RELATION OF THE STRENGTH DISTRIBUTION OF NB3SN TO THE CRITICAL-CURRENT OF A PRESTRESSED MULTIFILAMENTARY COMPOSITE SUPERCONDUCTOR

From an analysis of the change in the superconducting critical current at 4.2 K caused by pre-stressing treatment at room temperature, the d istribution of tensile strength of Nb3Sn in multifilamentary bronze-pr ocessed composite wires heat-treated at 973 K for 8.6 (sample A), 43 ( B) and 260 ks (C) is estimated using the two-parameter Weibull distrib ution function. The shape parameters of the Weibull distribution are 7 .2, 12. and 14, and the average strengths of the Nb3Sn are 1.3, 1.0 an d 0.79 GPa for samples A, B and C respectively for the present specime n length of 25 mm. This means that the scatter of strengths and also t he average strength for specimens of length 25 mm decrease with increa sing heat-treatment time. An attempt to predict the length dependence of the critical current of pre-stressed specimens from the data for sh ort specimens is presented. It seems that the permissible pre-stress b elow which there is no breakage of Nb3Sn (for all Nb3Sn transport curr ents) is reduced by 20-30% when the specimen is lengthened from 25 mm to 300 m, while the average strength of Nb3Sn is reduced by 50-70%.