Showing papers in "Smart Materials and Structures in 2005"

Ionic polymer–metal composites: IV. Industrial and medical applications

Abstract: This paper, the last in a series of four review papers to appear in this journal, presents some critical applications using ionic polymer?metal composites?(IPMCs). Industrial and biomedical applications of IPMCs are identified and presented along with brief illustration.

Finite-dimensional piezoelectric transducer modeling for guided wave based structural health monitoring

Abstract: Among the various schemes being considered for structural health monitoring (SHM), guided wave (GW) testing in particular has shown great promise. While GW testing using hand-held transducers for non-destructive evaluation (NDE) is a well established technology, GW testing for SHM using surface-bonded/embedded piezoelectric wafer transducers (piezos) is relatively in its formative years. Little effort has been made towards a precise characterization of GW excitation using piezos and often the various parameters involved are chosen without mathematical foundation. In this work, a formulation for modeling the transient GW field excited using arbitrary shaped surface-bonded piezos in isotropic plates based on the 3D linear elasticity equations is presented. This is then used for the specific cases of rectangular and ring-shaped actuators, which are most commonly used in GW SHM. Equations for the output voltage response of surface-bonded piezo-sensors in GW fields are derived and optimization of the actuator/sensor dimensions is done based on these. Finally, numerical and experimental results establishing the validity of these models are discussed.

A biomimetic undulatory tadpole robot using ionic polymer metal composite actuators

Abstract: The development of a wireless undulatory tadpole robot using ionic polymer–metal composite (IPMC) actuators is presented. In order to improve the thrust of the tadpole robot, a biomimetic undulatory motion of the fin tail is implemented. The overall size of the underwater microrobot prototype, shaped as a tadpole, is 96 mm in length, 24 mm in width, and 25 mm in thickness. It has one polymer fin tail driven by the cast IPMC actuator, an internal (wireless) power source, and an embedded controller. The motion of the tadpole microrobot is controlled by changing the frequency and duty ratio of the input voltage. Experimental results show that this technique can accurately control the steering and swimming speed of the proposed underwater tadpole robot.

Modeling and simulation of dielectric elastomer actuators

Abstract: Dielectric elastomers are used as base material for so-called electroactive polymer (EAP) actuators. A procedure and a specific constitutive model (for the acrylic elastomer VHB 4910) are presented in this work for finite element modeling and simulation of dielectric elastomer actuators of general shape and set-up. The Yeoh strain energy potential and the Prony series are used for describing the large strain time-dependent mechanical response of the dielectric elastomer. Material parameters were determined from uniaxial experiments (relaxation tests and tensile tests). Thereby the inverse problem was solved using iterative finite element calculations. A pre-strained circular actuator was built and activated with a predefined voltage. A three-dimensional finite element model of the circular actuator was created and the electromechanical activation process simulated. Simulation and actual measurements agree to a great extent, thus leading to a validation of both the constitutive model and the actuator simulation procedure proposed in this work.

Modeling and control of magnetorheological fluid dampers using neural networks

Abstract: Due to the inherent nonlinear nature of magnetorheological (MR) fluid dampers, one of the challenging aspects for utilizing these devices to achieve high system performance is the development of accurate models and control algorithms that can take advantage of their unique characteristics. In this paper, the direct identification and inverse dynamic modeling for MR fluid dampers using feedforward and recurrent neural networks are studied. The trained direct identification neural network model can be used to predict the damping force of the MR fluid damper on line, on the basis of the dynamic responses across the MR fluid damper and the command voltage, and the inverse dynamic neural network model can be used to generate the command voltage according to the desired damping force through supervised learning. The architectures and the learning methods of the dynamic neural network models and inverse neural network models for MR fluid dampers are presented, and some simulation results are discussed. Finally, the trained neural network models are applied to predict and control the damping force of the MR fluid damper. Moreover, validation methods for the neural network models developed are proposed and used to evaluate their performance. Validation results with different data sets indicate that the proposed direct identification dynamic model using the recurrent neural network can be used to predict the damping force accurately and the inverse identification dynamic model using the recurrent neural network can act as a damper controller to generate the command voltage when the MR fluid damper is used in a semi-active mode.

