Local variations in the critical current degradation of Ag/Bi2223 tape by tensile and bending strains

Using two types of tensile testing device, we have measured the strain dependence of I-c in AgMgNi-sheathed Bi-2223 57 core tapes at 77 K. The I-c degradation onset strains for the specimens with a short gauge length of 17.6 mm varied from 0.40 to 0.78%. In the relatively long samples with seven voltage terminals 5 mm apart along the gauge length of 60 mm, the smallest strain for the initiation of I-c degradation for the sections was 0.26%. The I-c in some other sections, however, began to degrade at larger strains and no degradation was found in another section even at 0.36%, which by far exceeds the strain corresponding to the yield stress of this tape. The local elongation in the first damaged section occupies most of the total elongation in the sample, while the net strain in the latter section is thought to be significantly less than average and no damage occurred there. The bending tests using several mandrels showed considerably consistent I-c degradation behaviour with increasing strain in all the sections, indicating that the strain distribution is uniform in the longitudinal direction.