Table of contents

A review of the vibration energy harvesting literature has been undertaken with the goal of establishing scaling laws for experimentally demonstrated harvesting devices based on electromagnetic transduction. Power density metrics are examined with respect to scaling length, mass, frequency and drive acceleration. Continuous improvements in demonstrated power density of harvesting devices over the past decade are noted. Scaling laws are developed from observations that appear to suggest an upper limit to the power density achievable with current harvesting techniques.

A theoretical model is proposed to study the ME effect in the layered ME composite with nanoscale thickness, which taking into account the surface effect. The layered ME composites nano structure is treated as a bulk core plus two surface layers with zero thickness. The influence on the structure overall properties resulted from the surface effect is modeled by a spring force exerting on the boundary of the bulk core. Using the derived equations, the so-called effective Miller-Shenoy coefficient, static and electromechanical resonance (EMR) properties of the nanoscale thickness ME composite for the extensional-bending coupling deformations are analyzed theoretically. At the same time, the effect of the substrate on ME effect is theoretically studied by altering the thickness ratio of the substrate. Numerical results shows the effective properties and the static and EMR properties of the composites are size-dependent, and surface effect have non-ignored effects on the ME effect. Besides, the EMR frequency in nano-thickness composites are expected to occur at very low frequencies compared to nominal dimensional composites. The EMR frequency shows an increase with increasing substrate thickness, and predicts a maximum in the EMR ME coefficient at small but nonvanishing substrate thickness.

Mirror-scanning mechanisms are a key component in optical systems for diverse applications. However, the applications of existing piezoelectric scanners are limited due to their small angular travels. To overcome this problem, a novel two-axis mirror-scanning mechanism, which consists of a two-axis tip-tilt flexure mechanism and a set of piezoelectric actuators, is proposed in this paper. The focus of this research is on the design, theoretical modeling, and optimization of the piezoelectric-driven mechanism, with the goal of achieving large angular travels in a compact size. The design of the two-axis tip-tilt flexure mechanism is based on two nonuniform beams, which translate the limited linear output displacements of the piezoelectric actuators into large output angles. To exactly predict the angular travels, we built a voltage-angle model that characterizes the relationship between the input voltages to the piezoelectric actuators and the output angles of the piezoelectric-driven mechanism. Using this analytical model, the optimization is performed to improve the angular travels. A prototype of the mirror-scanning mechanism is fabricated based on the optimization results, and experiments are implemented to test the two-axis output angles. The experimental result shows that the angular travels of the scanner achieve more than 50 mrad, and the error between the analytical model and the experiment is about 11%. This error is much smaller than the error for the model built using the previous method because the influence of the stiffness of the mechanical structure on the deformation of the piezoelectric stack is considered in the voltage-angle model.

Here, a strip-shaped 2-2 cement/polymer-based piezoelectric composite was designed and fabricated. The dielectric, piezoelectric and electromechanical coupling properties of the composite were investigated as well as the coupling effects between the thickness and lateral modes of the piezoelectric composites. The dielectric and piezoelectric properties of the composites can be greatly influenced by variations of the piezoelectric ceramic volume fraction and the structural dimensions of the composites. Excellent properties have been achieved for ultrasonic transducer applications in civil engineering monitoring fields, such as large piezoelectric voltage constants, high thickness electromechanical coupling coefficients and low acoustic impedance. The damping property of the composites was especially studied. The maximum damping loss factor of the composites is between 0.28–0.32, and the glass transition temperature is between 55°–66 °C.

Stabilized dissipated energy is an effective parameter on the fatigue life of shape memory alloys (SMAs). In this study, a formula is proposed to directly evaluate the stabilized dissipated energy for different values of the maximum and minimum applied stresses, as well as the loading frequency, under cyclic tensile loadings. To this aim, a one-dimensional fully coupled thermomechanical constitutive model and a cycle-dependent phase diagram are employed to predict the uniaxial stress-strain response of an SMA in a specified cycle, including the stabilized one, with no need of obtaining the responses of the previous cycles. An enhanced phase diagram in which different slopes are defined for the start and finish of a backward transformation strip is also proposed to enable the capture of gradual transformations in a CuAlBe shape memory alloy. It is shown that the present approach is capable of reproducing the experimental responses of CuAlBe specimens under cyclic tensile loadings. An explicit formula is further presented to predict the fatigue life of CuAlBe as a function of the maximum and minimum applied stresses as well as the loading frequency. Fatigue tests are also carried out, and this formula is verified against the empirically predicted number of cycles for failure.