Performance of a piezoelectric bimorph for scavenging vibration energy

Abstract: We analyze the performance of a piezoelectric bimorph in the flexural mode for scavenging ambient vibration energy and evaluate the dependence of the performance upon the physical and geometrical parameters of the model bimorph. The analytical solution for the flexural motion of the piezoelectric bimorph shows that the output power density increases initially, reaches a maximum, and then decreases monotonically with increasing load impedance, which is normalized by a parameter that is a simple combination of the physical and geometrical parameters of the scavenging structure, the bimorph, and the frequency of the ambient vibration, underscoring the importance for the load circuit to have the impedance desirable by the scavenging structure. The numerical results illustrate the considerably enhanced performances achieved by adjusting the physical and geometrical parameters of the scavenging structure.

Calibration of piezo-impedance transducers for strength prediction and damage assessment of concrete

Abstract: This paper presents a new approach for the non-destructive evaluation of concrete, covering both strength prediction and damage assessment, using the electro-mechanical impedance technique. A new empirical method is proposed to determine in situ concrete strength non-destructively using admittance signatures of surface-bonded piezo-impedance transducers. This is followed by the 'identification' of appropriate impedance parameters for concrete. The identified parameters are found to be sensitive to structural damages as well as to concrete strength gain during curing. Comprehensive tests were conducted on concrete specimens up to failure to empirically calibrate the 'identified' system parameters with damage severity. An empirical fuzzy probabilistic damage model is proposed to quantitatively predict damage severity in concrete based on variation in the identified equivalent stiffness.

Helical dielectric elastomer actuators

Abstract: This paper presents a new type of contractile polymer-based electromechanical linear actuator. The device belongs to the class of dielectric elastomer actuators, which are typically capable of undergoing large deformations induced by an applied electric field. It is based on a novel helical configuration, suitable for the generation of electrically driven axial contractions and radial expansions. The architecture, the principle of operation, a fabrication method and results of a preliminary prototype testing of the new device are described. An axial strain of −5% at about 14 V µm −1 was obtained from first prototypes. (Some figures in this article are in colour only in the electronic version)

A superelastic alloy microgripper with embedded electromagnetic actuators and piezoelectric force sensors: a numerical and experimental study

Abstract: This paper presents the analysis, design, and characterization of a superelastic alloy (NiTi) microgripper with integrated electromagnetic actuators and piezoelectric force sensors. The microgripper, fabricated by electro-discharge machining, features force sensing capability, large force output, and large displacements to accommodate objects of various sizes. The design parameters for the embedded electromagnetic actuators were selected on the basis of finite element sensitivity analysis. In order to make the microgripper capable of resolving gripping forces, piezoelectric force sensors were fabricated and integrated into the microgripper. The performance of the microgripper, the integrated force sensors, and the electromagnetic actuators was experimentally evaluated. A satisfactory match between experimental results and finite element simulations was obtained. Furthermore, comparison studies demonstrated that the superelastic alloy (NiTi) microgripper was capable of producing larger displacement than a stainless steel microgripper. Finally, experimental results of optical fiber alignment and the manipulation of tiny biological tissues with the superelastic microgripper were presented.

Topology optimization of smart structures: design of piezoelectric plate and shell actuators

Abstract: In this paper, a novel approach to the design of piezoelectric plate and shell actuators using topology optimization is described A new piezoelectric material model PEMAP-P (piezoelectric material with penalization and polarization) is proposed, which is an extension of the SIMP (solid isotropic material with penalization) model used for elastic materials In addition to the pseudo-density ρ1, which describes the 'amount' of piezoelectric material in each finite element, a new design variable ρ2 is introduced for the polarization of the piezoelectric material The optimization problem consists in distributing the piezoelectric actuators in such a way as to achieve a maximum output displacement in a given direction at a given point of the structure, while simultaneously minimizing the structural compliance Sequential linear programming (SLP) is used to solve the optimization problem Examples are given demonstrating the potential of the proposed approach for the optimal design of piezoelectric actuators for multi-layer plate and shell structures

