Microscopic Fractures and Transport Degradation in ITER Type ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strands

The cable-in-conduit cabling schemes chosen for conductors inherently have distributed axial, bending, and contact loads acting on the multifilamentary strands under the operating conditions of the magnet. Due to the brittle nature of filaments, such mechanical loads can lead to fractures in the filaments and cause a permanent degradation in the conductor performance, depending on the chosen cabling pattern, aspect ratio and void fraction. Therefore, the knowledge of filament fracture mechanisms and the corresponding degradation in strand´s transport property is an important factor in optimizing the design of Nb₃Sn strands and cabled conductors. Previously, we have developed a metallographic technique to resolve microscopic cracks in a multifilamentary strand and the crack density growth with applied axial tensile strain was investigated. Separately, the evolution of strand´s voltage-current characteristics at zero applied strain after various tensile strains loads was measured. Here, we present a numerical model of a strand by a three-dimensional electrical network and simulation results by introducing a crack in the system.