Characterization of Multi-field Behaviors on Fatigue Damage Due to High Cyclic Loading in YBCO-Coated Conductors Fabricated by the IBAD-PLD Technology

Abstract

In practice, yttrium barium copper oxide (YBCO) tapes often experience different types of fatigue loadings including continuous winding stress, repeated thermal cycles, and periodic electromagnetic force, consequently resulting in a poor performance. Based on cyclic loading tests in the structural design of some large YBCO superconducting devices, a 95% critical current (I_c) retention tensile stress criterion was proposed. Although the relationship between critical current and stress/strain has been extensively studied by some research teams, the effects of fatigue loading on macro-behavior, microstructure, and electromechanical responses have not been much reported. In this paper, the tapes were made using ion-beam-assisted deposition combined with pulsed laser deposition. Under the self-field, they were subjected to many cycles of axial loading to test their fatigue behavior. In this work, first, we report the effects of fatigue numbers on YBCO tapes’ tensile responses (e.g., Tensile Strengths) and electromechanical behaviors. Macro-behavior measurements showed that the mechanical behaviors and their electromechanical degradation of YBCO superconducting tapes depended on the number of fatigue loading cycles. Then, fracture surface morphologies of YBCO tapes’ superconducting layer and Hastelloy layer were also investigated with scanning electron microscope and energy-dispersive X-ray spectroscopy. It was found that the width of scratch lines and the size of fatigue defects on Hastelloy layer were increased with the number of fatigue cycles, which was the main cause of the degradation of mechanical properties. Moreover, observations of the microstructure conducted on the YBCO layer demonstrated that it was the crack motion and evolution that led to the current degradation under fatigue loading. During the process of fatigue loading, the small fatigue cracks become big with the increasing fatigue number. Lastly, a critical current-strain model of the fatigued HTS tapes, that combines the Ekin power-law formula and the Weibull distribution function, is proposed. This model can predict the electromechanical property of fatigued YBCO tapes under uniaxial tensile strain well.