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In this paper free vibration analysis of cracked composite beams subjected to coupled bending-torsion loads are presented. The composite beam is assumed to have an open edge crack. A first order theory is applied to count for the effect of the shear deformations on natural frequencies as well as the effect of coupling in torsion and bending modes of vibration. Local flexibility matrix is used to obtain the additional boundary conditions of the beam in the crack area. After obtaining the governing equations and boundary conditions, GDQ method is applied to solve the obtained eigenvalue problem. Finally, some numerical results are given to show the efficacy of the method. In addition, to count for the effect of coupling on natural frequencies of the cracked beams, different fiber orientations are assumed and studied.

The long composite blades on large wind turbines experience tremendous stresses while in operation. There is an interest in implementing structural health monitoring (SHM) systems inside wind turbine blades to alert maintenance teams of damage before serious component failure occurs. This paper proposes using an energy harvesting device inside the blade of a horizontal axis wind turbine to power a SHM system. The harvester is a linear induction energy harvester placed radially along the length of the blade. The rotation of the blade causes a magnet to slide along a tube as the blade axis changes relative to the direction of gravity. The magnet induces a voltage in a coil around the tube, and this voltage powers the SHM system. This paper begins by discussing motivation for this project. Next, a harvester model is developed, which encompasses the mechanics of the magnet, the interaction between the magnet and the coil, and the current in the electrical circuit. A free fall test verifies the electromechanical coupling model, and a rotating test examines the power output of a prototype harvester. Copyright © 2013 John Wiley & Sons, Ltd.

A nonlinear piezoelectric rotary transducer is developed that makes compact low speed wind generators realizable. Compact wind generators provide power to sensor nodes in remote or hard to reach locations. Since the locations of the sensor nodes are not optimized in terms of wind speed, the compact wind generators should be able to produce power from low speed wind. At smaller scales piezoelectric transduction becomes more effective than electromagnetic transduction. Therefore one way of realizing the compact wind turbines is by replacing the electromagnetic generator with a piezoelectric transducer. This work presents a novel piezoelectric transducer where the rotation of the blades results in large oscillations of piezoelectric beams. The piezoelectric bimorphs are made bi-stable by incorporation of repelling magnetic force. The Magnetic force is due to interaction of permanent magnets at the tip of the beams with permanent magnets rotating with the blades. Since the magnetic force changes with blade rotation, the dynamics of the beams changes in time and the system is thus parametrically excited. Two configurations are presented one called tangential configuration and the other is radial configuration. An 80 mm × 80 mm × 175 mm nonlinear piezoelectric wind generator can generate milliwatts of power from wind as slow as 2 ms −1 . The proposed compact wind generators are experimentally investigated in two steps. First the piezoelectric transducer is examined through constant rotational speed tests. Second wind tunnel experiments are performed to characterize the entire wind generator. An analytical model is developed for the piezoelectric rotational transducer. The model is verified with the experimental results. The nonlinear phenomena captured by the experimental investigations are explained using the analytical model. The model is also used for more case studies identifying specifically the effect of parametric excitations.

This article examines the concept and design of a multifunctional composite sandwich structure for simultaneous energy harvesting and vibration control. The intention is to design a composite wing spar for a small unmanned aerial vehicle which is able to harvest energy itself from ambient vibrations during normal flight along with available sunlight. If the wing experiences any strong wind gust, it will sense the increased vibration levels and provide vibration control to maintain its stability. The proposed multifunctional composite wing spar integrates a flexible solar cell array, piezoelectric wafers, a thin film battery, and an electronic module into a composite sandwich structure. The piezoelectric wafers act as sensors, actuators, and harvesters. The basic design factors are discussed for a beam-like multifunctional wing spar with energy harvesting, strain sensing, and self-controlling functions. The configurations, locations, and operating modes of piezoelectric transducers are also discussed for optimal power generation. The equivalent electromechanical representations of a multifunctional wing spar is derived theoretically and simulated numerically. Special attention is given to the self-contained gust alleviation with the goal of using available energy harvested from ambient vibrations. A reduced energy control law is implemented to reduce the actuation energy and the dissipated heat. This law integrates saturation control with a positive strain feedback controller and is represented by a positive feedback operation amplifier and a voltage buffer operation amplifier for each mode. This study builds off of our previous research and holds promise for improving unmanned aerial vehicle performance in wind gusts. Here, we also include, but not use, a flexible solar panel in our modeling.

Aircraft morphing with regard to UAVs has recently gained incredible momentum; however, only a limited amount of research has been conducted on its effect on tailless aircraft. This is partly due to aerodynamic compromises such as directional instabilities that arise in the absence of a vertical stabilizer. Yet birds readily adapt to adverse flight conditions without vertical stabilizers and are unhindered with respect to stability and maneuvering due to their smooth continuous shape change and rapid muscle response. This research, motivated by the discrepancy between manmade and natural flight designs, investigates the aerodynamic effects of a smart morphing horizontal tail exhibiting bending-twisting coupling for yaw control on a bio-inspired aircraft. The structural response due to actuation was determined using Abaqus and coupled with a Reynolds-averaged-Navier-Stokes turbulence model for a low-Reynolds-number fluid analysis of the deformed shape. The morphing tail was simulated as piezoelectric Macro Fiber Composites with oriented PZT rods. Directional moment and stability derivative are presented to gain insight into the effect of the morphing horizontal tail on yaw control.

