Recent problems in superconducting devices operating at the high magnetic field regime include the possibility of substantial critical current, Ic, degradation due to the delamination of the coated conductor (CC) tapes. The delamination may originate from excessive tensile radial stress and cleavage stress developed due to epoxy curing, cool-down and Lorentz force generated. Therefore, it is necessary to investigate the Ic response under transverse loading and the delamination behaviour in REBCO CC tapes to predict its performance in practical device applications. In this study, using the anvil test method, the delamination strength and the transverse tensile stress response of Ic in GdBCO CC tapes were investigated. Under transverse tensile stress, the Ic degradation in GdBCO CC tapes showed both abrupt and gradual behaviours. A reversible Ic degradation behaviour was also observed which was similar to the case under uniaxial tension. The electro-mechanical and mechanical delamination strengths were determined, wherein both increased when a smaller-sized anvil was used during the delamination test. The electro-mechanical delamination strength was approximately half of the mechanical delamination strength in the GdBCO CC tapes tested. Four delamination modes were observed and well correlated with electro-mechanical delamination strength and Ic degradation behaviour. Lastly, delamination sites within the CC tapes were also correlated with the Ic degradation behaviour that had been observed.

Nowadays, the superior electrical and mechanical properties of second generation (2G) practical coated conductor (CC) tapes make them viable options for various applications such as power cables, magnets and coils. The CC tapes are composite conductors where the constituent layers are directly affected by the mechanical and the electromechanical properties of the superconducting layer under operating conditions. It is therefore important to investigate the strain effect on the critical current ( I _c ) and to determine the irreversible limits for I _c degradation in CC tapes. However, standards or codes for testing the electromechanical properties of high-temperature superconductors are lacking. Therefore, the establishment of a test method or procedure for determining the electromechanical properties of 2G CC tapes in both engineering and practical aspects is needed. This study is a standardization activity, wherein the strain/stress sensitivity of I _c and the reversible strain/stress limits for I _c degradation of practical CC tapes using uniaxial tension tests were evaluated at 77 K and self-field. The reversible strain/stress limits of CC tapes were defined and the reversible recovery of I _c during loading–unloading was measured when using the 99% I _c ( ε ) recovery, 99% I _c0 recovery and 95% I _c0 retention criteria. The electromechanically determined irreversible limits were also compared to the mechanically determined stress and strain at the yield point. The obtained reversible stress limits showed a similar value regardless of criteria adopted, excepting Sample 2. However, the irreversible strain limit value obtained using the 99% I _c ( ε ) recovery criterion showed the largest value as compared to the cases using other criteria. Through repeated tests, the electromechanical test procedure of the CC tapes was evaluated based on statistical estimates. The obtained results can provide a fundamental basis for establishing test procedures comparing the criterion that should be considered in determining irreversible limits, whether normalized to the original state I _c value, I _c0 or determined using the reversible degradation of I _c ( ε ) at residual strain, when unloaded.

Development of a coated conductor tape joint with good quality and low joint resistivity, R _sj , in terms of transport and mechanical properties, was attempted by direct bonding at the interface of the Cu–Cu stabilizers in overlapped GdBCO CC tapes. In this study, we attempted to achieve a low R _sj by introducing hybrid joining, soldering and ultrasonic welding (UW), and its mechanism was analyzed theoretically. Coated conductor tapes were experimentally joined using various methods of soldering, UW, and combinations of the two. As a result, a much lower R _sj of about 57 nΩ · cm ² was obtained for RCE-DR-processed GdBCO CC tape joints using the hybrid joining method. The mechanical properties of the jointed CC tapes were also evaluated at room temperature and 77 K under self-field. Load–displacement curves of joined CC tapes followed the curve of the single CC tape. Critical current and joint resistance, R _j, of hybrid-joined CC tape were retained after double bending at room temperature up to 20 mm bending diameter.

In applications employing high-temperature superconducting conductors, various cyclic loading (fatigue) conditions produced by mechanical, thermal, or periodic electromagnetic forces are inevitable. Applying coated conductor (CC) tapes under fatigue loading conditions is expected to critically affect the long-term reliability of its superconducting performance. Most studies evaluating the mechanical and electromechanical characteristics use quasi-static uniaxial tensile tests. Few have focused on the characterization of CC tapes under fatigue loading. In this study, the electromechanical property characterization of Cu-stabilized GdBa ₂ Cu ₃ O _y (GdBCO) CC tapes including fatigue behaviors were investigated at 77 K. High-cycle uniaxial fatigue tests were carried out on GdBCO CC tapes 4 and 12 mm in width, and the two were compared in terms of mechanical and electromechanical aspects at a stress ratio of 0.1. The mechanical and electrical fatigue limits of the CC tapes were determined at 77 K. The 4 mm wide CC tapes showed less fatigue limits when compared to the 12 mm wide ones. However, regardless of the CC tape width, the sequence in the obtained characteristic strengths at 77 K was the same: yield strength > irreversible stress limit > mechanical fatigue limit > electrical fatigue limit. Fracture surface morphologies were observed using scanning electron microscopy-energy dispersive x-ray spectroscopy and electron probe micro-analysis to clarify the fatigue fracture mechanism and to examine the influence of the architecture of the CC tapes on fatigue behaviors. Damage along the edges, caused by slitting during fabrication of the 4 mm wide CC tapes, generated a stress concentration, eventually resulting in earlier crack initiation not only on the substrate, reducing mechanical fatigue strength, but also on the superconducting layer, degrading the measured critical current.