Critical Current and Fracture Behavior of Nb3Al Superconducting Strands under Cyclic Electromagnetic Force

Abstract

To obtain basic information on fracture behavior and its critical current properties in superconducting strands with long-term periodic excitation, a unique experiment of cyclic strain loading was carried out. The strain was applied to a superconducting filament directly at 4.2 K using an electromagnetic (Lorenz) force instead of the usual mechanical load. The sample strands were Jelly-roll process Nb3Al with a copper ratio of 2.1. A new magnet system to evaluate long-term cyclic strain loading was set up. It was able to generate external fields up to 14 T with 58 mm bore. Cyclic strain was applied by the sample current control: 500 A, 0.1 Hz. In the experiment, critical current measurements and applied cyclic strain were carried out in alternation and the variation of critical current was obtained during the cyclic test. In this paper, the experimental results of cyclic strain loading on the critical current of Nb3Al strand and mirostructural observation of fracture surface of the filaments is described. Our results show that degradation of critical current properties was not observed at cycle number n=1500 under the applied axial strain of 0.17%. A noxious sample, from which the outside copper stabilizer was deliberately removed, broke after n 10, and crack propagation was observed at the fracture surface of the Nb3Al filaments. Under a microstructural observation of the break sample, the fracture points of the filaments were seen to be different in a longitudinal direction. A striped pattern like a beach mark due to crack propagation was also observed at the Nb3Al fracture surface. It was found that the cross-sectional homogeneity of the strand is important to avoid the damage and breaking from cyclic loading.