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We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb/Nb3Sn wires at room temperature and at 7 K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7 K, respectively. From the obtained data, we estimated the deviatoric strain. At 7 K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain [variant_greek_epsilon]pb=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative evaluation of the deviatoric strain dependence of the upper critical field for the practical Nb3Sn wire. This is very important for the understanding of the general axial strain dependence of the superconducting properties. [ABSTRACT FROM AUTHOR]

Abstract: The strain dependence of critical current I c of a commercial RABiTS (Rolling-Assisted Biaxially Textured Substrates) YBCO coated conductor tape with Cu stabilizing layers on both surfaces has been studied by two kinds of bending apparatus and a tensile apparatus at 77K and 293K. According to the voltage measurements using multi-taps in the longitudinal direction, local I c degradation was also observed. In the continuous bending-tension test at 77K, in which YBCO layer was subjected to the tensile strain, I c began to decrease rapidly beyond the strain of 0.6% after initial moderate degradation. In the continuous bending-compression tests, the I c began to decrease rapidly beyond the strain of 0.4%. The strain for the beginning of degradation of I c obtained by tensile tests at 77K was almost the same as that by the bending-tension tests. Bending test at RT using FRP holders showed I c degradation not so different from that by the continuous bending tests at 77K. In all tests, the I c initially degraded locally irrespective of the position in the specimens. Furthermore, I c degradation by repeated bending was also studied. [Copyright &y& Elsevier]

It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. We have used our novel experimental setup, which utilizes the closed electric circuit concept for critical current and stability measurements of multi-stand superconducting cables. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. Significant degradation in the critical current of the cable was observed when the average compressive stress was about 20 MPa. This degradation was found irreversible after unloading. We tested the cable with epoxy or ice molds as well. No degradation was observed in the molded cables. We also tested the cable with smaller void fraction. In this case, significant degradation in critical current was observed. [ABSTRACT FROM AUTHOR]

We developed a novel critical current and stability experimental setup utilizing a closed electric circuit with a multi-stand superconducting cable. The feature of this setup is transverse mechanical loading implied to the multi-strand cable in the transverse direction. It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. Furthermore, these phenomena were only observed in the ITER full size conductors with large Lorentz forces under high magnetic field. The advantage of our setup is a critical current measurement with comparable mechanical stress under high magnetic field. Employing an inductive critical current measurement technique, we conducted the experiment with transport current of around 10 kA without any power supply nor current lead. As experimental results, we observed significant degradation due to compressive stress of around 30 MPa. This degradation was found irreversible, when it was unloaded. [ABSTRACT FROM AUTHOR]

Abstract: In order to supplement the VAMAS round robin test (RRT) on the bending strain dependence of critical current, I c, of Ag/Bi-2223 tapes conducted in 2001, a miniature bending RRT at 77K and room temperature, RT, participated by three laboratories has been conducted. Three devices developed by Goldacker of Karlsruhe, which allow continuous change of the bending radius of the sample and measurements of I c at 77K have been used. The onset strains for steep I c degradation, ε irr , at 77K, 0.55%, was larger than that at RT, 0.37%. This is due to an additional pre-compression in Bi-2223 filaments for the bending at 77K. The comparison of the present results bent at RT with those in the VAMAS-RRT showed that the scatters in the degradation behaviors were substantially small in the present results. ε irr of the former was larger than that of the latter. These appear to be explained by the different thermal histories. Based on the results, some attempts to reduce the scatter of bending test results were tried. [Copyright &y& Elsevier]

Bending strain dependence of critical current, Ic, of Ag/Bi-2223 57 filament tapes was investigated at 77 K and room temperature, RT, using a device invented by Goldacker. The strain where Ic start to degrade steeply at 77 K, 0.48%, was larger than that at RT, 0.36%. This is an indication of an additional pre-compression in filaments to be bent at 77 K compared with that at RT. According to the voltage measurements using multiple taps in the longitudinal direction, the Ic degradation behaviors at both temperatures varied depending on the section in the tape, due presumably to the difference in the crack propagation characteristics associated with the non-uniformity of the micro-structure. The onset strain for Ic degradation was more precisely determined. The degradation characteristics were shifted to the smaller strain in the present result as compared with that obtained by bending tests using G10 sample holders of different bending radius. The difference can be explained by the different thermal history on setting specimens to the holder and the constraints during cooling. The repeated bending tests at 77 K showed that the discrimination between the gradual and steep degradation stages was clear and the degradation was controlled by both the maximum strain and the strain range. [Copyright &y& Elsevier]

In order to investigate the compressive mechanical characteristics of Sm 123 single-grain bulk superconductors fabricated by TSMG method, compressive tests in the direction parallel and perpendicular to the c-axis have been carried out by using small specimens cut from the bulk samples. Stress-strain curves in the direction perpendicular to the c-axis were almost linear. The Young's moduli measured were 155-165 GPa. In the case of loading parallel to the c-axis, the curves were exponential in the low stress region. As the stress increased, they tended to a linear relation. The apparent Young's modulus in this region was 75-110 GPa. These characteristics are mainly ascribed to the closure behavior of pre-existing micro-cracks perpendicular to the c-axis. The average fracture strength in the direction of c-axis, 350-400 MPa, was larger than that perpendicular to it, 235-275 MPa. Specimens in the former were generally broken into great many small pieces while those in the latter were broken into some parts by lateral cracking. The compressive Young's modulus and the fracture strength for each direction were higher than those evaluated by the tensile loading, respectively. The effects of specimen length on the compressive mechanical properties were also discussed. [ABSTRACT FROM AUTHOR]

