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Axial compression test on ITER-TFMC conductors at room temperature
Fusion Engineering and Design 84 (2009) 780–783
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Axial compression test on ITER-TFMC conductors at room temperature H. Fillunger a,∗ , R.K. Maix a , R. Prokopec a , A. Danninger b a b
Vienna University of Technology, Atomic Institute of the Austrian Universities, 1020 Vienna, Austria Vienna University of Technology, Institute of Materials Science and Technology, Austria
a r t i c l e
i n f o
Article history: Available online 25 January 2009 Keywords: Superconductor Heat treatment Thermal expansion Young’s modulus Double spring system
a b s t r a c t The superconductor used for the ITER TF Model Coil (TFMC) consists of 720 twisted Nb3 Sn-strands and 360 copper strands, which are cabled around a central spiral and are surrounded by a stainless steel jacket. The conductor is heat-treated at 650 ◦ C in a stainless steel mould. After cool down to room temperature the conductor is found to be elongated by 0.45 mm/m, which can be attributed to the lower shrinkage of Nb3 Sn. This means that the jacket is under tensile stress and the cable under longitudinal compression after cool down. This effect aggravates upon cooling further to the operating temperature. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. In order to evaluate and possibly predict such elongations, it is necessary to know the Young’s modulus of the cable. Therefore, we investigated samples cut from a dummy conductor, a not heat-treated and a heat-treated TFMC conductor by mechanically compressing them. From the stress–strain diagram the elastic modulus of the cables was determined and found to be about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cables turned out to be fairly independent of the state of the cable as all three cables show almost the same modulus. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket. © 2008 Elsevier B.V. All rights reserved.
1. Introduction After heat treatment in a stainless steel mould at 650 ◦ C and cool down to room temperature the TFMC conductor showed an elongation of about 0.45 mm/m, which was found to be about the same for all pancakes [1]. The superconductor consists of 720 twisted Nb3 Sn-strands and 360 copper strands, which are cabled around a centre spiral and surrounded by a stainless steel jacket (see Fig. 1a). As the conductor was wound against the inner contour of the groove of the reaction mould of race track shape it had to follow the expansion of the mould during warm-up, in spite of the lower expansion coefficient of Niobium. Therefore, the cable was stretched while the jacket remained stress free during the reaction treatment. During formation of the Nb3 Sn filaments, all components of the cable relaxed to be also stress free. During cool down the conductor shrunk less than the mould, which resulted in an elongation. This means that the jacket is under tensile stress, while the cable is under compression, which aggravates upon cooling further to operating temperature and results in critical current degradation.
∗ Corresponding author. Tel.: +43 1 58801 14123; fax: +43 1 58801 14199. E-mail address: fi[email protected] (H. Fillunger). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.023
If the elongation is calculated assuming all components of the cable being fully bonded and using the material parameters according to the ITER material data collection, one ends up with about 1 mm/m, which is distinctly more than found during TFMC manufacture. In order to evaluate and possibly predict such conductor elongations with a double spring model, the cable’s compressive modulus must be known, which obviously is considerably lower than calculated. Therefore, samples prepared from three different conductors received from CEA Cadarache (see Fig. 1b) were investigated: • Three samples cut from a piece of a dummy conductor. • Three samples cut from a piece of a not heat-treated superconductor. • Three samples cut from a piece of the heat-treated conductor. The samples were cut by a wire-electro-erosion process making the surfaces plane and parallel. The compaction tests were performed using Zwick/Roell Z250 test machine. The loads were measured by a load cell with 250 kN nominal force and accuracy class 0.5 (for forces from 2 kN upward) according to EN ISO 376, the displacement was picked up from the motion of the crosshead by a sensor with an accuracy class of 0.5 according to EN ISO 9513. The compliance of the machine frame was determined by a dummy measurement of the bare compression piston.
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
781
Fig. 2. Typical diagrams obtained on the dummy conductor and the TFMC conductor before and after the reaction heat treatment.
2. Results Some of the samples showed displacement curves that were shifted to higher compressive loads, which could be explained by uneven surfaces of the cable underneath the stamp. This could be corrected by setting the zero-displacement-point numerically to a compressive load of 500 N. Fig. 2 shows typical load–displacement curves of the three types of conductor. The region of interest is restricted to compressive forces up to the order of the stretching force acting on the stainless steel jacket of the TFMC conductor at room temperature. Taking the cross-sections of all strands and referring the displacements to the original sample lengths, the load–displacement diagrams were converted into stress–strain diagrams. 3. Determination of the Young’s modulus of the cables
Fig. 1. (a) Build-up of the TFMC conductor. (b) Compression samples and a trial sample between the expanded fixture assembly.
