The Mechanism of Buckling in the High-Strength Bi-2223 Conductor Due to Release of Bending Strain

The tensile stress tolerance of a Bi-2223 conductor has been doubled by the use of a mechanically strong Ni-alloy reinforcement material and a pretension technique. It is possible that, however, in the case of overheating due to a thermal runaway in a coil, release of bending strain in the mechanically strong reinforcement tape buckles the conductor itself. The present paper experimentally investigates the basic mechanism of such a buckling phenomenon. If the bending diameter of the coil is small, typically ≤50 mm, the release of the bending strain of the reinforcements instantaneously buckles the conductor itself when the solder between the reinforcements and the Bi-2223/Ag composite is melted. This causes fractures of the Bi-2223 filaments and a permanent degradation of the coil performance. The buckling is prevented by applying a radial compressive stress to the conductor; a smaller diameter coil winding requires a higher amount of compressive stress.