超電導巻線用エポキシGFRPの熱・機械特性

抄録

A tightly constructed, rigid structure of the conductor winding and GFRP spacers in the coil case are essential elements for a superconducting magnet that is to operate at cryogenic temperature and sustain large magnetic forces. However, the contact pressures on the conductor, spacers and coil case imposed during the fabrication process may be greatly reduced by relaxation at room temperature and thermal contraction in the GFRP spacers during the cool-down process. We therefore studied 61 kinds of commercial and test GFRPs and established a basis for suitable GFRP spacer material to be used in superconducting magnet windings. Glass transition temperature, T_g, of the impregnating resin plays an important roll in the transverse creep deformation of GFRPs. GFRP spacers with T_g above 423K can maintain 80% of the initial pressure in a winding for two years at room temperature. This result was obtained by utilizing the transverse creep moduli of GFRPs at different temperatures and the time-temperature superposition procedure. Transverse thermal contraction from 293 to 4K decreases uniformly in all GFRPs as the resin weight content, R_c, decreases. Also, contraction is smaller than that of the 304 stainless steel used for the coil case when R_c is less than 15%. As a result, pressure decrease in the winding can be prevented during cool-down. The elastic modulus in the transverse direction of a GFRP is calculated by dividing the elastic modulus of the impregnating resin by R_c. Fractures of the GFRPs at low temperatures are primarily in shear mode under four-point bending, in-plane compression, transverse compression, or interlaminar shear loading. Moreover, the transverse-compressive fatigue limit of plain-woven GFRP at low temperature depends on the maximum compressive stress of the cyclic loading under high mean-compressive stress and the stress range under low mean-compressive stress.