [1–8]. The purpose of the RRT was to identify parameters affecting Ic of BSCCO tape superconductors subjected to bending strain by accumulating and comparing the measurement results reported from the participant laboratories, and eventually present a preliminary recommendation on the bending strain effect measurement method. In this paper, we report the outline of the test procedure and results of the RRT implemented in 2000–2001 and discuss possible error sources of the measurements.
2. Round robin test procedure Three types of samples, referred to as VAM-1, VAM-2 and VAM-3 were prepared as test samples
whose specifications are shown in Table 1. All samples are Ag (alloy) sheathed Bi-2223 multifilamentary tapes that were fabricated by the powder-in-tube technique and commercially available. Samples were different from each other in dimensions, number of filaments and Ic at liquid nitrogen temperature, 77 K. VAM-1 was exclusively used for the RRT, while VAM-2 and VAM-3 were only for complementary measurements. In this paper we simply defined the bending strain, e as the strain at the outer surface of the specimen e ¼ t=D;
ð1Þ
where t is the thickness of the superconducting tape and D is the diameter of the bending base plate. It was assumed in the definition that the neutral axis of bending remains at the center of thickness of the tape, although e may actually be different from the definition due to asymmetric deformation of the conductor during bending and cooling. The bending device used in the RRT was an assembly of a curved cover plate (female die) and a curved base plate (male die) as schematically illustrated in Fig. 1. Both plates were made of G-10. The test specimen, 80 mm in length, was placed on the base plate, pushed down with the cover plate, and then sandwiched between the plates at room temperature. The base plate had current terminals at both ends of the plate and served as a specimen holder for the Ic measurement. Distance between voltage taps was 30 mm. Six bending devices with curvatures corresponding e ¼ 0:2%, 0.4%, 0.6%, 0.8% and 1.0% for VAM-1 sample were prepared at the TWA office and distributed to each of participants together with several pieces of VAM-1 sample.
Table 1 Sample specifications Superconductor Matrix Width (mm) Thickness (mm) No. of filaments Ic at 77 K (A)
VAM-1
VAM-2
VAM-3
Bi(Pb)-2223 Ag/AgMg-alloy 3:7 0:02 0:27 0:02 57 50 1
Bi(Pb)-2223 Ag alloy 2:95 0:03 0:182 0:007 19 28 1
Bi(Pb)-2223 Ag alloy 3:14 0:04 0:254 0:008 37 42 2
K. Itoh et al. / Physica C 382 (2002) 7–11
9
3. Results and discussion
Fig. 1. Illustration of a typical bending device used in the RRT.
The Ic measurements, requested to be carried out according to the directions of the IEC standard [9], were conducted at 77 K and zero external field in this RRT. The bending and subsequent Ic measurement was started from 0% strain, and repeated on a single specimen up to 1.0%. Twelve laboratories participated in the RRT as listed in Table 2.
In the RRT, each participant laboratory made Ic measurements on three specimens of VAM-1 sample and reported their results. Fig. 2 shows the distribution histogram of the critical current for e ¼ 0%, Ic0 determined at Ic criterion of 1 lV/cm. The average Ic0 and standard deviation (S.D.) are 48.54 and 1.38 A, respectively. The average Ic0 is somewhat smaller than the catalog value shown in Table 1 while the S.D. is a little larger. It is seen in Fig. 2 that the distribution is not symmetric about the average. By ignoring the four lowest data points, we can obtain a more symmetric distribution with the average and S.D. of 48.92 and 0.89 A, respectively, closer to the catalog values. This means that the Ic homogeneity among specimens is fairly good and experimental errors in Ic measurements among the participants are rather small. In Fig. 3, Ic =Ic0 for all the data reported is plotted against e. It can be seen that the typical Ic =Ic0 is almost unity for e up to 0.2–0.35% while it drops steeply for higher eÕs. There are three curves (the dotted lines in Fig. 3) showing extraordinarily low values even at e ¼ 0:2%. They are reported from one of the participants and should be excluded from the statistics because those specimens
Table 2 List of participants K. Katagiri, Iwate University, Japan T. Kiss, Kyushu University, Japan T. Kuroda, NIMS, Japan T. Matsushita, Kyushu Institute of Technology, Japan K. Noto, Iwate University, Japan S. Ochiai, Kyoto University, Japan
W. Haessler, IFW, Germany B. ten Haken, University Twente, Netherlands A. Nyilas, FZK, Germany J. Sosnowski, Electrotechnology Institute, Poland H.S. Shin, Andong National University, Korea H. Weijers, NHMFL, USA Fig. 2. Distribution of Ic0 for all the data reported.
