This paper motivates a one-dimensional thermo-magneto-mechanical free energy model for NiMnGa. Following a
discussion of material behavior and modeling purpose, we present what might be referred to as a meso-scale model,
incorporating micro-scale physics while striving for macro-scale simplicity. Development of the model begins with the
construction of a free energy landscape for the material, with strain and magnetization as its order parameters. This
landscape includes paraboloidal energy wells - isolated from each other by energy barriers - to represent stable states of
the material. The energy well positions and barrier heights are allowed to vary as functions of stress, magnetic field, and
temperature. The resulting equations are employed within the theory of thermally activated processes to find the phase-fraction
evolution of a sample. Previous results demonstrating the potential of the modeling approach are included.
Shape memory alloy (SMA) beams are used in a variety of applications such as, e.g., stents or microactuators. In this
paper, a free energy based SMA model is implemented into the commercial finite element (FE) code ANSYS, and the
bending problem of a SMA cantilever beam is studied. In order to determine optimal parameter settings for the finite
element analysis, we systematically study convergence behavior and accuracy for different convergence criteria as well
as other relevant parameters such as element, substep and integration point number. The simulation results demonstrate
that the SMA model can represent both shape memory effect and superelasticity of SMA materials seamlessly.
Two micromechanisms including a microactuator of a shape memory alloy (SMA) and a retaining system are presented, which are implemented in a microvalve to maintain a closed condition while no power is supplied. In one design, the retaining system is realized by a pseudoelastic SMA microspring coupled to the SMA microactuator. Alternatively, a pressure compensation mechanism is developed based on two mechanically coupled membranes, which are located above and below the SMA microactuator. The mechanical, electrical and thermal behaviors of the SMA microactuator are simulated by a coupled finite element program. Based on force-displacement characteristics of microspring and microactuator, a design of the two micromechanisms is developed. The investigation reveals several advantages of the pressure-compensation mechanism. In particular, pressure compensation allows a maximum controllable pressure difference of more than 500 kPa compared to 100 kPa for the microspring mechanism. Furthermore, a larger actuation stroke close to the maximum possible design value is achieved. Dynamic flow measurements reveal similar time constants for both mechanisms of 15 and 55 ms for opening and closing, respectively.
A shape memory alloy (SMA) microgripper for manipulation of microparts is presented, which is made of a monolithic SMA device of 2 × 5.8 × 0.1 mm3 size comprising various integrated functional units like two microactuators for active closing and opening, gripping jaws and an optical slit for position sensing. Recently, the design and fabrication technology of the SMA microgripper have been developed. The present work concentrates on the mechanical and thermal performance. A 3D macromodel and its implementation in a coupled finite element (FEM) routine is introduced, which allows simulation of mechanical, electrical and thermal fields in shape memory actuators of arbitrary shape. The mechanical behavior is described by a two-phase macromodel taking into account material nonlinearity and history effects. The spatial distribution of electrical heating power is determined from simulation of the electrical potential distribution. For simulation of temperature profiles a heat transfer model is used, which takes the distribution of electrical heating power and the effects of latent heat, of heat conduction and of heat convection into account.
The simulation results are compared with experimentally determined characteristics. Mechanical tests reveal spring constants of the microactuators in austenitic condition of 1600 N/m, which are confirmed by the simulations. The stroke of the gripping jaws is between 250 and 300 μm depending on the maximum prestrain. The maximum gripping force is determined to 35 mN. Typical heat transfer times are about 100 ms upon heating and 150 ms upon cooling, which are in quantitative agreement with experimentally determined time constants.
Recently, a variety of manufacturing methods have enabled us to produce fine or thin shapes or unique structure of TiNi-based shape memory alloys: e.g., (a) rolled thin plates with a thickness less than 100 μm, (b) drawn fine wires with a diameter less than 50 μm, (c) drawn microtubes with an outer diameter less than 350 μm, (d) sputter-deposited thin films with a thickness less than 1 μm, (e) rapidly solidified ribbons of several tens μm thickness and (f) sintered porous structured materials. The characteristics and applications of these TiNi-based alloys are reviewed.
This paper describes experimental results of using various microlithography techniques to fabricate a range of microactuator devices from NiTi shape memory alloys. The range of products includes: planar double-beams form rolled foils etched form both sides; tapered double-beams; planar double beams from sputter-deposited films etched rom one side; a tubular test piece. Such photofabrication in not easily achieved and problems discussed in this paper include: achieving acceptable edge profiles through the thickness of the materials while maintaining high etch factors; tapering foil microactuators by means of chemical micro milling; coating NiTi tubes with electrophoretic photoresist; imaging a curved surface with a small radius of curvature; control of etching parameters for a constant rate of etch; the influence of NiTi oxide coatings on etching and; technical comparisons with other potential manufacturing processes.
A characterization method based on beam-bending experiments and finite element simulations has been developed. Calibrated microweights are applied to double-beam test devices with typical beam widths of 100 micrometers and lengths of 2 mm to determine load-dependent displacements with a maximum resolution of about 2 (mu) N and 2 micrometers , respectively. The resulting characteristics are used to determine the corresponding stress-strain characteristics, transformation temperatures and stress-rates of the material. The method has been used to study test devices of TiNi thin sheets with thicknesses d between 160 and 20 micrometers and of sputtered thin films (d equals 8 micrometers ) microfabricated by laser cutting or electrolytic photoetching. The measurements did not show any influence of the microfabrication processes. An investigation of size effects in test devices with decreasing thicknesses revealed enhanced transformation hystereses for thicknesses in the order of the grain size.
A shape memory membrane actuator has been developed, which consists of stress-optimized microfabricated NiTi beam-cantilever devices of 100 micrometers thickness. For stress- optimization, the lateral widths of the beams have been designed for homogeneous stress distributions without local stress maxima, allowing a maximum use of active material for bending actuation. As a result, higher work outputs for a given stress level and smaller hystereses are obtained compared to non-optimized actuators with parallel beam-cantilever devices. Thus, improved hysteresis control is possible and better lifetime characteristics are expected. For a strain limit of 2% the achieved work output is about 24 (mu) Nm.
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