Experimental Study of Helical Shape Memory Alloy Actuators: Effects of Design and Operating Parameters on Thermal Transients and Stroke

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.