A New Unified Approach for Flow Analysis of Magneto-Rheological Fluids

The focus of this study is to develop a new universal approach for the flow analysis of magneto-rheological (MR) fluids through channels. An experimental study is conducted to investigate the relationship between the pressure loss of a MR fluid as a function of the applied magnetic field strength, volumetric flow rate, and surface roughness without utilizing the assumption of shear yield stress. A relation for nondimensional friction factor is developed in terms of Mason number and dimensionless surface roughness. It is demonstrated that the pressure loss across the MR fluid flow channel is significantly affected by the magnetic field and the surface roughness.

Impact Performance of Magnetorheological Fluids

As part of an emerging effort in what is now termed the area of mechamatronics [1], an effort was begun to assess the suitability of MR (magnetorheological) material based devices for impact energy management applications. A fundamental property of MR materials is that their yield stress alters almost instantaneously (and proportionally) to changes in the strength of an applied magnetic field. Based on this property, MR based devices, if found suitable, would be desirable for impact energy management applications because of attendant response tailorability. However, it was identified that prior to adopting MR based devices for impact energy management applications several key issues needed to be addressed. The present study focused on one of the most significant of these, the verification of the tunability of the response of such devices at stroking velocities representative of vehicular crashes. Impact tests using a free-flight drop tower facility were conducted on an MR based energy absorber (shock absorber) for a range of impact velocities and magnetic field strengths. Results demonstrated that over the range of impact velocities tested — 1.0 to 10 m/s — the stroking force/energy absorption exhibited by the device remained dependent on and thus could be modified by changes in the strength of the applied magnetic field.

Modelling Tow Compression in Textile Preforms During Composites Processing

Fiber assemblies, in the form of woven, braided, nonwoven or knitted structures, are used as reinforcements in composites. These textile structures are subjected to in-plane membrane stresses such as tensile and shear, and out-of-plane stresses such as bending and transverse compression. Amongst various modes of deformation, transverse compaction behaviour is the least understood mode; however this mode is very important for composites processing using vacuum forming, resin transfer moulding, thermoforming and hot compaction methods. The present paper reports a computational approach to predicting the load-deformation behaviour of textile structures under compressive loading. During the compression of a random fiber assembly, fibers are subjected to kinematic displacements, bending and finally transverse compression of individual fibres. In the case of interlaced architectures, such as woven and braided structures, it is convenient to deal with deformations at meso-scale involving yarns or tows, and deal with inter-fiber friction and fibre compression at yarn/tow level. It can be seen from the load deformation graphs that the initial part is dominated by bending energy and the final part by compression energy. A combined yarn bending and compression model was in good agreement with the experimental curve during the entire load-deformation cycle. On the other hand, an elastica-based bending model predicts well during the initial part while tow compression model predicts well during the final part. Inter-fiber friction was initially ignored — this is being introduced in the refined model for both the dry and wet states.

An Antagonistic Flexural Unit Cell for Design of Shape Morphing Structures

An antagonistic flexural unit cell (AFC) concept for the design and fabrication of novel 2-D and 3-D lightweight shape morphing structures is introduced. A fully reversible flexural shape changing cell utilizing opposing one-way shape memory alloy (SMA) actuators is shown to require no spring-like bias elements. The SMA actuating elements are arranged such that the actuation (contraction) of one of them stretches the other one in the cell, preparing it to be actuated later to reverse a flexural displacement. This antagonistic operation allows fully reversed cyclic operation. The focus of this paper is an assessment of performance at the single cell level. The cell logically provides four possible configurations in different stages of its cycle. Two of them are of particular interest because they provide two different fixed shapes for the cell that can be maintained without the continuous supply of external energy. The final deformations of the cell and equilibrium stresses in the SMA elements depend on the amount of stored shape memory strain in each element, external forces and cell geometry. A model is developed, which allows a full characterization of the AFC. The model is used to study NiTi SMA-based AFCs and the results are therefore directly applicable to the design of shape morphing structures using such unit cells.

Theoretical Modeling for Electroactive Polymer-Ceramic Hybrid Actuation Systems

An electroactive polymer-ceramic hybrid actuation system (HYBAS) was recently developed. The HYBAS demonstrates significantly-enhanced electromechanical performance by utilizing advantages of cooperative contributions of the electromechanical responses of an electrostrictive copolymer and an electroactive single crystal. The hybrid actuation system provides not only a new type of device but also a concept to utilize different electroactive materials in a cooperative and efficient method for optimized electromechanical performance. In order to develop an effective procedure to optimize the performance of a hybrid actuation system (HYBAS), a theoretical model has been developed, based on the elastic and electromechanical properties of the materials utilized in the system and on the configuration of the device. The model also evaluates performance optimization as a function of geometric parameters, including the length of the HYBAS and the thickness ratios of the constituent components. The comparison between the model and the experimental results shows a good agreement and validates the model as an effective method for the further development of high performance actuating devices or systems for various applications.