Recent breakthrough development of the magnetic shape memory effect in Ni-Mn-Ga alloys

Abstract: Magnetic shape memory (MSM) alloys or ferromagnetic shape memory alloy (FSMA) materials discovered by Ullakko et al (1996 Appl. Phys. Lett. 69 1966–8) have received increasing interest, since they can produce a large strain with rather high frequencies without a change in the external temperature. These materials have potential for actuator and sensor applications. MSM materials exhibit giant magnetic field induced strain (MFIS) based on the rearrangements of the crystallographic domains (twin variants). The magnetization energy of the material is lowered when such twin variants that have the easy axis of magnetization along the field start to grow due to twin boundary motion. Currently, the best working MSM materials are the near-stoichiometric Ni2MnGa Heusler alloys in which the properties are highly composition dependent. Their modulated martensitic structures, 5M and 7M, show 6% or 10% response respectively in a magnetic field less than 800 kA m−1. The MSM service temperature of the 5M alloys is between 150 and 333 K, and the optimal frequency region is up to 500 Hz. The fatigue life of the MSM elements has been shown to be at least 50 × 106 shape change cycles. This paper reviews the research work carried out at Helsinki University of Technology on MSM materials since 1998.

Smart passive system based on magnetorheological damper

Abstract: Magnetorheological (MR) dampers are one of the most promising control devices for civil engineering applications to earthquake hazard mitigation, because they have many advantages such as small power requirement, reliability, and low price to manufacture. To reduce the responses of the controlled structure by using MR dampers, a control system including a power supply, controller, and sensors is needed. However, when a lot of MR dampers are applied to large-scale civil structures, such as cable-stayed bridges and high-rise buildings, the control system becomes complex. Thus, it is not easy to install and to maintain the MR damper-based control system. In this paper, to resolve the above difficulties, a smart passive system is proposed, which is based on an MR damper system. The smart passive system consists of an MR damper and an electromagnetic induction (EMI) system that uses a permanent magnet and a coil. According to the Faraday law of induction, the EMI system that is attached to the MR damper produces electric energy. The produced energy is applied to the MR damper to vary the damping characteristics of the damper. Thus, the smart passive system does not require any power at all. Furthermore, the output of electric energy is proportional to input loads such as earthquakes, which means the smart passive system has adaptability by itself without any controller or corresponding sensors. Therefore, it is easy to build up and maintain the proposed smart passive system. To verify the effectiveness of the proposed smart passive system, the performance is compared with that of the normal MR damper-based control system. The numerical results show that the smart passive system has comparable performance to the normal MR damper-based control system.

Sensor validation using principal component analysis

Abstract: For a reliable on-line vibration monitoring of structures, it is necessary to have accurate sensor information. However, sensors may sometimes be faulty or may even become unavailable due to failure or maintenance activities. The problem of sensor validation is therefore a critical part of structural health monitoring. The objective of the present study is to present a procedure based on principal component analysis which is able to perform detection, isolation and reconstruction of a faulty sensor. Its efficiency is assessed using an experimental application.

A shape memory alloy adaptive tuned vibration absorber: design and implementation

Abstract: In this paper a tuned vibration absorber (TVA) is realized using shape memory alloy (SMA) elements. The elastic modulus of SMA changes with temperature and this effect is exploited to develop a continuously tunable device. A TVA with beam elements is described, a simple two-degree-of-freedom model developed and the TVA characterized experimentally. The behaviour during continuous heating and cooling is examined and the TVA is seen to be continuously tunable. A change in the tuned frequency of 21.4% is observed between the cold, martensite, and hot, austenite, states. This corresponds to a change in the elastic modulus of about 47.5%, somewhat less than expected. The response time of the SMA TVA is long because of its thermal inertia. However, it is mechanically simple and has a reasonably good performance, despite the tuning parameters depending on the current in a strongly nonlinear way.

