Recently, superconducting devices are getting larger as can be seen in magnets for plasma containment in CTR, MHD generator and energy storage. Multifilamentary composites consisting of superconducting Nb-Ti filaments in a normal metal matrix are hopeful for such large scale applications because of good superconducting properties and workability of Nb-Ti alloy. The mechanical and electrical behaviour of Nb-Ti superconductor itself and composites at ambient and low temperatures should be understood for the successful use of superconducting wires in large magnets. From this stand point, this paper reviews the current research on stress effects in superconducting Nb-Ti wires as follows. (1) mechanical properties of Nb-Ti superconducting alloy (2) tensile and fatigue properties and characteristic behaviour of Nb-Ti multifilamentary composite wires (3) critical current degradation under several stresses and resultant strains (4) proposed mechanism of the critical current degradation (5) influence of stress effects on coil performances (6) effect of cyclic stress (strain) on electrical resistivity of matrix metals as stabilizer Problem areas have been cleared but much research needs to be done.
A review of the stress effects in superconducting compound wires is summarized in the following. (1) A stress level of 0.7-2×108Pa and a strain level of 0.1-0.2% represents a threshold of significant critical current degradation. (2) The peak effects in critical current and critical temperature of superconducting composite wires appear to be associated with the intrinsic strain (compression) of 0.2-0.3%. (3) The decrease in critical current is almost recoverable up to strains of about 1% upon load removal. But the first 20% decrease in critical current is reversible only at strains within about 0.5%. (4) In the reversible strain region and the strain amplitude within about 0.15%, no additional degradation in critical current happens owing to fatigue. In the irreversible region, however, a damage due to fatigue occurs, and most of the additional Ic degradation occurs in the first 10 cycles. (5) At high strains where the degradation is irreversible, the effect is well accounted for by cracks of over 100mm observed in the compound layer. At the reversible region, the effect may be probably explained either by a change in the intrinsic critical temperature and field, or by an alternation of the vortex-pinning structure. (6) The effect of stress on the type A-15 superconducting compound is considered similar to that on the other compounds of type B-1 or C-15 etc. (7) In design conditions at high loads, operating currents of magnet should be determined using a stress-dependent set of Ic-H characteristics rather than the usual zero-stress Ic-H characteristics.
An apparatus for investigating superconducting properties of conductors under stress, to be used for superconducting Tokamak Toroidal magnet, is been constructing in JAERI. The main parameters of the apparatus is as follows. maximum load capability 10ton maximum magnetic field strength 5.5T maximum transport current to sample 2, 000A
Recent research activities on stress effects in various kinds of high-field superconductors at NRIM are briefly reported. Improved high-field superconductors usable under large mechanical stress are being developed. The specification of stress-critical current measuring facility installed in NRIM is described.
We have made experiments to decide the optimum winding tension in constructing high field magnets, using N3Sn tape of I. G. C. as material. The samples have been stressed uniaxially at room temperature and measured at liquid He temperature. When those tapes are cooled down to liquid He temperature there the thermal stress (or strain) are derived from the difference of the thermal expansion of components materials (Nb, Nb3Sn, Solder, Cu) Nb3Sn layer is under compressional stress for this effect and experimences degradation of Tc. The degradation coefficient of Tc (∂Tc/∂p) was found to be -0.16×10-3K/Atm. The degradation of Ic on the tensile stress are observed even in elastic region and it is conjectured that the region of this effect is exsistence of the solder layers and the lower elastic limit of Cu layers compared with that. Guided by the above obtained results we have manufactured a magnet with the low winding tension (-60kg/cm2) and have already achived field of 11.7T.
Experimental apparatus for measurements of the stress-induced change in critical currents of the monofilamentary Nb3Sn wires is described, and also shown is an example of the critical current characteristics as a function of tensile strain. The sample used is a round Nb3Sn wire with the diameter of 0.25mm, the thickness of Nb3Sn layer of about 7μm and the bronze to Nb ratio of 3.4.
The critical current density Jc of superconducting V3Ga tape is studied in the magnetic field up to 10T as a function of tensile stress which is applied at 4.2K. An increase in Jc with small stresses was observed and considerable degradation followed at higher stress region.
The stress effects in various multifilamentary Nb3Sn superconductors have been investigated for a large scale, high field magnet. The experimental apparatus utilizes a 6T splitpair magnet with the sample in a straight geometry. The results show that at stresses above 250-800MPa the critical current is significantly degraded, with the magnitude of the reduction dependent on reinforcement techniques and construction in the conductor. The effects are related to mechanical properties, especially 0.2% yield strength and apparent Young's modulus of Nb3Sn composites at 4.2K.
This paper summarizes our works on mechanical properties and Ic degradations in multifilamentary stranded compound wires and multifilamentary compound tapes, including discussions on stress effects of compound superconductor and coil performances.
The tensile behaviour at R. T., 77K, 4.2K and the degradation of the critical current with stress have been measured on multifilamentary Ti-Nb-Zr-Ta alloy superconductors. The assembly of the stress effect apparatus is as follows; At the center of the 60KOe superconducting solenoid coil, sample wire is hold around an FRP spool and the wire ends are gripped to the load train. Current is supplied through helium vapourcooled flexible leads up to 2000A. It was clear that a definite degradation of the critical current with stress was not observed up to the stress equal to one third of the fracture stress at 4.2K. This stress value should be defined the maximum allowable stress of alloy superconductors examined from stress effects.
An experimental apparatus for the research of the stress effects on superconducting wires has been developed. The apparatus is capable of loading tensile force up to one ton on a superconducting wire specimen with current up to 2000A under 5.5T magnetic field perpendicular to it at 4.2K. Experimental results of critical current degradation of NbTi and Nb3Sn superconducting wires under tensile stress are presented. It is confirmed that Nb3Sn superconducting wire is more sensitive to tensile stress than NbTi superconducting wire. A plan of the experiment hereafter is also described.