Investigation of torsional shape memory alloy actuators

When a shape memory alloy rod or tube is pre-twisted then heated, part of the initial rotation is recovered as a result of shape memory effect. If the pre-twisted SMA rod or tube is constrained from rotating, an enormous blocking torque is generated. As a shape memory alloy rod is twisted the central region remains elastic while the outer layer undergoes martensitic transformation. This central elastic region can be thought of as an internal bias spring. Because of this self-biasing mechanism, a torsional SMA actuator can also exhibit an 'intrinsic' two-way effect. Since the maximum recoverable shear strain is 70% more than the maximum recoverable tensile strain for shape memory alloys, a torsional SMA actuator can provide large recovery torque at high rotation output while maintaining excellent repeatability. This paper discusses both theoretical modeling and experimental testing of torsion SMA actuators. The behavior of the torsional SMA actuators to be addressed includes stress/strain relations, controlled recovery behavior, blocked recovery, free-recovery, and actuator repeatability.