This work aims to investigate how bio-inspired morphing wings built with state-of-the-art materials affect the aerodynamics and extend the range of flight conditions. In particular, this study investigates the aerodynamic effects of coupled airfoil and planform sweep morphing. The morphed geometries were chosen to resemble a current morphing design that uses Macro Fiber Composites (MFCs) and Shape Memory Alloy (SMA) wires. The primary mode of camber actuation is achieved using the MFCs which are supplemented using antagonistic SMA wires, forming a hinge ahead of the MFCs. The SMA hinge also allows for bi-directional actuation, resulting in a reflexed airfoil. Numerical simulations were conducted using a Reynolds-averaged-Navier-Stokes (RANS) turbulence model for low-Reynolds-number flow, in addition to wind tunnel experiments. Nine different wing configurations were considered consisting of combinations of 3 sweep angles and 3 airfoil profiles, including unactuated (baseline), monotonic camber actuation, and reflex actuation. These geometries were 3D printed on a high resolution printer. Tests were conducted in a 2 ft. × 2 ft. wind tunnel at the University of Michigan at a flow speed of 10 m/s, consistent with the flow regime expected for this scale of aircraft. The preliminary results suggest a definite improvement in flight performance associated with the proposed coupling.

Development of morphing systems is a fast evolving area of research with broad applications, including morphing aircraft. Classified based on the magnitude of the motion, there are generally two scales of morphing: local (small scale; e.g. airfoil morphing), and global (large scale; e.g. wing morphing), both requiring adaptive structures to provide morphing motion while maintaining structural rigidity. In this paper, a new design is presented for local morphing, inspired by the notion of minimal actuation effort. Based on the concept of multi-loop linkages, this design allows a morphing curve, represented by a series of points, to take up three distinct shapes, with a single actuation input. The underlying design is based on a network of four-bar linkages connected together to form a multi-loop linkage, referred to as the Curve Adaptive Linkage Array (CALA). A three-step method is developed and presented here to find the geometric dimensions of the CALA. Furthermore, a case-study for an airfoil morphing application is presented and solved using the proposed method. The presented method provides a means to reduce the number of actuators needed for shape morphing, and is generally applicable to any shape morphing application.

The research presented here explores a finite element model of metastructure with distributed vibration absorbers experiencing unidirectional axial vibrations. This work builds off of previous work using a lumped mass model. The proposed model is compared to a baseline model where both models have equal mass. First, it is verified that the finite element formulation produces similar trends as the lumped mass model, specifically, that using vibration absorbers of linearly varying natural frequencies leads to a larger bandgap of reduced vibrations. Second, the two comparison models generated using the finite element formulation have different effective stiffnesses and the effects of this is examined. The results show that a small number of vibration absorbers, less than three, leads to a significant change in stiffness which in turn results in a larger dynamic response of the structure.

Unmanned Aerial Vehicles (UAVs) can require adaptation to large changes in flight condition but are restricted by their size. Hybrid actuation systems that excel at both long timescale trim conditions and short timescale deviations, e.g. gusts, can be utilized to augment UAVs controllability. Previously, the Synergistic Smart Morphing Aileron (SSMA) system, a camber-morphing airfoil system utilizing two smart-material actuators operating over different timescales, was introduced and characterized without positional controllers. The combined SSMA system utilizes the comparatively high blocking force of shape-memory alloy (SMA) wires to augment static displacement under high aerodynamic loads while maintaining high-bandwidth tip displacement from the faster piezo-driven MFC actuator. Static aeroelastic simulations have shown that the concept can achieve superior control over flow separation through reflex actuation under aerodynamic loading. This current work further identifies performance capabilities of the SSMA concept that are synergistic in nature, and thus extended beyond that of the constituent actuators in isolation. Dynamic URANS simulations show that allowing the comparatively faster MFC-driven actuator to compensate for the slower SMA wires by actuating through reflex camber mimics the forces of a faster monotonic actuator. By implementing position controllers, an experimental demonstrator is used to initially validate both the static and dynamic benefits of synergistic force control through a hybrid morphing system. Although specific to this configuration, this study exposes how a multi-material, multi-timescale morphing concept can be leveraged to improve steady and unsteady flow control.

One of the major challenges in civil, mechanical, and aerospace engineering is to develop vibration suppression systems with high efficiency and low cost. Recent studies have shown that high damping performance at broadband frequencies can be achieved by incorporating periodic inserts with tunable dynamic properties as internal resonators in structural systems. Structures featuring these kinds of inserts are referred to as metamaterials inspired structures or metastructures. Chiral lattice inserts exhibit unique characteristics such as frequency bandgaps which can be tuned by varying the parameters that define the lattice topology. Recent analytical and experimental investigations have shown that broadband vibration attenuation can be achieved by including chiral lattices as internal resonators in beam-like structures. However, these studies have suggested that the performance of chiral lattice inserts can be maximized by utilizing an efficient optimization technique to obtain the optimal topology of the inserted lattice. In this study, an automated optimization procedure based on a genetic algorithm is applied to obtain the optimal set of parameters that will result in chiral lattice inserts tuned properly to reduce the global vibration levels of a finite-sized beam. Genetic algorithms are considered in this study due to their capability of dealing with complex and insufficiently understood optimization problems. In the optimization process, the basic parameters that govern the geometry of periodic chiral lattices including the number of circular nodes, the thickness of the ligaments, and the characteristic angle are considered. Additionally, a new set of parameters is introduced to enable the optimization process to explore non-periodic chiral designs. Numerical simulations are carried out to demonstrate the efficiency of the optimization process. The financial support for this research was provided by Qatar National Research Fund , QNRF (a member of Qatar Foundation) via the National Priorities Research Program (NPRP), Project number NPRP 6-526-2-218 . Daniel J. Inman's work is supported in part by the U.S. Air Force Office of Scientific Research under the Grant number FA9550-14-1-0246 Electronic Damping in Multifunctional Material Systems monitored by Dr. B.L. Lee. The statements made herein are solely the responsibility of the authors. Scopus