Topological design of miniaturized resonating structures capable of harvesting electrical energy from low frequency environmental mechanical vibrations encounters a particular physical challenge, due to the conflicting design requirements: low resonating frequency and miniaturization. In this paper structural static stiffness to resist undesired lateral deformation is included into the objective function, to prevent the structure from degenerating and forcing the solution to be manufacturable. The rational approximation of material properties interpolation scheme is introduced to deal with the problems of local vibration and instability of the low density area induced by the design dependent body forces. Both density and level set based topology optimization (TO) methods are investigated in their parameterization, sensitivity analysis, and applicability for low frequency energy harvester TO problems. Continuum based variation formulations for sensitivity analysis and the material derivative based shape sensitivity analysis are presented for the density method and the level set method, respectively; and their similarities and differences are highlighted. An external damper is introduced to simulate the energy output of the resonator due to electrical damping and the Rayleigh proportional damping is used for mechanical damping. Optimization results for different scenarios are tested to illustrate the influences of dynamic and static loads. To demonstrate manufacturability, the designs are built to scale using a 3D microfabrication method and assembled into vibration energy harvester prototypes. The fabricated devices based on the optimal results from using different TO techniques are tested and compared with the simulation results. The structures obtained by the level set based TO method require less post-processing before fabrication and the structures obtained by the density based TO method have resonating frequency as low as 100 Hz. The electrical voltage response in the experiment matches the trend of the simulation data.

The results of a study on the suitability of materials derived from cobalt ferrite for sensor and actuator applications are presented. The mechanism responsible for the superior sensor properties of Ge-substituted cobalt ferrite compared with Ti and other cation substituted cobalt ferrite materials is believed to be due to the tetrahedral site preference of Ge⁴⁺ and its co-substitution with Co²⁺. Results also showed that the higher strain derivative of Ge-substituted cobalt ferrite compared with Ti-substitution is due to a higher magnetostrictive coupling in response to applied field in the material.

Polymer composites based on permanent magnetic bonded powders exhibit immense potential for applications in microactuators and sensors with magnetic performances comparable to their fully dense counterparts. While fabrication and integration of magnetic devices based on bonded magnetic powders is challenging via conventional deposition and electrochemical growth techniques, hybrid fabrication offers a promising alternative. This paper presents the evolution of permanent magnetic materials into bonded magnetic powders, the magnetic performance figures of merit of permanent magnetic materials significant for the design and manufacture of polymer based sensors and actuators. A review of the hybrid fabrication techniques such as replica molding, squeegee coating, spin casting etc are reported. Critical factors affecting the fabrication of polymer magnetic composites such as filler particle size and effect of magnetic field during fabrication are discussed. Prior art based on polymer magnetic composites for the fabrication of hard magnetic films and hard magnetic actuators are presented.

This work proposes a new damper featuring magnetorheological fluid (MR damper) and presents its field-dependent damping forces due to high impact. To achieve this goal, a large MR damper, which can produce a damping force of 100 kN at 6 A, is designed and manufactured based on the analysis of the magnetic flux intensity of the damper. After identifying the field-dependent damping force levels of the manufactured MR damper, a hydraulic horizontal shock tester is established. This shock testing system consists of a velocity generator, impact mass, shock programmer, and test mass. The MR damper is installed at the end of the wall in the shock tester and tested under four different experimental conditions. The shock profile characteristics of the MR damper due to different impact velocities are investigated at various input current levels. In addition, the inner pressure of the MR damper during impact, which depends on the input's current level, is evaluated at two positions that can represent the pressure drop that generates the damping force of the MR damper. It is demonstrated from this impact testing that the shock profiles can be changed by the magnitude of the input current applied to the MR damper. It directly indicates that a desired shock profile can be achieved by installing the MR damper associated with appropriate control logics to adjust the magnitude of the input current.

In this work, a new high-load magnetorheological (MR) fluid mount system is devised and applied to control vibration in a ship engine. In the investigation of vibration-control performance, a new modified indirect fuzzy sliding mode controller is formulated and realized. The design of the proposed MR mount is based on the flow mode of MR fluid, and it includes two separated coils for generating a magnetic field. An optimization process is carried out to achieve maximal damping force under certain design constraints, such as the allowable height of the mount. As an actuating smart fluid, a new plate-like iron-particle-based MR fluid is used, instead of the conventional spherical iron-particle-based MR fluid. After evaluating the field-dependent yield stress of the MR fluid, the field-dependent damping force required to control unwanted vibration in the ship engine is determined. Subsequently, an appropriate-sized MR mount is manufactured and its damping characteristics are evaluated. After confirming the sufficient damping force level of the manufactured MR mount, a medium-sized ship engine mount system consisting of eight MR mounts is established, and its dynamic governing equations are derived. A new modified indirect fuzzy sliding mode controller is then formulated and applied to the engine mount system. The displacement and velocity responses show that the unwanted vibrations of the ship engine system can be effectively controlled in both the axial and radial directions by applying the proposed control methodology.