10
K. Itoh et al. / Physica C 382 (2002) 7–11
Fig. 4. Ic =Ic0 vs. e curves for RRT average, parallel measurement sequence and pull and wind bending method. Fig. 3. Ic =Ic0 vs. e curves for all the data reported.
might have suffered from heavy damage when bent to e ¼ 0:2%. Discarding those data, the average, S.D. and the coefficient of variation ðCOVÞ ð¼ S:D: divided by averageÞ are calculated and listed in Table 3. The COV starts to increase at e ¼ 0:4% and continues to gradually increase with increasing e and reaches 6.2% at e ¼ 1:0%. Besides the RRT measurements two kinds of complementary measurements were examined. Firstly, the ÔparallelÕ sequence was examined as an alternative measurement sequence, where Ic of a specimen was measured only twice; at first with the e ¼ 0% device, and then with one of the curved strain devices. In this sequence five specimens, each bent to e ¼ 0:2%, 0.4%, 0.6%, 0.8% or 1.0%, were necessary to get an Ic vs. e curve, while a single specimen was enough in case of the RRT sequence. As shown in Fig. 4, the Ic vs. e curve Table 3 Statistics of RRT results Ic =Ic0
Bending strain, e (%) 0.2
0.4
0.6
0.8
1.0
Average S.D. COV (%)
0.997 0.0078 0.8
0.927 0.041 4.5
0.816 0.041 5.0
0.738 0.042 5.7
0.669 0.041 6.2
obtained was in good agreement with that of RRT average, although the RRT specimens experienced more thermal cycles than those of the parallel sequence. Secondly, a Ôpull and windÕ method was examined as an alternative bending method, where the specimen was soldered at one end and bent along the mandrel surface via pulling the other end of the specimen with a constant force. As also shown in Fig. 4, there were no significant differences between the RRT and complementary bending methods. One of the purposes to obtain e dependence of Ic is to determine the bending strain limit, el i.e., the bending strain tolerance. el is defined here as the bending strain where Ic reaches 95% of Ic0 . In order to determine el , Eq. (2) is introduced as a fitting equation, k
Ic =Ic0 ¼ 1 aðe e0 Þ :
ð2Þ
Here, a, k and e0 are fitting parameters. a and k are related to the slope and shape of the curve, respectively. e0 ð< eÞ is the strain where Ic begins to fall. The curve is convex, straight and concave when k > 1, k ¼ 1 and k < 1, respectively. The fitting of the RRT data using Eq. (2) is excellent, and the correlation coefficient exceeds 0.998 in most cases. In fact, the average and S.D. of Ic =Ic0
K. Itoh et al. / Physica C 382 (2002) 7–11
11
comparison between the RRT and FZK methods will be helpful to clarify the origins of data scatter of the RRT results.
