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 (R) 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 R 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 R under LCF. Results reveal a clear R dependence of fatigue behavior in REBCO CC joints. Specifically, at a particular R, employing R = 0.1 results in a gradual degradation of critical current (I-c), while R = 0.5 induces a considerably faster and less recoverable I-c decline. The results highlight the interaction between R 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.