Critical current changes and fatigue damage after 4.2 K strain cycling of superconducting composites

A facility for investigating the effects of cyclic strains on the current-voltage relation (I-V plots) in a superconducting composite has been constructed, as part of the conductor test program directed by Oak Ridge National Laboratory for the Tokamak Experimental Power Reactor. The initial results for a ribbon composite containing 18 Nb-Ti twisted filaments in copper showed three effects: (1) a 3% decrease in Icat 40 kOe during the initial 200 cycles at 0.35% maximum strain amplitude, (2) current sharing between filaments and copper at I < Icafter cycling with maximum strain increased to 0.57% and (3) voltage steps at I > Icafter 400 cycles with 0.57% strain amplitude. The first effect is consistent with Icmeasurements in unidirectional testing and appears to be associated with elastic strain fields. The third effect, the voltage steps at I > Icintroduced by the higher strain amplitudes, decreased very significantly with increasing magnetic field and disappeared at H > 40 kOe. A Nb3Sn composite fractured during tension-compression cycling but no damage occurred in 3800 load-unload cycles at strains up to 0.2%.