Preliminary mechanical "training" andσ _{θ} -ε _{θ} curves of thin coils with rectangular conductors exhibiting linear work hardening

Abstract

Bending stress in conductors of small solenoids often exceeds thermal and Lorentz stresses. This stress is primarily responsible for heat generation due to plastic deformation during magnet energizing and strain degradation of the critical current in strain-sensitive superconductors. Stresses arising in components of a thin solenoid coil after its fabrication and subsequent mechanical "training" by means of an internal pressure are studied in the paper. It is assumed that theσ-ε diagram of a composite superconductor can be approximated by a diagram with linear work hardening whereas an interlayer insulation and impregnation are linear elastic. The case where one plastic zone develops in the conductor during its bending and a new zone does not appear after the removal of the bobbin is considered. It is shown that the mechanical training allows to decrease the bending stress, stretch the constituents and increase the limit for elastic deformation of the coil. To predict the results of the training, theσ _{θ} -ε _{θ} curve of the coil is derived. A training level which provides elastic deformation of a Cu/NbTi conductor during the first energizing is estimated.