4. Conclusions
Fig. 5. el Õs arranged in ascending order of intra-laboratory averages.
for e P 0:4% calculated using Eq. (2) are almost in agreement with the respective values shown in Table 3. The average, S.D. and COV of el calculated from el ¼ e0 þ ðð0:05Þ=aÞ1=k , are 0.37%, 0.05% and 13.9%, respectively. The COV of el thus obtained seems rather large and not acceptable from the engineering point of view. Fig. 5 shows the el Õs arranged in ascending order of its intra-laboratory averages. It is seen that intra-laboratory data scatter is much smaller than the scatter of the total data. It is also seen that el Õs of laboratories with a larger intra-laboratory average show a smaller intra-laboratory data scatter. This implies that laboratories with large el have measured under more specified and strain controlled conditions, which are not regulated in the IEC standard. It seems especially important to minimize any extra strains due to thermal contractions which may be introduced during bending, soldering, cooling and warming procedures. Recently a new sophisticated bending apparatus has been developed at FZK, Germany [10], which has no specimen support excepting grips at both ends of the specimen. Using such an apparatus, we can avoid the effects of thermal contraction and realize an ideal bending moment. In the future, a
1. The RRT on bending strain effect in Bi-2223 conductors has been successfully carried out with participation of 12 laboratories worldwide. 2. Results are compared by el and the scatter in el is found to be 13.9%, rather large and not acceptable from the engineering point of view. 3. The reason for the large scatter in el has not been clarified yet. From Fig. 5 it is suggested that measurements should be done under more specified and strain controlled conditions. 4. Comparison among different bending methods will be necessary to clarify the error sources and improve the reliability of measurements.
Acknowledgements The authors wish to express their appreciation to all the RRT participants. This work was supported by Special Coordination fund for the Promotion of Science and Technology, the Ministry of Education, Culture, Sports, Science and Technology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
J.W. Ekin et al., Appl. Phys. Lett. 61 (1992) 858. J. Yau, N. Savvides, Appl. Phys. Lett. 65 (1994) 1454. W.D. Lee et al., Physica C 247 (1995) 215. K. Katagiri et al., Cryogenics 36 (1996) 491. B. Ullmann et al., IEEE Trans. Appl. Supercond. 7 (1997) 2042. L. Bigoni et al., IEEE Trans. Appl. Supercond. 9 (1999) 2597. M. Polak et al., Adv. Cryog. Eng. 46 (2000) 793. ASM Handbook, vol. 8, ASM International, 2000, p. 172. IEC 61788-3: 2000, Superconductivity, Part 3; Critical current measurement-DC critical current of Ag-sheathed Bi-2212 and -2223 oxide superconductors. W. Goldacker et al., Adv. Cryog. Eng. 47/48 (2002), to be published.
VAMAS round robin test on bending strain effect measurement of Bi-2223 tapes K. Itoh *, T. Kuroda, H. Wada National Institute for Materials Science, Sakura 3-13, Tsukuba, 305-0003 Ibaraki, Japan
Abstract In the framework of Versailles project on advanced materials and standards (VAMAS) a round robin test (RRT) has been implemented to determine a standard test method for the bending strain effect measurement by using three kinds of Ag alloy sheathed Bi-2223 multifilamentary tapes and with the participation of 12 laboratories worldwide. In the RRT, specially designed bending devices having curvatures corresponding to 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% bending strains were prepared and delivered to the participants. Bending of a specimen was done at room temperature and the critical current of the specimen was measured at 77 K and zero magnetic field. The bending and subsequent critical current measurement were started from 0% strain, and repeated on a single specimen up to 1.0%. The critical current vs. bending strain curves obtained were compared among laboratories whereby the critical current was normalized to that for the 0% strain state. The coefficient of variation (COV) ( ¼ the average divided by the standard deviation) was very small for the bending strain of 0.2%, increased at 0.4% strain, and remained at the same level up to 1.0% strain. The COV was, however, kept within 6.2% for all strains. Results were further compared in terms of bending strain limit and the COV in bending strain limit among participants was found to be 13.9%, rather large. Possible error sources are discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.72.Hs; 74.80.Bj; 81.70.Bt Keywords: Bi-2223/Ag tape; Critical current; Bending strain; Round robin test
1. Introduction Due to many successful efforts to reinforce the mechanical property of Bi-base high temperature superconductors by multifilamentarization and application of hard sheath material such as Ag– Mg alloy, the mechanical strain tolerance of the conductors has been improved from 0.1% to 0.2% for the early stage conductors to more than 0.5%. *
Corresponding author. Tel.: +81-298-59-5081; fax: +81298-59-5023. E-mail address: [email protected] (K. Itoh).
