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IN THIS VOLUME


Aerospace

2005;():1-10. doi:10.1115/IMECE2005-79049.

Residual stresses are basically developed due to intrinsic and extrinsic strains that form during the processing of composite materials. The extrinsic strains can be determined using Coefficient of Thermal Expansion (CTE), material properties, geometry of the structure, and processing conditions. Finite Element Method (FEM) as an efficient alternative technique for stress and strain analysis of the micromechanical systems and structures, has been employed to numerically investigate the residual stresses developed in Metal-Core Piezoelectric Fibers (MPF) and Active Fiber Composites (AFC) (or Macro Fiber Composites (MFC)), during the processing. Here in this work, ANSYS Finite Element Analysis (FEA) software is used to develop three different 3-dimensional models for MPF and MFC structures and then each model is solved for strain and stress results. Next, the stress and strain components of these models are studied throughout the structures to identify the magnitude and type of the stresses and strains within the constituent materials and then compared.

Commentary by Dr. Valentin Fuster
2005;():11-19. doi:10.1115/IMECE2005-79092.

Ferromagnetic shape memory alloys in the Ni-Mn-Ga system exhibit large reversible deformations of up to 9.5% in response to magnetic fields. Prior experimental measurements by the authors demonstrated large reversible strains of −0.41% along the [001] crystal direction of a cylindrical Ni50 Mn28.7 Ga21.3 rod driven with a magnetic field along the same direction and no external restoring force. This represents an unusual configuration which can lead to solenoid transducers with enhanced energy density and bandwidth relative to standard electromagnet devices. The paper builds on a previous thermodynamic framework which accurately quantifies several reversible and irreversible effects in the polarization of ferroic materials. The switching between two variant orientations in the presence of Zeeman energy and pinning energy is formulated through a Gibbs energy functional for the crystal lattice. The presence of nonhomogeneous local interaction fields, nonhomogeneous pinning distributions, and complex crystallographic features in real Ni-Mn-Ga alloys is addressed through stochastic homogenization techniques. Attributes of the model are illustrated through comparison of model results with experimental data.

Commentary by Dr. Valentin Fuster
2005;():21-31. doi:10.1115/IMECE2005-79112.

Nonlinear stress-strain relationships (physical nonlinearity) may have a large effect on the structural response of piezoelectric sensors and actuators. The paper addresses the subject concentrating on cylindrical piezoelectric rods that experience vibrations as a result of an alternating electric field applied in the axial direction. The problem considered in the paper is important in connection with design of piezoelectric transducers. It is shown that neglecting the non-linear stress-strain relationship of the piezoelectric material can result in a significant error in the predicted response of the transducer.

Topics: Vibration , Rods
Commentary by Dr. Valentin Fuster
2005;():33-39. doi:10.1115/IMECE2005-79194.

Orientation between loading and material property directions is a concern for both polycrystalline and single crystal piezoelectric materials. The design of devices fabricated from piezoelectric materials emphasizes alignment between principal actuation direction and a specific coupling coefficient direction. However, loading and actuation directions may not always be aligned. Complex component geometry, multiple loading types, multiple loading paths and fabrication tolerances may result in misalignment between mechanical loading direction, principal actuation direction, electrical loading direction and material property orientation. In this work a computational study is presented that examines the effects of off-axis loading as well as geometric features for piezoelectric ceramics. An ASTM dog-bone shaped tensile specimen is modified by the addition of cut-out features to provide geometry stress concentrations at various angles to the primary mechanical loading direction. Polycrystalline PZT-5A material properties are used. Mechanical loading is applied as in a standard tensile strength test. Electrical loading direction is aligned with the mechanical loading direction. The tensile specimen is also subjected to sequential mechanical and electrical loadings. In the initial condition the d33 axis is aligned with the mechanical loading direction of the tensile specimen. Additional runs are made after rotating the material axes away from the principal mechanical loading axes of the tensile specimen. Stress patterns and location of maximum stress levels, indicating initial failure sites, are discussed in terms of the complex relationship between geometric features, material orientation and loading condition.

Commentary by Dr. Valentin Fuster
2005;():41-47. doi:10.1115/IMECE2005-79334.

A synthesis technique using a topology optimization scheme for an adaptive wing structure and mechanism design will be described. This enables the design of energy efficient adaptive structures with controllable deformation characteristics. This is accomplished by using a multi-objective function that minimizes strain energy and maximizes mutual potential energy to design the structure and mechanism simultaneously. To enable simultaneous design for structure and mechanism, the reference structure is composed of three layers; a membrane layer for skin, a frame element layer for structure, and truss element layer for an efficient mechanism. The attachment points between mechanism and structure are also identified with linear springs that are located between mechanism and structure layers. We focus on a simultaneous design of a wing structure and mechanism for large shape change applications. The geometrically large deformation analysis scheme is also added to the synthesis to capture nonlinear effects in design and it will be compared with linear synthesis results.

Commentary by Dr. Valentin Fuster
2005;():49-57. doi:10.1115/IMECE2005-79387.

Work is underway to develop high energy density active materials based upon biological processes. These materials utilize the controlled transport of charge and fluid across a selectively-permeable membrane to achieve bulk deformation in a process referred to in the plant kingdom as nastic movements. The nastic material being developed consists of synthetic membranes containing biological ion pumps, ion channels, and ion exchangers surrounding fluid-filled cavities embedded within a polymer matrix. In this paper the formulation of a biological transport model and its coupling with a hyperelastic finite element model of the polymer matrix is discussed. The transport model includes contributions from ion pumps, ion exchangers, solvent flux, and ion channels. This work will form the basis for a feedback loop in material synthesis efforts. The goal of these studies is to determine the relative importance of the various parameters associated with both the polymer matrix and the biological transport components.

Commentary by Dr. Valentin Fuster
2005;():59-63. doi:10.1115/IMECE2005-79579.

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.

Commentary by Dr. Valentin Fuster
2005;():65-72. doi:10.1115/IMECE2005-79587.

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.

Commentary by Dr. Valentin Fuster
2005;():73-80. doi:10.1115/IMECE2005-79622.

Ferroelectric materials exhibit spontaneous polarization and domain structures below the Curie temperature. In this study a cubic to tetragonal phase transformation and the evolution of domain structures in ferroelectric crystals are simulated by using the time-dependent Ginzburg-Landau equation. The effects of electric boundary conditions on the formation of domain patterns and field induced polarization switching are discussed. The phase field model is used to simulate the formation of domain structures, domain wall motion and the macroscopic response of ferroelectric materials under external fields.

Commentary by Dr. Valentin Fuster
2005;():81-89. doi:10.1115/IMECE2005-79706.