Metamaterial inspired structures, or metastructures, are structural members that incorporate periodic or nonperiodic inserts. Recently, a new class of metastructures has been introduced which feature chiral lattice inserts. It was found that this type of inserts has frequency bandgaps which can be tuned by altering the geometry of the chiral lattice. Previous studies have shown that inserting non-periodic chiral lattices inside a beam-like structure results in efficient vibration attenuation at low frequencies. In the study presented in this paper, a genetic algorithm based optimization technique is developed to automatically generate chiral lattices which are tuned to suppress vibration in a flexible beam-like structure. Several parameters are incorporated in the optimization process such as the radius of circular nodes and characteristic angle as well as the spacing and distribution of circular inserts. The efficiency of the proposed optimization technique is verified analytically by numerical simulations. The Society for Experimental Mechanics, Inc. 2016. Scopus

This paper examines the damping properties of digital materials from the Objet Connex 3D Printer from Stratsys. The Objet Connex printer is a unique 3D printer because in a single print job, ten different materials with different stiffness can be utilized. The printer utilizes two base materials and mixes them in different ratios to obtain varying stiffness levels. The first base material, VeroWhitePlusTM is a rigid opaque plastic and the second base material, TangoPlusTM is a rubbery translucent material. This 3D printer has a variety of applications including the development of metastructures for vibration suppression. It has been demonstrated that both the VeroWhitePlusTM and TangoPlusTM materials exhibit viscoelastic behavior but extensive studies have not be carried out to determine the Young’s Modulus and loss factor data in terms of frequency. This paper is the first step towards that goal. The paper begins by going into more detail about the properties of the printer and examines some previous work on determining the material properties. Next, the test method used it outlined and lastly the experimental set-up is described. In order to obtain data for a high range of frequencies, the material must be tested at various temperatures.

This paper presents an experimental study of optimized numerical simulations on a metastructure created for the purpose of broadband vibration suppression. The proposed structure uses a stiff outer frame with periodic inserts as internal resonators in a beam like assembly to achieve high damping broadband performance. With the rapid improvement of 3D printing capabilities, chiral lattice inserts can be quickly created from an elastomeric material for use in the structure. The stiff outer frame is used to maintain the integrity of the structure while the flexible chiral lattice and annular masses dissipate the energy. This combined assembly is characterized by high frequency bandgaps and tuned vibration attenuation at low frequencies. These inserts and masses have been previously optimized to find the optimal topology and tuning to reduce global vibration levels of the beam. Experimental results demonstrate the suppression capabilities of this structure for possible use in vibration absorption in load carrying structural members.

Optical membranes are currently pursued for their ability to replace the conventional rigid mirrors that are used in space-based telescopes. Among some of the many benefits of using optical membranes is their ability to considerably reduce the weight of the structure. Given the low density of these thin-film membranes, the lower end dynamics play a more significant role than in their rigid plate-like counterparts. Space-based mirrors are subjected to a series of disturbances. Among those encountered are thermal radiation, debris impact, and slewing maneuvers. Thus, being able to model the dynamics appropriately is essential for the adequate performance of thin-film membrane mirrors. With this in mind, the work presented herein uses an impedance based modeling approach to describe the coupled dynamics of a pressurized optical membrane mirror with the end goal of performing vibration suppression of a membrane through acoustic excitation. First the effects of mass loading due to air surrounding a membrane and energy loss due to sound radiation to the far field are modeled in the case of a single membrane. These results are compared to the case of a membrane in vacuum. Second, the membrane is then coupled to a cylindrical cavity where the modeling takes into account the structural acoustic coupling between a cylindrical membrane and a rigid cylindrical cavity, similar to a drum. The coupled model also takes into account the energy loss by sound radiation to the far field due to the membrane's vibration. Third, this paper also looks at using a positive position feedback controller for vibration suppression of the membrane. This is done using a centralized acoustic source at the base of the cavity as the method of actuation. The acoustic actuation is of great interest since it does not mass load the membrane in the conventional way, as most methods of actuation would.

The topic of multifunctional material systems using active or smart materials has recently gained attention in the research community. Multifunctional piezoelectric systems present the ability to combine multiple functions into a single active piezoelectric element, namely, combining sensing, actuation, or energy conversion ability with load-bearing capacity. Quantification of the bending strength of various piezoelectric materials is, therefore, critical in the development of load-bearing piezoelectric systems. Three-point bend tests are carried out on a variety of piezoelectric ceramics including soft monolithic piezoceramics (PZT-5A and PZT-5H), hard monolithic ceramics (PZT-4 and PZT-8), single-crystal piezoelectrics (PMN-PT and PMN-PZT), and commercially packaged composite devices (which contain active PZT-5A layers). A common 3-point bend test procedure is used throughout the experimental tests. The bending strengths of these materials are found using Euler-Bernoulli beam theory to be 44.9 MPa for PMN-PZT, 60.6 MPa for PMN-PT, 114.8 MPa for PZT- 5H, 123.2 MPa for PZT-4, 127.5 MPa for PZT-8, 140.4 MPa for PZT-5A, and 186.6 MPa for the commercial composite. The high strength of the commercial configuration is a result of the composite structure that allows for shear stresses on the surfaces of the piezoelectric layers, whereas the low strength of the single-crystal materials is due to their unique crystal structure, which allows for rapid propagation of cracks initiating at flaw sites. The experimental bending strength results reported, which are linear estimates without nonlinear ferroelastic considerations, are intended for use in the design of multifunctional piezoelectric systems in which the active device is subjected to bending loads.

