Enhanced critical axial tensile strain limit of CORC® wires: FEM and analytical modeling

Conductor on Round Core (CORC (R)) cables and wires are composed of spiraled high-temperature superconducting (HTS) rare-earth barium copper oxide (REBCO) tapes, wound in multiple layers, and can carry very high currents in background magnetic fields of more than 20 T. They combine isotropic flexibility and high resilience to electromagnetic and thermal loads. The brittle nature of HTS tapes limits the maximum allowable axial tensile strain in superconducting cables. An intrinsic tensile strain above about 0.45% will introduce cracks in the REBCO layer of straight HTS tapes resulting in irreversible damage. The helical fashion at which the REBCO tapes are wound around the central core allows tapes to experience only a fraction of the total axial tensile strain applied to the CORC (R) wire. As a result, the critical strain limit of CORC (R) wires can be increased by a factor of more than 10 that of REBCO tapes. Finite element (FE) and analytical models are developed to predict the performance of CORC (R) wires under axial tensile strain. A parametric analysis is carried out by varying the winding angle, the Poisson's ratio of the CORC (R) wire core, the core diameter, and the tape width. The results show that a small variation in winding angle can have a significant impact on the cable's axial tensile strain tolerance. While the radial contraction of the helically wound tapes in a CORC (R) wire under axial tensile strain depends on its winding angle, it is mostly driven by the Poisson's ratio of the central core, affecting the tape strain state and thus its performance. Contact pressure from multiple layers within the CORC (R) wire also affects the CORC (R) wire performance. The FE model can be used to optimize the cable design for specific application conditions, resulting in an irreversible strain limit of CORC (R) cables and wires as high as 7%.