Tensegrity structures have become of engineering interest in recent years, but very few have found practical use. This lack of integration is attributed to the lack of a well formulated design procedure. In this paper, a preliminary procedure is presented for developing morphing tensegrity structures that include actuating elements. To do this, the virtual work method has been modified to allow for individual actuation of struts and cables. A generalized connectivity matrix for a cantilever beam constructed from either a single 4-strut cell or multiple 4-strut cells has been developed. Global deflections resulting from actuation of specific elements have been calculated with or without external loads. Furthermore, the force density method is expanded to include a necessary upper bound condition such that a physically feasible structure can be designed. Finally, the importance of relative force density values on the overall shape of a structure comprising of multiple unit cells is discussed.

Topics: Design , Wings
Commentary by Dr. Valentin Fuster
2005;():91-99. doi:10.1115/IMECE2005-79765.

The transport of charge due to electric stimulus is the primary mechanism of actuation for a class of polymeric active materials known as ionomeric polymer transducers. Continuum-based models of ion transport have been developed for the purpose of understanding charge transport due to diffusion and migration. In this work a two dimensional ion hopping model has been built to describe ion transport in ionomeric polymer transducer (IPT) with Monte-Carlo simulation. In the simulation, cations are distributed on 50nm × 50nm × 1nm (or 50nm × 10 nm × 1nm) lattice cells of IPT while the same number of negative charges are uniformly scattered and fixed as background. In the simulation, thermally activated cations are hopping between multiwell energy structures by overcoming energy barriers around with a hopping distance of 1nm during each time step. A step voltage is applied between the electrodes of the IPT. In one single simulation step, coulomb energy, external electric potential energy and intrinsic energy of the material are calculated and added up for the energy wells around the cations. And then hopping rates in every potential hopping direction are obtained. Due to the random nature of the ion transitions, a weighting function from Monte-Carlo algorithm is added in to calculate the ion hopping time. Finally hopping time is compared, the minimum hopping time is chosen and one hopping event is completed. Both system time and ions distribution are updated before the next simulation loop. Periodic boundary conditions are applied when ions hop in the direction perpendicular to the electric field. The influence of the electrodes on both faces of IPT is presented by the method of image charges. The charge density at equilibrium state is compared with the result from a continuum-based model. The property of charge density has charge neutrality over the central part of the membrane and the charge imbalance over boundary layers close to the anode and cathode. Electric field distribution is obtained after charge distribution. After it is demonstrated that ion hopping model leads to the results qualitatively matching the property of IPT, the paper uses the model to analyze the polymer-metal interface when the electrode shape inside transducer varies.

Topics: Metals , Simulation , Polymers
Commentary by Dr. Valentin Fuster
2005;():101-109. doi:10.1115/IMECE2005-79974.

A microstructural model of the motion of particle pairs in MR fluids is proposed that accounts for both hydrodynamic and magnetic field forces. A fluid constitutive equation is derived from the model that allows prediction of velocity and particle structure fields. Results for simple shear and elongational flows are presented for cases where particle pairs remain in close contact so they are hydrodynamically equivalent to an ellipsoid of aspect ratio two. Additionally, only the magnetic force component normal to the vector connecting the centers of a particle pair affects motion. Shear flow results indicate particle pairs rotate continuously with the flow at low magnetic fields while a steady state is reached at high fields. For elongational flows, when the applied magnetic field is parallel to the elongation direction, particle pairs orient in the field/flow direction. Either orientation is possible when the field is perpendicular to the flow.

Commentary by Dr. Valentin Fuster
2005;():111-120. doi:10.1115/IMECE2005-80049.

Ferromagnetic Shape Memory Alloys (FSMAs) like Ni-Mn-Ga have attracted significant attention over the past few years. What makes these materials attractive as actuators is their high energy density, large stroke, and high bandwidth. Among other applications, these properties make FSMAs potentially candidates for developing lightweight Electro-Hydraulic Actuators (EHA). The role of the FSMA transducer is to provide the mechanical energy by the linear displacement in the EHA. In order to develop effective FSMA-based transducers, it is important to study their dynamic behavior. In this paper a dynamic model is presented for a Ni-Mn-Ga transducer. The transducer consists of the Ni-Mn-Ga material, a linear spring, Helmholtz coils, and a soft iron housing. An enhanced phenomenological model is also presented in this work to describe the strain output of the actuator in the response to the magnetic field strength. Using this model the effect of design parameters on the performance of the actuator is studied.

Commentary by Dr. Valentin Fuster
2005;():121-124. doi:10.1115/IMECE2005-80090.

Electrochemically formed graphite intercalation compounds (GICs) have many intrinsic properties well-suited for compact actuation in applications at high temperatures. GICs using ionic liquids are of interest because of their good thermal stability at elevated temperatures, high ionic conductivity, and low volatility. In this study we observed the potential and strain behavior of highly oriented pyrolytic graphite and 1-ethyl-3-methylimidazolium hexafluorophosphate subjected to a light compressive load and constant current. In situ measurements of the anode during intercalation showed a reversible strain of 2.5% to 4.5% from 100°C up to 250°C.

Commentary by Dr. Valentin Fuster
2005;():125-132. doi:10.1115/IMECE2005-80118.

This paper treats the question of adaptive control of a projectile fin using a piezoelectric actuator. The hollow projectile fin is rigid, within which a flexible cantilever beam with a piezoelectric active layer is mounted. The model of the fin-beam system includes the aerodynamic moment which is a function of angle of attack of the projectile. The rotation angle of the fin is controlled by deforming the flexible beam which is hinged at the tip of the rigid fin. It is assumed that the system parameters are completely unknown and that only the fin angle and its derivative are measured for synthesis. A linear combination of the fin angle and fin’s angular rate is chosen as the controlled output variable and an adaptive servoregulator is designed for the control of the fin angle and the rejection of the disturbance input (aerodynamic moment). In the closed-loop system, the fin angle asymptotically converges to the desired value and the elastic modes converges to their equilibrium values. Computer simulation is performed which shows that in the closed-loop system, the fin angle is precisely controlled in spite of uncertainties in the fin-beam parameters and the aerodynamic moment coefficients. Furthermore, a laboratory model of the projectile fin is developed and the adaptive controller is implemented for real-time control. Experimental results are presented which show that adaptive servoregulator accomplishes fin angle control.

Commentary by Dr. Valentin Fuster
2005;():133-144. doi:10.1115/IMECE2005-80217.

Functionally Graded Piezoceramics (FGP) offer performance similar to conventional piezoceramic actuators while reducing the problems associated with their bonded construction (high stress levels, large stress discontinuities, delamination, etc.). This paper presents the Dual Electro/Piezo Property (DEPP) gradient method and the tools necessary for designing, modeling, and producing DEPP FGP actuators including: material property gradient maps, a Micro-Fabrication by Co-eXtrusion (MFCX) process, and experimentally validated analytic and numeric performance and stress modeling methodologies that account for continuous and layered material gradients and complex electric field profiles. These models predict a dramatic internal stress reduction achieved by the DEPP method. Preliminary reliability testing confirm this with an increase in piezoelectric actuator lifetimes over 1010 cycles, improvement of almost four orders of magnitude compared to conventional piezoceramic actuation.

