Rare-earth barium copper oxide (REBCO) high-temperature superconducting (HTS) -coated conductor (CC) tapes are potential conductors for high-field magnets. In an operating high-field magnet, REBCO CC tapes are bent into coils and simultaneously subjected to hoop electromagnetic forces. Under these combined bending-tension loads, the critical current (Ic) of the REBCO CC tapes is at risk of irreversible degradation. Therefore, investigating the mechanical behavior and electromechanical properties of REBCO CC tapes under combined bending-tension loads is necessary. In this study, the mechanical behavior of REBCO CC tapes was analyzed using a finite element model (FEM). First, in the fabrication-cooling process of REBCO CC tapes, the thermal residual stress/strain accumulated in the constituent layers was analyzed. Then, considering the thermal residual stress/strain as the initial stress-strain state, the mechanical behavior of REBCO CC tapes under various mechanical loads, such as axial tension, bending, and combined bending‑tension at 77 K, was analyzed. Furthermore, a phenomenological model of the internal strain in the REBCO films dependence of Ic was developed for analyzing the electromechanical properties of REBCO CC tapes under combined bending-tension loads at 77 K. Calculated results showed that the compressive thermal residual strain in REBCO films at 77 K was -0.25%, and the internal strain in the REBCO films corresponding to the irreversible degradation of Ic was 0.45% under axial tension loads, which was verified by experimental results. Under bending and combined bending-tension loads, the distribution of the internal strain in the REBCO films along the width direction was non-uniform owing to the Poisson effect, and the onset of the irreversible degradation of Ic occurred at the outer edge of the REBCO films. The phenomenological model was experimentally verified to be effective in Ic degradation behavior prediction under combined bending-tension loads, and model predictions based on the FEM results indicated that the electromechanical properties were significantly influenced by the bending modes (tensile bending or compressive bending) of the REBCO CC tapes.
Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-field applications. In epoxy-impregnated REBCO superconducting windings, during the cooling process, the delamination induced by thermal mismatch stress significantly threatens stable operation due to the weak c-axial strength of REBCO CC tapes. In this study, a two-dimensional axisymmetric multilayer delamination finite element model with main layers of REBCO CC tapes and insulation materials was developed based on the bilinear cohesive zone model. Based on the proposed model, the stress distribution and delamination properties of an epoxy-impregnated REBCO winding induced during the cooling process were investigated. Furthermore, the effects on the structural configuration on the delamination and optimisation schemes of fabrication are discussed. The model results indicate that during the cooling process of the REBCO winding, when the radial tensile stress induced by the thermal mismatch stress is greater than the delamination strength, interface cracking will occur. Interface failure occurs in regions containing several turns of coils and not just a single-turn coil. Our analyses indicate that structural failure caused by delamination depends on the radius ratio of the outer to the inner winding, where a small radius ratio is preferred to reduce the risk of delamination. An excessive radius ratio can result in multiple delamination failures during the cooling process. The optimisation scheme, which divides the winding into several sub-windings with a consistently small radius ratio, is an effective method for mitigating the risk of delamination failure, which is consistent with available experimental results. Additionally, by reducing the thermal expansion coefficient of the epoxy resin, the risk of delamination failure during cooling can be significantly reduced. For the winding structure considered in this study, if the coefficient of thermal expansion of the epoxy resin is reduced to 5 (×10^6 K-1), then delamination failure caused by thermal mismatch will be eliminated. Our model results are consistent with those of several valuable experimental phenomena and numerical calculated in the literature.
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