We have started a new project to develop a cryogen-free 19 T superconducting magnet with a 52-mm room temperature bore. The magnet is designed to be consisting of three sections, i.e., the innermost high-Tc superconducting coil section, the middle Nb3 Sn section and the outer NbTi section. The electromagnetic hoop stress is applied to the supercon- ducting wire due to the magnetic field generation. It is described to the products of magnetic field, transport current density, and winding radius. The maximum electromagnetic stress is estimated to exceed 200 MPa in the middle Nbc Sn section and 50 Pa in the high-Ta section. It is important to explore the mechanical characteristics to design the magnet. In this study, mechanical properties, i.e., stress-strain characteristics and stress dependence of the critical current, were investigated for oxide superconducting tapes. [ABSTRACT FROM AUTHOR]

Abstract: An inter-comparison program (round-robin-test) aimed at the establishment of a standard test method for the bending strain effect on the critical current in Ag alloy-sheathed Bi-2223 tapes was implemented with participation of 12 laboratories around the world. Samples were bent at room temperature using bending devices that were specially designed for this RRT. The critical current was measured at 77K and in the self-field. After critical current measurement of an unbent sample, bending of the sample and subsequent critical current measurements were done from 0.2% to 1.0% in 0.2% steps of strains on a single sample. Reported data from participants showed that the critical current values start to decrease from 0.4% strain and continue to decrease to 1.0% strain. Moreover, the data scatter starts to increase stepwise at 0.4% strain, being correspondent to the variation in the critical current values with bending strain, and it continues to increase gradually to 1.0% strain. The coefficient of variation values are 2.5%, 5.1% and 6.4% for 0%, 0.4% and 1.0% bending strain, respectively. Results were also compared in terms of the irreversible bending strain. Possible sources of error are discussed. [Copyright &y& Elsevier]

Abstract: Since the high T c rare-earth based bulk superconductor is subjected to the tensile load in radial and circumferential direction by the Lorentz force generated in the magnetization process, the evaluation of the strength by the tensile test is indispensable. Ultimate tensile strength of the bulk superconductor depends on the defects in each sample. Many artificial specimens containing voids were generated for numerical stress calculations. The distribution of the voids diameter in each artificial specimen was based on the observations of Dy123 containing 25wt% Dy211 (abbreviated as Dy25). Furthermore, the effect of a center crack superposed to the field of the voids on the strength was analytically evaluated. The strength depends on both the size and the location of the voids. The maximum crack length which has eventually no effect on the strength was evaluated. By the evaluation method proposed in this study, it was found that if there had been no void in the Dy25 bulk sample, the tensile strength could have been estimated to be 63MPa. The voids increase the stress intensity factor at the crack tip. If there is a crack with 0.16mm or more in the Dy25 superconductor bulk with the porosity 10%, the fracture may not be originated around a void but at a crack tip. [Copyright &y& Elsevier]

Abstract: Bending tests of the as-grown and the annealed specimens cut from Dy123 bulk samples were carried out. The Young’s modulus of the annealed specimen was lower than that of the as-grown specimen. On the other hand, the fracture strength of the annealed specimen was slightly higher than that of the as-grown specimen. The fracture toughness of the annealed specimen was also higher. The smaller bulk showed excellent mechanical properties in comparison with the larger bulk. The mechanical properties of these bulks are discussed in terms of the porosity and the mechanical properties of the matrix evaluated by Vickers indentation tests. [Copyright &y& Elsevier]

Abstract: Superconductors carrying currents larger than several 10kA are required for magnetic confinement fusion systems. A cable-in-conduit-conductor (CICC), which has about 1000 of twisted thin superconducting strands in a ridged conduit, is a promising candidate for such large and high magnetic field superconducting system. A specific phenomenon in the CICC is so called avalanche-like quench (AQ). When some strands become resistive due to current saturation at critical current and/or a mechanical disturbance, the current re-distributes from the strands into neighboring ones, which are electro-magnetically coupled more strongly than the others. This causes current saturation in the neighboring strands. Finally, the whole cable becomes normal. We call this phenomenon an avalanche-like quench. Small multi-strand cables were tested by induced-current methods. Quench behaviors of both Nb3Sn and NbTi cables are observed and compared. In the experiment, for the Nb3Sn cable, the critical current measurement was stably carried out at high field without quench. In contrast, for the NbTi cable, we observed a number of AQs during inductive energization. Consequently, we suspected that the reason is based on the difference of critical temperature (T C) between Nb3Sn and NbTi. The difference of T C is several Kelvins, however, their macroscopic behaviors differ significantly. [Copyright &y& Elsevier]