Investigation of the electromechanical behaviors in Cu-stabilized GdBCO coated conductor tapes using high-cycle fatigue tests at 77 K and related fractographic observations

Abstract

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.

Rules of non-superconducting phase particles on crack propagation in YBCO coated conductors fabricated by the IBAD-MOCVD

“…Note that before etching the YBCO layer in this sample, no cracks were observed on the surface of the YBCO layer other than the cutting cracks. Figure 8(a) shows that significant cracks formed by loading are randomly distributed in the YBCO layer at almost perpendicular to the loading direction, observed using SEM after etching the YBCO layer for 80 s. Except for a few cutting cracks (labelled with red arrows in figure 8(a)) propagated inward at the slit edge, which is similar to the fatigue tests at 77 K [28], most cracks formed directly in the YBCO layer. Then the initiation of these cracks was investigated.…”

“…For these samples, no a-axis grains are observed and only non-superconducting phase particles are randomly distributed at its edge. However, compared with the edge of 12 mm wide YBCO-CCs without microcracks seen in figure 2(a), it is inevitable that microcracks at ∼ 40 • relative to the edge of conductor were introduced in 3 mm wide YBCO-CCs during the cutting of the samples [24,26,28], as shown in figure 2(b). Therefore, in order to avoid the influence of a-axis grains, the 3 mm wide cut samples of YBCO-CCs (labelled as B in figure 1(d)) were chosen for the following experiments.…”

“…In addition, nanoscale defects that can enhance the vortex pinning are introduced as an effective approach to obtain a higher J c [22,23]. Cracks that initiate under tensile stress or generate at the edges of the tape during the cutting process can cause the critical current degradation and decrease the delamination strength in YBCO-CCs [24][25][26][27][28]. The aforementioned studies have revealed that cracks are undesirable in YBCO-CCs, but a systematic investigation of the crack development mechanisms within YBCO-CCs has not yet been carried out.…”

Simulation of Fatigue in High-Temperature Superconductor using Findley criterion

“…609 MPa is the limit for 4 mm wide tape, and 679 MPa is the limit for 12 mm wide tape. Through fractographic studies, the electrical fatigue strength of a CC tape is discovered to be considerably influenced by the fracture behaviour in the SC layer, whereas the mechanical fatigue strength is mainly driven by the substrate [3]. At room temperature and 77 degrees Celsius, the monotonic tensile stress-strain curve for YBCO coated conductors were measured.…”

“…The normal stress sensitivity coefficient and limit factor are k and f, respectively. By setting σmax = 669 MPa, σmin = 66.9 MPa, and R (stress ratio) = 0.1, these values can be calculated [3]. It may be observed that, fig.…”