Wing twisting has been shown to improve aircraft flight performance. The potential benefits of a twisting wing
are often outweighed by the mass of the system required to twist the wing. Shape memory alloy (SMA) actuators
repeatedly demonstrate abilities and properties that are ideal for aerospace actuation systems. Recent advances
have shown an SMA torsional actuator that can be manufactured and trained with the ability to generate
large twisting deformations under substantial loading. The primary disadvantage of implementing large SMA
actuators has been their slow actuation time compared to conventional actuators. However, inductive heating of
an SMA actuator allows it to generate a full actuation cycle in just seconds rather than minutes while still . The
aim of this work is to demonstrate an experimental wing being twisted to approximately 10 degrees by using an
inductively heated SMA torsional actuator. This study also considers a 3-D electromagnetic thermo-mechanical
model of the SMA-wing system and compare these results to experiments to demonstrate modeling capabilities.
Shape memory alloys (SMAs) show size effect in their response. The critical stresses, for instance, for the start of martensite and austenite transformations are reported to increase in some SMA wires for diameters below 100 μm. Simulation of such a behavior cannot be achieved using conventional theories that lack an intrinsic length scale in their constitutive modeling. To enable the size effect, a thermodynamically consistent constitutive model is developed, that in addition to conventional internal variables of martensitic volume fraction and transformation strain, contains the spatial gradient of martensitic volume fraction as an internal variable. The developed theory is simplified for 1D cases and analytical solutions for pure bending of SMA beams are presented. The gradient model captures the size effect in the response of the studied SMA structures.
Shape memory alloy (SMA) pipe couplers use the shape memory effect to apply a contact pressure onto the surface of
the pipes to be coupled. In the current research, a SMA pipe coupler is designed, fabricated and tested. The thermally
induced contact pressure depends on several factors such as the dimensions and properties of the coupler-pipe system.
Two alloy systems are considered: commercially-available NiTiNb couplers and in-house developed NiTi couplers. The
coupling pressure is measured using strain gages mounted on the internal surface of an elastic ring. An axisymmetric
finite element model including SMA constitutive equations is also developed, and the finite element results are compared
with the experimental results.
Coupling between the electric field, magnetic field, and strain of composite materials is achieved when electro-elastic (piezoelectric) and magneto-elastic (piezomagnetic) particles are joined by an elastic matrix. Although the matrix is neither piezoelectric nor piezomagnetic, the strain field in the matrix couples the E field of the piezoelectric phase to the B field of the piezomagnetic phase. This three-phase electro-magneto-elastic composite should have greater ductility and formability than a two-phase composite in which E and B are coupled by directly bonding two ceramic materials with no compliant matrix. A finite element analysis and homogenization of a representative volume element is used to determine the effective electric, magnetic, mechanical, and coupled-field properties of an elastic (epoxy) matrix reinforced with piezoelectric and piezomagnetic fibers. The effective magnetoelectric moduli of this three-phase composite are, however, less than the effective magnetoelectric coefficients of a two-phase piezoelectric/piezomagnetic composite, because the epoxy matrix is not stiff enough to transfer significant strains between the piezomagnetic and piezoelectric fibers.
Modeling ionic diffusion in electrolytes requires the simultaneous solution of the Nernst-Planck (electro-diffusion) equation and Gauss' law. Unfortunately, the Nernst-Planck equation is not applicable in the ionic double layer that forms at an electrode/electrolyte interface. Furthermore, the large gradients of the electric potential in the double layer can cause numerical instabilities. The double layer is usually modeled using the Gouy-Chapman theory, a steady state solution, which predicts an exponential decay of the electric potential and ion concentration in the direction normal to the electrode. In the present paper we present a novel theory in which the Gouy-Chapman equation, a three-dimensional theory, is replaced by an interface (2-D) theory of the double layer. The effects of the double layer are then modeled as boundary conditions applied to the Nernst-Planck equation and Gauss' law. Interfacial equations are derived for the species mass balances, the conservation of charge, Gauss's law, and the quasi-static form of Faraday's law. Each of these physical principles is derived for both a regular (or single) interface and a double interface representing an electric double layer. The standard interfacial variables are augmented with an electric charge, electric potential, electric field, electric polarization, and electric displacement, whereas conventional electrostatics includes only interfacial charge.
Large arrays of MEMS with programmable electrodes and electromagnets are used to achieve microscale positioning of particles, whiskers, and fibers in polymer matrix materials. Arrays of MEMS are placed above and beneath thin layers of a random particle-filled liquid polymer. Microscale variations in the electric and magnetic fields are then used to control body forces that move the piezoelectric and piezomagnetic particles. The body forces are due to the gradients in the E and B fields. Such body forces are generally small on a macroscopic scale. However, standard microfabrication methods enable the generation of very high gradients in E and B on the microscale, therefore generating body forces large enough to overcome microscopic sedimentation forces, viscous forces, and the mutual attraction of particles. We discuss the free body diagram of a particle, the design of MEMS arrays using a finite element code (ANSYS) to determine the electric and magnetic fields, and the fabrication of the MEMS arrays. Currently there is no way to affordably arrange the particles in the optimal microscale pattern in composite materials. Ideally, the current method will provide an affordable and versatile method of patterning the microstructure of multi-functional composite materials, sensors, actuators.
A reflective type Fresnel lens using an array of micromirrors is designed and fabricated using the MUMPs® surface micromachining process. The focal length of the lens can be rapidly changed by controlling both the rotation and translation of electrostatically actuated micromirrors. The rotation converges rays and the translation adjusts the optical path length difference of the rays to be integer multiples of the wavelength. The suspension spring, pedestal and electrodes are located under the mirror to maximize the optical efficiency. Relations are provided for the fill-factor and the numerical aperture as functions of the lens diameter, the mirror size, and the tolerances specified by the MUMPs® design rules. The fabricated lens is 1.8mm in diameter, and each micromirror is approximately 100mm x 100mm. The lens fill-factor is 83.7%, the numerical aperture is 0.018 for a wavelength of 632.8nm, and the resolution is approximately 22mm, whereas the resolution of a perfect aberration-free lens is 21.4μm for a NA of 0.018. The focal length ranges from 11.3mm to infinity. The simulated Strehl ratio, which is the ratio of the point spread function maximum intensity to the theoretical diffraction-limited PSF maximum intensity, is 31.2%. A mechanical analysis was performed using the finite element code IDEAS. The combined maximum rotation and translation produces a maximum stress of 301MPa, below the yield strength of polysilicon, 1.21 to 1.65GPa. Potential applications include adaptive microscope lenses for scanning particle imaging velocimetry and a visually aided micro-assembly.