Evaluating fatigue and electromechanical degradation in REBCO CC tapes at cryogenic temperature using optical fiber sensors

Abstract Preliminary demonstrations show Rayleigh scattering-based optical frequency-domain reflectometry distributed optical fiber sensors (DOFSs) effectively detect and locate fatigue damage in rare-earth barium copper oxide (REBCO) coated conductor (CC) tape by reconstructing time-series macro-strain data. While significant degradation of critical current ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) may occur prior to mechanical failures when these tapes are subjected to cyclic stresses or strains. To address this issue, a comprehensive study was conducted on the degradation mechanism and critical current level of REBCO CC tapes under cyclic fatigue loading. A new method has been developed to estimate fatigue damage on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> degradation by reconstructing time-series macro-strain data using DOFSs. The damage variable was introduced to quantitatively characterize impact on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> degradation of REBCO CC tape under cyclic fatigue loading. Additionally, the fatigued specimens were analyzed using scanning electron microscopy for microstructural changes, aiming to verify the feasibility of the method and elucidate the mechanism behind <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> degradation. Experimental results demonstrate that the introduced damage variable exhibits consistency with the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> degradation, which has been further adapted to evaluate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> degradation of REBCO CC tapes in combination with Weibull probability distribution function. This provides a reliable and real-time measurement methodology for I c degradation assessments in REBCO CC tapes under actual operating conditions, facilitating to provide critical reference values for optimizing designs and ensure the stable and safe operation of superconducting structures.