Intelligent tires equipped with sensors as well as the monitoring of the tire/road contact conditions are in demand for improving vehicle control and safety. With the aim of identifying the coefficient of friction of tire/road contact surfaces during driving, including during cornering, we develop an identification scheme for the coefficient of friction that involves estimation of the slip angle and applied force by using a single lightweight three-axis accelerometer attached on the inner surface of the tire. To validate the developed scheme, we conduct tire-rolling tests using an accelerometer-equipped tire with various slip angles on various types of road surfaces, including dry and wet surfaces. The results of these tests confirm that the estimated slip angle and applied force are reasonable. Furthermore, the identified coefficient of friction by the developed scheme agreed with that measured by standardized tests.

A multi-branch thermoviscoelastic-themoviscoplastic finite deformation constitutive model incorporated with structural and stress relaxation is developed for a thermally activated shape memory polymer (SMP) based syntactic foam. In this paper, the total mechanical deformation of the foam is divided into the components of the SMP and the elastic glass microballoons by using the mixture rule. The nonlinear Adam-Gibbs model is used to describe the structural relaxation of the SMP as the temperature crosses the glass transition temperature (T_g). Further, a multi-branch model combined with the modified Eying model of viscous flow is used to capture the multitude of relaxation processes of the SMP. The deformation of the glass microballoons could be split into elastic and inelastic components. In addition, the phenomenological evolution rule is implemented in order to further characterize the macroscopic post-yield strain softening behaviors of the syntactic foam. A comparison between the numerical simulation and the thermomechanical experiment shows an acceptable agreement. Moreover, a parametric study is conducted to examine the predictability of the model and to provide guidance for reasonable design of the syntactic foam.

In this paper, we demonstrate flexible fiber-based Al–NaOCl galvanic cells fabricated using fiber drawing process. Aluminum and copper wires are used as electrodes, and they are introduced into the fiber structure during drawing of the low-density polyethylene microstructured jacket. NaOCl solution is used as electrolyte, and it is introduced into the battery after the drawing process. The capacity of a 1 m long fiber battery is measured to be ∼10 mAh. We also detail assembly and optimization of the electrical circuitry in the energy-storing fiber battery textiles. Several examples of their applications are presented including lighting up an LED, driving a wireless mouse and actuating a screen with an integrated shape-memory nitinol wire. The principal advantages of the presented fiber batteries include: ease of fabrication, high flexibility, simple electrochemistry and use of widely available materials in the battery design.

Auxetics are materials showing a negative Poisson's ratio. This characteristic leads to unusual mechanical properties that make this an interesting class of materials. So far no systematic approach for generating auxetic cellular materials has been reported. In this contribution, we present a systematic approach to identifying auxetic cellular materials based on eigenmode analysis. The fundamental mechanism generating auxetic behavior is identified as rotation. With this knowledge, a variety of complex two-dimensional (2D) and three-dimensional (3D) auxetic structures based on simple unit cells can be identified.

Magnetoelectric (ME) coupling for PZT/Ni composites with PZT embedded in Ni is investigated under combined magnetic and compressive loadings. As the applied compressive stress increases, the ME coefficient (α_ME) rises monotonically. Incorporating the size effect of the composites, we present a linear relationship between the peak value of α_ME and the applied compressive stress. The phenomenon of the four-state ME coupling remains under the applied compressive stresses as that without the stress, namely, the polarization direction of PZT reverses when the applied static magnetic field (H_S) exceeds a critical value. Finite-element simulations are carried out to quantify the effect of compressive stresses on the ME coupling for the embedded composites. The calculation agrees well with the experimental data. It indicates that the linear enhancement of α_ME depends on the increase of the piezomagnetic coefficient d_11,m under the combined magnetic and compressive loadings. The strengthened ME effect via planar compressive stress is promising for applications such as four-state memory devices, magnetic field sensors, transformers, etc.

This paper presents a new design for a single active finger ionic polymer metal composite (IPMC) microgripper. This design has one stationary finger and one actuating finger. The gripper is tested in comparison with a two fingered gripper (2FG) on its ability to perform pick and place operations. The grippers each use IPMC strips in three widths: 1.25 mm, 2.5 mm and 5.0 mm. The single fingered gripper shows success rates of 86.2%, 89.2%, and 75% respectively versus 78.5%, 93.9% and 75% for a 2FG. The single fingered gripper performance is nearly equivalent to that of a 2FG. Even though a single finger produces half the force, its ability to carry objects is as good as or better than a 2FG. In addition, the stationary finger is considerably stiffer than an active IPMC finger, which helps in positional accuracy. Using half the IPMC, the single fingered gripper is the economical choice.