To know the effects of stress/strain on the superconducting properties and to be able to estimate the strain tolerance is very important practically for engineers developing superconducting devices such as coils, transformers, motors and power transmission lines, since superconductors in such applications are subjected to high mechanical loads that can significantly degrade the superconducting properties. There are three test methods routinely applied: uniaxial tensile test, transverse load test and bending test. We can estimate the strain tolerance of a conductor loaded with hoop stresses by the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 1 8 7 - 5
8
K. Itoh et al. / Physica C 382 (2002) 7–11
uniaxial tensile test, where the critical current, Ic is measured as a function of tensile strain at cryogenic temperatures. In the transverse load test where Ic is measured under compressive transverse strain, we can estimate the transverse strain limit in designing superconducting coils. The bending test is effective to estimate the strain tolerance when transferring a conductor from drum to drum and when winding it on a coil former. Versailles project on advanced materials and standards (VAMAS)/technical working area 16 (TWA16, superconducting materials) decided in 1998 to start a project of the prestandardization work on the bending test via a round robin test (RRT). The reason for the selection of the bending test was that the bending test is rather simple compared to other tests and many people can participate in the project. A working plan for the RRT was proposed in 1999 based on preliminary studies and a survey of the literature [1–8]. The purpose of the RRT was to identify parameters affecting Ic of BSCCO tape superconductors subjected to bending strain by accumulating and comparing the measurement results reported from the participant laboratories, and eventually present a preliminary recommendation on the bending strain effect measurement method. In this paper, we report the outline of the test procedure and results of the RRT implemented in 2000–2001 and discuss possible error sources of the measurements.
2. Round robin test procedure Three types of samples, referred to as VAM-1, VAM-2 and VAM-3 were prepared as test samples
whose specifications are shown in Table 1. All samples are Ag (alloy) sheathed Bi-2223 multifilamentary tapes that were fabricated by the powder-in-tube technique and commercially available. Samples were different from each other in dimensions, number of filaments and Ic at liquid nitrogen temperature, 77 K. VAM-1 was exclusively used for the RRT, while VAM-2 and VAM-3 were only for complementary measurements. In this paper we simply defined the bending strain, e as the strain at the outer surface of the specimen e ¼ t=D;
ð1Þ
where t is the thickness of the superconducting tape and D is the diameter of the bending base plate. It was assumed in the definition that the neutral axis of bending remains at the center of thickness of the tape, although e may actually be different from the definition due to asymmetric deformation of the conductor during bending and cooling. The bending device used in the RRT was an assembly of a curved cover plate (female die) and a curved base plate (male die) as schematically illustrated in Fig. 1. Both plates were made of G-10. The test specimen, 80 mm in length, was placed on the base plate, pushed down with the cover plate, and then sandwiched between the plates at room temperature. The base plate had current terminals at both ends of the plate and served as a specimen holder for the Ic measurement. Distance between voltage taps was 30 mm. Six bending devices with curvatures corresponding e ¼ 0:2%, 0.4%, 0.6%, 0.8% and 1.0% for VAM-1 sample were prepared at the TWA office and distributed to each of participants together with several pieces of VAM-1 sample.
Table 1 Sample specifications Superconductor Matrix Width (mm) Thickness (mm) No. of filaments Ic at 77 K (A)
VAM-1
VAM-2
VAM-3
Bi(Pb)-2223 Ag/AgMg-alloy 3:7 0:02 0:27 0:02 57 50 1
Bi(Pb)-2223 Ag alloy 2:95 0:03 0:182 0:007 19 28 1
Bi(Pb)-2223 Ag alloy 3:14 0:04 0:254 0:008 37 42 2
K. Itoh et al. / Physica C 382 (2002) 7–11
9
3. Results and discussion
Fig. 1. Illustration of a typical bending device used in the RRT.