Commentary by Dr. Valentin Fuster
2005;():145-154. doi:10.1115/IMECE2005-80220.

Ionomeric polymer transducers have received considerable attention in the past ten years due to their ability to generate large bending strain and moderate stress at low applied voltages. Bending transducers made of an ionomeric polymer membrane sandwiched between two flexible electrodes deform through the expansion of one electrode and contraction of the opposite electrode due to cation displacement. This is similar to a bimorph type actuation. In this study we report actuation through the thickness of the membrane, leading to the potential of a new actuation mechanism for ionomeric polymer materials. Several experiments are performed to compare the bending actuation with the extensional actuation capability. The direct assembly method previously developed by the authors is used to fabricate ionic polymer transducers with controlled electrode dimensions and morphology. Electrodes with varying thickness are used to alter thickness of the active material. In the first experiment, the actuators are cut in beam shape and are allowed to bend in cantilever configuration. In the second configuration, bending is constrained by sandwiching the membranes between two solid metal plates and force is measured across the thickness of the actuator. A bimorph model is used to assess the effect of electrode thickness on the strain. An electromechanical coupling model presented by Leo et al. [1] determined the strain in the active areas as a function of the charge. This model is presented with a linear and a quadratic term that produces a 1st harmonic response for a sine wave actuation input. The quadratic term in the strain generates a zero net bending moment for ionic polymer transducers with symmetric electrodes. The linear term is also canceled in extensional actuation for symmetric electrodes. Experimental results demonstrates strain on the order of 110 μstrain in the thickness direction compared to 1700 μstrain peak to peak on the external fibers for the same transducer when it is allowed to bend under +/−2V potential at 0.5 Hz.

Topics: Polymers , Transducers
Commentary by Dr. Valentin Fuster
2005;():155-159. doi:10.1115/IMECE2005-80256.

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.

Commentary by Dr. Valentin Fuster
2005;():161-174. doi:10.1115/IMECE2005-80335.

The objective of this research is to address some of the important design issues of the recently developed piezoelectric resonant actuation system (RAS) concept. The RAS is achieved through both mechanical and electrical tailoring. With mechanical tuning, the resonant frequencies of the actuation system (includes the piezoelectric actuator and the related mechanical and electrical elements for actuation) can be adjusted to the required actuation frequencies. This obviously will increase the authority of the actuation system. To further enhance controllability and robustness, the actuation resonant peak can be significantly broadened and flattened with electrical tailoring through the aid of an electric network of inductance, resistance, and negative capacitance. Therefore, one can achieve a high authority actuator without the negative effects of resonant systems. In this investigation, the RAS is analyzed and compared to an equivalent mechanical system to provide better physical understandings. Design guidelines of the RAS are derived in a dimensionless form, and the optimal values of the electrical components are explicitly determined. A method of implementing the actuator circuitry is proposed and realized via a digital signal processor (DSP) system. Performance of the resonant actuation system is analyzed and verified experimentally on a full-scale piezoelectric tube actuator for light class helicopter rotor control. The electric power consumption of the RAS is analyzed and discussed in terms of the power factor and apparent power. It is demonstrated that a piezoelectric resonant actuation system with proper tunings not only yields high authority with a broad frequency bandwidth but also is electrically efficient in terms of power consumption.

Commentary by Dr. Valentin Fuster
2005;():175-182. doi:10.1115/IMECE2005-80394.

This paper focuses on the development of a model which characterizes the nonlinear and hysteretic stress-dependence inherent to magnetic transducer materials operating in high drive regimes. The model builds upon a previous ferromagnetic characterization framework based on energy analysis at the lattice level in combination with stochastic homogenization techniques. Aspects of the stress-dependent magnetomechanical model are illustrated through comparison with experimental steel data.

Topics: Stress , Transducers
Commentary by Dr. Valentin Fuster
2005;():183-191. doi:10.1115/IMECE2005-80396.

Application of Rotational Isomeric State (RIS) theory to the prediction of Young’s modulus of a solvated ionomer is considered. RIS theory directly addresses polymer chain conformation as it relates to mechanical response trends. Successful adaptation of this methodology to the prediction of elastic moduli would thus provide a powerful tool for guiding ionomer fabrication. The Mark-Curro Monte Carlo methodology is applied to generate a statistically valid number of end-to-end chain lengths via RIS theory for a solvated Nafion case. The distribution of chain lengths is then fitted to a Probability Density Function by the Johnson Bounded distribution method. The fitting parameters, as they relate to the model predictions and physical structure of the polymer, are studied so that a means to extend RIS theory to the reliable prediction of ionomer stiffness may be identified.

Commentary by Dr. Valentin Fuster
2005;():193-198. doi:10.1115/IMECE2005-80404.

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 NiMn2 Ga 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.

Commentary by Dr. Valentin Fuster
2005;():199-207. doi:10.1115/IMECE2005-80569.

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.

Commentary by Dr. Valentin Fuster
2005;():209-218. doi:10.1115/IMECE2005-80789.

Conventional Finite-Element programs are able to compute the vibration response of mechanical structures. Increasingly also so-called multi-field problems can be solved. For piezoelectric actuators and sensors, electrical degrees of freedom apart from the mechanical ones have to be considered too. The pure actuator effect can also be modelled using the coefficients of thermal expansion. But regarding the optimal placement of flat piezoceramic modules, which couple in the mechanical part through the d31-effect, it proves to be advantageous to consider them after doing the computational complex modal analysis. In this paper, this modal coupling approach is described in detail. It introduces an additional modelling error, because the effect of the stiffness and mass of the modules is not considered in the construction process of the functional space, from which modal shapes are derived. But due to the comparatively small contribution to the global mass and stiffness of such flat devices, this additional error can generally be accepted. Furthermore this error can be reduced to an arbitrarily small amount, if the number of retained eigenmodes is increased and the gain in computational speed is significant. For the calculations, self-written triangle elements with full electro-mechanical coupling have been used, being coded completely in MATLAB. Finally the optimization procedure for the placement of the piezoceramic modules including their mass and stiffness is demonstrated for a test structure.

Commentary by Dr. Valentin Fuster
2005;():219-226. doi:10.1115/IMECE2005-80790.

Quality monitoring in microelectronics becomes more and more important because of the constantly rising complexity and miniaturization of modern microelectronic devices. This paper describes a model based method for online quality monitoring in ultrasonic wire bonding. This new approach aims to reconstruct the metallophysical processes within the bonding zone by the aid of a validated analytical model of the bonding system. The model comprises a 2-DOF electromechanical analogous circuit representing the ultrasonic transducer, a phase-locked-loop controller for frequency control and time varying spring-damper elements representing the bond process. It will be shown how faulty bonds can clearly be identified by this method.