We study the magneto-rheological response of hybrid-magnetic elastomer composites consisting of two different magnetic filler particles at fixed overall concentration. Thereby, we focus on an optimization of mechanical and magnetic properties by combining highly reinforcing magnetic nano-particles (MagSilica) with micro-sized carbonyl-iron particles (CIP), which exhibit high switch ability in a magnetic field. We observe a symbiotic interaction of both filler types, especially in the case when an orientation of the magnetic filler particles is achieved due to curing in an external magnetic field. The orientation effect is significant only for the micro-sized CIP particles with high saturation magnetization, indicating that the induced magnetic moment for the nano-sized particles is too small for delivering sufficient attraction between the particles in an external magnetic field. A pronounced switching behavior is observed for the non-cross-linked melts with 15 and 20 vol.% CIP, whereby the small strain modulus increases by more than 50%. For the sample without the coupling agent silane, one even observes a relative modulus increase of about 140%, which can be related to the combined effect of a higher mobility of the particles without a silane layer and the ability of the particles to come in close contact when they are arranged in strings along the field lines. For the cross-linked samples, a maximum switching effect of about 30% is achieved for the system with pure CIP. This magneto-sensitivity decreases successively if CIP is replaced by MagSilica, while the tensile strength of the systems increases significantly. The use of silane reduces the switching effect, but it is necessary for a good mechanical performance by delivering strong chemical bonding of the magnetic filler particles to the polymer matrix.

This paper proposes a novel technique for predicting a repulsive force in a haptic interface. The goal of many haptic systems is to reflect a desired repulsive force to an operator. However, there is no way of describing a repulsive force mathematically. This means that the force-reflection performance of haptic systems cannot be simulated at the design process. Even though reflecting a repulsive force to an operator is the purpose of the systems, many haptic systems have been designed without knowing how an operator perceives a repulsive force during manipulation. Such design process unavoidably entails a lot of trials and errors and increases development time and costs. Here we show that the repulsive force can be predicted by establishing an optimal controller. 1-Degree of freedom lever system is designed with light and heavy inertia in order to reflect inertial variation. The dynamics of the system is derived and an optimal controller is established based on the system dynamics. The optimal controller predicts the repulsive forces under three different position trajectories. After manufacturing the lever system, actual repulsive force data is collected under the same position trajectories. The predicted repulsive forces are then compared with the actual repulsive forces. To demonstrate the effectiveness of the proposed method, a correlation coefficient between the predicted repulsive force and the actual one is presented. In addition, the mean value and standard deviation of the force error are provided. After showing that a repulsive force can be predicted by an optimal controller, a steering-wheel simulator is designed and manufactured to show that the proposed method is applicable to a haptic system's design as well.

The magnetoelastic sensor is a wireless, passive sensor platform typically comprised of a strip of magnetoelastic material that exhibits a mechanical vibration when under the excitation of a magnetic ac field. At the resonant frequency, the vibration of the sensor is most prominent, generating a significant secondary magnetic field that can be detected with a remotely located coil. Biological and chemical sensing can be realized by functionalizing a mass- or elasticity-changing coating on the magnetoelastic sensor, causing a shift in the resonant frequency when exposed to the target analyte. To date, most magnetoelastic sensors are rectangular and are designed to sense a uniform coating over the entire sensor surface. This paper presents a new magnetoelastic sensor design with higher sensitivity, achieved by applying non-uniform coatings and altering the sensor to a triangular shape. In addition, the new design allows the magnetoelastic sensor to form a sensor array that requires only a fraction of sample volume for multi-parameter sensing compared to the current sensor design.

An analytical model predicting the in-plane Young's and shear moduli of a shape memory polymer filled honeycomb composite is presented. By modeling the composite as a series of rigidly attached beams, the mechanical advantage of the load distributed on each beam by the infill is accounted for. The model is compared to currently available analytical models as well as experimental data. The model correlates extremely well with experimental data for empty honeycomb and when the polymer is above its glass transition temperature. Below the glass transition temperature, rule of mixtures is shown to be more accurate as bending is no longer the dominant mode of deformation. The model is also derived for directions other than the typical x and y allowing interpolation of the stiffness of the composite in any direction.

Free-standing electroded piezoelectric thick-films are straightforwardly fabricated thanks to the association of the low-cost screen-printing technology to the sacrificial layer method. After subsequent printing and drying of a stack of sacrificial, Au, PZT and Au layers on an alumina substrate, the final firing is performed at 900 °C. Then, the partial or total releasing step of the Au/PZT/Au is achieved in a diluted acidic solution. Bridges (3.3 × 3.3 × 0.080 mm³) and cantilevers (8 × 2 × 0.09 mm³) are directly attached to the alumina substrate on top of which they are processed. Studies of the electromechanical behavior of these components show the influence of both the releasing and the densification processes on the piezoelectric properties of the final component. Cantilevers fabricated with this method exhibit favourable properties for sensing applications.

We study the effect of semiconduction on mechanical-to-electrical energy conversion through a theoretical analysis on the thickness-extensional vibration of a piezoelectric semiconductor plate driven mechanically. An analytical solution is obtained. A ZnO plate is used as a numerical example. Results show that both the electrical output power and the energy conversion efficiency are sensitive to semiconduction at a moderate carrier density of 10¹⁵ m⁻³, and that the effect of the dissipation due to semiconduction can be comparable to the effect of material damping when the material quality factor is in the usual range of 10²–10³.

