Optimisation of Ni-Ti shape memory alloy response time by transient heat transfer analysis

Nickel-Titanium (Ni-Ti) shape memory alloys (SMAs) are commonly applied in commercial actuator design due to the high associated fatigue and tensile strengths, low cost and high activation temperature. Consequently, Ni-Ti SMAs provide an opportunity for the development of novel electromechanical actuators. However, the cooling response time is typically of significantly larger duration than the associated heating response time. The applicability of SMA actuators would be significantly greater if the cooling response time was reduced to allow a symmetric, high speed activation profile. This work provides insight into the opportunities associated with enhancing thermal heat transfer efficiency to achieve this objective. An explicit model of the temperature of Ni-Ti SMA wire is developed to estimate the temperature-time profile during resistive heating. A finite-difference equation is developed to predict the associated temperature during cooling. These models are used to confirm that for a typical scenario, the cooling stage dominates the total response time, and that lagging with a highly conductive media can be used to dramatically reduce the cooling response time. The finite-difference equation is validated against steady state data, and extended to provide insight into the effects of SMA lagging, including the effects of periodic excitation on cooling rate and the minimum observed SMA temperature during a heating cycle. The outcomes of this work are generally applicable to any axisymmetric transient heat transfer optimisation problem.