Commentary by Dr. Valentin Fuster
2005;():227-237. doi:10.1115/IMECE2005-80896.

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.

Topics: Elastomers , Membranes
Commentary by Dr. Valentin Fuster
2005;():239-248. doi:10.1115/IMECE2005-80900.

A fully coupled model considering both sensing and actuation for composite plates with embedded piezoelectrics is constructed using the variational asymptotic method. Without invoking any ad hoc kinematic assumptions, we take advantage of the geometric small parameter inherent in the structure to mathematically split the original three-dimensional, geometrically nonlinear, piezoelectricity problem into: a coupled, linear, one-dimensional piezoelectric through-the-thickness analysis and a coupled, geometrically nonlinear, two-dimensional piezoelectric plate analysis. Two asymptotically correct models of multi-layer plates are developed for two different types of electrode arrangements. The constructed models are of the form of classical plate theory having a layerwise displacement and electric potential distribution. The present theory is implemented using the finite element method into the computer program VAPAS (variational-asymptotic plate and shell analysis). Simple examples are used to demonstrate the application of the present theory.

Commentary by Dr. Valentin Fuster
2005;():249-258. doi:10.1115/IMECE2005-80916.

Rotary screw-type stick-slip actuators driven by smart materials offer the attractive characteristics of large stroke, fine resolution, high power-off holding force, and relative simplicity to manufacture. However, the actuation speed of current rotary stick-slip actuators limits their applications. The research presented in this study focuses on rapid positioning applications and involves the development of two mechanism designs. Both designs utilize piezoelectric stacks driven by sawtooth waves to provide stick-slip contact between two partially threaded jaws and a 1/4-80 screw. The first mechanism design utilizes two piezoelectric stacks in a push-pull configuration. The motion of the two stacks is coupled through a lever amplification mechanism. The second design uses a single stack to directly drive the jaws without amplification. However, a second stack is oriented perpendicular to the first stack and driven with a triggered square wave. This stack varies the normal and thus frictional force between the jaws and screw, permitting more efficient slipping and sticking of the actuator. This allows for potential increases in both output speed and force. Existing actuators were modeled using both theoretical and finite element analysis techniques. These results were compared with experimental data to validate the models.

Commentary by Dr. Valentin Fuster
2005;():259-268. doi:10.1115/IMECE2005-80974.

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.

Commentary by Dr. Valentin Fuster
2005;():269-275. doi:10.1115/IMECE2005-81057.

This work discusses the development and characterization of rectangular shaped Epoxy Composite Laminated Piezoelectric Stress-Enhanced actuators (ECLIPSE). ECLIPSE actuators are unimorph type d31 actuators that are manufactured with a lead zirconate titanate (PZT) plate sandwiched between unidirectional Kevlar 49/epoxy composite layers with dissimilar coefficients of thermal expansion in orthogonal directions. Cooling the actuator from an elevated curing temperature resulted in a residual stress gradient through the actuator, a compressive stress on the brittle piezoelectric plate, and a large out-of-plane deformation. Extended classical lamination theory (ECLT) is used to model the residual stress state and curvature of the actuator. The model results are compared to the classical lamination theory. The ECLT was developed by Hyer to explain the non-linear behavior of unsymmetric cross-ply laminates [1-3]. Three actuator layups were fabricated and characterized: [90/PZT/90/0], [90/90/PZT/90/0/0], and [90/90/90/PZT/90/0/0/0]. It is shown that geometric non-linearity is important to consider when modeling ECLIPSE actuators.

Topics: Actuators
Commentary by Dr. Valentin Fuster
2005;():277-284. doi:10.1115/IMECE2005-81213.

Aerodynamic performance of aircraft can be changed by moving separate surfaces which are mechanically connected to the main wing and moved with complex linkages. A possible alternative method of changing performance with less mechanical complexity is presented. Instead of separate control surfaces, the shape of complete aerodynamic structures can be changed with shape memory alloy (SMA) materials as part of the structure. In this work SMA wire is wrapped around a simple test wing. When activated by heating, the wire contracts which results in twisting the wing. The angle of attack along the wingspan changes which changes the aerodynamic forces on the wing. This could be used to optimize the flight condition. Results are presented from initial wind tunnel experiments which show the change in lift due to twisting. Aerodynamic models that account for the variable angle of attack along the span are also developed. The results from the experiments and aerodynamic model are compared.

Topics: Aerodynamics , Wings
Commentary by Dr. Valentin Fuster
2005;():285-293. doi:10.1115/IMECE2005-81366.

Plants have the ability to develop large mechanical force from chemical energy available with bio-fuels. The energy released by the cleavage of a terminal phosphate ion during the hydrolysis of bio-fuel assists the transport of ions and fluids in cellular homeostasis. Materials that develop pressure and hence strain similar to the response of plants to an external stimuli are classified as nastic materials. Calculations for controlled actuation of an active material inspired by biological transport mechanism demonstrated the feasibility of developing such a material with actuation energy densities on the order of 100kJ/m3 by Sundaresan et. al [2004]. The mathematical model for a simplified proof of concept actuator referred to as micro hydraulic actuator uses ion transporters extracted from plants reconstituted on a synthetic bilayer lipid membrane (BLM). Thermodynamic model of the concept actuator discussed in Sundaresan et. al [2005] predicted the ability to develop 5% normalized deformation in thickness of the micro-hydraulic actuator. Our experimental demonstration of controlled fluid transport through AtSUT4 reconstituted on a 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt) (POPS), 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-Phosphoethanolamine (POPE) BLM on lead silicate glass plate having an array of 50 μm holes driven by proton gradient is discussed here.

Topics: Mechanisms
Commentary by Dr. Valentin Fuster
2005;():295-301. doi:10.1115/IMECE2005-81428.

Magnetorheological (MR) fluids possess the unique ability to undergo dramatic and nearly completely reversible changes in their rheological properties under the application of a magnetic field. These controllable fluids can serve as quiet, rapid interfaces between electronic controls and mechanical systems. One area of application is to use these fluids in torque transfer devices, such as clutches and brakes. After determining MR fluid properties and behavior using a rheometer, a parallel disk type MR clutch was successfully developed, which utilized a stationary electromagnetic coil. Finite element analysis was used to design the coil and clutch assembly in order to maximize the magnetic field generated within the MR fluid. The resulting magnetic field was uniform over the active portion of the clutch, easily controllable by adjusting the current passing through the coil, and provided a large range of field strength values. The experimentally measured output torque was generally in good agreement with predicted values. This work will detail the design considerations and methodology used to develop this clutch, which can be extended to the design of other MR devices.

Topics: Torque , Design
Commentary by Dr. Valentin Fuster
2005;():303-311. doi:10.1115/IMECE2005-81448.