This work uses dynamic shearing experiments to examine S- (soft magnetic) and H- (hard magnetic) magnetoactive elastomers of four different types. The aligned materials had larger shear stiffness, and all materials but the unaligned H material displayed increased stiffness with magnetic field. All four types showed generally the same damping ratio with no strong trends across material types. We discuss these results in terms of shearing forces arising due to magnetic torques.

Cement-based piezoelectric composites are employed as the sensing elements of a new smart traffic monitoring system. The piezoelectricity of the cement-based piezoelectric sensors enables powerful and accurate real-time detection of the pressure induced by the traffic flow. To describe the mechanical-electrical conversion mechanism between traffic flow and the electrical output of the embedded piezoelectric sensors, a mathematical model is established based on Duhamel's integral, the constitutive law and the charge-leakage characteristics of the piezoelectric composite. Laboratory tests show that the voltage magnitude of the sensor is linearly proportional to the applied pressure, which ensures the reliability of the cement-based piezoelectric sensors for traffic monitoring. A series of on-site road tests by a 10 tonne truck and a 6.8 tonne van show that vehicle weight-in-motion can be predicted based on the mechanical-electrical model by taking into account the vehicle speed and the charge-leakage property of the piezoelectric sensor. In the speed range from 20 km h⁻¹ to 70 km h⁻¹, the error of the repeated weigh-in-motion measurements of the 6.8 tonne van is less than 1 tonne. The results indicate that the embedded cement-based piezoelectric sensors and associated measurement setup have good capability of smart traffic monitoring, such as traffic flow detection, vehicle speed detection and weigh-in-motion measurement.

Structural component ductility and energy dissipation capacity are crucial factors for achieving reinforced concrete structures more resistant to dynamic loading such as earthquakes. Furthermore, limiting post-event residual damage and deformation allows for immediate re-operation or minimal repairs. These desirable characteristics for structural 'resilience', however, present significant challenges due to the brittle nature of concrete, its deformation incompatibility with ductile steel, and the plastic yielding of steel reinforcement. Here, we developed a new composite material system that integrates the unique ductile feature of engineered cementitious composites (ECC) with superelastic shape memory alloy (SMA). In contrast to steel reinforced concrete (RC) and SMA reinforced concrete (SMA-RC), the SMA-ECC beams studied in this research exhibited extraordinary energy dissipation capacity, minimal residual deformation, and full self-recovery of damage under cyclic flexural loading. We found that the tensile strain capacity of ECC, tailored up to 5.5% in this study, allows it to work compatibly with superelastic SMA. Furthermore, the distributed microcracking damage mechanism in ECC is critical for sufficient and reliable recovery of damage upon unloading. This research demonstrates the potential of SMA-ECC for improving resilience of concrete structures under extreme hazard events.

The mechanical response of a bent shape memory alloy (SMA) wire is a key point for the understanding of the process of the creation of confining effects in a wrapped concrete cylinder for example. The objective of the present study is to model the phenomena involved in the bending of a martensitic SMA wire. The mechanism of martensite reorientation is considered in the model, which also takes into account the asymmetry between tension and compression. For validation purposes, experiments were performed on Ni–Ti wires: measurement of residual curvatures after bending release and tensile tests on pre-bent wires. In particular, the analysis shows a variation in axial stiffness as a function of the preliminary curvature. This result shows the necessity of modelling the distributions of the state variables within the wire cross-section for the simulation of confinement processes using SMA wires. It also opens prospects to potential application to the bending of SMA fibres in smart textiles.

The finite element method (FEM) model of a piezoelectric macro fiber composite (MFC) is presented. Using a specially developed numerical model, the complete set of macroscopic values of elastic compliance and piezoelectric tensors is computed. These values are useful in numerical FEM simulations of more complex systems such as noise and vibration suppression devices or active acoustic metamaterials, where the MFC actuator can be approximated by a plate-like uniform piezoelectric material. Using this approach, a great reduction of the FEM model complexity can be achieved. The computed numerical macroscopic values of the MFC actuator are compared with MFC manufacturerʼs data and with data obtained using different computational methods. A demonstration of active tuning of effective elastic constants of the piezoelectric MFC actuator by means of a shunt electric circuit is presented. The effective material constants are computed using the FEM model developed. The effect of the shunt circuit capacitance on the effective anisotropic Youngʼs moduli is analyzed in detail. A method for finding the proper shunt circuit adjustment that yields the maximum values of the MFC actuator Youngʼs modulus is shown. Possible applications to noise and vibration suppression are discussed.