The fixture assembly consists of special rings for placing the samples in the centre of the testing machine and a steel piston for transmitting the compressive force to the cable inside the jacket. The automatic test runs were structured in 10 steps. Before starting them, the samples were preloaded with 50 N (Table 1).
The stress–strain curves of each sample were subdivided into several branches. The linear trend curves of each branch give the medium values of Young’s modulus. As an example, the stress–strain diagram with added trend curves of a reacted conductor is shown in Fig. 3. In Table 2, the values of the Young’s modulus calculated from measured data, based on the sum of the cross-section of all strands, and obtained from the linear trend curves of the stress–strain diagrams are listed and compared with those calculated from the fully bonded model. The measured values are of the same order of magnitude as those measured by P. Decool et al. reported in an internal report of CEA Cadarache (Cable 40 KA CEA-MIC1, measure exper-
Table 1 Steps of the automatic test runs. Steps
Movement of crosshead
Limit
Speed
Comment
1 2
Compression (=downwards) Decompression (=)upwards
0.5 mm 5000 N
0.5 mm/min 0.5 mm/min
For all samples For all samples
3
Compression
0.8 mm
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
4, 6, 8, 10
Decompression
5000 N
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
5
Compression
65,000 N
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
7, 9
Compression
4.0 mm
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
782
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
Fig. 3. Stress–strain diagram with added linear trend curves of a reacted conductor sample.
imentale des characteristique mechanique compte rendu d’essai, report No. P/EM/93-38, 1993). The stiffness of the cables during the compaction phase (first loading) is nearly equal for all three different types of cables: the dummy cable, the not heat-treated cable and the TFMC cable. The stiffness is governed by the shape of the cable, the void fraction and the twist pitch. The influence of the different materials in the cable is of secondary importance. We are presently preparing investigations on samples with a reduced void fraction, in order to evaluate its effect. In the first load release and during the load restoring phase, which is the most relevant phase with respect to the conductor acting as a double spring system, the Young’s modulus corresponds to about one-tenth of that calculated according to the fully bonded model (see Table 2). At the operating point of the conductor, the Young’s modulus measured during the second and third load release and the load restoring phase is no longer relevant for the status of the cable, because the permanent deformation is already too high. During the cool down from the heat treatment temperature to room temperature no movement can take place between strands and the jacket. However, this is not possible during a compression test. The role of friction cannot be separated in the compaction measurement. Nevertheless by taking the average of the moduli measured during up- and down-loading this effect may be taken into account. 4. The double spring system of jacket and cable The TFMC conductor represents a double spring system during cool down to room temperature. The tension spring is represented by the jacket, while the cable is the counteracting compression spring. At a strain of 0.45 mm/m the intrinsic force level is 16985 N (see Fig. 4). The double spring is in equilibrium when Es × εs × As = Ec × εc × Ac (E = elastic modulus, ε = strain, A = crosssection; s stands for stainless steel and c for cable). The cable is compressed by the internal spring force according to the experimental characteristic.
Fig. 4. Double spring system consisting of the jacket being the tension spring and the cable being the compression spring.
In this situation the superconducting cable has already experienced a compression of 3.8 × 10−03 which is about 8.5 times as much as the jacket elongation and is beyond the acceptable deformation of the cable during winding procedures (0.2%). External stretching forces like electromagnetic forces acting on the double spring will only lead to a small release of the cable’s precompression. 5. Acoustic emission during compression of samples cut from the heat-treated conductor Only the samples of the reacted conductor containing Nb3 Sn filaments produced a clearly audible cracking noise when reaching a compression force of more than 15,000 N. The cracking noise became intensive at compacting forces of more than 18,000 N and ended at a displacement of about 2.5 mm. As seen in the load–displacement diagrams, the starting value of the cracking noise is below the value of the remaining spring force (16,785 N) in the reacted TFMC conductor at room temperature. 6. Conclusions The Young’s modulus of the cable inside the jacket of the TFMC conductor is about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cable turns out to be fairly independent of the state of the conductor, since dummy, unreacted and reacted conductors show nearly identical values. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket. In order to evaluate and possibly predict the elongation of a conductor, its compressive modulus should be known. It is a function of the materials involved, their geometry and the void fraction. As the void fraction of the ITER conductors is now considerably lower than that of the TFMC conductor, we are preparing tests on conduc-
Table 2 Young’s modulus (N/m2 ) calculated from measured data and extracted from the linear trend curves of related sections of the stress–strain diagram in comparison to the fully bonded model.