This article proposes intrinsic polymer fiber sensors for the performance-based assessment and health monitoring of civil infrastructure systems. Such sensors would allow the dynamic measurement of large strains as required for structures during earthquake loading. Furthermore, the interferometric nature of the sensor permits high accuracy for such measurements. However, the use of the polymer fiber sensors at large strain magnitudes is not without significant challenges as compared to conventional silica optical fiber sensors due to the finite deformation of the fiber and nonlinear photoelastic effects. This article analyzes the implications of the large deformations on the opto-mechanical response of the sensors, derives the sensor opto-mechanical properties to be obtained through sensor calibrations, and demonstrates the data acquisition method to be utilized for the new sensors.

Commentary by Dr. Valentin Fuster
2005;():313-322. doi:10.1115/IMECE2005-81528.

This paper presents an enhanced phenomenological model for shape memory alloy wires. Shape Memory Alloys (SMAs) are a group of materials with unique phase transformation related characteristics. They have been applied in both active and passive ways for actuation, vibration suppression, and sensing. SMA phenomenological models are widely used for engineering applications due to their simplicity and ease of simulation. A phenomenological model normally has two parts: a constitutive model based on free energy analysis and a phase transformation kinetics model based on experimental results. The existing phenomenological models are formulated to qualitatively predict the behavior of SMA systems for simple loadings. In this study, we have shown that there are certain situations in which these models are either not correctly formulated, and therefore are not able, to predict the behavior of SMA wires or the formulation is not straightforward for engineering applications. Such cases mostly happen when the temperature and the stress of the SMA wires change simultaneously. The phenomenological models discrepancy is studied experimentally using a dead-weight that is actuated by a SMA wire. An enhanced formulation is presented along with a modeling methodology for SMA systems with complex thermomechanical loadings.

Commentary by Dr. Valentin Fuster
2005;():323-332. doi:10.1115/IMECE2005-81597.

The present study explores both structural and controller design to attenuate vibration in large membrane structures especially due to low-frequency harmonic excitations. It is very difficult for membrane structures to suppress the low-frequency vibration induced by flexible support structures, because a lightly pre-stressed membrane has extremely low mode frequencies and little damping effect. The present study proposes the use of web-like perimeter cables around a membrane, and the application of simple and lightweight active controllers only along the web cables. This strategy successfully suppresses the membrane vibration when the web-cable configuration is appropriately tailored. Both linear and nonlinear finite-element analyses exhibit a clear tradeoff between structural mass and control efficiency.

Commentary by Dr. Valentin Fuster
2005;():333-346. doi:10.1115/IMECE2005-81700.

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.

Topics: Actuators
Commentary by Dr. Valentin Fuster
2005;():347-352. doi:10.1115/IMECE2005-81781.

Time reversal processing for measured sensor signal can be applied to detect not only structural damage but also impact loading of the structure. In this study, a model based impact detection method for structural health monitoring is proposed. Impact identification capability of the numerical time reversal processing is demonstrated by using a rectangular aluminum plate with nine surface bonded piezoceramic sensors. Illustrative numerical simulation results indicate that the proposed method can be successfully identifying both exact impact point and time.

Commentary by Dr. Valentin Fuster
2005;():353-363. doi:10.1115/IMECE2005-82088.

Advances in technology for Unmanned Air Vehicles (UAVs) have given rise to increasingly smaller UAVs and a growing need for adaptable UAV wing structures. Deployable, or collapsible, wings offer space savings and broaden the number of possible applications for UAVs. Aerial surveillance applications, such as military reconnaissance, law enforcement, forestry, map making, pipeline surveillance, and border patrol typically demand a UAV that can be carried and deployed by a single user in an isolated field. This paper provides a framework of deployable wing concepts using compliant mechanisms. Compliant mechanisms offer reduced part count, decreased need for lubrication, efficient manufacturability, and space savings compared to traditional joint systems. The design space for compliant deployable wings is explored, concepts are categorized into classes based on motion type, and opportunities are identified for creating new concepts. A method is presented for selecting and evaluating concepts for specific applications. A product development project illustrates selection of a deployable wing design for a particular application, and the paper concludes with suggestions for further work.

Commentary by Dr. Valentin Fuster
2005;():365-374. doi:10.1115/IMECE2005-82203.

Inspired by actuation mechanisms in plant structures and motivated by recent advances in electro-chemically driven micro-pumps, this paper is concerned with a novel concept for active materials based on distributed hydraulic actuation. Due to the similarity of the actuation principles seen in plants undergoing nastic motion, we refer to this class of active materials as nastic materials. We present a mechanical modeling approach for nastic materials representing the effects of pressure generation and fluid transport by incompressible eigenstrains. This model is embedded into a two-level macro/micro topology optimization procedure. On a macroscopic level, the integration of nastic material into a structural system is optimized. The placement and distribution of nastic material on a flexible substrate are optimized to generate target displacement and force distributions. On a microscopic level, the stress and strain generation is tailored to desired macroscopic material properties by optimizing the layout of vascular fluid channels embedded in an elastic matrix. For the layout optimization of vascular fluid channels, a novel topology optimization procedure is presented that models the effects of pressure along the fluid channels via an analogy with thermal conduction and convection. For this purpose an auxiliary heat transfer problem is solved. The macro-scale optimization procedure is studied for plate structures patterned by nastic materials in order to generate target bending and twist deformations. The results show the significant differences of the optimal distributions of active material depending on the strain model used for representing the actuation concept. The micro-scale vascular design methodology is verified with plane-stress examples. The results show that the layout of fluid channels can be optimized such that target strains are generated.

Commentary by Dr. Valentin Fuster
2005;():375-380. doi:10.1115/IMECE2005-82592.

A typical magnetorheological (MR) flow mode damper consists of a piston attached to a shaft that travels in a tightly fitting hydraulic cylinder. The piston motion makes fluid flow through an annular valve in the MR damper. An electro-magnet applies magnetic field to the MR fluid as it flows through the MR valve, and changes its yield stress. An MR fluid, modeled as a Bingham-plastic material, is characterized by a field dependent yield stress, and a (nearly constant) postyield plastic viscosity. Although the analysis of such an annular MR valve is well understood, a closed form solution for the damping capacity of a damper using such an MR valve has proven to be elusive. Closed form solutions for the velocity and shear stress profile across the annular gap are well known. However, the location of the plug must be computed numerically. As a result, closed form solutions for the dynamic range (ratio of field on to field off damper force) cannot be derived. Instead of this conventional theoretic procedure, an approximated closed form solution for a dampers dynamic range, damping capacity and other key performance metrics is derived. And the approximated solution is used to validate a rectangular duct simplified analysis of MR valves for small gap condition. These approximated equations are derived, and the approximation error is also provided.

Commentary by Dr. Valentin Fuster
2005;():381-390. doi:10.1115/IMECE2005-82593.