This study demonstrates that standard polymer optical fibers (POF) can be directly integrated into composites from 3D orthogonal woven preforms during the weaving process and then serve as in-situ sensors to detect damage due to bending or impact loads. Different composite samples with embedded POF were fabricated of 3D orthogonal woven composites with different parameters namely number of y-/x-layers and x-yarn density. The signal of POF was not affected significantly by the preform structure. During application of resin using VARTM technique, significant drop in backscattering level was observed due to pressure caused by vacuum on the embedded POF. Measurements of POF signal while in the final composites after resin cure indicated that the backscattering level almost returned to the original level of un-embedded POF. The POF responded to application of bending and impact loads to the composite with a reduction in the backscattering level. The backscattering level almost returned back to its original level after removing the bending load until damage was present in the composite. Similar behavior occurred due to impact events. As the POF itself is used as the sensor and can be integrated throughout the composite, large sections of future 3D woven composite structures could be monitored without the need for specialized sensors or complex instrumentation.

Harvesting ambient vibration energy is a promising method for realizing self-powered autonomous operation for low-power electronic devices. Most energy harvesters developed to date employ bending-beam configurations and work around the resonant points. There are two critical problems that have hindered the widespread adoption of energy harvesters: insufficient power output and narrow working bandwidth. To overcome these problems, we proposed a novel energy harvester, called a high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH). The HC-PEH delicately synthesizes the merits of the force amplification effect of the flexural motion and the dynamic properties of elastic beams, and thus is capable of high power output with wide working bandwidth. In this paper, theoretical and experimental studies were performed on the HC-PEH. Taking nonlinear stiffness, nonlinear damping, and nonlinear piezoelectricity into account, we developed an analytical model that provides comprehensive insight into the nonlinear mechanical and electrical behaviors of the system. The analytical results closely render the experimental data and demonstrate great performance enhancement. In the experiment, a maximum power output of 54.7 mW is generated at 26 Hz under an acceleration of 4.9 m s⁻², which is over one order of magnitude higher than other state-of-the-art systems.

Modern compact and low power sensors and systems are leading towards increasingly integrated wearable systems. One key bottleneck of this technology is the power supply. The use of energy harvesting techniques offers a way of supplying sensor systems without the need for batteries and maintenance. In this work we present the development and characterization of two inductive energy harvesters which exploit different characteristics of the human gait. A multi-coil topology harvester is presented which uses the swing motion of the foot. The second device is a shock-type harvester which is excited into resonance upon heel strike. Both devices were modeled and designed with the key constraint of device height in mind, in order to facilitate the integration into the shoe sole. The devices were characterized under different motion speeds and with two test subjects on a treadmill. An average power output of up to 0.84 mW is achieved with the swing harvester. With a total device volume including the housing of 21 cm³ a power density of 40 μW cm⁻³ results. The shock harvester generates an average power output of up to 4.13 mW. The power density amounts to 86 μW cm⁻³ for the total device volume of 48 cm³. Difficulties and potential improvements are discussed briefly.

A fluidically driven microactuator that generates supersonic, pulsed microjets has been implemented with smart materials to actively and precisely control the frequency of the microjets in a closed-loop manner. Since this actuator relies on a number of microscale flow and acoustic phenomena to produce the pulsed microjets, its resonant frequency is determined by its geometry and other flow parameters. The design discussed in this paper integrates piezoelectric stacks by connecting them to movable sidewalls within the actuator such that the microactuatorʼs internal geometry can be controlled by varying the voltage across the piezo-stacks. An open-loop control scheme demonstrates the frequency modulation capabilities that are enabled with this design: very large frequency deviations (up to $\pm 500\;{\rm Hz}$) around the actuator design frequency are attained at very high rates (up to 1 kHz). Closed-loop control of the microactuatorʼs frequency was also demonstrated, and the results indicate that (combined with appropriate sensors) this actuator could be used effectively for active, feedback control in high-speed, resonance-dominated flowfields. This proof of concept study clearly illustrates the ability of this robust and compact actuator to produce perturbations that can be modulated and controlled based on the desired control objective.

The confluence of advancements in microelectronic components and vibrational energy harvesting has opened the possibility of remote sensor units powered solely from the motion of their hosts. There are numerous applications of such systems, including the development of modern wildlife tracking/data-logging devices. These 'bio-logging' devices are typically mass-constrained because they must be carried by an animal. Thus, they have historically traded scientific capability for operational longevity due to restrictions on battery size. Recently, the precipitous decrease in the power requirements of microelectronics has been accompanied by advancements in the area of piezoelectric vibrational energy harvesting. These energy harvesting devices are now capable of powering the type of microelectronic circuits used in bio-logging devices. In this paper we consider the feasibility of employing these vibrational energy harvesters on flying vertebrates for the purpose of powering a bio-logging device. We show that the excess energy available from birds and bats could be harvested without adversely affecting their overall energy budget. We then present acceleration measurements taken on flying birds in a flight tunnel to understand modulation of flapping frequency during steady flight. Finally, we use a recently developed method of estimating the maximum power output from a piezoelectric energy harvester to determine the amount of power that could be practically harvested from a flying bird. The results of this analysis show that the average power output of a piezoelectric energy harvester mounted to a bird or bat could produce more than enough power to run a bio-logging device. We compare the power harvesting capabilities to the energy requirements of an example system and conclude that vibrational energy harvesting on flying birds and bats is viable and warrants further study, including testing.

