Functional fatigue of polycrystalline Cu-Al-Mn superelastic alloy bars under cyclic tension

This paper studies the response of Cu-Al-Mn superelastic alloy (SEA) bars to repeated tensile load at room temperature. The test specimens are three polycrystalline and one single crystal SEA bars of 11 mm diameter and 100 mm length. Strain-controlled tensile load is applied for 1,000 cycles with target strain amplitudes of 6-7%. In the test, variations are monitored in the stress-strain curve, recovery strain, elastic modulus, forward transformation stress, and damping ratio. All bars showed functional fatigue, or gradual loss of superelasticity, while they did not fracture by the end of the loading cycles. Full strain recovery was observed up to 50-100 cycles without any training beforehand, while a slight decrease of forward transformation stress was observed during these cycles. Then, a gradual loss of stress plateau and gradual increase of residual strain were observed up to 200-400 cycles. Afterward, a nearly linear response was observed. Microstructure and texture analyses suggest that both the grain sizes larger than the bar diameter and the uniform distribution of crystallographic orientations are possible reasons for avoiding premature fracture, which is often seen in polycrystalline Cu-based SEAs.