Destruction of superconductivity in current carrying NbTi fibers by strain energy

Individual thin fibers of NbTi (46.5 wt % Ti) have been driven from the superconducting to the normal state by tensile force while immersed in liquid helium and carrying subcritical currents. The additional force for successive excursions from the superconducting to the normal state has been measured and found to saturate. This "mechanical training" can be reduced to the starting state for a fiber while at zero load by exceeding the critical current. Estimates are made of the mechanical energy to initiate quenching and since the tensile machine is "hard" estimates have also been made of the rise in temperature while subjected to short bursts of current greater than critical. There would appear to be structural changes which are stress level dependent and of a "shape memory" nature.