Thermally and mechanically induced residual strain and strain tolerance of critical current in stainless steel-laminated Bi2223/Ag/Ag alloy composite superconductors

Abstract

It is known that the tensile strain tolerance of the critical current of Bi2223 composite superconductors is improved when the compressive residual strain of Bi2223 filaments in the current transport direction (sample length direction) is enhanced. In the present work, the thermally and mechanically induced residual strain accumulation process of a stainless steel-laminated Bi2223/Ag/Ag alloy superconducting composite tape fabricated at American Superconductor Corporation was studied. A calculation procedure is presented for a description of the strain change of the constituents (Bi2223, Ag, Ag alloy and stainless steel) during cooling and heating, and during lamination, followed by the stress relaxation. As the input values, the mechanical property values of the constituents and the residual strain of Bi2223 filaments in the laminated composite tape at room temperature, measured with x-rays, were used. Such an approach revealed the change of residual strain of each constituent in the thermal and mechanical process in the laminated composite tape. Then, from a comparison of the residual strain accumulation in the laminated tape with that in the insert tape, the effects of the stainless steel lamination on the enhancement of the compressive residual strain of Bi2223 filaments and the improvement of the tensile strain tolerance and strain tolerance window of critical current at 77 K of the composite tape are discussed.

Effect of fatigue loading on critical current in stainless steel–laminated DI-BSCCO superconducting composite tape

“…3 compares the relationship between the stress and strain at 77 K, and that between the normalized critical current at the tively. These values correspond well with previously reported results where the fracture strain of the filament almost doubled owing to lamination [9][10][11]. In the next section, the maximum fatigue stress for SUS3ply tapes is selected according to the irreversible stress under the loading step, r l,irr95% , of 342 MPa, and that under the unloading step, r u,irr99% , of 315 MPa.…”

“…It is also reported that the lamination suppresses the fracture of Bi2223 filaments. Consequently, the fracture strain of Bi2223 filaments in the laminated composite tapes is twice as much as that of filaments in the non-laminated composite tapes [9][10][11].…”

“…Indeed, the tensile strain tolerance of the critical current in Bi2223/Ag/Ag alloy tapes is dramatically improved when the tapes are laminated with stainless steel [5][6][7][8]. This is because the compressive residual strain of Bi2223 filaments is increased by reinforcing the composite tape with materials having high modulus and low coefficient of thermal expansion [9]. Additional compressive residual strain is generated in the Bi2223 filaments when the mechanical pre-tension externally imposed by the reinforcing materials is removed after lamination.…”

Applicability of Weibull Distribution to Description of Distributed Normalized Critical Current of Bent-Damaged Bi2223 Composite Tape

“…Because the " r is negative (compressive) in the Bi2223 composite tape, as has been demonstrated by X ray diffraction analysis, 7,8) the actual fracture strain of the filaments in the composite tape is by À" r higher than the intrinsic fracture strain " f of the filaments alone. 2,4,[6][7][8]11,14) The " f À " r value is different from location to location within a specimen as well as from specimen to specimen. In the preceding works, 11,12) the heterogeneous damage behavior was formulated by the distributed " f À " r values and was correlated to the distribution of critical current values.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Heterogeneous damage occurs, and the damage evolution behavior differs from specimen to specimen and from location to location within a specimen. [9][10][11][12][13][14] Accordingly, when a number of specimens are tested, the critical current differs from specimen to specimen.…”