This article presents a summary of passive, semipassive, semiactive, and active control methods for schemes using harvested energy as the main source of energy to suppress vibrations via piezoelectric materials. This concept grew out of the fact that energy dissipation effects resulting from energy harvesting can cause structural damping. First, the existing equivalent electromechanical modeling methods are reviewed for vibration-based energy harvesters using piezoelectric transducers. Modeling of base excitation cantilever beam ranges from lumped to distributed parameter formulations. The commonly used electrical power conditioning circuits and their optimization are also summarized and discussed. The energy dissipation from harvesting induces structural damping, and this leads to the concept of purely passive shunt damping. This article reviews the literature on vibration control laws along the lines of purely passive, semipassive, semiactive, and active control. The classification of pervious results is built on whether external power is supplied to the piezoelectric transducers. The focus is placed on recent articles investigating semipassive and semiactive control strategies derived from synchronized switching damping. However, whether or not the harvested energy is large enough to satisfy a vibration suppression requirement has become an important topic of research but has not yet specifically been addressed in previous studies. Hence, this survey also reviews the possible control methods aiming for less control energy consumption and addresses the potential application for simultaneous vibration control and energy harvesting.

One of the main objectives of the structural health monitoring community is to continuously monitor the integrity of a structure by embedding sensors and actuators into the structure. A piezoelectrically driven beam is considered in the present work and the effect of the growth of a single crack on the vibratory characteristics of the beam is studied. It is shown that in order to combine the effect of a crack and the piezoceramic (PZT) actuator into the modeling, available formulations of the mode shapes of the cracked beam, such as modeling the beam as a bi-section connected with a rotational spring, are not applicable as they are not twice differentiable. A new formulation of the mode shapes of the cracked beam is introduced here by applying the Rayleigh–Ritz method. The effect of location and depth of the crack on the vibratory response of the beam is modeled through formulating the loss of energy due to the presence of the crack, which is approximated here as a massless rotational spring. It is shown that the proposed crack modeling provides a close approximation of the mode shapes of the cracked beam. This modeling approach is beneficial as it provides twice-differentiable mode shapes for the cracked beam and can be used as trial functions in the assumed mode approximations. Numerical analysis is also provided for various design parameters, such as crack depth and position and the location of the PZT patch.

In a structural system with piezoelectric actuators, the damping effect can be achieved by properly switching the voltage on the actuators. Semi-active synchronized switch damping (SSD) approaches are typical switched-voltage control methods, which have recently been a topic of active research. In this study, the energy conversion of a SSD control system with an arbitrary switching frequency is investigated theoretically and validated numerically. First the general expression of the switched voltage on the piezoelectric actuator is derived. The results show that maximum voltage magnitude is obtained on the piezoelectric actuator when piezoelectric is switched at every odd number of displacement extrema. Next the average converted energy per vibration cycle is derived for an arbitrary switching frequency. The results show that the efficiency of energy conversion is reduced drastically even if the switching frequency is slightly deviated from the optimal frequency. Finally numerical simulation and experiments were carried out to validate the theoretical results.

This paper presents an electromechanical comparison of the transduction capabilities of a clamped-free Macro-Fiber Composite (MFC) actuated unimorph to other unimorphs employing monolithic piezoceramic devices. Four cantilevered unimorph beams, employing PSI PZT-5A, PSI PZT-5H, MIDE QP10N and MFC 8528-P1 type devices, are analyzed in the comparison. The unimorphs are evaluated for their actuation performance via tip displacement induced with harmonic voltage excitation, energy harvesting via electrical power output on a resistive shunt induced by harmonic base acceleration and linear feedback control effectiveness with the application of a Positive Position Feedback controller. The constitutive nonlinearities arising from large strain and electric field amplitudes are identified and briefly discussed. The MFC 8528-P1 type unimorph showed the highest mass-normalized actuation authority and the lowest energy harvesting capability.

The unique aerodynamic nature of the ducted-fan configuration makes it difficult to represent in terms of traditional nondimensional coefficients. Analysis of wind-tunnel data for an axisymmetric generic ducted-fan configuration has led to a new nondimensional modeling scheme. Force coefficients are plotted versus advance ratio for a range of angles of attack, yielding several observations. Each angle of attack yields a linear trend versus advance ratio, with all lines converging through a single point. This fulcrum point shares the same thrust coefficient value as hover tests but at a nonzero advance ratio. This suggests that a hovering ducted fan self-induces a freestream flow, even though the vehicle is stationary. The complicated pitching moment behavior is captured succinctly through modeling the center of pressuremovement. The fan power required was also successfully modeled using the figure of merit, typically a hover performance metric, over the entire flight regime to relate power required to thrust generated. A new wind-tunnel velocity correction for ducted fans is also presented. The new statistical modeling technique attains very high correlation for present and legacy data. It is possibly the most concise representation of ducted-fan aerodynamics to date, using 12 nondimensional coefficients.

DOI: 10.2514/1.C031542 This paper presents the investigation of a multifunctional energy harvesting and energy-storage wing spar for unmanned aerial vehicles. Multifunctional material systems combine several functionalities into a single device in order to increase performance while limiting mass and volume. Multifunctional energy harvesting can be used to provide power to remote low-power sensors on unmanned aerial vehicles, where the added weight or volume of conventional harvesting designs can hinder flight performance. In this paper, a prototype self-charging wing spar containing embedded piezoelectric and battery elements is modeled, fabricated, and tested to evaluate its energy harvesting and storage performance. A coupled electromechanical model based on the assumed modes method is developedtopredictthevibrationresponseandvoltageresponseofacantileveredwingsparexcitedunderharmonic base excitation. Experiments are performed on a representative self-charging wing spar, and the results are used to verify the electromechanical model. The power-generation performance of the self-charging wing spar is investigatedindetailforharmonicexcitationinclamped–freeboundaryconditions.Experimentsarealsoconducted to demonstrate the ability of the wing spar to simultaneously harvest and store electrical energy in a multifunctional manner.Itisshownthat,foraninputbaseaccelerationlevelof0:25 gat28.4Hzatthebaseofthestructure,1.5mW of regulated dc power isdelivered from the piezoelectric layers to the thin-film battery, resulting in a stored capacity of 0.362 mAh in 1 h.

