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