Fatigue analysis of shape memory alloys: energy approach

Abstract: The purpose of this paper is to define a low cycle fatigue criterion for NiTi shape memory alloys (SMAs) in order to perform numerical calculations necessary for designing structures made of SMA and subjected to cyclic loading. For this purpose, fatigue tests are performed in a temperature and deformation regime in which the alloy exhibits pseudoelasticity. A behavior similar to plastic shakedown is observed; during the first cycles a hysteresis loop with a varying size which eventually stabilizes is obtained. Therefore, by analogy with plastic fatigue (low cycle fatigue), it is shown that the dissipated energy of the stabilized cycle is a relevant parameter for the estimation of lifetime.

Unseating prevention for multiple frame bridges using superelastic devices

Abstract: Unseating of bridge spans due to excessive relative hinge opening is a common problem for bridges subjected to strong ground motion. Various unseating prevention devices have been developed in both the United States and Japan to try to reduce the likelihood of collapse due to unseating. This paper presents the results of the evaluation of unseating prevention devices using nitinol shape memory alloys (SMAs). Superelastic SMAs have the ability to remain elastic under very large deformations, due to a solid-state martensitic transformation. This unique property leads to enhanced performance of the adaptive superelastic unseating prevention device, compared with conventional devices used in the United States and Japan. To assess the effectiveness of the devices, nonlinear time history analyses are performed on a typical multiple frame reinforced concrete box girder bridge using a suite of representative ground motions. The results show that for multiple frame reinforced concrete box girder bridges the adaptive superelastic devices are very effective in limiting the relative hinge displacement and preventing unseating, compared with the conventional steel cable restrainers.

An innovative magnetorheological damper for automotive suspension: from design to experimental characterization

Abstract: The development of a powerful new magnetorheological fluid (MRF), together with recent progress in the understanding of the behavior of such fluids, has convinced researchers and engineers that MRF dampers are among the most promising devices for semi-active automotive suspension vibration control, because of their large force capacity and their inherent ability to provide a simple, fast and robust interface between electronic controls and mechanical components. In this paper, theoretical and experimental studies are performed for the design, development and testing of a completely new MRF damper model that can be used for the semi-active control of automotive suspensions. The MR damper technology presented in this paper is based on a completely new approach where, in contrast to in the conventional solutions where the coil axis is usually superposed on the damper axis and where the inner cylindrical housing is part of the magnetic circuit, the coils are wound in a direction perpendicular to the damper axis. The paper investigates approaches to optimizing the dynamic response and provides experimental verification. Both experimental and theoretical results have shown that, if this particular model is filled with an 'MRF 336AG' MR fluid, it can provide large controllable damping forces that require only a small amount of energy. For a magnetizing system with four coils, the damping coefficient could be increased by up to three times for an excitation current of only 2 A. Such current could be reduced to less than 1 A if the magnetizing system used eight small cores. In this case, the magnetic field will be more powerful and more regularly distributed. In the presence of harmonic excitation, such a design will allow the optimum compromise between comfort and stability to be reached over different intervals of the excitation frequencies.

Active and passive damping of noise and vibrations through shape memory alloys: applications and mechanisms

Abstract: Recent achievements have been analysed in designing and application of shape memory alloys as high-damping elements, utilizing pseudoelastic hysteresis, transient damping effects in the two-phase state and damping capacity of the martensitic phase. Dealing with intrinsic damping capacity of martensitic phases, several new observations are described, like 'universal' low-temperature high-damping properties of ternary Cu-based alloys, high non-linear damping capacity of a binary NiTi in R phase and high linear damping of binary hydrogen-charged NiTi. Based on the analysis of results of recent studies of damping in NiTi (B 19' martensite, R phase) and Cu-based families of alloys (Cu–Al–Ni, Cu–Zn–Al, Cu–Al–Be), we try to introduce a guideline relating desired damping properties of a SMA with its structural characteristics. Among the parameters determining the contribution of specific defect species to damping we suggest considering density of specific type of defects (intervariant boundaries and internal defects of variants like dislocations and twins); their mobility (determined by crystallography and geometrical factors, like accommodation and size of martensitic variants); concentration and type of obstacles impeding the motion of defect species and, thus, producing damping (concentration, mobility and distribution of quenched-in/point-like defects, precipitates, etc). The importance of distinguishing linear and non-linear components of damping is emphasized, since, in a general case, they can be related to different elements of defect microstructure of martensite.