Nonlinear piezoelectric effects in flexural energy harvesters have recently been demonstrated for drive amplitudes well within the scope of anticipated vibration environments for power generation. In addition to strong softening effects, steady-state oscillations are highly damped as well. Nonlinear fluid damping was previously employed to successfully model drive dependent decreases in frequency response due to the high-velocity oscillations, but this article instead harmonizes with a body of literature concerning weakly excited piezoelectric actuators by modeling nonlinear damping with nonconservative piezoelectric constitutive relations. Thus, material damping is presumed dominant over losses due to fluid-structure interactions. Cantilevers consisted of lead zirconate titanate (PZT)-5A and PZT-5H are studied, and the addition of successively larger proof masses is shown to precipitate nonlinear resonances at much lower base excitation thresholds while increasing the influence of higher-order nonlinearities. Parameter identification results using a multiple scales perturbation solution suggest that empirical trends are primarily due to higher-order elastic and dissipation nonlinearities and that modeling linear electromechanical coupling is sufficient. This article concludes with the guidelines for which utilization of a nonlinear model is preferred.

A unified approximation method is derived to illustrate the effect of electro-mechanical coupling on vibration-based energy harvesting systems caused by variations in damping ratio and excitation frequency of the mechanical subsystem. Vibrational energy harvesters are electro-mechanical systems that generate power from the ambient oscillations. Typically vibration-based energy harvesters employ a mechanical subsystem tuned to resonate with ambient oscillations. The piezoelectric or electromagnetic coupling mechanisms utilized in energy harvesters, transfers some energy from the mechanical subsystem and converts it to an electric energy. Recently the focus of energy harvesting community has shifted toward nonlinear energy harvesters that are less sensitive to the frequency of ambient vibrations. We consider the general class of hybrid energy harvesters that use both piezoelectric and electromagnetic energy harvesting mechanisms. Through using perturbation methods for low amplitude oscillations and numerical integration for large amplitude vibrations we establish a unified approximation method for linear, softly nonlinear, and bi-stable nonlinear energy harvesters. The method quantifies equivalent changes in damping and excitation frequency of the mechanical subsystem that resembles the backward coupling from energy harvesting. We investigate a novel nonlinear hybrid energy harvester as a case study of the proposed method. The approximation method is accurate, provides an intuitive explanation for backward coupling effects and in some cases reduces the computational efforts by an order of magnitude.

This paper summarize the authors previous effort on inverse eigenvalue problem and applies the theory to the model updating problem. Comments are made on how their procedure may be used to solve the damage detection problem.

Among structural health monitoring (SHM) techniques, Lamb waves is frequently used as diagnostic tools to detect damage in plate-like structures. Temperature variation can cause significant changes in guided-wave propagation and transduction for SHM. In this paper, controlled experiments examine changes in Lamb wave propagation and transduction using PZT-5A piezoelectric wafers under quasi-statically varying temperature (from 5°C to 60°C). The baseline selection method and baseline signal stretch method are used to compensate the temperature influence on Lamb wave propagation. The results of the experiments demonstrate the effectiveness of the temperature compensation approach and the simulated damage on the plate can be detected effectively under elevated temperatures environment.

This article presents a simple and versatile measurement system for structural health monitoring (SHM) based on the electromechanical impedance (EMI) technique. The proposed system allows real-time data acquisition from multiple sensors and includes compensation for temperature effects, eliminating one cause of incorrect diagnoses in structural monitoring. Besides the low cost compared to conventional impedance analyzers, the hardware and the software are simple and easier to implement than other measurement systems that have been recently proposed. Tests were carried out on an aluminum beam under different temperatures and on an aluminum plate with nine PZT (lead zirconate titanate) patches. The experimental results show conclusively that the proposed methodology is efficient and feasible for real-time SHM.

Bolted joints are a form of mechanical coupling largely used in machinery due to their reliability and low cost. Failure of bolted joints can lead to catastrophic events, such as leaking, train derailments, aircraft crashes, etc. Most of these failures occur due to the reduction of the pre-load, induced by mechanical vibration or human errors in the assembly or maintenance process. This article investigates the application of shape memory alloy (SMA) washers as an actuator to increase the pre-load on loosened bolted joints. The application of SMA washer follows a structural health monitoring procedure to identify a damage (reduction in pre-load) occurrence. In this article, a thermo-mechanical model is presented to predict the final pre-load achieved using this kind of actuator, based on the heat input and SMA washer dimension. This model extends and improves on the previous model of Ghorashi and Inman [2004, “Shape Memory Alloy in Tension and Compression and its Application as Clamping Force Actuator in a Bolted Joint: Part 2 – Modeling,” J. Intell. Mater. Syst. Struct., 15:589–600], by eliminating the pre-load term related to nut turning making the system more practical. This complete model is a powerful but complex tool to be used by designers. A novel modeling approach for self-healing bolted joints based on curve fitting of experimental data is presented. The article concludes with an experimental application that leads to a change in joint assembly to increase the system reliability, by removing the ceramic washer component. Further research topics are also suggested.

A new approach to analyze the response from a piezoelectric wafer in an impedance-based structural health monitoring (SHM) method is proposed. It is shown that the time-domain response of a piezoceramic wafer provides information on the electromechanical impedance (EMI) variation when a monitored structure is damaged. Practical analysis was carried out using wavelet transform in two different levels. This approach simplifies EMI based SHM and the results show that it is more sensitive to damage than methods based on impedance measurements in the frequency domain. The efficiency of this new approach is demonstrated through experiments using an aluminum plate. The piezoelectric wafer was excited using a chirp signal and its response was analyzed using both frequency response functions (FRF) and the proposed method. The results confirm that this new approach is more sensitive to detect damage than FRF based methods.