A magnetorheological (MR) fluid, modeled as a Bingham-plastic material, is characterized by a field dependent yield stress, and a (nearly constant) postyield plastic viscosity. Based on viscometric measurements, such a Bingham-plastic model is an idealization to physical magnetorheological behavior, albeit a useful one. A better approximation involves modifying both the preyield and postyield constitutive behavior as follows: (1) assume a high viscosity preyield behavior over a low shear rate range below the yield stress, and (2) assume a power law fluid (i.e., variable viscosity) above the yield stress that accounts for the shear thinning behavior exhibited by MR fluids above the yield stress. Such an idealization to the MR fluid’s constitutive behavior is called a viscous-power law model, or a Herschel-Bulkley model with preyield viscosity. This study develops analytical quasi-steady analysis for such a constitutive MR fluid behavior applied to a flow mode MR damper. Closed form solutions for the fluid velocity, as well as key performance metrics such as damping capacity and dynamic range (ratio of field on to field off force). Also, specializations to existing models such as the Herschel-Bulkley, the Biviscous, and the Bingham-plastic models, are shown to be easily captured by this model when physical constraints (idealizations) are placed on the rheological behavior of the MR fluid.

Topics: Viscosity , Dampers
Commentary by Dr. Valentin Fuster
2005;():391-400. doi:10.1115/IMECE2005-82721.

An analytical micromechanics approach is presented to model the effective longitudinal mechanical properties of Metal-Core Piezoelectric Fibers (MPF). The model assumes general orthotropic material properties for the piezoelectric as well as the core material. Next, the general orthotropic solution is reduced to transversely isotropic for the piezoelectric fiber and isotropic for the metal-core. This MPF system is also modeled using finite element analysis (FEA) and the results from the analytical solution and FEA are compared for verification purpose. Next, the Metal-Core Piezoelectric Fiber (MPF) is embedded inside a metal or a polymer and the resulting longitudinal mechanical properties of these Active Fiber Composite (AFC) systems are given analytically.

Commentary by Dr. Valentin Fuster
2005;():401-410. doi:10.1115/IMECE2005-82751.

A parallel genetic algorithm is developed for the design of morphing aircraft structures using tendon actuated compliant truss. The wing structure in this concept is made of solid members and cables. The solid members are connected through compliant joints so that they can be deformed relatively easily without storing much strain energy in the structure. The structure is actuated using cables to deform into a required shape. Once the structure is deformed, the cables are locked and hence carry loads. Previously an octahedral unit cell made of cables and truss members was developed to achieve the required shape change of a morphing wing developed at NASA. It was observed that a continuously deformable truss structure with required morphing capability can be achieved by a cellular geometry tailored to local strain deformation. A wing structure made of these unit cells was sized for a representative aircraft and was found to be suitable. This paper describes the development of new unit cell designs that fit the morphing requirements using topology optimization. A ground structure approach is used to set up the problem. A predetermined set of points is selected and the members are connected in between the neighboring nodes. Each member in this ground structure has four possibilities, 1) a truss member, 2) a cable that morphs the structure into a required shape, 3) a cable that is antagonistic and brings it back to the original shape 4) a void, i.e., the member doesn’t exist in the structure. This choice is represented with a discrete variable. A parallel genetic algorithm is used as an optimization approach to optimize the variables in the ground structure to get the best structural layout. The parallelization is done using a master slave process. A fitness function is used to calculate how well a structural layout fits the design requirements. In general, a unit cell that has lesser deflection under external loads and higher deflection under actuation has a higher fitness value. Other requirements such as having fewer cables and achieving a required morphing shape are also included in the fitness function. The finite element calculations in the fitness function can be done using either linear or nonlinear analysis. The paper discusses the different ways of formulating the fitness function and the results thereof.

Commentary by Dr. Valentin Fuster
2005;():411-420. doi:10.1115/IMECE2005-82771.

There is a demand for hybrid actuation systems which combine actuation and valving systems in a compact package. MR fluids can be used in valves to control the motion of an output cylinder. Such a valving system will have no moving parts and thus can be used in applications where there is high centrifugal loading. In the current setup, MR valves are configured in the form of a Wheatstone bridge where the two arms form the high and low pressure sides of the output cylinder. The actuation is performed using a compact piezoelectric stack driven actuator. The frequency rectification of the piezo stack motion is done using reed valves. This actuator and valve configuration form a compact hydraulic system with electro-mechanical valves. The advantages of such systems are that part count is low, fewer moving parts and the ability to control the motion of the output cylinder by controlling the fluid flow through the MR valves. By the application of different magnetic fields in the arms of the bridge (by applying different currents to the magnetic circuits), we can control the differential pressure seen by the output cylinder. This allows us to design different controllers for the system. The two systems in this configuration have been separately evaluated. The piezo pump system was first tested for its performance and efficiency with conventional hydraulic fluid and MR fluid. At this stage, the MR valve setup has not been added to isolate the actuating system from the valve system and the MR fluid acts merely as a transmission fluid. The Wheatstone bridge setup was then added and the efficiency of the MR valve was tested against a dummy mechanical valve. The modeling of the valve was done on the basis of standard rheological models like Bingham Plastic and bi-viscous models. Data for bi-directional actuation of the output cylinder is presented and assessed analytically.

Topics: Pumps , Valves , Pistons
Commentary by Dr. Valentin Fuster
2005;():421-428. doi:10.1115/IMECE2005-82786.

Nastic structures are potentially high-energy density smart materials that will be capable of achieving controllable deformation and shape change due to internal microactuation that functions on principles found in the biological process of nastic motion. In plants, nastic motion is accomplished through osmotic pressure changes causing a respective increase or decrease in cell volume, thereby causing net movement. In nastic structures, osmotic pressure is increased by moving fluid from low concentration to high concentration areas by means of active transport, powered by adenosine triphosphate (ATP) hydrolysis. Power analysis involves calculating possible ranges of actuation as a result of interior pressure exchanges and hydraulic flux rates which will determine the speed of actuation. Because pressure inside the actuating cylinder is uniform, the cylinder undergoes deformation in all the three dimensions. Predicting the work-energy balance involves considering the factors that determine the total volumetric change, including cylinder wall expansion, surface bulging and stretching, and outside forces that oppose the actuation. The hydraulic flux rates determine both the force magnitude and the actuation speed. Energy analysis considers the pressure variation range needed to accomplish the desired actuation deflection, and the energy required for active transport mechanisms to move the volume of fluid into the nastic actuator. Nonlinear effects are present, as the pressure inside the actuation cylinder increases, it takes more energy for active transport to continue moving fluid into it. The chemical reaction of ATP hydrolysis supplies the energy for active transport, which is related to the ratio of the reactants, to the products, as well as to the pH level. As the pH lowers, more energy is released through ATP hydrolysis. Therefore, as pH decreases, ATP Hydrolysis releases more energy, enabling active transport to move more fluid into the actuation cylinder, thereby increasing the internal osmotic pressure and causing material deformation work and actuation.

