<title>Study of damping capacities of nitinol shape-memory alloys in martensite, austenite, and martensite-austenite co-existence phases</title>

Abstract

It is well known that both martensite and austenite (superelastic) Nitinol Shape Memory Alloys (SMAs) have damping capacities, benefiting from their hysteretic stress-strain relationships. In general, for SMA devices for passive vibration control, martensite SMA has a larger damping capacity; however, it requires external heat to cause a phase transformation to restore its original shape. On the other hand, superelastic SMA has less damping capacity, but it has a strong re-centering force to restore its initial shape and there is little residual strain of the superelastic SMAs. This paper researches the damping capacity of Nitinol in martensite and austenite co-existence phase. Nitinol with the co-existence of both martensite and austenite phases combines advantages of martensite SMAs and superelastic SMAs and has a large damping capacity with self-shape restoring ability. To quantitatively study the damping effect of Nitinol in martensite and austenite co-existence phase, a setup is designed and fabricated. This setup involves a cantilevered steel beam with pre-stressed SMA wires attached to each surface at the remote end of the beam. The SMA wires function as a damper to the cantilevered beam. A piezoceramic patch sensor attached to the beam near its cantilevered end is used to record the data of the vibration of beam and the data is then used to estimate the damping ratio of the system. The percentage of the martensite phase in the Nitinol SMA wires is controlled by electrically heating the wires via a closed-loop control system. Experimental results verify that the Nitinol wires with the co-existence of the both martensite and austenite have the best damping property for vibration suppression. For practical implementation, the transformation temperate of the SMA wire damper can be chosen as the room temperature so that both martensite and austenite co-exist.