Characterization of cyclic properties of superelastic monocrystalline Cu–Al–Be SMA wires for seismic applications

Abstract

Although Ni–Ti has been recognized as a promising type of shape memory alloys (SMAs) for seismic response mitigation devices in civil structures, its temperature-dependent mechanical behavior prevents its practical use in cold temperature environment. This study experimentally characterizes the cyclic properties of monocrystalline (also known as single-crystal) Cu–Al–Be SMA wires. The emphasis is put on those properties of common interest in seismic applications, e.g. “yield” stress, energy dissipation capability, stabilization of hysteretic shapes (also known as training effect), sensitivity to loading frequency and ambient temperature, large-strain fatigue, and so on. The testing results of another two types of SMA wires, namely Ni–Ti and polycrystalline Cu–Al–Be wires, are also presented for comparison. The monocrystalline Cu–Al–Be specimens show great superelastic strain of up to 23%. Insignificant degradation of transformation stress or accumulation of residual deformation is observed with increasing number of loading cycles. Meanwhile, their cyclic properties show minimal sensitivity to the variation of applied loading frequency or ambient temperature. The tested specimens maintain stable superelasticity down to −40°C. Compared with Ni–Ti SMAs, the monocrystalline Cu–Al–Be SMA wires are found to be superior in both superelastic capacity and cold-temperature performance and have comparable performance in terms of fatigue, training effect and energy dissipation. Moreover, these wires also have significantly higher superelastic capacity than polycrystalline Cu–Al–Be or other copper-based SMAs. This experimental study proves that monocrystalline Cu–Al–Be SMA has good potential for seismic applications, which is particularly favorable in outdoor environment with cold winter. Additionally, the hysteresis of monocrystalline Cu–Al–Be wires exhibits remarkable dependence on strain amplitude and complex internal loops. This fact necessitates the future development of more sophisticated constitute models for their complex superelastic behavior.