A theoretical and experimental analysis regarding the correct size of PZT (Lead Zirconate Titanate) transducers used in structural health monitoring (SHM) systems based on the electromechanical impedance (EMI) technique is presented. Tests were carried out on aluminum structures using 5A and 5H PZT patches of various sizes. The results show that the sensitivity of SHM systems for damage detection can be optimized if the transducers are correctly sized according to the proposed procedure.

The research study presented here examines four conventional vibration suppression control laws and four hybrid modifications of these laws using a switching method. The motivation is to determine which of these eight controllers results in the least amount of power flow to the actuator to have the same settling time under free vibrations. The reason to look at reduced energy controllers is the idea that in some applications, very little energy is available for control, yet passive and semi-active methods cannot meet performance demands. In particular, the eventual goal is to reduce transient vibrations of smart structures using energy obtained from harvesting and/or low-power storage devices (batteries or super capacitors), as often desirable in aerospace systems. The four conventional active control systems compared in this study are Positive Position Feedback (PPF) control, Proportional Integral Derivative (PID) control, non-linear control, and Linear Quadratic Regulator (LQR) controls. A hybrid version of each controller is obtained by implementing a bang-bang control law (on-off control). The bang-bang control algorithm switches the control voltage between an external voltage supply and the feedback signal provided by the PPF, PID, non-linear, or LQR controllers. The purpose of combining the bang-bang control law with the aforementioned controllers is to reduce the power requirement for vibration suppression by providing an active controller with limited voltage input. Free vibrations of a thin cantilevered beam with a piezoceramic transducer are controlled by these eight controllers with a focus on the fundamental transverse vibration mode. Experimental results exhibit that the system with hybrid bang-bang-non-linear controller requires 67.3% less power than its conventional version. The hybrid versions require significantly less power flow compared to their conventional counterparts for the PPF, PID, and LQR controllers as well. Experiments also reveal the presence of substantial piezoelectric non-linearities in the transducer. The voltage-dependent behavior of the electromechanical coupling coefficient is identified empirically and represented by a curve-fit expression. A real-time adaptive control algorithm is developed to account for the voltage-dependent behavior of the coupling coefficient, enabling good agreement between the simulation and experimental results.

An analytical electromechanical model is proposed to predict the deflection, voltage, and the power output of a proposed low-frequency micro-harvesting structure. The high natural frequencies of the existing designs of micro-scale vibrational energy harvesters are serious drawbacks. A zigzag design is proposed to overcome this limitation. First, the natural frequencies and the mode shapes of the zigzag structure are calculated. The piezoelectric direct and reverse effect equations, together with the electrical equations, are used to relate the voltage output of the structure to the base vibrations magnitude and frequency. The closed-form solution of the continuous electromechanical vibrations gives the power output as a function of base acceleration spectrum. The usefulness of the design is proved by the significant increase of the power output from the same base accelerations, providing a method of designing a micro-scale harvester with low natural frequency. The optimal mechanical and electrical conditions for power generation are investigated through the case studies.

In this work, the pyroelectric coefficient for macro fiber composites (MFCs) was characterized by first modeling the secondary pyroelectric coefficient using material properties of MFCs, and then experimentally computing the total pyroelectric coefficient using two separate thermal chambers. The computed total secondary pyroelectric coefficient was 29.88 μC m−2 K−1. The primary pyroelectric coefficient will be higher due to the primary effect having a more dominate influence in ferroelectric materials. The sum of the primary and secondary effects is the total pyroelectric coefficient. The MFC was tested in thermal chambers at NASA Langley and at University of Michigan where the temperature was ramped to 70 °C, as well as thermally cycled between 40 °C and 100 °C. The P2 MFC type yielded a higher pyroelectric coefficient than the P1 type at about −343 versus −82 μC m−2 K−1. The higher the pyroelectric coefficient is, the more energy it produces for given temperature fluctuations with time. This leads to future work in energy harvesting using the pyroelectric effect and further advancing potential novel applications using MFCs.

The electromechanical impedance (EMI) technique has been successfully used in structural health monitoring (SHM) systems on a wide variety of structures. The basic concept of this technique is to monitor the structural integrity by exciting and sensing a piezoelectric transducer, usually a lead zirconate titanate (PZT) wafer bonded to the structure to be monitored and excited in a suitable frequency range. Owing to the piezoelectric effect, there is a relationship between the mechanical impedance of the host structure, which is directly related to its integrity, and the electrical impedance of the PZT transducer, which is obtained through the ratio between the excitation and sensing signals. This work investigates the influence of the type of excitation signal and voltage level on impedance-based monitoring systems. To illustrate our conjecture, tests have been carried out on an aluminum specimen with different health conditions and the results show conclusively that the excitation signal has influence on system performance and power dissipation in the PZT transducer.

The purpose here is to enable peak-to-peak actuation of piezocomposite devices to perform shape control of structures, specifically bimorph variable-camber control surfaces on small aircraft. One of the major drawbacks in implementing control with piezocomposite actuation is its asymmetry in the positive and negative voltage directions. To remedy this situation, a novel, solid-state electrical circuit employing diodes and resistors is proposed. The circuit allows a single bipolar amplifier to control a bimorph composed of two piezoceramic composite actuators known as Macro-Fiber Composite (MFC). The MFCs have an asymmetric actuation range of -500 to 1500 V; hence an MFC bimorph with conventional serial or parallel electrical connection limits the actuation to the -500 to 500 V range. The proposed circuit allows the division of the input voltage so that: (1) The MFC that is in extension receives 1500 V, and (2) The MFC that is in compression receives -500 V. The circuit also allows the actuation in the opposite direction without any physical changes (i.e., not using any electromechanical switches) The input-output relationship of the circuit is characterized experimentally. An MFC bimorph is used to further demonstrate the capability of the circuit.

