Fatigue-Induced Electrical Degradation of Composite High-Purity/High-Strength Aluminum Rings at 4 K

One design scenario for the Anchorage Municipal Light & Power superconducting magnetic energy storage (SMES) system uses Al to thermally stabilize and structurally support the magnet. Composite Al rings were fabricated from high-purity (99.995%) Al and high-strength Al alloy (6061-T6) to simulate the mechanical and electrical characteristics of the stabilizer during fatigue tests in liquid He. To simulate the magnet operating conditions, as the coil is periodically charged and discharged, one test was conducted where a composite ring was subjected to fatigue at 4 K, with the peak hoop stress held constant during the test and the ring’s electrical resistance periodically measured at 0 and 4 T. For comparison, a second fatigue test was conducted on a similar ring where peak hoop strain was held constant. In both tests the initial peak strain was 0.21%. In the constant-strain test, the increase in resistivity at 4 T after 4000 fatigue cycles was 40% greater than for the constant-stress test, indicating a significant strengthening of the composite through work-hardening of the high-purity Al. These results, applied to the design of Al-stabilized magnets, allow a substantial reduction in the required quantity of high-purity-Al stabilizer and in the magnet’s cost.