Ybco Superconductor Characterization under Shear Strain

Abstract

YBCO based high temperature superconductors in practical applications are subjected to shear strain. `Critical current characteristics’ of such high temperature superconductors are known to get degraded in strain state. In this work, shear stresses resulting from the finite twisting of HTS tape having varying widths have been modeled using FEA analysis for different twisting angles. Supporting the findings of this model, an American Superconductor Corporation (AMSC) 2G YBCO tape of a given width has been twisted and experimentally investigated in self field for a given current ramp-rate. Under uniform twist of the YBCO tape at 77 K, the degradation in the current carrying ability up to 30% was observed. The irreversibility in the current carrying ability of HTS tape was also observed beyond the twisting angle per unit length of 25 degree/cm. The superconductor to resistive transition index, `n’ is found to behave in an identical manner to the critical current as a function of twisting angle. Such degradation is largely attributed to the torsional shear strain resulting from the twisting. High temperature second generation (2G) commercial grade YBCO coated conductors are available now-a-days in long length for extensive uses in electrical power cable (Rutherford cable) in the form of stacked twisted HTS conductors [1-3]. Even the proposal of coated conductor in conduit cable (CCICC) employs the idea of stacked HTS conductor twisted over different transposition lengths [4]. In such applications individual HTS tape subjects to twist induced shear stress/strain. The strain beyond a certain limit can critically affect the transport property as well as can modify the super-current carrying path in the coated conductor [5]. Hence, it has become necessary to ensure the maximum tolerable shear stress/strain applied to HTS which could induce recoverable damage. Previous works on the torsoinal strain dependence of Bi-2223, YBCO coated conductor shows experimental findings on Ic degradation behavior. Finite element analyses along with theoretical co-relation also have been surveyed on torsion experiments of high temperature superconducting tapes [6-8]. In case of pure torsion, the strain induced on the tape cross-section is non-uniform and complex. However, the maximum torsional shear strain could be determined at the midpoint of the width side of the cross-section using the formula (et = tθ/L) [8-9]. In the present study, FEA modeling for shear stress developed on 2G YBCO tape of different widths has been considered. Further, the shear strain dependence of the critical current of coated conductor has been investigated experimentally for a high current ramp-rate. In this paper, the FEA analysis is discussed in section-II. The experimental results and conclusion are discussed in section-III & section-IV respectively.