The Ic measurements, requested to be carried out according to the directions of the IEC standard [9], were conducted at 77 K and zero external field in this RRT. The bending and subsequent Ic measurement was started from 0% strain, and repeated on a single specimen up to 1.0%. Twelve laboratories participated in the RRT as listed in Table 2.
In the RRT, each participant laboratory made Ic measurements on three specimens of VAM-1 sample and reported their results. Fig. 2 shows the distribution histogram of the critical current for e ¼ 0%, Ic0 determined at Ic criterion of 1 lV/cm. The average Ic0 and standard deviation (S.D.) are 48.54 and 1.38 A, respectively. The average Ic0 is somewhat smaller than the catalog value shown in Table 1 while the S.D. is a little larger. It is seen in Fig. 2 that the distribution is not symmetric about the average. By ignoring the four lowest data points, we can obtain a more symmetric distribution with the average and S.D. of 48.92 and 0.89 A, respectively, closer to the catalog values. This means that the Ic homogeneity among specimens is fairly good and experimental errors in Ic measurements among the participants are rather small. In Fig. 3, Ic =Ic0 for all the data reported is plotted against e. It can be seen that the typical Ic =Ic0 is almost unity for e up to 0.2–0.35% while it drops steeply for higher eÕs. There are three curves (the dotted lines in Fig. 3) showing extraordinarily low values even at e ¼ 0:2%. They are reported from one of the participants and should be excluded from the statistics because those specimens
Table 2 List of participants K. Katagiri, Iwate University, Japan T. Kiss, Kyushu University, Japan T. Kuroda, NIMS, Japan T. Matsushita, Kyushu Institute of Technology, Japan K. Noto, Iwate University, Japan S. Ochiai, Kyoto University, Japan
W. Haessler, IFW, Germany B. ten Haken, University Twente, Netherlands A. Nyilas, FZK, Germany J. Sosnowski, Electrotechnology Institute, Poland H.S. Shin, Andong National University, Korea H. Weijers, NHMFL, USA Fig. 2. Distribution of Ic0 for all the data reported.
10
K. Itoh et al. / Physica C 382 (2002) 7–11
Fig. 4. Ic =Ic0 vs. e curves for RRT average, parallel measurement sequence and pull and wind bending method. Fig. 3. Ic =Ic0 vs. e curves for all the data reported.
might have suffered from heavy damage when bent to e ¼ 0:2%. Discarding those data, the average, S.D. and the coefficient of variation ðCOVÞ ð¼ S:D: divided by averageÞ are calculated and listed in Table 3. The COV starts to increase at e ¼ 0:4% and continues to gradually increase with increasing e and reaches 6.2% at e ¼ 1:0%. Besides the RRT measurements two kinds of complementary measurements were examined. Firstly, the ÔparallelÕ sequence was examined as an alternative measurement sequence, where Ic of a specimen was measured only twice; at first with the e ¼ 0% device, and then with one of the curved strain devices. In this sequence five specimens, each bent to e ¼ 0:2%, 0.4%, 0.6%, 0.8% or 1.0%, were necessary to get an Ic vs. e curve, while a single specimen was enough in case of the RRT sequence. As shown in Fig. 4, the Ic vs. e curve Table 3 Statistics of RRT results Ic =Ic0
Bending strain, e (%) 0.2
0.4
0.6
0.8
1.0
Average S.D. COV (%)
0.997 0.0078 0.8
0.927 0.041 4.5
0.816 0.041 5.0
0.738 0.042 5.7
0.669 0.041 6.2
obtained was in good agreement with that of RRT average, although the RRT specimens experienced more thermal cycles than those of the parallel sequence. Secondly, a Ôpull and windÕ method was examined as an alternative bending method, where the specimen was soldered at one end and bent along the mandrel surface via pulling the other end of the specimen with a constant force. As also shown in Fig. 4, there were no significant differences between the RRT and complementary bending methods. One of the purposes to obtain e dependence of Ic is to determine the bending strain limit, el i.e., the bending strain tolerance. el is defined here as the bending strain where Ic reaches 95% of Ic0 . In order to determine el , Eq. (2) is introduced as a fitting equation, k
Ic =Ic0 ¼ 1 aðe e0 Þ :
ð2Þ
Here, a, k and e0 are fitting parameters. a and k are related to the slope and shape of the curve, respectively. e0 ð< eÞ is the strain where Ic begins to fall. The curve is convex, straight and concave when k > 1, k ¼ 1 and k < 1, respectively. The fitting of the RRT data using Eq. (2) is excellent, and the correlation coefficient exceeds 0.998 in most cases. In fact, the average and S.D. of Ic =Ic0
K. Itoh et al. / Physica C 382 (2002) 7–11
11
comparison between the RRT and FZK methods will be helpful to clarify the origins of data scatter of the RRT results.
