Microscopic fracture of filaments and its relation to the critical current under bending deformation in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀composite superconducting tapes

Abstract

The strain dependence of the critical current, I c , of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (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 I c with the increase in the Bi2223 tape curvature was calculated from the volume fraction of the broken filaments. The calculated I c 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.

Theory analysis of critical-current degeneration in bended superconducting tapes of multifilament composite Bi2223/Ag

“…It was revealed that the critical-current in the superconducting composite tapes under bending deformation decreases seriously beyond an irreversible strain, and once the applied strain exceeds such the irreversible strain, the critical-current never returns to the original value even when the applied strain is released [4][5][6]. The influence of strain on the damage of superconductor filaments in Bi2223/Ag composite tapes was also observed in the experiment study by the scanning electron microscopy (SEM) [7,8]. The micrographs showed that damage of filaments was not detected for the case of small deformation, while clear cracks were observed in the tensile side of the tape when the applied strain exceeds an irreversible strain.…”

“…The mechanical properties are among the most important parameters for HTS tapes [1][2][3]. The relation of deformation and fracture behavior of multi-filamentary Bi2223/Ag composite tapes to the superconducting property has been studied widely [4][5][6][7][8][9]. It was revealed that the critical-current in the superconducting composite tapes under bending deformation decreases seriously beyond an irreversible strain, and once the applied strain exceeds such the irreversible strain, the critical-current never returns to the original value even when the applied strain is released [4][5][6].…”

Prediction of critical current–bending strain relation of Bi2223 composite tape using residual strain of filaments, load–strain curve and geometry of cross-section

“…It has been known that the critical current of Bi2223/Ag/Ag alloy composite tapes decreases with increasing applied tensile (e T ) and bending (e B ) strains beyond the irreversible strain due to the damage evolution of the Bi2223 filaments [1][2][3][4][5][6][7][8][9][10][11][12][13]. In the damage evolution process under the applied tensile strain, the residual strain e Bi,r of the filaments, which arises during cooling due to the difference in coefficient of thermal expansion among the Bi2223 filaments, Ag and Ag alloy [3,7,9,10,12] and tensile fracture strain e Bi,f of the filaments in the sample length direction (current transport direction) play a dominant role in the damage process.…”

“…In the damage evolution process under the applied tensile strain, the residual strain e Bi,r of the filaments, which arises during cooling due to the difference in coefficient of thermal expansion among the Bi2223 filaments, Ag and Ag alloy [3,7,9,10,12] and tensile fracture strain e Bi,f of the filaments in the sample length direction (current transport direction) play a dominant role in the damage process. Under the applied bending strain, they also play a dominant role since the damage is caused by the tensile strain in the sample length direction [10,13].…”

Direct measurement of mechanical properties of Bi2223 filament using Ag alloy removed tape

“…However, detailed understanding of stress and strain states of Bi2223 filaments in deformed composite tape is difficult. This is because, in addition to the presence of thermal residual stresses that arise during processing, a composite tape has several components whose stress-strain curve is unknown, and Ag and Ag alloy deforms both in elastic and plastic states [5][6][7][8][9][10]. Present methods involve analyzing the stress-strain behavior of each component from the composite stress-strain behavior using the rule of mixture [5,[8][9][10].…”