Keiji Fukushi

Dielectric breakdown strength of GFRPs under mechanical stresses in cryogenic liquids

The dielectric strength of GFRPs (glass-fiber-reinforced plastics) under combined application of electrical and mechanical stresses was investigated in liquid He, liquid N/sub 2/, and silicone oil at room temperature. With the combined application of tensile and electrical stresses in cryogenic liquids, the dielectric breakdown of GFRPs occurred prior to mechanical breakdown when the applied electrical stress was lower than the DBS (dielectric breakdown strength) of GFRPs without tensile stress. In the case of tensile stress application under constantly applied electrical stress, the DBS and GFRPs decreased markedly. The reason for the large DBS decrease of GFRPs was the occurrence of microcracks in matrix resins because of their brittleness at cryogenic temperatures. Combining GFRP with polyimide film in cryogenic liquids lessened the decrease in DBS. Compared with tensile stress application, the influence of compressive stress on the DBS of GFRPs was very small.<<ETX>>

Mechanical Properties of Low Thermal Contraction GFRP

Liquid helium duct spacers in large superconducting magnets must have high compressive strength to withstand large electromagnetic forces. They must also show a low thermal contraction, in the direction perpendicular to the laminations, to maintain windings stability at liquid helium temperature. To attain higher glass content or higher density, we adopted glass roving cloth with mineral fillers and unidirectional woven glass cloths. E-,S-, and R-glass cloths were examined as reinforcements and epoxy and bismaleimide triazine, as matrix resins.