It has been claimed that embedding piezoceramic devices as structural diagnostic systems in advanced composite structures may introduce mechanical impedance mismatches that favor the formation of intralaminar defects. This and other factors, such as cost and their high strain sensitivity, have motivated the use of thin-film piezopolymer sensors. In this paper, we examine the performance of sandwich composite panels fitted with embedded piezopolymer sensors. Our experiments examine both how such thin-film sensors perform within a structure and how the inclusion of sensor films affects structural performance. Strain-controlled tests on sandwich panels subjected to three-point bending under wide-ranging static and dynamic strains lead us to conclude that embedding thin piezopolymer films has no marked reduction on the tensile strength for a wide range of strain loading paths and magnitudes, and that the resilience of the embedded sensor is itself satisfactory, even up to the point of structural failure. Comparing baseline data obtained from standard surface-mounted sensors and foil gauges, we note that whereas it is possible to match experimental and theoretical strain sensitivities, key properties—especially the pronounced orthotropic electromechanical factor of such films—must be duly considered before an effective calibration can take place.

This paper presents a new wideband electromagnetic vibration energy harvester (VEH) composed of a magnetic core embedded into the coil axis. The magnetic core generates a nonlinear magnetic force, which gives rise to the nonlinearity in the behavior of the VEH. Moreover, the magnetic core increases the flux linkage with the coil. These features improve the operational bandwidth and output power of the VEH. Numerical analysis and experimental measurements reveal that the operational bandwidth of the proposed VEH is over 30 Hz in which the output power is kept about 0.1 mW. Moreover, the proposed VEH operates by complicated oscillation due to nonlinear forces acting on the oscillator. Evaluation of the Lyapunov exponent for the measured oscillation suggests that the proposed VEH produces chaotic oscillation.

In this work we present a novel approach to designing responsive structures by segmentation of monolithic plates into an assembly of topologically interlocked building blocks. The particular example considered is an assembly of interlocking osteomorphic blocks. The results of this study demonstrate that the constraining force, which is required to hold the blocks together, can be viewed as a design parameter that governs the bending stiffness and the load bearing capacity of the segmented structure. In the case where the constraining forces are provided laterally using an external frame, the maximum load the assembly can sustain and its stiffness increase linearly with the magnitude of the lateral load applied. Furthermore, we show that the segmented plate with integrated shape memory wires employed as tensioning cables can act as a smart structure that changes its flexural stiffness and load bearing capacity in response to external stimuli, such as heat generated by the switching on and off an electric current.

This paper presents analytical modeling to study the seismic response of bridge systems with conventional and advanced details. For validation, a 33 m quarter-scale model of a four-span bridge incorporating innovative materials and details seismically tested on the shake tables at the University of Nevada, Reno was taken. The bridge specimen involved use of advanced materials and details to reduce damage at plastic hinges and minimize residual displacements. A three-dimensional, nonlinear model incorporating the response of the innovative materials was developed to study the bridge response using the finite-element software OpenSees. Existing finite-element formulations were used to capture the response of the advanced materials used in the bridge. The analytical model was found to be able to reproduce comparable bent displacements and bent shear forces within reasonable accuracy. The validated model was further used to study different types of bridges under suite of scaled bi-directional near-fault ground motions. Comparisons were made on behavior of five different bridge types, first conventional reinforced concrete bridge, second post-tensioned column bridge, third bridge with elastomeric rubber elements at the plastic hinge zone, fourth bridge with nickel–titanium superelastic shape memory alloy (SMA) reinforcing bar and fifth bridge with CuAlMn superelastic SMA reinforcing bar. Both the SMA used bridges also utilized engineered cementitious composite element at the plastic hinge zone. The results showed effectiveness of the innovative interventions on the bridges in providing excellent recentering capabilities with minimal damage to the columns.

A gradient enhanced method is proposed to extract a probability distribution of damage size based on damage images from structural health monitoring. The method provides comprehensive information about damage size and enables prediction of remaining useful life (RUL) of aircraft plate-like structures. A three-step procedure is designed to construct a likelihood function about damage size from intensity image, and a gradient function is employed a priori to obtain a narrow distribution of damage size in the Bayesian framework, providing the empirical probability density function of damage size, from which the probability of damage size larger than critical crack size can be calculated. RUL of plate-like structures can be obtained by calculating the cycles after which the crack size would reach a critical value by a damage growth model. The proposed method converts an ultrasonic damage imaging result to probability density function of damage size, with potential to provide accurate and precise estimation of RUL.

