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Effect of transverse stress applied during reaction heat treatment on the stiffness of Nb3Sn Rutherford cable stacks and Nb3Sn and Cu loading stress evolution in Rutherford cable stacks as a function of externally applied pressure
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Europe/Zurich
112/R-028 (CERN)
Description
Abstract
In Nb3Sn superconducting accelerator magnets the knowledge of the stress-strain behaviour of the coils is necessary to optimize the magnet design and the assembly parameters. An experimental parametric study has been performed to measure the stiffness of 11 T Rutherford cable stacks as a function of the compaction during manufacturing in three different conditions. The cable stack stiffness was compared to that of a conductor block extracted from an already powered coil coming from a short model of the 11 T magnets for the LHC High Luminosity Upgrade Project. The stress-strain behaviour of the coil block can be represented with a ten-stack sample that has a similar epoxy volume fraction. The transversal conductor block stiffness increases with increasing applied load, and with increasing compression during the reaction heat treatment. The axial conductor block stiffness is approximately 95 GPa, which is about twice of that in transverse direction.
In order to understand better the Nb3Sn coil stress-strain behaviour and the effect of the epoxy impregnation, we have measured for the first time directly the Nb3Sn and Cu elastic strain evolution in coil segments upon monotonic and cyclic compressive loading. For this purpose a load frame that enables rotation of the sample load axis with respect to the neutron scattering geometry was installed in the StressSpec beamline at the neutron source Heinz Maier-Leibnitz FRM II. The neutron diffraction data suggests that the magnet coil wound of Rutherford type cables can be considered as fibre reinforced composite material. In axial load direction the load is mainly carried by the Nb3Sn filaments. The composite stiffness can be estimated by the rule of mixtures assuming iso-strain conditions. Under transverse compression the fully annealed Cu provides an isotropic pressure around the Nb3Sn filaments. The transverse load stresses in the Cu and Nb3Sn are similar, indicating iso-stress behaviour of the coil constituents under transverse compression. The broadening of the Nb3Sn diffraction peaks above critical stress values indicates reversible and irreversible Nb3Sn loading stress effects.