Fatigue behavior of REBCO coated conductors under through-thickness tensile stress

Fujikura Ltd has developed production techniques of REBa 2 Cu 3 O X (REBCO, RE = rare earth) coated conductors (CCs) using large-area ion-beam-assisted-deposition and hot-wall type pulsed-laser-deposition. We have provided high-performance REBCO CCs with long length and high homogeneity of critical current. The CCs are required to maintain their superconducting properties under mechanical stresses for a long-term. Therefore, it is important to evaluate the fatigue properties of the CCs. Although there are many investigations on cyclic tensile fatigue in the longitudinal direction of the CCs, few studies on fatigue properties in the through-thickness direction have been reported. In the case of the through-thickness stress, i.e. delamination stress, the stress is applied to the REBCO ceramic layer without substrate reinforcement, static fatigue fracture may occur by subcritical crack growth peculiar to ceramics. In this work, we carried out dynamic fatigue tests under delamination stresses at room temperature (RT) and liquid nitrogen (LN 2 ) temperature 77 K to determine the fatigue coefficient N characterizing the static fatigue. As a result, we found that the N- value was relatively low ( N ∼ 20) at RT, while the N -value was extremely high ( N ∼ 150) at LN 2 temperature. This result shows that the crack growth of the REBCO layer under the through-thickness tensile stress is extremely slow at low temperatures as well as that reported for bulk ceramics such as SiO 2 glass. Therefore, it is expected that the static fatigue in the through-thickness direction of REBCO CC hardly occurs at low temperatures.