A Variational-Asymptotic Theory of Smart Slender Structures

The goal of the present work is to develop an efficient simulation tool with high-fidelity to help the engineers design and analyze smart slender structures with embedded piezoelectric materials. Actuation and sensing capabilities of piezoelectric material embedded in smart beam including geometric nonlinearity will be explored. The dimensional reduction process has been carried out using the powerful Variational Asymptotic Method. Starting from the exact three-dimensional electric-mechanically coupled enthalpy functional, the asymptotical analysis is done on the functional itself with respect to the naturally occurring small parameters. The original three-dimensional electric-mechanical problem of the slender structure is decomposed into two separate problems: a two-dimensional analysis over the cross section and a one-dimensional analysis over the beam reference line. The coupled cross-sectional analysis is being implemented in VABS, a versatile cross-sectional analysis code.

Electroactive Paper Materials Coated With Carbon Nanotubes and Conducting Polymers

Electro-Active Paper (EAPap) materials based on cellulose are attractive for many applications because of their low voltage operation, lightweight, dryness, low power consumption, bio-degradable. The construction of EAPap actuator has been achieved using the cellulose paper film coated with thin electrode layers. This actuator showed a reversible and reproducible bending movement. In order to improve both force and displacement of this, EAPap actuator efforts are made to construct the device using increasing number of complementary conducting polymer layers and carbon nanotubes. A hybrid EAPap actuator is developed using single-wall carbon nanotubes (CNT)/Polyaniline (PANi) electrodes, as a replacement to gold electrodes. It is expected that the use of CNT can enhance the stiffness of the tri-layered actuator, thus improving the force output. Furthermore, the presence of the CNT may increase the actuation performance of the EAPap material. CNT is dispersed in NMP(1-Methyl-2-pyrrolidine), and the resulting solution is used as a solvent for PANi. The CNT/PANi/NMP solution is then cast on the EAPap by spin coating. The coated EAPap is dried in an oven. The effect of processing parameters on the final performance of the CNT/PANi electrodes is assessed. The final performance of the electrodes is quantified in terms of the electrical conductivity under dc and ac measurement conditions. The actuation output of the CNT/PANi/EAPap samples is tested in an environmental chamber in terms of free displacement and blocked force. The performance of the hybrid actuators is also investigated in terms of frequency, voltage, humidity and temperature to help shed light on the mechanism responsible for actuation. Comparison of these results in that of the EAPap with PANi and gold electrodes are also accomplished. EAPap materials are bio-degradable that is important property for artificial muscle actuators for biomimetic with controlled properties and shape.

Modeling and Control of a 1-D Membrane Strip With an Integrated PZT Bimorph

Ultra-lightweight, ultra-large and deployable satellite technology is at the forefront of research efforts for future on-orbit reconnaissance missions. The minimal mass and stowage volume associated with the technology are attractive traits for getting larger bandwidth satellites on-orbit. One of the key components for such a satellite is the membrane lens or aperture for optical or radar applications, and understanding the membrane’s dynamics is critical for mission success. As either an optical reflector or radar antenna, the vibration levels of the membrane must be minimized and eliminated. This work examines the possibility of integrating a PZT bimorph near the boundary of a strip sample to eliminate detrimental vibration. By starting with a 1-D model, the dominant governing phenomena of the system dynamics can be established and used to build more complex models with confidence. A physics-based finite element (FE) model of a thin strip of Kapton HN material with a monolithic PZT bimorph bonded near a boundary is developed in a MatLab environment and verified experimentally. The membrane strip under tension is modeled as a beam under axial load. In doing so, the FE model is able to capture the relevant transverse dynamics of the experimental setup. Having verified the FE model, an LQR controller is developed and simulated to demonstrate effective control over the transverse dynamics of the membrane sample.