Analytical and Experimental Investigation of Different Synthetic Jet Actuator Configurations

The ultimate goal of this project is to actively control the flow over a micro air vehicle using smart materials. MAVs are a new type of aircraft operating at Reynolds numbers of about 50,000 that are one to two orders of magnitude lower than encountered in larger aircraft. The intention is to implement smart structures and couple them with fluids to improve the deteriorated aerodynamics of MAVs and help improve efficiency, stability and maneuverability of such vehicles. The actuators used in this work for artificially controlling the boundary layer are piezoelectrically driven synthetic jets. We theoretically investigated and predicted the behavior of the synthetic jet as we changed the geometry and material property parameters of the actuator. Analytical results were then compared to the results obtained from the experiments. It is crucial to be able to accurately design a strong unimorph to be implemented as an active component of a synthetic jet actuator and design the geometry configuration of the cavity that will best couple with the chosen membrane. A condenser microphone, a constant temperature anemometer (CTA) and a laser vibrometer were used to quantify actuator performance. It was observed that the size of the cavity and the size and shape of the exit nozzle were related and the performance of the actuator increased when the structure was tuned such that the resonant frequency of the diaphragm and that of the cavity were close to matching. A square unimorph made of PZT-5H and bonded to a 0.20- mm brass shim maximized jet velocity for the actuators studied. Optimum direction of change in the volume and the dimensions of the nozzle will strongly depend on the resonant frequency of the membrane in use. In this situation, increasing either the volume of the cavity or the thickness of the nozzle made the two frequencies move away from each other producing reduction in jet velocity. Increasing the area of the nozzle, made the structure behave more as needed and was taken as a key parameter for tuning the base geometry of the device.

Piezoelectric Modal Sensors for Two-Dimensional Structures

Using the orthogonal property of eigenfunctions, piezoelectric modal sensors for one-dimensional members were created by shaping electrode patterns proportional to modal strains. However, it is not easy to apply the same concept to two-dimensional structures due to the difficulty in implementing location weight needed for signals. Therefore, nonlinear optimization scheme is employed in this paper to design modal sensors for two-dimensional structures. For a given electrode pattern, the signal contributed from each mode is found by integrating the corresponding free surface charges on the sensing electrode. Then the modal sensor is obtained by modifying electrode pattern to achieve required relative signal strength for different modes. Sensors that are capable to sense or filter out the signal generated by a specific mode can be developed. A two-dimensional aluminum plate coated with PZT layer is adopted as an example. Mode shapes are found with finite element analysis. Modal sensors are designed successfully and mode- reject filters are also demonstrated.

On the Shortcomings of Shape Memory Alloy Phenomenological Models

The phenomenological models for SMAs, consisting of a thermodynamics based- constitutive and a phase transformation kinetics model, are the most widely used models for engineering applications. The existing phenomenological models are able to predict the behavior of SMA-actuated systems in most cases, except for cases arising from a simultaneous change in temperature and stress of the SMA elements, as is documented in this study. For such cases, the existing models fail to adequately predict the behavior of SMA elements undergoing complex thermomechanical loadings. A rotary SMA-actuated robotic arm is modeled using the existing constitutive models, in order to document the conditions under which the models fail. The model is verified against the experimental results, to document that under certain conditions, the model is not able to predict the behavior of the SMA-actuated manipulator. The phenomenological models discrepancy is also studied experimentally using a dead-weight that is actuated by an SMA wire.

Properties of Electro-Active Paper and Its Potential as a Bio-Inspired Actuator for Special Applications

On September 26, 2002, NASA announced that a consortium of six universities including: The University of Maryland, Virginia Tech, The University of Virginia, North Carolina A&T State University, North Carolina State University, and Georgina Tech had submitted the winning proposal for a National Institute of Aerospace. The Institute began formal operations in January of 2003 in Hampton, VA, and its mission included research, education, outreach, and technology transfer. One important focus of the NIA was to stimulate research among its member universities of potential benefit to NASA and to develop additional partnerships to further NIA focus areas. The work described in this paper is such an activity in bio-inspired actuator materials. This work was originally advocated and developed at Inha University, and it is being extended by teams from Inha University, North Carolina A&T State University, and NASA Langley so that the potential for these actuators as devices for special applications is better understood. This paper focuses on important performance characteristics of electro-active paper (EAPap) actuators and the potential of thes actuators to propel autonomous devices. EAPap is a paper that produces large displacement with small force under an electrical excitation. EAPap is made with chemically treated papers with electrodes on both outer surfaces. When electrical voltage is applied to the electrodes, a tip displacement is produced. One drawback in such actuators is that the actual power produced is variable, and the displacement is relatively unstable. Further, the performance tends to degrade in time and as a function of how the papers are processed. Environmental factors also impact the performance of the product including temperature and humidity. The use of such materials in ambulatory devices requires attention to these concerns and further research is needed to find what initial applications are most congruent with EAPap performance and service lift. In this paper, we have extended the knowledge base of EAPap to include additional ranges of temperature and humidity. We have also looked beyond the current tests on cantilevered beam actuators to segmented plate sections and have tested the ability of these actuators to perform as oscillatory devices both in and out of phase, and to chart their performance vs. time humidity and temperature thus emulating a rudimentary wing or walking assembly.

A Rate-Independent Constitutive Model for Square Honeycomb Core

All-metal sandwich construction with square honeycomb core holds promise for significant improvements in stiffness, strength and blast resistance for built-up plate structures. Analysis of the performance of sandwich plates under various loads, static and dynamic, requires modeling of face sheets and core with some fidelity. While it is possible to model full geometric details of the core for a few selected problems, this is unnecessary and unrealistic for larger complex structures under general loadings. A continuum constitutive model is recently developed as alternative means of modeling the cores. The constitutive model falls within the framework of a compressible rate-independent, anisotropic elastic-plastic solid. A multi-scale computing approach based on it has been developed for modeling the sandwich structures. The validity of the approach is established by comparing numerical finite element simulations using the model with those obtained by a full three-dimensional meshing of the core geometry for a clamped sandwich plate with square honeycomb core subject to uniform pressure load. Limitations of the model are also discussed.