Commentary by Dr. Valentin Fuster
2005;():429-438. doi:10.1115/IMECE2005-82849.

This paper examines a double adjustable magnetorheological (MR) damper that can produce high damper force and substantial dynamic force range (the ratio of the field-on to the field-off damper force), particularly over a high speed piston range. To this end, a double adjustable MR damper is configured and its hydraulic model is theoretically constructed. From this hydraulic model, the governing equation for the double adjustable MR damper is derived using the nonlinear pressure-flow relationship. The damper characteristics of the double adjustable MR damper are theoretically evaluated and compared with those of a conventional flow-mode MR damper. To investigate the effectiveness of the double adjustable MR damper in mitigating shock of a gun recoil system, a mechanical model of the recoil system using a double adjustable MR damper is theoretically constructed and the dynamic equation for the recoil system is derived. The shock mitigation performance of the MR gun recoil system with no-field and constant magnetic field inputs is numerically evaluated. A simple on-off control algorithm is proposed to improve the shock mitigation performance of the passive MR gun recoil systems. Finally, the shock mitigation performance of the MR gun recoil system using no-field, constant field, and on-off control algorithm is analyzed and compared with those of a conventional gun recoil system under nominal firing forces, as well as a 30% perturbation from nominal.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2005;():439-442. doi:10.1115/IMECE2005-79282.

Classical molecular dynamics (CMD) simulation is an important technique for analyzing custom-designed nanostructured materials and nano-sized systems such as nanowires and nanobelts. This research focuses on analyzing the strength of Fe2 O3 +Al energetic nanocomposites using CMD. A generic potential form is used to describe the behavior of the Fe+Al+Fe2 O3 +Al2 O3 system. The potential is able to describe bulk single crystal behavior of Fe, Al, Fe2 O3 , Al2 O3 as well as interfacial transitions among them. The nanostructures analyzed include polycrystalline Aluminum, Fe2 O3 as well as their composites with two different volume fractions (0.6/0.4 and 04/0.6). The polycrystalline structures are generated using voronoi tessellation. Quasi-static strength analyses are carried out using a massively parallel CMD code for both tension and compression. The analyses reveal that reverse Hall-Petch (H-P) effect is operative for polycrystalline Al under both tension and compression. However, for polycrystalline Fe2 O3 reverse H-P effect is operative under tension only. Compression still shows direct H-P effect. This effect transcends into the strength of both composites at all grain sizes. In addition, we also observe tension-compression strength asymmetry in the all polycrystalline systems. This framework offers an important tool for nanoscale design of advanced nanocomposite materials.

Commentary by Dr. Valentin Fuster
2005;():443-449. doi:10.1115/IMECE2005-80999.

Modeling at the structural scale most often requires the use of beam and shell elements. This paper compares two finite element formulations based on first-order shear deformation theory undergoing thermo-mechanical loading. One formulation is a two node beam element employing static condensation based on the work of Chakraborty et al. The second formulation follows a more traditional route using FSOD theory for a three node beam element. Both formulations are used to investigate the behavior of a functionally graded beam under axial and through-the-thickness temperature gradients. Both formulations work well for a constant uniform mechanical or temperature loading. However, for beam structures containing a thermal gradient in the axial direction, the two node beam element performs poorly as compared to the three node element in terms of transverse shearing stress calculated from the equilibrium equation.

Commentary by Dr. Valentin Fuster
2005;():451-456. doi:10.1115/IMECE2005-81075.

This work illustrates the manufacturing and tensile testing of a novel concept of honeycomb structures with hexagonal and auxetic (negative Poisson’s ratio) topology, made of shape memory alloy (SMA) core material. The honeycombs are manufactured using Nitinol ribbons having 6.40 mm of width and 0.2 mm of thickness. The ribbons were inserted in a special dye using cyanoacrilate to bond the longitudinal strips of the unit cells. The ribbons were subjected to tensile test at room temperature (martensite finish) and austenite finish temperature. Tensile tests at room temperature were performed on the honeycomb. The stress-strain curve obtained from the test on a single ribbon at room temperature was then used to develop nonlinear Finite Element beam elements using a commercial code. The beam elements were then used to model the honeycomb samples under tensile loading. Good agreement is observed between numerical nonlinear simulations and the experimental results.

Commentary by Dr. Valentin Fuster
2005;():457-461. doi:10.1115/IMECE2005-81123.

A new experimental technique was developed to characterize the mechanical properties of LIGA (an acronym from German words for lithography, electroplating, and molding) materials. An advanced imaging capability, scanning electron microscopy (SEM), with an integrated loading stage allows the acquisition of in situ microstructural images at the micro scale during loading. The load is measured directly from a load cell, and the displacement field is calculated from the SEM images based on the digital image correlation (DIC) technique. The DIC technique is a full-field deformation measurement technique which obtains displacement fields by comparing random speckle patterns on the specimen surface before and after deformation. The random speckle patterns are typically generated by applying a thin layer of material with high contrast to a specimen surface. Alternatively, DIC can also be applied using the microstructural features of a surface as texture patterns for correlation. DIC technique is ideally suited to characterize the deformation field of MEMS structures without the need to generate a random speckle pattern, which can be very challenging on the micro and nanoscale. In this paper, the technique is experimentally demonstrated on a LIGA specimen. The digital images showing LIGA surface features acquired during the loading can serve as random patterns for the DIC method. Therefore, full-field displacement and strain can be obtained directly on the specimen and the errors incurred by the testing system can be eliminated.

Commentary by Dr. Valentin Fuster
2005;():463-469. doi:10.1115/IMECE2005-81636.

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.

Commentary by Dr. Valentin Fuster
2005;():471-478. doi:10.1115/IMECE2005-81858.

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.

Commentary by Dr. Valentin Fuster
2005;():479-482. doi:10.1115/IMECE2005-81863.

Different from conventional metal foams, the mixed open/closed cell characteristic of the sintered metallic hollow sphere (MHS) materials offers excellent specific stiffness and energy absorption capacity. Based on our experimental study on two types of MHS specimens we reported before, a theoretical model is presented here to predict the mechanical behavior of the MHS materials in large strains (i.e. in the plateau phase). It is anticipated that the HCP packing configuration dominates in the actual MHS material structure, so that this configuration is taken in the idealized model. Based on the hypothesis of a periodic repeatability of a representative block, the large deformation of hollow spheres compressed by rigid balls is numerically simulated to identify the characteristics of deformation mechanism. Then, a rigid-plastic analysis is adopted to model this large deformation mechanism and to predict the stress-strain behavior of the representative block. Thus, the variation of the stress with strain is obtained for the idealized material of certain relative density, resulting in a relationship between the average stress in large strains (i.e. the plateau stress) and the relative density of the actual material. The predicted stresses are found in good agreement with the experimentally measured characteristic stresses of the two types of MHS materials.