Development of an Antagonistic SMA Actuator for Instar Rifle Stabilization System

Shape memory alloys are notoriously slow and suffer from creep and controllability issues [1,2]. This paper presents three methods to address these issues: a high-stress cyclic conditioning regime to reduce creep to operationally insignificant levels, an unconventional pulse-width-modulated duty cycle with heatsink to increase frequency to the ten hertz range, and simple position feedback control strategy for motion control. These methods are discussed within the context of a simple antagonistic leveraged SMA actuation system developed for an INertially STAbilized Rifle (INSTAR). An overview of design and basic parameter models for the L-Lever is provided along with benchtop experimental characterization of the quasistatic and dynamic behavior. The actuator was integrated into a one degree of freedom INSTAR platform to demonstrate the insitu methods via barrel control. The methods discussed in this paper led to a fast, low-creep, controllable actuator with outstanding authority resulting in precise barrel control with capabilities to greatly increase shooter accuracy.

Dynamic Actuation of Electro-Elastic Spherical Membranes Using Dielectric Elastomers

This paper presents dynamic results for spherical dielectric elastomer actuators subject to an inflating mechanical pressure and an applied voltage. Different equilibria modes arise during dynamic operation due to inertial effects. In previous work, the inertial effects have been studied for the limited case of a constant applied pressure during membrane deformation [1]. Here, novel results are presented in which the dynamic response of spherical dielectric elastomer actuators to a pressure-time loading history as well as a more realistic constant gas flow rate are considered. The results are calculated for both the damped and the zero-damped cases. The spherical membrane is assumed to follow the Mooney material model where various inflation modes arise depending on the material parameters. The range of Mooney material parameters considered, the driving pressure and the applied voltage all affect the dynamic response.

Shock-Induced Chemical Reactions in Multi-Functional Structural Energetic Intermetallic Nanocomposite Mixtures

Shock induced chemical reactions of intermetallics or mixtures of metal and metal-oxides are also used to synthesize new materials with unique phases and microstructures. These materials are also of significant interest to the energetics community because of the significant amount of heat energy released during chemical reactions when subjected to shock and/or thermal loading. Binary energetic materials are classified into two categories— metal/metal oxides and intermetallics. When these materials are synthesized at a nano level with binders and other structural reinforcements, the strength of the resulting mixture increases. Thus, these materials can be used as dual-functional binary energetic structural materials. In this paper, we study the shock-induced chemical reactions of intermetallic mixtures of nickel and aluminum of varying volume fractions of the constituents. The chemical reaction between nickel and aluminum produces different products based on the volume fraction of the starting nickel and aluminum. These chemical reactions along with the transition state are modeled at the continuum level. In this paper, the intermetallic mixture is impact loaded and the subsequent shock process and associated irreversible processes such as void collapse and chemical reactions are modeled in the framework of non-equilibrium thermodynamics. Extended irreversible thermodynamics (EIT) is used to describe the fluxes in this system and account for the associated irreversible processes. Numerical simulations of the intermetallic mixture are carried out using finite difference schemes.

Analytical and Numerical Predictions of Thermoelastic Properties of Carbon Single-Walled Nanotubes

The objective of this paper is to develop constitutive models to predict thermoelastic properties of carbon single-walled nanotubes using analytical, asymptotic homogenization, and numerical, finite element analysis, methods. In our approach, the graphene sheet is considered as a non-homogeneous network shell layer which has zero material properties in the regions of perforation and whose effective properties are estimated from the solution of the appropriate local problems set on the unit cell of the layer. Our goal is to derive working formulas for the entire complex of the thermoelastic properties of the periodic network. The effective thermoelastic properties of carbon nanotubes were predicted using asymptotic homogenization method. Moreover, in order to verify the results of analytical predictions, a detailed finite element analysis is followed to investigate the thermoelastic response of the unit cells and the entire graphene sheet network.