Dummy cable TFMC cable untreated TFMC cable heat-treated
Fully bonded model
1. Loading
1. Load release and load restoring
2. Load release and load restoring
3. Load release and load restoring
1.3000 × 10+11 1.3126 × 10+11 1.3400 × 10+11
0.9676 × 1010 ± 1% 0.974 × 1010 ± 1.05% 0.955 × 1010 ± 3%
1.287 × 1010 ± 0.4% 1.341 × 1010 ± 1.5% 1.171 × 1010 ± 22%
1.241 × 1010 ± 2.6% 1.190 × 1010 ± 3.2% 1.029 × 1010 ± 23%
1.226 × 1010 ± 2.7% 1.142 × 1010 ± 4.3% 0.957 × 1010 ± 25%
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
tor samples with different reduced void fractions to possibly obtain correction factors for the cable modules calculated according to the fully bonded model. The cracking noise during the compression beginning at about 15,000 N, i.e. below the intrinsic force level of 16,985 N, may be interpreted as breaking of Nb3 Sn filaments. It is obvious that such events must be avoided, for instance by a drastic increase of the stiffness ratio of conduit to cable, which on the other hand would lead to an increased elongation. However, the possible variation of design parameters of CICCs is reduced to the void fraction inside the cable and to the wall thickness of the jacket.
783
Acknowledgements CEA Cadarache kindly provided the sample material. The technical assistance of Mr. H. Hartmann preparing the samples is acknowledged. Reference [1] A. Ulbricht, J.L. Duchateau, W.H. Fietz, D. Ciazynski, H. Fillunger and S. Fink, et al., The ITER toroidal field model coil project, FED 73/2-4, 2002.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Axial compression test on ITER-TFMC conductors at room temperature H. Fillunger a,∗ , R.K. Maix a , R. Prokopec a , A. Danninger b a b
Vienna University of Technology, Atomic Institute of the Austrian Universities, 1020 Vienna, Austria Vienna University of Technology, Institute of Materials Science and Technology, Austria
a r t i c l e
i n f o
Article history: Available online 25 January 2009 Keywords: Superconductor Heat treatment Thermal expansion Young’s modulus Double spring system
a b s t r a c t The superconductor used for the ITER TF Model Coil (TFMC) consists of 720 twisted Nb3 Sn-strands and 360 copper strands, which are cabled around a central spiral and are surrounded by a stainless steel jacket. The conductor is heat-treated at 650 ◦ C in a stainless steel mould. After cool down to room temperature the conductor is found to be elongated by 0.45 mm/m, which can be attributed to the lower shrinkage of Nb3 Sn. This means that the jacket is under tensile stress and the cable under longitudinal compression after cool down. This effect aggravates upon cooling further to the operating temperature. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. In order to evaluate and possibly predict such elongations, it is necessary to know the Young’s modulus of the cable. Therefore, we investigated samples cut from a dummy conductor, a not heat-treated and a heat-treated TFMC conductor by mechanically compressing them. From the stress–strain diagram the elastic modulus of the cables was determined and found to be about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cables turned out to be fairly independent of the state of the cable as all three cables show almost the same modulus. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket. © 2008 Elsevier B.V. All rights reserved.
1. Introduction After heat treatment in a stainless steel mould at 650 ◦ C and cool down to room temperature the TFMC conductor showed an elongation of about 0.45 mm/m, which was found to be about the same for all pancakes [1]. The superconductor consists of 720 twisted Nb3 Sn-strands and 360 copper strands, which are cabled around a centre spiral and surrounded by a stainless steel jacket (see Fig. 1a). As the conductor was wound against the inner contour of the groove of the reaction mould of race track shape it had to follow the expansion of the mould during warm-up, in spite of the lower expansion coefficient of Niobium. Therefore, the cable was stretched while the jacket remained stress free during the reaction treatment. During formation of the Nb3 Sn filaments, all components of the cable relaxed to be also stress free. During cool down the conductor shrunk less than the mould, which resulted in an elongation. This means that the jacket is under tensile stress, while the cable is under compression, which aggravates upon cooling further to operating temperature and results in critical current degradation.
