Two different mechanisms of fatigue damage due to cyclic stress loading at 77 K for MOCVD-YBCO-coated conductors

Tensile fatigue tests were carried out at 77 K for YBCO- coated conductors fabricated by metal - organic chemical vapor deposition ( MOCVD). The S - N relationship, variation of critical current ( I-c) during cyclic loading and microscopic fatigue damage were investigated. Fatigue strength at 10(6) cycles was evaluated to be sigma(max) = 1300 MPa and 890 MPa under the stress ratios of 0.5 and 0.1. Two different mechanisms of fatigue damage, depending on the number of stress cycles to failure, were observed. In one of the fracture mechanisms, fatigue behavior is characterized by overall fracture which occurs at 10(4) - 10(5) cycles. For these specimens, Ic after unloading does not degrade before overall fracture. Although only shallow slip bands were found at the Ag surface, fatigue cracks were found on the Hastelloy C- 276 surface of the fractured specimen. These results suggest that overall fracture due to cyclic stress was caused by fatigue of the Hastelloy substrate. In the other fracture mechanism, even though overall fracture did not occur at 10(6) cycles, a slight decrease of I-c was detected after 10(5) cycles. No fatigue crack was found on the Hastelloy surface, while deep slip bands corresponding to the initial stage of fatigue crack were observed on the Ag surface. From these results, we concluded that I-c degradation at a high cycle number is attributed to the fatigue of the Ag stabilizing layer.