Abstract
Shape memory alloy actuators’ strokes can be increased at the expense of recovery force via heat treatment to form compressed springs in their heat-activated, austenitic state. Although there are models to explain their behaviour, few investigations present experimental results for support or validation. The aim of the present paper is to determine via experimentation how certain parameters affect a helical shape memory alloy actuator’s outputs: its transformation times and stroke. These parameters include wire diameter, spring diameter, transition temperature, number of active turns, bias force and direct current magnitude. Six investigations were performed: one for each parameter manipulation. For repeatability and to observe thermo-mechanical training effects, the springs were cyclically activated. The resultant patterns were compared with results predicted from one-dimensional models to elucidate the findings. Generally, it was observed that the transformation times and strokes converged at changing stress levels; the convergence is likely the peak where the summation of elastic stroke and transformation stroke has reached its maximum. During cyclic loading, the actuators’ strokes decreased to a converged value, particularly at larger internal stresses; training should therefore be performed prior to the actuator’s implementation for continual use applications.
Highlights
Shape memory alloys (SMAs) got their name from their intrinsic ability to remember their shape.Using heat treatment techniques, a SMA actuator can be programmed to be a specific shape in its heat activated austenite phase; some shapes are more useful than others to perform mechanical work.Popular shapes used for mechanical mechanisms include wires, bars, torsion springs, helical springs and, more recently, wave springs [1]
For the heat times, it was found that the patterns projected by the derived heat transfer equation that accounted for superelasticity (Equation 6) were generally correct, but greatly underestimated the true heat times
For the cooling times, the patterns projected by the derived heat transfer equation (Equation 7) were generally correct, but most values were within 10% of the experimental values, indicating that it can provide a good estimate of the true reaction time
Summary
Shape memory alloys (SMAs) got their name from their intrinsic ability to remember their shape.Using heat treatment techniques, a SMA actuator can be programmed to be a specific shape in its heat activated austenite phase; some shapes are more useful than others to perform mechanical work.Popular shapes used for mechanical mechanisms include wires, bars, torsion springs, helical springs and, more recently, wave springs [1]. A SMA actuator can be programmed to be a specific shape in its heat activated austenite phase; some shapes are more useful than others to perform mechanical work. SMAs in wire form are most popular, as they are readily available, low cost and generally less difficult to model. SMA wires provide the highest recovery force, but have a low stroke; the recovery strain is typically less than 5%. The limited stroke can be improved by using mechanical amplification mechanisms, such as levers or adjusting curvature, as was done in Phillip Beezley’s Hylozoic Ground [2], but the mechanisms may require significant space along with a sacrifice in recovery force. Even with mechanical amplification, a great length of wire is needed for adequate stroke magnitudes. Sufficient space must be provided for both the length of wire and the amplification mechanism
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