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This article presents the material modeling and experimental characterization of SMA rod and tube actuators undergoing pure torsional deformations. The investigation of the torsional characteristics of SMAs is carried out in order to gain a fundamental understanding of the behavior of SMA torsional actuators. The proposed application is to alter the twist distribution of a tiltrotor blade from hover to forward flight modes, providing improved flight efficiency in both modes of flight. To describe the behavior of the actuator, a torsional model involving the extension of the one-dimensional (uniaxial) formulation of SMA phenomenology is presented. As an example, Brinson's model is chosen as the representative uniaxial model; however, the approach used here to extend the uniaxial model into the torsional domain is applicable to nearly any uniaxial SMA model. The parameters for the uniaxial model are derived from extensional testing of an SMA rod or tube, and are then used to predict the torsional characteristics of the same material. A major advantage of this method compared to prior art in torsional modeling is that the simplicity of deriving and implementing the parameters from uniaxial testing is carried over into the torsional domain. This simplicity derives from the fact that the measured properties (forces and displacements) in tensile loading are more readily suited to determining the model parameters than the measured properties in torsional loading which have been used by previous researchers (angles and torques which are, in fact, integrated quantities from strains and stresses). This article also presents a comprehensive experimental investigation and model correlation with both tension and torsional behavior of SMA rods and tubes. The model is compared with experimental data including torsional actuation tests against torsional springs. It is shown that the theoretical model demonstrates good agreement with the experimental data over a wide range of thermomechanical conditions. In addition, experimental phenomena associated with the torsional behavior of the SMA actuator such as the effects of heat treatment, twist rate, and loading pattern are examined.

This paper describes a modeling approach to incorporate the effects of nonquasistatic loading on the extensional behavior of an SMA wire. Experimental results under a variety of loading conditions indicate that the instantaneous temperature of the material is closely related to the applied strainrate in the material. In order to predict this behavior, a coupled thermo-mechanical analysis with the rate form of SMA constitutive models is formulated. The temperatures and temperature rates are not prescribed, but are derived from energy conservation of the material. The stress rate is simultaneously derived using the rate form of SMA constitutive models. Important parameters governing heat transfer such as specific heat, heat transfer and latent heat are modeled and validated with experimental data. For the material tested (0.38 mm diameter SMA wire), the quasistatic strain rate below which no significant deviation in material characteristics are observed was empirically determined to be about 0.0005/s. At a strain rate of 0.01/s, the transformation stresses in the material are increased by approximately 0.5 × 10 8 Pa, accompanied by an increase in temperature of about 2°C over the quasistatic values. The predictions for the stress-strain curves are compared with experimental data at different strain rates over a range of environmental temperatures, and are found to be in good qualitative agreement. The model is also shown to be in good qualitative agreement with the experimental behavior under more complex loading conditions involving two different strain rates of loading. Parametric variation of the model coefficients is discussed. Although the qualitative aspects of the model are in good agreement with experimental data, it is argued that more comprehensive estimation of the model parameters is required to assess the quantitative aspects of the model. The model described in the paper uses the rate forms of the Brinson model. However, it is equally applicable with a modified rate-dependant form of any other quasistatic model describing SMA behavior.