Study of Filament Cracking Under Uniaxial Repeated Loading for ITER TF Strands

In a tokamak, such as ITER, superconducting strands suffer from bending and uniaxial strain due to Lorentz force loading/unloading and thermal cool down which may degrade the performance over time due to specific cable-in-conduit conductor (CICC) design choice. Under repeated uniaxial loading the Cu(Sn) matrix which surrounds the brittle Nb3Sn filaments allows the possibility of some elastic-plastic deformation that can initiate filament cracking. Here we present a metallographic study of filament cracking under increasing uniaxial loading cycles (0, 1000, 10,000 and 30,000 cycles) for one ITER Toroidal field (TF) bronze-process strand (tested at 0.4%, 0.6% and 1% strain) and one ITER TF internal tin strand (tested at 0.4%, 0.6% and 0.7% strain). Significant cracking of Nb3Sn filaments was found at close to the respective fracture limits (strain at which strand breaks under uniaxial tensile loading) for both strands. After 0.6% strain, filament cracking in the bronze-process strand tends to increase with increasing number of loading cycles up to 10,000 and then remains almost constant after increasing the loading cycles from 10,000 to 30,000. The internal tin strand on the other hand showed an increase in filament cracking with increasing loading cycles to 10,000 up to 0.6% strain. For both types of strand and in all conditions the cracks were most likely to be found adjacent to voids.