The research presented here employs solid-state actuators for flow separation delay or for forced attachment of separated flow seen in airfoils at low Reynolds numbers. To reduce separation, periodic excitation to the flow around the leading edge of the airfoil is induced by Macro-Fiber Composite actuated clamped-free unimorph benders. An electromechanical model of the unimorph is briefly presented and parametric study is conducted to aid the design of a unimorph to output high deformation at a desired frequency. The optimum frequency and amplitude for lift improvement at post-stall angles are identified experimentally. Along with aerodynamic force and structural displacement measurements, helium bubble flow visualization is used to verify existing separated flow, and the attached flow induced by flow control. The lift enhancement induced by several flow control techniques is compared. A symmetric and non-uniform (3D) flow excitation results in the maximum lift enhancement at post-stall region at the lowest power consumption level. A maximum lift coefficient increase of 27.5% (in the post-stall region) is achieved at 125 Hz periodic excitation, with the 3D symmetric actuation mode at 5 m/s and the reduced frequency of 3.78. Cl, max is increased 7.6% from the baseline.

Structural Health Monitoring (SHM) is the science and technology of monitoring and assessing the condition of aerospace, civil and mechanical infrastructures using a sensing system integrated into the structure. Impedance-based SHM measures impedance of a structure using a PZT (Lead Zirconate Titanate) patch. This paper presents a low-power wireless autonomous and active SHM node called Autonomous SHM Sensor 2 (ASN-2), which is based on the impedance method. In this study, we incorporated three methods to save power. First, entire data processing is performed on-board, which minimizes radio transmission time. Considering that the radio of a wireless sensor node consumes the highest power among all modules, reduction of the transmission time saves substantial power. Second, a rectangular pulse train is used to excite a PZT patch instead of a sinusoidal wave. This eliminates a digital-to-analog converter and reduces the memory space. Third, ASN-2 senses the phase of the response signal instead of the magnitude. Sensing the phase of the signal eliminates an analog-to-digital converter and Fast Fourier Transform operation, which not only saves power, but also enables us to use a low-end low-power processor. Our SHM sensor node ASN-2 is implemented using a TI MSP430 microcontroller evaluation board. A cluster of ASN-2 nodes forms a wireless network. Each node wakes up at a predetermined interval, such as once in four hours, performs an SHM operation, reports the result to the central node wirelessly, and returns to sleep. The power consumption of our ASN-2 is 0.15 mW during the inactive mode and 18 mW during the active mode. Each SHM operation takes about 13 seconds to consume 236 mJ. When our ASN-2 operates once in every four hours, it is estimated to run for about 2.5 years with two AAA-size batteries ignoring the internal battery leakage.

The future in terms of autonomous devices is about to experience an important breakthrough because of the development of energy-harvesting systems. This paper presents a new vibration energy-harvesting electrical interface for optimizing the power extracted from piezoelectric elements. Based on piezoelectric energy-harvesting properties, the proposed circuit allows an optimization of the extracted energy independently from the structure's parameters and without any tuning required. Contrary to classical converter topologies (for instance, step-down or buck-boost), the proposed architecture does not require large inductors, allowing a high-integration potential, while requiring a very little power consumption.

Previously, a sensor diagnostics method has been developed for the impedance-based structural health monitoring technique. Impedance techniques utilize piezoelectric patches bonded to the structure of interest for inference of damage. Measuring the slope of piezoelectric susceptance allows unhealthy sensor to be identified. While this sensor diagnostics technique is very useful in detecting damaged sensors bonded to a structure, the method is also susceptible to temperature variations. The object of this study is to accurately provide sensor diagnostics at any temperature. The model developed should be accurate and easy to implement on health monitoring hardware. A frame structure is fabricated to simulate a real structure with complex boundary conditions for experimental testing in various thermal environments. A model predicting piezoelectric susceptance slope at any temperature is generated and validated on the frame structure in an extended temperature range.

The recent proliferation of microscale devices has raised the issue of energy harvesting for replacing batteries that present maintenance and recycling problems. Particularly, piezoelectric seismic microgenerators offer the advantages of easy maintenance and high power output, but are very sensitive to frequency drifts that can dramatically decrease their performance. The purpose of the present article is to expose a technique to ensure that the harvester resonance frequency matches the base motion frequency, without any external intervention. The principles of the proposed method rely on ultralow-cost frequency sensing combined with an energy-efficient stiffness tuning, through the use of an additional actuator. Experimental results carried out to validate the model show that such an approach permits increasing the effective bandwidth of the structure by a factor of 4 in terms of mechanical vibrations and having a 100% frequency band gain in terms of total power output of the device (i.e., taking into account the energy spent by the actuation). The total energy produced by the harvesting device, taking into account the actuation cost, is discussed as well.

The modified positive position feedback (MPPF) controller, an active vibration control method that uses collocated piezoelectric actuator actuators and sensors, is developed using an adaptive controller. The adaptive mechanism consists of two main parts: (1) frequency adaptation and (2) adaptive controller. Frequency adaptation only tracks the frequency of vibrations using fast Fourier transforms. The obtained frequency is then fed to MPPF compensators and the adaptive controller. This provides a unique feature for MPPF by extending its domain of capabilities from controlling tonal vibrations to broadband disturbances. The adaptive controller mechanism consists of a reference model that is of the same order as the MPPF system and its compensators. The adaptive law provides the additional control force that is needed for controlling frequency changes caused by broadband vibrations. The experimental results show that the frequency adaptation method that is derived has worked quite well. The results also indicate that the MPPF can provide significant vibration reduction on a cantilever beam that is used throughout the experiments.

A two-stage power conditioning circuit consisting of an AC-DC converter followed by a DC-DC converter is proposed for a vibration-based energy harvesting system. The power conditioning circuit intends to maximize the amount of power extracted from a piezoelectric energy harvester by matching the source impedance with the circuit by adaptively adjusting the duty cycle. An equivalent electrical circuit representation derived from a distributed-parameter piezoelectric energy harvester model is adapted to enable the impedance matching method proposed here. For a given piezoelectric energy harvester, there is a theoretical maximum power output that is determined by the mechanical damping, base acceleration, and the effective mass of the harvester structure under base excitation. Experimental results are given to validate the effectiveness of the proposed resistive impedance matching circuit around the first resonance frequency of a cantilevered piezoelectric energy harvester.

