Compatibility Evaluation of Glass/Epoxy Insulation for Use in ITER’s Central Solenoid Superconducting Electromagnets

A variety of glass fiber types are being considered for use as insulation material in the central solenoid superconducting electromagnets for the International Thermonuclear Experimental Reactor (ITER). Before these materials can be used, a compatibility of these glass products with an epoxy matrix has to be characterized. In this study, three candidate glass types (1581, 7781, and 38050) were considered and their compatibility with a DGEBF epoxy matrix was determined via three-point flexure tests at room temperature (295K) and liquid nitrogen temperature (77K) and constant load flexure creep tests at room temperature. The material variables among these glass products were – type of weave pattern (8-harness satin versus plain), fiber surface treatment (epoxy compatible surface treatment versus no treatment), and cloth density. Based on the stiffness and strength data at both 77K and 295K, the 7781/epoxy composites showed the highest values followed closely by 1581/epoxy. However, 38050/epoxy performed quite poorly. Evidence in an increase of interfacial strength as the temperature was lowered to 77K was seen through examination of the failure pattern. The correlations of the glass cloth density and fiber volume fraction with stiffness and strength at both temperatures were almost linear. It was also found that the failure modes of samples having fibers with epoxy compatible surface treatment differed from those having no surface treatment. Constant load flexural creep tests showed that the 7781/epoxy composites performed the best with the least amount of strain at higher loads than the other types of composites. The 1581/epoxy composite followed with the 38050 again performing quite poorly.