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.
Manufacturing a single long-length rare-earth barium copper oxide (REBCO) coated conductor (CC) tape with uniform critical current (Ic) is still a challenge; therefore, joining between multiple-length CC tapes is needed in the production of longer CC tapes for superconducting cables, coils, and magnets. Various joining techniques have been developed to achieve acceptably low joint resistance (Rj) with no Ic degradation and good electromechanical properties. The authors established ultrasonic welding (UW) and hybrid welding (HW) methods for joining Cu-stabilized CC tapes with different configurations. However, these methods have yet to be applied to Ag-stabilized CC tapes to produce compact joints and longer tapes during in-line production. This study used the UW and HW methods to fabricate Ag-stabilized CC joints to have low Rj without Ic degradation through a direct connection at the overlapped Ag layers between both CC tapes. Two CC tape samples with different thicknesses of ~2 µm and ~6 µm Ag stabilizers were supplied to produce UW and HW Ag-stabilized CC joints. These were compared with soldered CC joints fabricated by a mechanically controlled soldering method. In the case of UW, the design of experiment (DOE) using the Taguchi method was used to systematically determine the optimized weld parameter combination that yields a lower Rj without any Ic degradation. This study is a systematic attempt to evaluate the applicability of UW for solid-state joining between thin Ag layers, unlike the conventional joining of thick Cu stabilizers. Moreover, the electromechanical properties of differently processed Ag-stabilized REBCO CC joints were evaluated using both lap-shear and double-bending tests. As a result, the optimum UW parameter combinations were obtained to achieve Ag-stabilized CC joints. However, some variations were probably due to the differences in the production batch and thickness of the Ag layers. The UW Ag-stabilized CC joints showed superior joint characteristics and electromechanical properties compared to soldered CC joints. Good solid-state bonding of the Ag layers was observed through microscopic observation of the cross-section at the joint region of the UW CC joint. The joint characteristics and electromechanical properties of the UW Ag-stabilized CC joints can be further improved using the HW method.
Various test techniques have been established to investigate the electromechanical properties (EMPs) of CC tapes under external loads. The most conventional method is to examine variations in critical current, Ic, by repeatedly measuring the V-I curves while intermittently applying a load or deformation to the CC tape. The conventional methods for obtaining EMPs, such as the reversible limits for Ic degradation, require repeated measures of Ic in a loading-unloading scheme, and this entails considerable time and effort therefore, they must be improved for practical and engineering reasons. We recently developed an easy-to-use system that can continuously measure variations in Ic while applying a load or deformation to the CC tape, thereby evaluating its EMPs. The main advantages of the new measurement system are real-time monitoring of Ic behaviors during loading and allowing reduced the test time. While it uses a conventional test configuration, this new system continuously measures Ic through effective feedback control of the electrical-field voltage induced in the CC tape specimen during loading. Through this feedback control system, the Ic degradation behaviors in CC tapes resulting from possible cracking in the superconducting layer during loading are depicted. The reversible limits for Ic degradation were also determined. To assess the effectiveness of this newly developed measuring system, the applicability of the method was identified by evaluating the EMPs of various commercially available CC tapes. By comparing the results with those achieved using conventional testing, we found this to effectively evaluate the EMPs of CC tapes. The results showed that this system provides a simple way of evaluating the EMPs of HTS CC tapes by simultaneously measuring variations in Ic under load or deformation. It is much faster at depicting Ic degradation behaviors, and it elaborately determines the reversible limits of Ic induced in the CC tape during testing.
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