Dissertation submitted to the University of Cambridge for the degree of Doctor of Philosophy, March, 1998
Although it has been recognised that SMA materials have a significant potential for deployment actuators, the number of applications of SMA-based actuators to the present day is still quite small, since a deeper understanding of the thermomechanical behaviour of SMA and how it might be exploited in the design of working actuators is necessary.
In order to get a complete picture of the thermomechanical behaviour of Nitinol, one type of SMA, two kinds of experiment, purely thermal tests and thermomechanical tests, were carried out on Nitinol wire and bar.
In the purely thermal tests, a Differential Scanning Calorimeter was used to determine the phase transformation/temperature relation of Nitinol wire with diameter of 1 mm and Nitinol bar. The thermomechanical tests, including tension tests at different temperatures, tension tests under different strain rates, response to suddenly applied loads, thermal cycling under different loads and thermal cycling with fixed length were carried out on Nitinol wire. Torsional tests, thermal cycling under different torques, and normal tension and thermal cycling tests were carried out on Nitinol bar with diameter of 6.5 mm.
We have developed a thermo-micromechanical model based on complementary free energy and micromechanical transformation system to investigate the behaviour of shape memory under uniaxial load cycling and thermal cycling. Experimentally observed phenomena, such as $V$-shape of critical stress, non-symmetrical behaviour in tension and compression, transformation front behaviour, and multi-phase transformation, were explained by this model.
We present a phenomenological model which is based on tension test, carried out at different constant temperatures, and thermal cycling test under different constant loads. We have shown that our model can reproduce accurately the stress-strain-temperature relationship for all the quasi-static tests we have carried out on Nitinol wires. This model has no difficulty in dealing with incomplete loading or thermal cycling. This model has been used to simulate thermal cycling behaviour with fixed length, strain rate effects, and phase transformation front behaviour.
Three Nitinol wire-based rotatory actuators, a one-way actuator, a biased actuator, and a two-way actuator, have been designed and tested. The behaviour of these actuators has been simulated by using the phenomenological model that we have developed. The power consumption to operate all actuators was measured. Two simple approaches were proposed to estimate the average power requirements. A preliminary study of a torsional actuator has been made. The general design procedure for Nitinol-based actuators is summarised.
The broad aim of this dissertation is also to provide a user's guide to the design of SMA-based actuators.