Recent advancement in materials processing enabled us to grow large single-grain bulk RE-Ba-Cu-O superconductors (RE: rare earth elements) with high critical current densities. These superconductors can exhibit a large electromagnetic force with the interaction of external magnetic fields. Various devices have been developed by utilizing such a force: magnetic bearings, flywheels for energy storage, load transport, hysteresis motors, and several levitation devices. A large magnetic field can also be trapped by bulk superconductors, which can function as a quasipermanent magnet. Trapped field values have already reached 10T, thus leading to many novel applications of high trapped field magnets. The final target will be a second-generation Maglev train.
Resin cracking, a cause of coil quench in superconducting magnets, occurs when a resin contains small flaws and sustains high thermal stress caused by coil restraint during cooling. Seven epoxy resins were chosen to evaluate thermomechanical properties, small flaw strength, and creep deformation at low temperatures. Although the plain specimen strengths consistently increased as the temperature decreased, the fracture toughness values resulting from large cracks reached maximum values at about 80K, then decreased to the lower values at 4K. The loss factor during cyclic loading behaved similarly because of low-temperature relaxation of the resin, having a maximum value at about 150K. The strengths corresponding to small flaws leveled off to those of plain specimens; i.e., they deviated from linear fracture mechanics. Thermal stress in the epoxy resin caused by coil restraint was experimentally measured by simulating the coil-molding process. These stresses were close to the calculated ones obtained by using the elastic moduli and the coefficients of linear thermal expansion of the resin. However, they were slightly lower because of stress relaxation at a