Development of a universal cryogenic test facility

Abstract

Magnets for High Energy Physics applications built to date are generally superconducting magnets, which operate at cryogenic temperatures. The reliability and safety of the applications are entirely dependent on good design which in turn rely heavily on predictable materials performance. Where at these low temperatures the fracture toughness is of importance to be known (alongside mechanical properties such yield and tensile strength). At CERN at the materials and engineering department (EN-MME-MA) a testing facility is being commissioned for the measurement of mechanical at cryogenic temperatures. A tensile test facility has been realised, yet no such set-up is available for fracture toughness measurement. The aim of this thesis was to develop a test set-up for the measurement of the fracture toughness in order to realise a universal cryogenic testing system, which can be used for both tensile tests and fracture toughness test at low temperatures. A design for a set-up is proposed for the measurement of the fracture toughness which can be employed within the current cryostat. The design of the tooling and cryostat have been extensively verified using numerical methods taking into account thermal effects (such as conduction and contraction) and the varying material properties at these low temperatures. A modified C(T) specimen is proposed for more robust and reliable set-up. For this modified specimen it is shown with numerical methods that the modification are expected to have a negligible impact on the fracture toughness measurements. Tests have been performed using the proposed design with four specimen fabricated from two different materials (SS316L and Ti6Al4V), at both room temperature and at 4 K (using liquid helium). The set-up is shown to provide sufficient data for the characterisation of the fracture toughness for both Linear Elastic Fracture Mechanics tests as well as Elastic Plastic Fracture mechanics tests.