Fracture measurements on insulation for high-field superconducting magnets: Relationship to processing regimes and test temperature

The next European dipole project aims to develop the technology required for the next generation of superconducting dipole magnets for particle physics and other application areas. Development of these magnets is limited by manufacturing issues connected with the insulation systems, and we report work directed towards resolving these issues. In design of superconducting magnets in the past, a lack of knowledge of fracture properties at cryogenic temperatures has lead to the requirement to make customised test pieces simulating regions of high-stress, particularly in the non-metallic insulation. The current work aims to provide fracture properties information to be used in stress calculations, for example by the finite element method, and so allow better optimisation of magnet design. We report early measurements of mode 1 interlaminar fracture toughness on sample insulation materials, focussing on the effects of the heat treatment cycle which forms the niobium–tin superconductor and to which its insulation is subjected.