Bending Strain Sensitivity of Critical Current in REBCO Coated Conductor Winding for Coil Application at 77 K

High-temperature superconducting (RE)Ba2Cu3 O7-x (REBCO)-coated conductor (CC) tapes are known for their capacity to tolerate high tensile stress under uniaxial tension, which is typically in the range of 600-700 MPa at 77 K. However, when used in a coil, there is a limit to the allowed strain before degradation of the critical current (I-c) occurs. This is attributed to the combined strain due to bending and axial tension which was caused during fabrication, cool down, and operation of coils and magnets and corresponds to the irreversible strain limit of I-c degradation. Applied strain along the CC tape's edgewise direction (hard bending strain) also affects the irreversible strain limits of I-c considerably. With such conditions, degradation of the REBCO coil performance can be expected. Therefore, the current study aims to systematically investigate the mechanism for such I-c degradation in REBCO coils. Linear superposition of strains induced in different modes of easy and hard bending were studied analytically and experimentally. The I-c degradation behaviors were examined in IBAD/REBCO CC tapes in windings with different pitches on mandrels with different diameters. The investigation was done at 77 K and in self-field conditions using an apparatus designed for coil testing. The bend strain parameters in coils were analyzed, including the radius of curvature and pitch between CC tape turns. Through the linear superposition of strains induced in the REBCO coils, an appropriate approach is suggested to effectively handle the I-c degradation under winding including edgewise bending.