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Thin-film shape memory alloys (SMAs) have become excellent candidates for microactuator fabrication in MEMS. We develop a material model based on a combination of free energy principles in combination with stochastic homogenization techniques. In the first step of the development, we construct free energies and develop phase fraction and thermal evolution laws for homogeneous, single-crystal SMAs. Second, we extend the single-crystal model to accommodate material inhomogeneities and polycrystalline compounds. The combined model predicts rate-dependent, uniaxial SMA deformations due to applied stress and temperature. Moreover, the model admits a low-order formulation that is suitable for subsequent control design. We illustrate aspects of the model through comparison with thin-film NiTi superelastic hysteresis data.

We develop a model that quantifies constitutive nonlinearities and hysteresis inherent to ferroelastic compounds, with emphasis placed on shape memory alloys. We formulate the model in two steps. First, we use the Landau theory of phase transitions to characterize the effective Gibbs free energy for both single-crystal and polycrystalline ferroelastics. The resulting nonlinear equations model equilibrium material behavior in the absence of impurities. Second, we incorporate pinning losses to account for the energy required to move domain walls across material inclusions. The full model is analogous to those developed by Jiles and Atherton for ferromagnetic compounds and Smith and Hom for ferroelectric materials. We illustrate aspects of the model through numerical simulations and comparisons with experimental stress-strain data.Keywords: Ferroelastic hysteresis; shape memory alloy; domain wall theory; ferroelastic domain; superelasticity; Landau theory of phase transitions;anyhysteretic strain; domain wall pinning; pinning sites. Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

This paper summarizes techniques for modeling geometric properties of THUNDER actuators which arise in the fabrication process. These actuators are high performance composites comprised of layers of piezoceramics in combination with aluminum, stainless steel, brass or titanium bonded with hot-melt adhesive. During the construction process, the assembly is heated under pressure to high temperatures, cooled and repoled to restore the actuator capabilities. This process provides the actuators with the robustness necessary to withstand the high voltages required for large displacement and force outputs. The process also provides the actuators with their characteristic curved shape. In this paper, relations between the thermal and electrostatic properties of the material and the nal geometric con guration are quanti ed. This provides an initial model that can be employed in control applications which employ THUNDER actuators.

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