Since 1991, co-operative studies have been conducted toward the standardization of the low-cycle fatigue testing method under fully reversed axial-strain control at the liquid helium temperature (4K), Nine organizations were involved, and the low-cycle fatigue tests have been shared to determine whether or not rules for 4K-testing methods were needed. Materials used were SUS316L, the strain hardening type stainless steel, and JN-1, the strain softening type high strength stainless steel. The first viewpoints were the strain rate dependency of the serration behavior and the maximum degree of the strain control error due to such serrations at the given strain range, which were assumed to result from the typical response of stress-strain curves at 4K. The second viewpoints were the strain rate dependency of fatigue lives and the specimen temperature, the variations of which were considered to be the sources of the serrations. The third viewpoints were to verify the general definition of fatigue lives such as N0 and N25, which were used for high-temperature testing based on relations between the tensile peak stress and the crack size. Results obtained so far indicated that the typical rules needed for 4K-testing were strain control error within ±4% and a strain rate of 0.4%/s or less at which the minimum temperature rise was assured to be 1K or less. Also it was indicated that N0 and N25 were applicable for 4K-testing. This year, supplementary studies have been planned and a draft for JIS-code will be prepared in 1995.
Since oxide high-Tc superconductors were discovered in 1986, there were reports of Y-Ba-Cu-O superconductor with Tc in the 90-K class, followed by a 110-K class Bi-Sr-Ca-Cu-O superconductor and a 125-K class Tl-Ba-Ca-Cu-O superconductor. There have been many efforts towards developing superconducting wires using these oxide superconductors. Many processing technologies including solid reaction process, liquid process and film process have been investigated. To make the application of oxide superconducting wires practical, they should be long (-1, 000m), carry a high critical current density, of at least 10, 000A/cm2 at the minimum value under a relevant magnetic field, and not be strain-sensitive so that they can withstand the winding process, which is inevitable in practical applications. At present, a 1, 080m-long wire has been achieved using bismuth-based compounds with silver-sheathing processing technology. Critical current density of oxide superconducting wires has been improved through the understanding of how microstructures are related to Jc and can be controlled. In addition, multifilamentary wires proved to have good strain-resistant properties. Prototypes for practical applications have been fabricated using oxide superconducting wires. Among these applications, two categories proved to be promising. The first one is large current conductor application, such as current leads for magnets, bus bars and power transmission cables. The second one is magnet applications. Since oxide superconductors have a wide critical surface, we expect that these magnets can be used under a variety of cooling conditions with different coolants.
To achieve a further increase in critical current densities of NbTi multifilamentary superconducting wires for AC use, several wires having the theoretically designed Nb island-type artificial pins introduced into NbTi superconducting filaments were fabricated. Oversaturation phenomena were observed whereby Fp increased in the wide range of magnetic field and the position of peak Fp shifted systematically from the low magnetic field side to the high-field side as the reduction rate of artificial pins increased. High critical current densities, such as 1.42×1010A/m2 at 1T (obtained by wire with a filament diameter of the 0.1μm class) 8.7×109A/m2 at 2T and 6.1×109A/m2 at 3T (obtained by wire with a filament diameter of 0.4μm class) were achieved. Moreover, it was clarified that the field-position of peak Fp was decided by pin interval, ds, if the correlation length I66 was smaller than half of the fluxoid spacing, af. As a result, helpful guideline in designing critical current densities by introducing artificial pins was derived.