Bending and axial strain dependence of the critical current in superconducting BSCCO tapes

Abstract

A comparison is made between the critical current (Ic) versus bending strain and axial strain of superconducting multi-filamentary Bi2Sr2Ca2Cu3Ox (Bi-2223) AgMg sheathed tapes. For the bending strain measurement the tape is sandwiched between a curved base and cover plate. Six sets of bending plates introduce bending strains ranging from 0% to 1.0%. The measurements show a slight decrease in Ic after the first bending step after which the degradation becomes more pronounced. The Ic in a bent conductor is calculated assuming a linear axial strain profile inside the conductor. For this calculation the Ic degradation determined in an axial compression and elongation experiment is used. The model predicts an immediate decrease of Ic, caused by the compressive strain dependence. There is a good agreement (within 5%) between the measured data and the calculated values. Based on this good agreement it can be concluded that a possible shift in the neutral line or the formation of additional cracks due to bending has no significant influence on the Ic degradation. It is concluded that the influence of thermal contraction is crucial for a good calculation.

Prediction of variation in critical current with applied tensile/bending strain of Bi2223 composite tape from tensile stress-strain curve

“…Tensile damage was given to the samples directly at 77 K and the critical current was measured at the same temperature, as stated in B above. For estimation of the damage strain parameter f − r that determines directly the irreversible tensile strain T,irr for critical current, [1][2][3][4][5][6][7][8][10][11][12][15][16][17][18][19] the stress-strain curve was measured at 77 K.…”

“…In the process of critical currentbending strain relation measurement, when the specimens are cooled to 77 K, compressive strain is added to the Bi2223 filaments in the current transport direction due to the higher coefficient of thermal expansion of metallic constituents ͑Ag and Ag alloy͒ than that of Bi2223 filaments. [1][2][3][4][5][6][7][8][10][11][12][13][14][15][16][17]24,[26][27][28] As the damage of filaments in the bent specimens is caused by the tensile strain in the current transport direction, the filaments have been damaged by the bending-induced tensile strain at room temperature. The additionally exerted compressive strain on filaments at 77 K due to cooling does not cause further damage.…”

“…The damage of the filaments at room temperature is conserved at 77 K, and hence, the critical current at 77 K is determined by the damage at room temperature. 7,16,17,24 FIG. 1.…”

“…It is noted here that, as the Ag in the composite tapes is soft, it is yielded in tension at room temperature in the initial state where the samples are cooled from the heat-treatment temperature to room one due to the difference in coefficient of thermal expansion among the constituents. [1][2][3][4][5][6][7][8][10][11][12][13][14][15][16][17][18]24,[26][27][28] When the samples are further cooled to 77 K, Ag keeps the tensileyielded state also at 77 K. When the sample is warmed from 77 K, Ag comes to be yielded in compression on the way from 77 K to room temperature. Accordingly, once the samples are cooled to 77 K and then warmed to room temperature, Ag is yielded in compression at room temperature, 15,16 whereas, it is yielded in tension at room temperature in the initial state.…”

“…The residual strain r is introduced during cooling from heat-treatment temperature in processing due to the difference in coefficient of thermal expansion among the constituents ͑Bi2223, Ag, and Ag alloy͒. [1][2][3][4][5][6][7][8][10][11][12][13][14][15][16][17][18]24,[26][27][28] In case of application of bending strain B to samples, the tensile strain in the sample length direction plays a dominant role for damage evolution. 7,8,19,20,[24][25][26][27][28][29] On this point, the tensile damage behavior is related to the bending one, and f − r is a common key parameter for damage evolution under applied tensile and bending strains.…”