Especially for ageing aircraft the development of fatigue cracks at fastener holes due to stress concentration and varying loading conditions constitutes a significant maintenance problem. High frequency guided waves offer a potential compromise between the capabilities of local bulk ultrasonic measurements with proven defect detection sensitivity and the large area coverage of lower frequency guided ultrasonic waves. High frequency guided waves have energy distributed through all layers of the specimen thickness, allowing in principle hidden (2nd layer) fatigue damage monitoring. For the integration into structural health monitoring systems the sensitivity for the detection of hidden fatigue damage in inaccessible locations of the multi-layered components from a stand-off distance has to be ascertained. The multi-layered model structure investigated consists of two aluminium plate-strips with an epoxy sealant layer. During cyclic loading fatigue crack growth at a fastener hole was monitored. Specific guided wave modes (combination of fundamental A₀ and S₀ Lamb modes) were selectively excited above the cut-off frequencies of higher modes using a standard ultrasonic wedge transducer. Non-contact laser measurements close to the defect were performed to qualify the influence of a fatigue crack in one aluminium layer on the guided wave scattering. Fatigue crack growth monitoring using laser interferometry showed good sensitivity and repeatability for the reliable detection of small, quarter-elliptical cracks. Standard ultrasonic pulse-echo equipment was employed to monitor hidden fatigue damage from a stand-off distance without access to the damaged specimen layer. Sufficient sensitivity for the detection of fatigue cracks located in the inaccessible aluminium layer was verified, allowing in principle practical in situ ultrasonic monitoring of fatigue crack growth.

Prestressed structures experience limited tensile stresses in concrete, which limits or completely eliminates the occurrence of cracks. However, in some cases, large tensile stresses can develop during the early age of the concrete due to thermal gradients and shrinkage effects. Such stresses can cause early-age cracks, termed 'pre-release cracks', which occur prior to the transfer of the prestressing force. When the prestressing force is applied to the cross-section, it is assumed that partial or full closure of the cracks occurs by virtue of the force transfer through the cracked cross-section. Verification of the closure of the cracks after the application of the prestressing force is important as it can either confirm continued structural integrity or indicate and approximate reduced structural capacity. Structural health monitoring (SHM) can be used for this purpose. This paper researches an SHM method that can be applied to prestressed beam structures to assess the condition of pre-release cracks. The sensor network used in this method consists of parallel long-gauge fiber optic strain sensors embedded in the concrete cross-sections at various locations. The same network is used for damage detection, i.e. detection and characterization of the pre-release cracks, and for monitoring the prestress force transfer. The method is validated on a real structure, a curved continuous girder. Results from the analysis confirm the safety and integrity of the structure. The method and its application are presented in this paper.

A novel analytical model for magnetoelectric (ME) laminate composites made of piezoelectric (PE) and piezomagnetic (PM) phases is proposed. The multiphysics equations are applied to all four possible laminate configurations (TT, LT, TL, and LL), with appropriate boundary conditions. Closed form, explicit formulas are derived for the calculation of the intrinsic ME charge coefficient, ME voltage coefficient, and ME coupling factor as a function of material properties of both phases and the PM volume fraction. The predicted ME voltage coefficient is in agreement with previous work and experimental data. A new approach is proposed to take into account the conductivity of the PM phase resulting in calculated ME charge coefficients within 30% of experimental data, which is a major departure from the available approaches that either require to impose an additional constraint on the model or simply ignore the conductivity of the PM phase. To assess the conversion of magnetic work into electric work, a novel approach is developed to calculate the ME coupling factor in closed form by using the calculated properties of the ME composite structure, thus avoiding the equivalent circuit assumption, and furthermore novel coupling factor formulas are developed for all four polarization/magnetization configurations and taking into account the strain coupling in both inplane directions. Using actual material properties, conclusions are drawn regarding the optimal configuration and PM volume fraction necessary to achieve maximum charge, voltage, and work conversion.

Predicting the residual fatigue life of a material is not a simple task and requires the development and association of many variables that as standalone tasks can be difficult to determine. This work develops a modulated nonlinear elastic wave spectroscopy method for the evaluation of a metallic components residual fatigue life. An aluminium specimen (AA6082-T6) was tested at predetermined fatigue stages throughout its fatigue life using a dual-frequency ultrasound method. A modulated nonlinear parameter was derived, which described the relationship between the generation of modulated (sideband) responses of a dual frequency signal and the linear response. The sideband generation from the dual frequency (two signal output system) was shown to increase as the residual fatigue life decreased, and as a standalone measurement method it can be used to show an increase in a materials damage. A baseline-free method was developed by linking a theoretical model, obtained by combining the Paris law and the Nazarov–Sutin crack equation, to experimental nonlinear modulation measurements. The results showed good correlation between the derived theoretical model and the modulated nonlinear parameter, allowing for baseline-free material residual fatigue life estimation. Advantages and disadvantages of these methods are discussed, as well as presenting further methods that would lead to increased accuracy of residual fatigue life detection.