Cyclic Deformation and Degradation in Shape Memory Effect of Ti-Ni Wire

摘要

Recently, the robot actuator worked by the driving recovery-force in thermoelastic martensitic transformation of shape memory alloys (SMA) is under development. In general, such a SMA actuator necessitates a number of cyclic repeated motions, so that the investigation of the gradual decrease of recovery force with repeated motion cycles as well as the prevention of such a degradation of shape memory effect (SME) are very important for the actual use as a robot actuator. However, such research and discussions about the degradation of SME are very few up to the present. Therefore, in the present study, the most basic, single axial SMA actuator of Ni-Ti wire is made, and the cyclic motion (i.e. fatigue) test by the alternative, heating up and cooling down of SMA Ni-Ti wire is performed. Heating up of wire is done by a direct current method, and cooling down by fan in air. The characteristics on the cyclic motion and its degradation of SME is discussed. As a result, the degradation in SME can be devided in two factors; the cyclic deformation by martensitic-trasformation and the gradual elongation of Ni-Ti wire. From the investigation in the changes of these two factors with repeated motion cycles, it is found that the latter factor contributes to the degradation to a much greater extent than the former under the present fatigue test condition of Ti-Ni wire. The efficiency of training before the use of SMA material is confirmed as one of the useful measures to prevent the degradation of SME in engineering applications.