Thermomechanical Characterization of Epoxy Composites Used in High Field Magnet Coils

In the framework of the High-Luminosity Large Hadron Collider project and the planned Future Circular Collider, prototype magnet coils based on Nb3Sn superconductors are under development at CERN. The coils, which during magnet operation are cooled to 1.9 K, are impregnated with S2-glass reinforced epoxy resin, providing the brittle conductors with electrical insulation and mechanical protection, strength and toughness – the latter property having been found to be a critical factor for the magnet performance. Pure and glass-fibre reinforced specimens of the epoxy systems CTD101K, MY750, and Mix61 have been produced and thermomechanically characterized through quasi-static uniaxial tensile tests at room temperature (RT), in quasi-static three-point bending at RT and 77 K, and dynstat notched impact tests at RT. These measurements have been coupled with measurements of the glass transition temperature and thermal shrinkage to describe the ductile-to-brittle transition of the thermosetting epoxies. The pure and reinforced epoxy resins, which exhibited different characteristics under loading at room temperature, were found to have converging elastic stiffness properties at cryogenic temperatures. In the cryogenic domain, the unreinforced MY750 and Mix61 are capable of withstanding significantly higher loads than the CTD101K, which with fibre-reinforcement also shows signs of damage onset at much lower stresses compared to the other epoxy systems. The former epoxy systems exhibit generally higher toughnesses both up to fracture at ambient temperature and 77 K and up to the onset of the first energy-relasing damage at 77 K. The anisotropic properties of glass-fibre reinforced CTD101K composites have also been measured, and the results are discussed in regard to classical lamination theory. Comparison of both the tensile toughness and the dynstat notched impact strength to the flexural toughness revealed slight correlations within experimental uncertainties. A possible correlation of the short-beam toughness to the flexural toughness in an approximate 1:1 ratio was found at ambient temperature and 77 K for pure and fibre-reinforced samples that exhibited similar fracture modes in the two tests.