4. Conclusions
Fig. 5. el Õs arranged in ascending order of intra-laboratory averages.
for e P 0:4% calculated using Eq. (2) are almost in agreement with the respective values shown in Table 3. The average, S.D. and COV of el calculated from el ¼ e0 þ ðð0:05Þ=aÞ1=k , are 0.37%, 0.05% and 13.9%, respectively. The COV of el thus obtained seems rather large and not acceptable from the engineering point of view. Fig. 5 shows the el Õs arranged in ascending order of its intra-laboratory averages. It is seen that intra-laboratory data scatter is much smaller than the scatter of the total data. It is also seen that el Õs of laboratories with a larger intra-laboratory average show a smaller intra-laboratory data scatter. This implies that laboratories with large el have measured under more specified and strain controlled conditions, which are not regulated in the IEC standard. It seems especially important to minimize any extra strains due to thermal contractions which may be introduced during bending, soldering, cooling and warming procedures. Recently a new sophisticated bending apparatus has been developed at FZK, Germany [10], which has no specimen support excepting grips at both ends of the specimen. Using such an apparatus, we can avoid the effects of thermal contraction and realize an ideal bending moment. In the future, a
1. The RRT on bending strain effect in Bi-2223 conductors has been successfully carried out with participation of 12 laboratories worldwide. 2. Results are compared by el and the scatter in el is found to be 13.9%, rather large and not acceptable from the engineering point of view. 3. The reason for the large scatter in el has not been clarified yet. From Fig. 5 it is suggested that measurements should be done under more specified and strain controlled conditions. 4. Comparison among different bending methods will be necessary to clarify the error sources and improve the reliability of measurements.
Acknowledgements The authors wish to express their appreciation to all the RRT participants. This work was supported by Special Coordination fund for the Promotion of Science and Technology, the Ministry of Education, Culture, Sports, Science and Technology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
J.W. Ekin et al., Appl. Phys. Lett. 61 (1992) 858. J. Yau, N. Savvides, Appl. Phys. Lett. 65 (1994) 1454. W.D. Lee et al., Physica C 247 (1995) 215. K. Katagiri et al., Cryogenics 36 (1996) 491. B. Ullmann et al., IEEE Trans. Appl. Supercond. 7 (1997) 2042. L. Bigoni et al., IEEE Trans. Appl. Supercond. 9 (1999) 2597. M. Polak et al., Adv. Cryog. Eng. 46 (2000) 793. ASM Handbook, vol. 8, ASM International, 2000, p. 172. IEC 61788-3: 2000, Superconductivity, Part 3; Critical current measurement-DC critical current of Ag-sheathed Bi-2212 and -2223 oxide superconductors. W. Goldacker et al., Adv. Cryog. Eng. 47/48 (2002), to be published.