The most common superconductors used in medium and high field superconducting magnets are Nb3Sn multi-tilamentary composite wires. The mechanical properties of these wires can have a strong bearing on the capabilities of the magnets because their superconducting properties are strongly dependent on the strain state within the Nb3Sn. The wires can be subjected to high levels of mechanical stress during fabrication and when the magnet is energised. The effect that this stress has on the Nb3Sn strain state is dependent on the stress-strain behaviour of the wires. A tensile testing procedure was developed and extensive measurements of stress-strain curves of particular superconducting wire were made at room temperature and 77 K. The effect of the reaction heat-treatment process on the stress-strain behaviour was investigated and this showed that it would not be possible to improve the mechanical properties, while at the same time maintaining satisfactory superconducting properties, by optimisation of the heat-treatment duration. The heat-treatment process changes the physical dimensions of wires and this can influence the stress and strain that the wires experience. A study of these dimensional changes was made and the implications were discussed. Another part of the research was the development of a finite element model to predict the stress-strain behaviour of Nb3Sn multi-filamentary composite wire. Considerable attention was paid to the accurate representation of the constituent material properties. A very good match between the F. E. and experimental results was obtained, although a number of aspects of the model remain uncertain. To provide further experimental data to validate the F. E. model, a novel experimental procedure was developed. This involved reducing the diameter of wires by etching in acid and measuring the effect on axial strain and stress-strain behaviour. This provided data that was directly related to the material within separate radial zones within the wire cross-section. Axial residual stresses were obtained as well as stress-strain curves for the material within the individual radial zones. In general, the results showed that the F. E. model provides a good representation of the wire.
Permanent link to this resource: https://doi.org/10.24384/9pgv-zw73
Harvey, David A.
Supervisors: Fellows, Neil; Durodola, John
Faculty of Technology, Design and EnvironmentSchool of Engineering, Computing and Mathematics
Year: 2009
Engineering and Physical Sciences Research Council : GrantOxford Instruments Superconductivity Ltd. : Funder
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