∗ Corresponding author. Tel.: +43 1 58801 14123; fax: +43 1 58801 14199. E-mail address: fi[email protected] (H. Fillunger). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.023
If the elongation is calculated assuming all components of the cable being fully bonded and using the material parameters according to the ITER material data collection, one ends up with about 1 mm/m, which is distinctly more than found during TFMC manufacture. In order to evaluate and possibly predict such conductor elongations with a double spring model, the cable’s compressive modulus must be known, which obviously is considerably lower than calculated. Therefore, samples prepared from three different conductors received from CEA Cadarache (see Fig. 1b) were investigated: • Three samples cut from a piece of a dummy conductor. • Three samples cut from a piece of a not heat-treated superconductor. • Three samples cut from a piece of the heat-treated conductor. The samples were cut by a wire-electro-erosion process making the surfaces plane and parallel. The compaction tests were performed using Zwick/Roell Z250 test machine. The loads were measured by a load cell with 250 kN nominal force and accuracy class 0.5 (for forces from 2 kN upward) according to EN ISO 376, the displacement was picked up from the motion of the crosshead by a sensor with an accuracy class of 0.5 according to EN ISO 9513. The compliance of the machine frame was determined by a dummy measurement of the bare compression piston.
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
781
Fig. 2. Typical diagrams obtained on the dummy conductor and the TFMC conductor before and after the reaction heat treatment.
2. Results Some of the samples showed displacement curves that were shifted to higher compressive loads, which could be explained by uneven surfaces of the cable underneath the stamp. This could be corrected by setting the zero-displacement-point numerically to a compressive load of 500 N. Fig. 2 shows typical load–displacement curves of the three types of conductor. The region of interest is restricted to compressive forces up to the order of the stretching force acting on the stainless steel jacket of the TFMC conductor at room temperature. Taking the cross-sections of all strands and referring the displacements to the original sample lengths, the load–displacement diagrams were converted into stress–strain diagrams. 3. Determination of the Young’s modulus of the cables
Fig. 1. (a) Build-up of the TFMC conductor. (b) Compression samples and a trial sample between the expanded fixture assembly.
The fixture assembly consists of special rings for placing the samples in the centre of the testing machine and a steel piston for transmitting the compressive force to the cable inside the jacket. The automatic test runs were structured in 10 steps. Before starting them, the samples were preloaded with 50 N (Table 1).
The stress–strain curves of each sample were subdivided into several branches. The linear trend curves of each branch give the medium values of Young’s modulus. As an example, the stress–strain diagram with added trend curves of a reacted conductor is shown in Fig. 3. In Table 2, the values of the Young’s modulus calculated from measured data, based on the sum of the cross-section of all strands, and obtained from the linear trend curves of the stress–strain diagrams are listed and compared with those calculated from the fully bonded model. The measured values are of the same order of magnitude as those measured by P. Decool et al. reported in an internal report of CEA Cadarache (Cable 40 KA CEA-MIC1, measure exper-
Table 1 Steps of the automatic test runs. Steps
Movement of crosshead
Limit
Speed
Comment
1 2
Compression (=downwards) Decompression (=)upwards
0.5 mm 5000 N
0.5 mm/min 0.5 mm/min
For all samples For all samples
3
Compression
0.8 mm
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
4, 6, 8, 10
Decompression
5000 N
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
5
Compression
65,000 N
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
7, 9
Compression
4.0 mm
0.5 mm/min 5.0 mm/min
For the samples no. 1 and no. 2 For the sample no. 3
782
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
Fig. 3. Stress–strain diagram with added linear trend curves of a reacted conductor sample.
