Remarkable weakness against cleavage stress for YBCO-coated conductors and its effect on the YBCO coil performance

► Cleavage strength for YBCO-coated conductor is extremely low, typically 0.5MPa. ► The remarkable weakness is due to cracks on the slit edge of the conductor. ► The cleavage stress appears on YBCO double pancake coils impregnated with epoxy. ► The cleavage stress should be avoided in the coil winding. Cleavage strength for an YBCO-coated conductor at 77K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor; cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils.