Transverse-stress limit for irreversible degradation of Nb3Sn cables
In plain words
Nb3Sn is brittle, and the huge magnetic forces in a high-field magnet squeeze the cable until its ability to carry current is permanently damaged; where that threshold lies and what breaks first is uncertain.
Precise statement
Rutherford cables of Nb3Sn filaments in epoxy-impregnated coils at fields up to $\sim 1.6e5\,\mathrm{G}$ are under transverse stress of order $1.5e9 \text{ to } 2e9\,\mathrm{dyn}/\mathrm{cm}^{2}$ (approximate). Determine the stress at which the critical current degrades irreversibly, its dependence on filament layout, impregnation and temperature cycling, and the microscopic failure mode (filament cracking, void collapse, debonding), as a function that fixes the maximal practical dipole field.
What would settle it
Critical-current measurements of cable stacks under calibrated transverse stress, combined with post-test microscopy of filament cracks and a model that reproduces the onset stress.