High-Tc superconductor (HTS) is expected to be applied to various fields because of its high critical temperature and usability in liquid N2. HTS is a ceramic and mechanically brittle so it should be handled very carefully so as not to break when it is applied to a large-scale system. Moreover, damage by electromagnetic forces is an important problem in the case of applications to energy systems. Therefore, the mechanical properties, strength, rigidity, and toughness of HTS must be improved. On the other hand, there are fiber reinforcement methods to improve the mechanical properties of structural ceramics, such as SiC/SiC and Al2O3/Al2O3. We consider this method and its applicability to enhance the mechanical properties of HTS. In this study, the problems and necessary conditions of the fibers were clarified to develop fiber-reinforced HTS. Bi1.85Pb0.35Sr1.90Ca2.05Cu3.05Oy (BPSCCO) was adopted as the HTS matrix and short fibers of Al2O3, ZrO2⋅Y2O3, ZrO2, SiC, Si-Ti-C-O, ZnO, K2O⋅6TiO2 were added into the BPSCCO bulks individually as 5% of the volume. Additionally, a no-fiber BPSCCO sample was made as a reference. The sintering temperature was 1, 068-1, 118K and the sintering time was 90ks. The supercondutivity of these samples was investigated, and only Al2O3/BPSCCO and ZrO2⋅Y2O3/BPSCCO demonstrated superconductivity at 77K. The superconductivity of these fiber/BPSCCO, which were sintered at high temperature, was inferior to no-fiber BPSCCO. It is considered that some compounds showing electrical insulation would be formed by reaction of the short fiber and the BPSCCO matrix during the sintering process. The mechanical properties of Al2O3/BPSCCO, ZrO2⋅Y2O3/BPSCCO and no-fiber BPSCCO were investigated by three-point bending tests at room temperature. The bending strengths of Al2O3/BPSCCO, ZrO2⋅Y2O3/BPSCCO, no-fiber BPSCCO were 38-53, 38-74 and 58-88MPa, respectively. From SEM observation results of Al2O3/BPSCCO and ZrO2⋅Y2O3/BPSCCO after the bending tests, it is recognized that bond strength at the interface between the short fibers and BPSCCO matrix is rather weak, and that the fibers do not function as reinforcements but act as defects. Therefore, fibers which do not react with BPSCCO and have a good interface with the BPSCCO matrix should be studied.
This paper investigates the use of the small-punch (SP) test to estimate the elastic-plastic fracture toughness (JIC) of structural alloys and weldments for superconducting magnets in fusion energy systems. SP testing was performed with thin-plate specimens of 10×10×0.5mm at liquid helium temperature (4K). Correlations between SP energy, equivalent fracture strain, and JIC were assessed. All JIC data were obtained using 25-mm-thick compact specimens that followed the standard test method for JIC. Finite element analysis was also performed to convert the experimentally measured load-displacement data into useful engineering information. The criterion for fracture used is the strain-energy density (strain energy absorbed per unit volume) required to produce crack initiation in a solid, uncracked specimen. The fracture strain-energy density was calculated and correlated with JIC.