An automatic impact monitor for a composite panel employing smart sensor technology

Abstract: Impacts can inflict serious damage on composite structures. For structures on the surface of an aircraft this has potentially critical consequences. The problem is exacerbated by the fact that the damage is internal, with little or no indication on the surface of the structure, making it difficult to detect. A potential solution to the problem would be an impact monitoring system that could detect when an impact occurred. This idea was explored using a composite panel, inside which a SMART Layer was embedded. Two techniques were studied that were able to estimate the locations of impacts on the panel from the measurements provided by the piezoelectric strain sensors in the SMART Layer. The first technique employed artificial neural networks and the second used a triangulation procedure incorporating a genetic algorithm. The knowledge acquired during the study made it possible to develop a simple and efficient prototype impact monitoring system for the composite panel.

An enhanced SMA phenomenological model: I. The shortcomings of the existing models

Abstract: This paper provides an enhanced phenomenological model for shape memory alloys (SMAs), to better model their behavior in cases where the temperature and stress states change simultaneously. 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 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 most often occur when the temperature and stress of the SMA wire change simultaneously, such as the case of rotary SMA actuators. To this end, a rotary SMA-actuated robotic arm is modeled using the existing constitutive models. The model is verified against the experimental results to document that the model is not able to predict the behavior of the SMA-actuated manipulator, under certain conditions.

Analytical development of single crystal Macro Fiber Composite actuators for active twist rotor blades

Abstract: A Macro Fiber Composite (MFC) is a piezoelectric fiber composite which has an interdigitated electrode, rectangular cross-section and unidirectional polycrystalline piezoceramic (PZT) fibers embedded in the polymer matrix. A MFC actuator has much higher actuation performance and flexibility than a monolithic piezoceramic actuator. Moreover, the single crystal piezoelectric material exhibits much higher induced strain levels, energy density and coupling than those of polycrystalline piezoceramic materials. Thus, the performance of an MFC can be improved by using single crystal piezoelectric fiber instead of polycrystalline piezoceramic fiber. This study investigates the analytical modeling, material properties and actuation performance of an MFC using single crystal piezoelectric material (single crystal MFC). For single crystal MFC, the mechanical properties are calculated by the classical lamination theory, and the uniform fields model (UFM) is adopted to predict piezoelectric strain constants. In addition, the actuation performance of the single crystal MFC with the active twist rotor blade is studied. The material properties and actuation performance of single crystal MFC are compared with those of standard MFC.

The matching pursuit approach based on the modulated Gaussian pulse for efficient guided-wave damage inspection

Abstract: The success of the guided-wave damage inspection technology depends not only on the generation and measurement of desired waveforms but also on the signal processing of the measured waves, but less attention has been paid to the latter. This research aims to develop an efficient signal processing technique especially suitable for the current guided-wave technology. To achieve this objective, the use of a two-stage matching pursuit approach based on the Gabor dictionary is proposed. Instead of truncated sine pulses commonly used in waveguide inspection, Gabor pulses, the modulated Gaussian pulses, are chosen as the elastic energy carrier to facilitate the matching pursuit algorithm. To extract meaningful waves out of noisy signals, a two-stage matching pursuit strategy is developed, which consists of the following: rough approximations with a set of predetermined parameters characterizing the Gabor pulse, and fine adjustments of the parameters by optimization. The parameters estimated from measured longitudinal elastic waves can be then directly used to assess not only the location but also the size of a crack in a rod. For the estimation of the crack size, in particular, Love's theory is incorporated in the matching pursuit analysis. Several experiments were conducted to verify the validity of the proposed approach in damage assessment.

Equivalent modeling for ionic polymer–metal composite actuators based on beam theories

Abstract: Equivalent beam and equivalent bimorph beam models for IPMC (ionic polymer–metal composite) actuators are described in the ensuing paper. Important physical properties of IPMCs including Young's modulus and electro-mechanical coupling coefficient were determined using the rule of mixture, bimorph beam equations, and measured force–displacement data of a cantilevered IPMC actuator. By using a beam equation with estimated physical properties, the actuation displacements of a cantilevered IPMC actuator were calculated to show an excellent agreement between the computed tip displacements and the measured data. Finite element analysis (FEA), along with the predetermined physical properties, was used to predict the force–displacement relationship of an IPMC actuator, which is key data to effectively design many engineering devices of interest. Indicated by the results from the FEA agreeing with the measured data, the proposed models can be adopted for modeling of IPMC actuators with advanced shapes and other boundary conditions.

