Mode I type delamination fracture toughness of YBCO coated conductor with additional Cu layer

► A fracture toughness test method was developed for a YBCO coated conductor with an additional Cu layer. ► Mode I type tests were carried out using double cantilever beam (DCB) specimens. ► Delamination propagated into the YBCO layer, and sometimes reached the Ag/YBCO interface. ► The fracture toughness for YBCO was about 10J/m2. ► That for Ag/YBCO interface was about 100J/m2. Although interlaminar fracture at a YBa2Cu3O7−δ (YBCO)/CeO2 interface was reported for YBCO coated conductors, this has not yet been investigated by a fracture mechanical approach. In the present study, we developed a mode I type fracture toughness test method for a YBCO coated conductor with an additional Cu layer using double cantilever beam (DCB) specimens. Fracture mechanism was investigated by microscopic observation by a scanning electron microscope (SEM), together with composition analysis by an energy dispersive X-ray spectroscope (EDS). A pre-crack introduced at the YBCO/CeO2 interface deviated from the interface, and propagated into the YBCO layer, and sometimes reached the Ag/YBCO interface. The fracture toughness, GR, for YBCO and the Ag/YBCO interface was evaluated to be 7–10J/m2 and 80–120J/m2, respectively. The complex stress intensity factor ratio, K2/K1, at YBCO/CeO2 interface was evaluated to be −0.19, and this ratio controlled the formation of microcracks in the YBCO layer. The main crack propagated into the YBCO layer accompanied with the formation of microcracks.