imentale des characteristique mechanique compte rendu d’essai, report No. P/EM/93-38, 1993). The stiffness of the cables during the compaction phase (first loading) is nearly equal for all three different types of cables: the dummy cable, the not heat-treated cable and the TFMC cable. The stiffness is governed by the shape of the cable, the void fraction and the twist pitch. The influence of the different materials in the cable is of secondary importance. We are presently preparing investigations on samples with a reduced void fraction, in order to evaluate its effect. In the first load release and during the load restoring phase, which is the most relevant phase with respect to the conductor acting as a double spring system, the Young’s modulus corresponds to about one-tenth of that calculated according to the fully bonded model (see Table 2). At the operating point of the conductor, the Young’s modulus measured during the second and third load release and the load restoring phase is no longer relevant for the status of the cable, because the permanent deformation is already too high. During the cool down from the heat treatment temperature to room temperature no movement can take place between strands and the jacket. However, this is not possible during a compression test. The role of friction cannot be separated in the compaction measurement. Nevertheless by taking the average of the moduli measured during up- and down-loading this effect may be taken into account. 4. The double spring system of jacket and cable The TFMC conductor represents a double spring system during cool down to room temperature. The tension spring is represented by the jacket, while the cable is the counteracting compression spring. At a strain of 0.45 mm/m the intrinsic force level is 16985 N (see Fig. 4). The double spring is in equilibrium when Es × εs × As = Ec × εc × Ac (E = elastic modulus, ε = strain, A = crosssection; s stands for stainless steel and c for cable). The cable is compressed by the internal spring force according to the experimental characteristic.
Fig. 4. Double spring system consisting of the jacket being the tension spring and the cable being the compression spring.
In this situation the superconducting cable has already experienced a compression of 3.8 × 10−03 which is about 8.5 times as much as the jacket elongation and is beyond the acceptable deformation of the cable during winding procedures (0.2%). External stretching forces like electromagnetic forces acting on the double spring will only lead to a small release of the cable’s precompression. 5. Acoustic emission during compression of samples cut from the heat-treated conductor Only the samples of the reacted conductor containing Nb3 Sn filaments produced a clearly audible cracking noise when reaching a compression force of more than 15,000 N. The cracking noise became intensive at compacting forces of more than 18,000 N and ended at a displacement of about 2.5 mm. As seen in the load–displacement diagrams, the starting value of the cracking noise is below the value of the remaining spring force (16,785 N) in the reacted TFMC conductor at room temperature. 6. Conclusions The Young’s modulus of the cable inside the jacket of the TFMC conductor is about one-tenth of that calculated assuming all components of the cable being fully bonded. The stiffness of the cable turns out to be fairly independent of the state of the conductor, since dummy, unreacted and reacted conductors show nearly identical values. The cable and the jacket can be interpreted as a double spring system, the jacket being the expansion spring and the cable being the compression spring. The mechanical compression of the cable is about 8.5 times larger than the measured elongation of the jacket. In order to evaluate and possibly predict the elongation of a conductor, its compressive modulus should be known. It is a function of the materials involved, their geometry and the void fraction. As the void fraction of the ITER conductors is now considerably lower than that of the TFMC conductor, we are preparing tests on conduc-
Table 2 Young’s modulus (N/m2 ) calculated from measured data and extracted from the linear trend curves of related sections of the stress–strain diagram in comparison to the fully bonded model.
Dummy cable TFMC cable untreated TFMC cable heat-treated
Fully bonded model
1. Loading
1. Load release and load restoring
2. Load release and load restoring
3. Load release and load restoring
1.3000 × 10+11 1.3126 × 10+11 1.3400 × 10+11
0.9676 × 1010 ± 1% 0.974 × 1010 ± 1.05% 0.955 × 1010 ± 3%
1.287 × 1010 ± 0.4% 1.341 × 1010 ± 1.5% 1.171 × 1010 ± 22%
1.241 × 1010 ± 2.6% 1.190 × 1010 ± 3.2% 1.029 × 1010 ± 23%
1.226 × 1010 ± 2.7% 1.142 × 1010 ± 4.3% 0.957 × 1010 ± 25%
H. Fillunger et al. / Fusion Engineering and Design 84 (2009) 780–783
tor samples with different reduced void fractions to possibly obtain correction factors for the cable modules calculated according to the fully bonded model. The cracking noise during the compression beginning at about 15,000 N, i.e. below the intrinsic force level of 16,985 N, may be interpreted as breaking of Nb3 Sn filaments. It is obvious that such events must be avoided, for instance by a drastic increase of the stiffness ratio of conduit to cable, which on the other hand would lead to an increased elongation. However, the possible variation of design parameters of CICCs is reduced to the void fraction inside the cable and to the wall thickness of the jacket.
783
Acknowledgements CEA Cadarache kindly provided the sample material. The technical assistance of Mr. H. Hartmann preparing the samples is acknowledged. Reference [1] A. Ulbricht, J.L. Duchateau, W.H. Fietz, D. Ciazynski, H. Fillunger and S. Fink, et al., The ITER toroidal field model coil project, FED 73/2-4, 2002.