Commentary by Dr. Valentin Fuster
2005;():483-488. doi:10.1115/IMECE2005-82206.

The ability of a material containing a periodic arrangement of second-phase inclusions to prevent transmission of waves in certain frequency ranges is well known. This is true for all types of waves including acoustic, electromagnetic, and elastic. These forbidden regions are called band gaps. They arise as incident waves are effectively attenuated by interference among the scattered wave fields. Indeed much of current semiconductor technology revolves around band-gap engineering with regard to electron flow in the periodic potentials resulting from atoms in their lattice positions. The phenomena are also being heavily explored in the context of light via the development of photonic crystals. Things become more interesting if instead of thinking of periodic arrangements, one selectively removes some of the inclusions in the periodic geometry creating defects. If done right, this can result in a material microstructure that can guide waves through the material. Advances in nano and micromanufacturing technologies in the last couple of years have opened up the possibility to fabricate heterogeneous material systems with precise positional control of the constituent materials. For example, it is now possible to place thin-film materials precisely at a resolution of fractions of a micron. Depending on how it is done, one can envision designing a material so that a wave will be guided to a particular location and/or away from another and as a result damping or amplifying the wave locally. In this work we develop a topology optimization approach to design such nanostructured materials. We demonstrate the approach through the design of three multifunctional phononic composite materials composed of silicon and aluminum: i) a grating designed to stop wave propagation at a specified frequency, ii) a waveguide that bends the propagation path of an elastic wave, and iii) an elastic switch that switches an input signal between two output ports based on the state of the input signal.

Topics: Design
Commentary by Dr. Valentin Fuster
2005;():489-495. doi:10.1115/IMECE2005-82260.

Auxetic materials and structures exhibit the very unusual property of becoming wider when stretched and narrower when squashed (i.e. they have a negative ‘Poisson’s ratio’). This property results in many beneficial effects in the characteristics of the system that make auxetics superior to conventional systems in many practical and high tech applications, including aeronautics where, for example, auxetics are being proposed as potential components for the production of better quality lifting devices such as helicopter rotor blades or airplane wings. This work reviews and discusses the behaviour of known and novel cellular systems, which exhibit this unusual but highly desirable property.

Commentary by Dr. Valentin Fuster
2005;():497-502. doi:10.1115/IMECE2005-82348.

Multilayered films have emerged as an attractive choice of coating material in magnetic recording media and magneto-resistance device related applications. These films possess advantages such as excellent magnetic properties, good corrosion resistance and high chemical stability. Hence, it is imperative that their structural and magnetic properties be characterized so that they can be tailored for specific applications. The present study aims towards the development and characterization of Platinum (Pt)/Cobalt (Co) multilayer films for magnetic and structural properties. Pt/Co multilayered films were deposited on Silicon (Si) substrate using dual electron beam co-evaporation. Four layers of each Pt and Co were deposited on a Si substrate, forming a multilayer film. Controlled deposition rates were used in order to attain the desired thickness of Co and Pt layers. The films were characterized using scanning and transmission electron microscopy and surface profilometry. Magnetic property measurements were conducted on these films in both perpendicular and parallel orientations, in order to understand the correlation between structure of various constituent elements of the film and their magnetic behavior. Results are analyzed and compared with those obtained from previous studies on similar multilayer films, to gain a better understanding of the effect of various parameters on properties of the film.

Commentary by Dr. Valentin Fuster
2005;():503-510. doi:10.1115/IMECE2005-82404.

Combined heat treatment and applied mechanical strains have been employed to modify the structure and properties of polyurethane (PU) foams. Consequently, foams with a range of pore sizes, shapes and orientations have been produced, including those possessing auxetic (negative Poisson’s ratio) behaviour. Four conversion conditions were employed: triaxial, biaxial and uniaxial compression (to linear compression ratios of 0.9, 0.8 and 0.7) and uniaxial tension to (linear extension ratios of 1.1 and 1.2). The converted foams were then observed by either SEM or optical microscopy, as appropriate, to determine the pore structure. Mechanical properties were measured using tensile testing in conjunction with videoextensometry, allowing the values of Young’s modulus and Poisson’s ratio to be ascertained. Increased anisotropy in the pore structure and elastic properties was observed. The biaxial as well as the triaxial conversion routes led to auxetic behaviour for the PU foams.

Commentary by Dr. Valentin Fuster
2005;():511-515. doi:10.1115/IMECE2005-82501.

Magnesium composites containing nano-size silicon carbide (SiC) particulates were synthesized using powder metallurgy technique coupled with a novel microwave assisted rapid sintering. The sintered specimens were hot extruded and characterized in terms of microstructural, physical and mechanical properties. Microstructural characterization revealed minimal porosity and the presence of a continuous network of nano-size SiC particulates decorating the particle boundaries of the metal matrix. Thermal mechanical analysis revealed a marginal reduction in the average coefficient of thermal expansion (CTE) values of the matrix with the addition of nano-size SiC reinforcement. Mechanical characterization revealed that the addition of nano-size SiC particulates lead to an increase in microhardness, 0.2% yield strength (YS), ultimate tensile strength (UTS) and ductility of the matrix. Particular emphasis was placed to correlate the effects of nano-size SiC reinforcement on the microstructural, physical and mechanical properties of monolithic magnesium.

Commentary by Dr. Valentin Fuster
2005;():517-526. doi:10.1115/IMECE2005-82776.

Functionally Graded Materials (FGM) have continuous variation of material properties from one surface to another unlike a composite which has stepped (or discontinuous) material properties. The gradation of properties in an FGM reduces the thermal stresses, residual stresses, and stress concentrations found in traditional composites. An FGM’s gradation in material properties allows the designer to tailor material response to meet design criteria. For example, the Space Shuttle utilizes ceramic tiles as thermal protection from heat generated during re-entry into the Earth’s atmosphere. However, these tiles are prone to cracking at the tile / superstructure interface due to differences in thermal expansion coefficients. An FGM made of ceramic and metal can provide the thermal protection and load carrying capability in one material thus eliminating the problem of cracked tiles found on the Space Shuttle. This paper will explore analysis of shell panels under thermal loading and compare performance of traditional homogeneous materials to FGMs using ABAQUS [1] finite element software. First, theoretical development of FGMs is presented. Second, finite element modeling technique for FGMs is discussed for a thermal stress analysis. Third, homogeneous curved panels made of ceramic and metal are analyzed under thermal loading. Finally, FGM curved panels created from a mixture of ceramic and metal are analyzed. FGM performance is compared to the homogeneous materials in order to explore the effect continuously grading material properties has on structural performance.

Commentary by Dr. Valentin Fuster

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