Advances in low-power electronics and in energy harvesting technologies have enabled the conception of truly self-powered devices. Nevertheless, most studies rely on steady-state assumptions, which are rarely available in real environments. This article exposes, as a first step, a study of the transient stage of vibration energy harvesting systems initially at rest both mechanically and electrically (null initial displacement and voltages), when the harvesting device is submitted to a time-limited sine force at its resonance frequency. A particular focus will be placed on the total energy harvested as well as on the time of harvesting, allowing a better understanding of the dynamics of harvesting processes for systems subjected to time-limited excitations.

This study aims to enable solid-state aerodynamic force generation in high-dynamic-pressure airflow. A novel, high-load-output, bidirectional variable-camber airfoil employing a type of piezoceramic composite actuator known as a Macro-Fiber Composite is presented. The novel airfoil employs two active surfaces and a single four-bar (box) mechanism as the internal structure. The unique choice of boundary conditions allows variable and smooth deformation in both directions from a flat camber line. The paper focuses on actuation modeling and response characterization under aerodynamic loads. A parametric study of aerodynamic response is employed to optimize the kinematic parameters of the airfoil. The concept is fabricated by implementing eight Macro-Fiber Composite 8557-P1-type actuators in a bimorph configuration to construct the active surfaces. The box mechanism generates deflection and camber change as predicted. Wind-tunnel experiments are conducted on a 12.6% maximum thickness, 127 mm chord airfoil. Aerodynamic and structural performance results are presented for a flow rate of 15 m /s and a Reynolds number of 127,000. Nonlinear effects due to aerodynamic and piezoceramic hysteresis are identified and discussed. A lift coefficient change of 1.54 is observed purely due to voltage actuation. Results are compared with conventional, zero-camber NACA and other airfoils. A 72% increase in the lift-curve slope is achieved when compared with a NACA 0009 airfoil.

A novel method of guided wave-based structural health monitoring is developed in which no direct baseline data are required to identify structural damage. Conventional wave propagation structural health monitoring techniques involve the comparison of structural response data to a prerecorded baseline or reference measurement taken while the structure is in pristine condition. The need to compare new data to a prerecorded baseline can present several complications, including data management issues and difficulty in accommodating the effects of varying environmental and operational conditions on the data. To address the complications associated with baseline comparison, this new method accomplishes reference-free damage detection by acquiring what is referred to as an instantaneous baseline measurement for analysis. The instantaneous baseline technique is validated through both analytical and experimental testing. Analytical tests show that the instantaneous baseline method is able to correctly identify simulated damage. It is found experimentally that nonpermanent damage in the form of removable putty as well as permanent damage in the form of corrosion and cuts are all identifiable in thin aluminum plate test structures without direct comparison to baseline data when implementing the instantaneous baseline method.

The paper presents the modified Quadratic Compression Method (QCM) for both mass and stiffness model updating. The modeling error is defined in a parametric setup, i.e. with pre-specified principal submatrices multiplied by unknown scalar parameters. The optimal parameters are obtained by minimizing the error in a squared down version of the eigenvalue equation, and of the mass orthogonality condition, thus with reduced computation yet with no loss of information. The method has closed ties with Minimization of the Error in the Constitutive Equation (MECE), and in some cases is shown to belong to that class with a particular choice of the weighting matrix. Theoretical analysis of the propagation of the noise into the identified parameters, as well as extensive simulations, reveal that QCM has in some cases desirable noise filtering properties.

The traveling wave ultrasonic motor is considered for use in haptic devices where feedback forces are required, particularly in force-feel systems where a certain input— output relation is desired between the applied torque and the response. Owing to some of its unique characteristics, there are circumstances where the ultrasonic motor may be considered an appealing alternative to the more standard DC motor. However, the two types of motors are significantly different in their principles of operation, and are not necessarily interchangeable for all applications. The ultrasonic motor is limited in that the torque cannot be arbitrarily controlled under external loading. Moreover, direct control of the motor torque is difficult due to the complex nature of the contact mechanics. To accommodate these limitations, we investigated a method of model reference force-feedback control, in which the interaction torque is used as the reference source. Experimental results demonstrated that the closed-loop system is able to approximate simple second-order behavior, thus approximating the feel of a spring and damper.

The purpose of the research presented here is to exploit actuation via smart materials to perform shape control of an aerofoil on a small aircraft and to determine the feasibility and advantages of smooth control surface deformations. A type of piezoceramic composite actuator known as Macro-Fiber Composite (MFC) is used for changing the camber of the wings. The MFC actuators were implemented on a 30° swept wing, 0·76m wingspan aircraft. The experimental vehicle was flown using two MFC patches in an elevator/aileron (elevon) configuration. Preliminary flight and wind-tunnel testing has demonstrated the stability and control of the concept. Flight tests were performed to quantify roll control using the MFC actuators. Lift and drag coefficients along with pitch and roll moment coefficients were measured in a low-speed, open-section wind tunnel. A vortex-lattice analysis complemented the database of aerodynamic derivatives used to analyse control response. The research, for the first time, successfully demonstrated that piezoceramic devices requiring high voltages can be effectively employed in small air vehicles without compromising the weight of the overall system.

The authors' vision of the way forward in multifunctional materials and structures is presented. Their opinion reflects the growth of research activities in the subject area evolved from those of smart materials and adaptive structures. Their ultimate goal is to create the knowledge to enable a new concept of multifunctional materials and structures for autonomic systems, which captures a deeper level of integration of functions between control, transduction, and structure.