A hybrid piezoelectric/fiber optic diagnostic system for structural health monitoring

Abstract: A hybrid piezoelectric/fiber optic diagnostic system has been developed for quick non-destructive evaluation and long term health monitoring of aerospace vehicles and structures. The hybrid diagnostic system uses piezoelectric actuators to input a controlled excitation to the structure and fiber optic sensors to capture the corresponding structural response. The system consists of three major parts: a diagnostic layer with a network of piezoelectric elements and fiber gratings to offer a simple and efficient way to integrate a large network of transducers onto a structure; diagnostic hardware consisting of an arbitrary waveform generator and a high speed fiber grating demodulation unit together with a high speed data acquisition card to provide actuation input, data collection, and information processing; and diagnostic software to determine the condition of the structure. This paper presents key development issues related to the manufacturing of the hybrid piezoelectric/fiber optic diagnostic layer and integration of a highly portable diagnostic hardware. Validation and proof testing of this integrated diagnostic system are also presented.

Structural Health Monitoring and Intelligent Infrastructure

Abstract: This special issue collects together 19 papers that were originally presented at the First International Conference on Structural Health Monitoring and Intelligent Infrastructure (SHMII-1'2003), held in Tokyo, Japan, on 13–15 November 2003. This conference was organized by the Japan Society of Civil Engineers (JSCE) with partial financial support from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sport, Science and Technology, Japan. Many related organizations supported the conference. A total of 16 keynote papers including six state-of-the-art reports from different counties, six invited papers and 154 contributed papers were presented at the conference. The conference was attended by a diverse group of about 300 people from a variety of disciplines in academia, industry and government from all over the world. Structural health monitoring (SHM) and intelligent materials, structures and systems have been the subject of intense research and development in the last two decades and, in recent years, an increasing range of applications in infrastructure have been discovered both for existing structures and for new constructions. SHMII-1'2003 addressed progress in the development of building, transportation, marine, underground and energy-generating structures, and other civilian infrastructures that are periodically, continuously and/or actively monitored where there is a need to optimize their performance. In order to focus the current needs on SHM and intelligent technologies, the conference theme was set as 'Structures/Infrastructures Sustainability'. We are pleased to have the privilege to edit this special issue on SHM and intelligent infrastructure based on SHMII-1'2003. We invited some of the presenters to submit a revised/extended version of their paper that was included in the SHMII-1'2003 proceedings for possible publication in the special issue. Each paper included in this special issue was edited with the same quality standards as for any paper in a regular issue. The papers cover a wide spectrum of topics including smart and effective sensing technologies, reliable approaches to signal processing, rational data gathering and interpretation methods, advanced damage characterization, modeling feature selection and diagnosis methods, and system integration technologies, etc. This special issue contains the most up-to-date achievements in SHM and intelligent technologies and provides information pertaining to their current and potential applications in infrastructure. It is our hope that this special issue makes a significant contribution in advancing awareness and acceptance of SHM and intelligent technologies for the maintenance and construction of different kinds of infrastructure. We would like to express our sincere thanks to Professor Varadan (Editor-in-Chief), Professor Matsuzaki (Regional Editor), the Editorial Assistants and the staff at Institute of Physics Publishing for their great support and advice in publishing this special issue. Special thanks are due to all the reviewers for their willingness to share their time and expertise. Final but important thanks go to Ms Suzhen Li (Doctorate Candidate at Ibaraki University) for her assistance in editing this special issue.

