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In tokamak fusion reactors, such as ITER, superconducting strands are subjected to repeated Lorentz force loading and unloading which may degrade performance over time. In addition to Lorentz force when cooling from reaction heat treatment to cryogenic operation temperature and in any subsequent operational thermal cycles, the different thermal contraction between conduit material and strand bundle introduces further strain loading and unloading due to contraction of strands. The Cu matrix which surrounds the brittle Nb3Sn filaments allows the possibility of some elastic-plastic degradation that can initiate filament cracking and we are seeking to understand if there are design variables that might ameliorate such degradation. In this study we used advanced metallographic techniques to observe the effect of 1000 to 30,000 loading cycles on filament cracking at axial strains from 0.4% to 1.14%. The strands examined include both bronze process and internal tin ITER production strands. After fatigue testing at 77 K the strands were polished in longitudinal cross section and imaged by Scanning Electron Microscopy (SEM). Crack densities within filaments as a function of strain and number of fatigue cycles were quantified from large montages covering ≈ 20 mm length of strand. The observed filament crack density is highly sensitive to the Kirkendall voids produced in the Nb3Sn reaction. Although in bronze process strands we saw many cracks in filaments surrounded by voids at lower strains, we rarely observed filament cracks away from voids until the axial strain reached 1%. Similarly for internal tin process strands we found few cracks in filaments surrounded by voids at lower strains but we rarely observed filament cracks away from voids until the axial strain exceeded 0.6%. Contribution of void/filament cracks was found highest among all types of cracks. Moreover, significant cracking of Nb3Sn filaments always starts at axial strain close to the fracture limit in both bronze and internal tin route strands. Bronze process strands showed significant loading cycle effect on filament cracking as crack density increased significantly by increasing loading cycles from 1000 to 10,000. In contrast, internal tin strand did not show any significant loading cycle effect as compared to bronze process strands. A detailed comparison of the development of cracking under different axial strains and increasing loading cycles for different ITER strands will be presented.
A Thesis submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Master of Science.
Includes bibliographical references.
David Larbalestier, Professor Directing Thesis; Juan Carlos Ordonez, Committee Member; William S. Oates, Committee Member.
Florida State University
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