Preliminary mechanical training of thin coils

Bending with tension of composite superconductors during the coil winding induces a high and very nonuniform normal stress on their cross-sectional areas so that the constituent materials can be in the plastic state inside one or two extensive conductor zones already in the winding process. The removal of the bobbin from the coil results in stress redistribution. Stresses arising in components of a thin solenoidal coil after its winding, removal of the bobbin and subsequent mechanical "training" by means of internal pressure are studied. It is assumed that a multifilamentary composite superconductor is macroscopically homogeneous and its stress-strain diagram is analogous to that of a ductile material following the straight-line law in unloading and reloading whereas interlayer insulation and impregnation materials are linear elastic. The case where one plastic zone develops during the coil winding in the conductor of the rectangular cross section and a new zone does not appear after the bobbin removal operation is considered. The higher the winding pretension, the lower the tensile stress in the conductor after the removal of the bobbin. It is shown that by mechanical training of the coil it is possible to redistribute stresses between its components and decrease the conductor stress.