Magnetorheological elastomer-based smart sandwich beams with nonconductive skins

Abstract: The field-dependent dynamic flexural rigidity of a simply supported sandwich beam with a soft core composed of a magnetorheological elastomer (MRE) part and non-MRE parts is studied in this paper. The skins of the sandwich beam are nonconductive such that there are no magnetoelastic loads applied to the skins during vibration. The orientation of the chain-like structures inside the MRE part is perpendicular to the skins such that the MRE part operates in shear mode. Due to such a configuration, the dynamic flexural rigidity of the sandwich beam can be controlled by applied magnetic fields due to the field-dependent shear modulus of the MRE part. Based on the Hamilton principle, a dynamic model of the proposed sandwich beam is developed. A simply supported beam excited by a vertical force, distributed uniformly in a narrow region around the center of the beam is simulated. The anti-resonant frequencies are found to change with the shear modulus of the MRE part up to 40%, while the resonant frequencies change only slightly. Although MRE is an extremely soft material with a zero-field shear modulus about 0.4 MPa, the results from the current research indicate that the sandwich configuration can well utilize the controllable properties of MRE to realize applicable semi-active devices with controllable stiffness.

Milling workpiece chatter avoidance using piezoelectric active damping: a feasibility study

Abstract: During the milling of thin-walled and flexible structures such as those manufactured for the aerospace industries, workpiece chatter can become a key factor that limits productivity. Chatter is a form of self-excited vibration that can be stabilized by increasing the damping of the vibrating structure. In the present study, an experimental investigation is described which assesses the feasibility of using piezoelectric active vibration control to mitigate workpiece chatter in milling. A positive position feedback control strategy is used, and a series of milling tests is performed, which demonstrate a sevenfold improvement in the limiting stable depth of cut. The practical issues concerning the application of this technique are then discussed.

Constitutive models of electrorheological and magnetorheological fluids using viscometers

Abstract: A key aspect of application of electrorheological (ER) and magnetorheological (MR) fluids is the characterization of rheological properties. In this study, two rotational viscometers to measure the field-dependent flow behavior (shear stress versus shear rate) of ER/MR fluids are theoretically analyzed. One is a rotational coaxial cylinder viscometer, and the other is a rotational parallel disk viscometer. The equations between shear stress and torque as well as shear rate and angular velocity are derived on the basis of the Bingham-plastic, biviscous, and Herschel–Bulkley constitutive models. The shear stress for the rotational coaxial cylinder viscometer can be straightforwardly calculated from the measured torque. However, in order to determine the shear rate, three approximation methods are applied. Meanwhile, the shear stress and shear rate in the rotational parallel disk viscometer can be obtained directly from the torque and angular velocity data. In order to comprehensively understand the flow behavior of ER/MR fluids with respect to the constitutive models, nondimensional analyses are undertaken in this study.

Fiber optic health monitoring of civil structures using long gage and acoustic sensors

Abstract: Structural health monitoring with optical fibers provides practical sensing capabilities in many applications including in aeronautics and mechanical structures. A variety of optical fiber sensors have been used including Bragg gratings, intensity or amplitude sensors, and Fabry–Perot ones. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. Monitoring of strains, deformations, and deflections provides clues essential for evaluation of design parameters and behavior under service loads. Long gage distributed or multiplexed sensors are excellent candidates for such applications. On the other hand, detection of structural damage and anomalies such as cracking in concrete, splintering of fibers in composites, and fracturing of welds and connections are best accomplished by acoustic sensors. This paper describes principles involved in serial multiplexing of two kinds of optical fibers, namely long gage and acoustic sensors. Both sensor types offer promise in structural health monitoring of large civil structural systems. Representative examples are introduced and described in detail.

Simulation of a piezoelectrically actuated valveless micropump

Abstract: This paper numerically studies the performance of a piezoelectrically actuated valveless micropump with consideration of the three-way electro–mechanical–fluid couplings. Simulation of the piezoelectrically actuated valveless micropump (PVAM) indicates that both the pumping rate and the membrane deflection amplitude increase with the increase of the actuating frequency in a low frequency range ( 7.5 kHz). At even higher frequencies (>50 kHz), the pumping rate decreases further because the deflection amplitude decreases. This agrees with reported experimental results. The changing membrane deflection shapes at various frequencies clearly play an important role in the performance of the pump.