Effects of Stress Ratio on Low-Cycle Fatigue Performance of Various REBCO CC Lap Joints at 77 K

In high-performance applications, the REBCO-coated conductor (CC) tapes are frequently pushed to their limits, resulting in fatigue-related failures, particularly at the vulnerable CC joint—a critical weak point in the structure of REBCO coils and magnets. This study investigates the influence of stress ratio (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i>) on the electromechanical durability of ultrasonically welded (UW) joints in two commercially available REBCO CC tapes. Employing a systematic approach, the research investigates fatigue behavior and cumulative damage under varied <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> values (0.1 and 0.5). Low-cycle fatigue (LCF) tests at 0.02 Hz provide in-depth insights into the fatigue behavior of each CC tape joint. Utilizing a flux-free hybrid welding approach, the study analyzes joint enhancement, electromechanical properties at varying <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> under LCF. Results reveal a clear <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> dependence of fatigue behavior in REBCO CC joints. Specifically, at a particular <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i>, employing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> = 0.1 results in a gradual degradation of critical current (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>), while <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> = 0.5 induces a considerably faster and less recoverable <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> decline. The results highlight the interaction between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> and fatigue cycles in REBCO joints, emphasizing the need for additional investigation across a broader spectrum of materials to comprehensively understand and optimize their performance in CC coils and magnets.