An Equivalent Continuum-Atomistic Characterization Model for Nanographitic Materials

An equivalent continuum-atomistic algorithm is proposed for carbon-based structures such as nano-scale graphene platelets (NGPs) and carbon nanotubes (CNTs) individually or as stiffeners with polymers. This equivalent continuum-atomistic model will account for the nonlocal effect at the atomistic level and will be a highly accurate mean to determine the bulk properties of graphene-structured materials from its atomistic parameters. In the model, the equivalent continuum and atomic domains are analyzed by finite elements and molecular dynamics finite element-based where atoms stand as nodes in discretized form. Micromechanics idea of representative volume elements (RVE) will be used to determine averaged homogenized properties. In the procedure, a unit hexagonal cell will be the RVE. A minimum volume of material containing this RVE and the neighboring hexagonal cells will be chosen. The size of this volume should cover all the atoms, which have bonded, and nonbonded interaction with the atoms of the RVE unit cell. This minimum volume will be subjected to several load cases. Determination of the response of the RVE hexagonal unit cell contained within the minimum volume, and its potential energy density under the defined load cases, will lead to the determination of mechanical parameters of an equivalent, continuum geometrical shape. For a single layer NGP the thickness of the hexagonal continuum plate is assumed to be 0.34 nm, while in three-dimension and multilayered the actual thickness of layers can be implemented. Under identical loading on the minimum volumes, identical potential (strain) energies for both models will be assumed. Through this equivalence a linkage between the molecular force field constants and the structural elements stiffness properties will be established.

A Thermo-Magneto-Mechanical Model for Ferromagnetic Shape Memory Thin Film Actuators

The last decade has witnessed the discovery of materials combining shape memory behavior with ferromagnetic properties (FSMAs), see, e.g., James & Wuttig1, James et al.2. These materials feature the so-called giant magnetostrain effect, which, in contrast to conventional magnetostriction, is due to the motion of martensite twins. It was first observed in NiMn2Ga single crystals, Ullakko et al.3, but later discovered in polycrystals as well, see Ullakko4. This effect has motivated the development of a new class of active materials transducers, which combine intrinsic sensing capabilities with superior actuation speed and improved efficiency when compared to conventional shape memory alloys. The effect has also been found in thin films, Rumpf et al.5, and this technology is currently being developed intensively in order to pave the way for applications in micro- and nanotechnology. As an example, Kohl et al.6,7, recently proposed a novel actuation mechanism based on NiMnGa thin film technology, which makes use of both the ferromagnetic transition and the martensitic transformation allowing the realization of an almost perfect antagonism in a single component part. The implementation of the mechanism led to the award-winning development of an optical microscanner8. Possible applications in nanotechnology arise, e.g., by combination of smart NiMnGa actuators with scanning probe technologies. The key aspect of Kohl’s device is the fact that it employs electric heating for actuation, which requires a thermo-magneto-mechanical model for analysis. The research presented in this paper aims at the development of a model that simulates this particular material behavior. It is based on ideas originally developed for conventional shape memory alloy behavior, (Mueller & Achenbach9, Achenbach10, Seelecke11, Seelecke & Mueller12) and couples it with a simple expression for the nonlinear temperature-and position-dependent effective magnetic force. This early and strongly simplified version does not account for a full coupling between SMA behavior and ferromagnetism yet, and does not incorporate the hysteretic character of the magnetization phenomena either. It can however be used to explain the basic actuation mechanism and highlight the role of coupled magnetic and martensitic transformation with respect to the actuator performance. In particular will we be able to develop guidelines for desirable alloy compositions, such that the resulting transition temperatures guarantee optimized actuator performance.

Topology Optimization of Carbon Nanotube Reinforced Damping Layers

Topology optimization has been successfully used for improving vibration damping in constrained layer damping structures with viscoelastic materials. Reinforcing carbon nanotubes in a polymer matrix greatly influences the mechanical properties of the polymer. Such nanotube-reinforced polymers (NRP) can be used to further enhance the damping properties of the constrained layer structures. The inclusion of nanotubes into a polymer matrix provides a new design variable in the topology optimization studies on such structures. In this work, the topology optimization of structures using such NRP as the damping material is performed. The resulting structures show a phenomenal improvement in damping. Moreover, a more efficient method is used for the optimization process.