Stability of cyclic strain output of NI-TI shape memory alloy (SMA) actuator under different loading conditions and frequency bandwidths

This paper presents a study on the stability of cyclic strain output of the Ni-Ti SMA for up to 4×10 ⁴ cycles under different loading conditions and at different actuation frequency bandwidths. It is a part of research activity conducted at the Canadian Space agency (CSA) on applications of SMA, such as the applications for low frequency vibration damping of spacecraft deployable booms, flatness control of space-borne membrane SAR (Synthetic Aperture Radar) antenna and other potential space applications. Pulsed power supply is used to heat the SMA to produce typical cyclic actuations under different loading conditions. The stability of SMA is specifically studied in terms of the development of plastic strain (elongation) and degradation in amplitude of strain output over the number of cycles. The experimental result demonstrates that external loading condition has a significant effect on the stability of strain output of SMA actuator. For 4×10 ⁴ cycles of actuation tested, stable cyclic actuation can be achieved when the loading stress is in the range between 60 MPa to 130 MPa. Experiment also indicates that by increasing the actuation frequency, plastic strain of SMA actuator over the number of actuation cycles can be reduced. Particularly, in the case that the cyclic actuation at relatively high frequency involves only partial martensitic phase transformation, fatigue of SMA can significantly be minimized. Experiment on stability of SMA actuator under feedback control application is also performed and SMA tends to have good stability in this case.