Light-activated polymers are an exciting class of materials that respond mechanically when irradiated at particular
wavelengths. Recent demonstrations include two novel polymers developed by Scott et al (2006) and Lendlein et al
(2005). In these polymers, photochemistry alters the microstructure of the cross-linked polymer network, which is
further translated as light-induced deformation and when properly used light-induced shape memory effect. In this work,
we develop a model framework to simulate the photomechanical response of light-activated polymer systems. This
framework breaks down the observed macroscopic photomechanical phenomenon into four coupled sets of underlying
physics, which occur throughout the material during irradiation and mechanical deformation. In the context of this
framework, a basic photomechanical phenomenon involves simultaneously modeling photophysics, photochemistry,
chemomechanical coupling, and mechanical behavior. Furthermore, network alteration are accounted for through the
parallel decomposition of the cross-linked network into two components, an original network and a photochemically
altered network, which allows to capture the observed photomechanical behaviors demonstrated in these materials. One
of the principal strengths of this model framework is its generality as it can be applied to light activated polymer systems
with fundamentally different of photophysics, photochemistry, and chemomechanical behaviors simply by choosing
different field equations for the four sets of physics specific to a material system.
Shape memory polymers (SMPs) are receiving increasing attention because of their ability to store a temporary shape
for a prescribed period of time, and then when subjected to an environmental stimulus, recover an original programmed
shape. They are attractive candidates for a wide range of applications in microsystems, biomedical devices, deployable
aerospace structures, and morphing structures. In this paper we investigate the thermomechanical behavior of shape
memory polymers due to instrumented indentation, a loading/deformation scenario that represents complex multiaxial
deformation. The SMP sample is indented using a spherical indenter at a temperature T1 (>Tg). The temperature is then
lowered to T2 (g) while the indenter is kept in place. After removal of the indenter at T2, an indentation impression
exists. Shape memory is then activated by increasing the temperature to T1 (>Tg); during free recovery the indentation
impression disappears and the surface of the SMP recovers to its original profile. A recently-developed three-dimensional
finite deformation constitutive model for the thermomechanical behavior of SMPs is then used with the
finite element method to simulate this process. Measurement and simulation results are compared for cases of free and
constrained recovery and good agreement is obtained, suggesting the appropriateness of the simulation approach for
complex multiaxial loading/deformations that are likely to occur in applications.
We describe the design, fabrication, and testing of an electrostatic vertical actuator that exhibits a range of motion that covers the entire initial gap between the actuator and substrate and provides controllable digital output motion. This is obtained by spatially tailoring the electrode arrangement and the stiffness characteristics of the microstructure to control the voltage-deflection characteristics. The concept is based on the electrostatic pull down of bimaterial beams, via a series of electrodes attached to the beams by flexures with tailored stiffness characteristics. The range of travel of the actuator is defined by the post-release deformed shape of the bilayer beams, and can be controlled by a post-release heat-treat process combined with a tailored actuator topology (material distribution and geometry, including spatial geometrical patterning of the individual layers of the bilayer beams). Not only does this allow an increase in the range of travel to cover the entire initial gap, but it also permits digital control of the tip of the actuator which can be designed to yield linear displacement - pull in step characteristics. We fabricated these actuators using the MUMPs surface micromachining process, and packaged them in-house. We measured, using an interferometric microscope, full field deformed shapes of the actuator at each pull in step. The measurements compare well with companion simulation results, both qualitatively and quantitatively.
Exposure to elevated temperatures can cause permanent alteration of the shape of a multimorph MEMS device. In particular, the cantilever beam structure has been characterized extensively for the bimorph polysilicon and gold materials combination. Environments even briefly elevated above 110 C can induce change in structural displacement. That is, when observed at some reference temperature, the displacement is different from the original shape prior to baking, when monitored at the same reference temperature. Thermal exposure can cause the magnitude of displacement to increase in excess of two or threefold. Structural bifurcation can magnify the displacement of a plate-like structure. The process used to increase deformation of a multimorph MEMS structure will be described, and characterized according to the part's geometry. Such change in shape is not entirely permanent and is subject to relaxation. Change in deformation has been characterized throughout the time-span of approximately one-year. Lastly the implications of the so-called heat-treatment (or energy storage) mechanism are discussed in terms of MEMS device reliability, manufacture and packaging, as well as design.
The effect of crystallographic texture on the electroelastic moduli of piezoelectric polycrystals has been studied using micromechanical modeling that makes use of the uniform field concept. An orientational averaging scheme has been developed for textured piezoelectric polycrystals, which, when combined with the conventional self-consistent approach, provides an estimate of the effective electroelastic moduli in terms of texture. In the special situation where the polycrystal exhibits a fiber texture, a class of uniform fields exist under certain crystal symmetries, so that the effective electroelastic moduli can be determined exactly. This is confirmed by the coincidence of the corresponding upper and lower bounds. Numerical results are presented for both cases and compared to known theoretical predictions where possible.
Surface tension self-assembly of MEMS has been shown to be an excellent approach for assembling three-dimensional MEMS structures by allowing more precise alignments and vastly increased complexity. This paper investigates and addresses some of the factors that limit the precision of surface tension self-assembly. Each factor can be analyzed and addressed for a simple structure, but for more complex structures, these effects need to be addressed additively and in unison. This paper also discusses, the additive tolerance effects of these structures and present several methods of tolerance analysis. The toleranceing, in conjunction with the precision analysis, will in the future allow for the creation of extremely precise surface tension self-assembled structures, allowing for the creation of MEMS applications that were previously unobtainable using any existing fabrication or packaging process.
We study the viscoelectroelastic behavior of heterogeneous piezoelectric solids, focusing on the connection between heterogeneity and coupled mechanical and electrical relaxations. Our approach is based on the existence of a correspondence between quasistatic viscoelectroelasticity and static piezoelectricity when linear constitutive response exists. We couple this correspondence principle with micromechanics models to predict the overall behavior of heterogeneous piezoelectric solids in terms of microstructural details. We devote specific attention to a class of two-phase materials consisting of a lossless piezoelectric phase embedded in a lossy matrix and obtain closed form expressions for the effective complex electroelastic moduli. Numerical results are presented and discussed, and reasonable agreement with experiment is observed.
Exact relations are obtained between the effective thermoelectroelastic moduli of two-phase composite materials and their corresponding isothermal electroelastic moduli. The relations are a generalization of the well-known results of Levin and Rosen and Hashin to the inherently anisotropic coupling between the electric and elastic fields in thermoelectroelastic composites. The exact relations can be used to obtain the effective thermal expansion and pyroelectric coefficients of the composite when the effective electroelastic (elastic, piezoelectric, and dielectric) moduli are known, either by theory or experiment. Attention is focused on two- phase thermoelectroelastic laminates which admit an exact solution for the electroelastic, and thus the thermoelectroelastic, moduli. The results also provide a means to asses the internal consistency of any approximate micromechanics model that is proposed to estimate the thermoelectroelastic moduli of two-phase composite materials.
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