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Microscopic fracture of filaments and its relation to the critical current under bending deformation in (Bi,Pb)2Sr2Ca2Cu3O10 composite superconducting tapes

The strain dependence of the critical current, Ic, of (Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223)/Ag/Ag–Mg composite superconducting tapes has been studied both experimentally and analytically under bending deformation. Tests have been carried out for one type of tape used in the VAMAS bending round-robin programme. The complex stress–strain behaviour of each component was first analysed in tension. This was done by comparing the stress–strain curves of composite tapes with those of Ag and Ag–Mg alloy tapes. Here, the plastic deformation (work hardening) of Ag and Ag–Mg alloy, and the thermal residual strain due to the manufacturing process were taken into account. The fracture strain of Bi2223 filaments was inversely determined as 0.08% to meet the global tensile stress–strain curve of the composite tape. The calculated stress–strain curves finally agreed well with the experimental results when the as-supplied bending strain was taken into account. Then, the analysis was modified to fit the bending deformation. Here, the movement of the neutral axis due to the non-symmetric and elastic–plastic stress–strain curves of the components and their Bauschinger effect were taken into account. The relative decrease of Ic with the increase in the Bi2223 tape curvature was calculated from the volume fraction of the broken filaments. The calculated Ic agreed well with the experimental results when the movement of the neutral axis and the Bauschinger effect were taken into account. Microscopic observation of the spatial distribution of the filament fracture indicated that the damage occurred at the outermost layer on the tensile side when the curvature was small, and then the damage front shifted to the inside layers. The observed fracture behaviour of the Bi2223 filament agreed well with the estimated location based on the above analysis.