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This study investigates the nonlinear vibration of magnetoelectroelastic composite cylindrical nanoshells embedded in elastic media for the first time. The small-size effect and thermoelectromagnetic loadings are considered. Based on the nonlocal elasticity theory and Donnell’s nonlinear shell theory, the nonlinear governing equations and the corresponding boundary conditions are derived using Hamilton’s principle. Then, the Galerkin method is utilized to transform the governing equations into a nonlinear ordinary differential equation and subsequently the method of multiple scales is employed to obtain an approximate analytical solution to nonlinear frequency response. The present results are verified by the comparison with the published ones in the literature. Finally, an extensive parametric study is conducted to examine the effects of the nonlocal parameter, the external magnetic potential, the external electric potential, the temperature change, and the elastic media on the nonlinear vibration characteristics of magnetoelectroelastic composite nanoshells.

The nonlinear vibration characteristics of the piezoelectric circular cylindrical nanoshells resting on a viscoelastic foundation are analyzed. The small scale effect and thermo-electro-mechanical loading are taken into account. Based on the nonlocal elasticity theory and Donnell’s nonlinear shell theory, the nonlinear governing equations and the corresponding boundary conditions are derived by employing Hamilton’s principle. Then, the Galerkin method is used to transform the governing equations into a set of ordinary differential equations, and subsequently, the multiple-scale method is used to obtain an approximate analytical solution. Finally, an extensive parametric study is conducted to examine the effects of the nonlocal parameter, the external electric potential, the temper-ature rise, and the Winkler-Pasternak foundation parameters on the nonlinear vibration characteristics of circular cylindrical piezoelectric nanoshells.

In this study, the thermo-hygro-mechanical bending and vibration of functionally graded material (FGM) microbeams with microporosity defect are investigated. The mathematical model of the system is developed by using the sinusoidal beam theory and the modified couple stress theory. Based on Hamilton’s principle, the governing equations and boundary conditions of imperfect FGM microbeams are derived. Then, Navier’s method is utilized to calculate the deflections and natural frequencies of the system. Analytical results are presented to illustrate the effects of the microporosity volume fraction, the microporosity distribution type, the power-law index, the temperature and the moisture on the static bending and free vibration of imperfect FGM microbeams.

This paper performs nonlinear vibration analysis of metal foam circular cylindrical shells reinforced with graphene platelets. An improved Donnell nonlinear shell theory is employed to formulate the present model. The graphene platelet reinforced material properties are evaluated by the Halpin–Tsai equation. Different types of porosity and graphene platelet (GPL) distribution are taken into account. Governing equations are derived via Hamilton's principle and then they are transformed to ordinary differential equations using the Galerkin method. Afterwards, nonlinear frequencies of the system are solved by using the multiple scale method. Our findings demonstrate that GPL reinforced metal foam (GPLRMF) shells exhibit hardening-spring vibration characteristics. The nonlinear to linear frequency ratio of the shell closely relates to the porosity distributions and GPL patterns. The effect of geometrical size of graphene platelets on nonlinear vibration characteristics of GPLRMF cylindrical shells is also highlighted.

In the present work, nonlinear dynamics of fluid-conveying functionally graded material (FGM) sandwich nanoshells is investigated. In order to describe the large-amplitude motion, the von Karman nonlinear geometrical relations are taken into account. Compressibility and viscidity of the fluid are neglected, and the velocity potential and Bernoulli's equation are used to describe the fluid pressure acting on the nanoshells. Based on the classical shell theory and incorporating the surface stress effect, the governing equations are derived by using Hamilton's principle. After that, the Galerkin method is used to discretize the equation of motion, resulting in a set of ordinary differential equations with respect to time. The ordinary differential equations are solved analytically by utilizing the method of multiple scales. Results show that the surface stress plays important roles on the nonlinear vibration characteristics of fluid-conveying FGM sandwich thin-walled nanoshells. Furthermore, the fluid speed, the power-law index, the fluid mass density, the core thickness and the initial surface tension can also influence the vibration characteristics of fluid-conveying FGM sandwich nanoshells.

In this paper, free vibrations of cylindrical nanoshells made of nanoporous metal foam are investigated for the first time. Based on the modified couple stress theory and Love’s thin shell theory, the governing equations of the present system are derived by using Hamilton’s principle. Two types of nanoporosity distribution are considered in the construction of the nanoporous shells. Then, the Navier method and Galerkin method are utilized to solve natural frequencies of the nanoporous shells under different boundary conditions. Afterwards, a detailed parametric study is conducted. Results show that the nanoporosity type, the material length scale parameter, the porosity coefficient, the length-to-radius ratio, and the radius-to-thickness ratio play important role on the free vibrations of nanoporous shells. To check the validity of the present analysis, the results are compared with those in previous studies for the special cases.

In this paper, we propose a truss-based compressive-mode piezoelectric energy harvester to harness energy from low frequency vibrations with a wide bandwidth and a high-power output. The design is an integration of mainly three modules: separated excitation mechanism from transduction mechanism for low resonant frequency, truss mechanism for magnification of the applied force onto the piezoelectric element, and amplitude limit mechanism to induce impact forces. We then formulate the harvester as a two-degrees-of-freedom system featured by superposition of harmonic and impact-induced nonlinear responses. Based on our structure design, we fabricate a prototype to conduct experimental studies. The experimental results show that the harvester is capable of harnessing energy efficiently from vibrations at the resonant frequencies of 3.3 and 6.09 Hz. The total bandwidth is expanded to 4.2 Hz owing to the structure nonlinearity and bifurcation. The open circuit voltage reaches 83.3 V and maximum power gets up to 38.2 mW with the matching impedance. Also, the harvester exhibits excellent charging performance in terms of saturated voltage and charging time.

A nonlinear surface-stress-dependent nanoscale shell model is developed on the base of the classical shell theory incorporating the surface stress elasticity. Nonlinear free vibrations of circular cylindrical nanoshells conveying fluid are studied in the framework of the proposed model. In order to describe the large-amplitude motion, the von Karman nonlinear geometrical relations are taken into account. The governing equations are derived by using Hamilton's principle. Then, the method of multiple scales is adopted to perform an approximately analytical analysis on the present problem. Results show that the surface stress can influence the vibration characteristics of fluid-conveying thin-walled nanoshells. This influence becomes more and more considerable with the decrease of the wall thickness of the nanoshells. Furthermore, the fluid speed, the fluid mass density, the initial surface tension and the nanoshell geometry play important roles on the nonlinear vibration characteristics of fluid-conveying nanoshells.

The free thermal vibration of functionally graded material (FGM) cylindrical shells containing porosities is investigated. Both even distribution and uneven distribution are taken into account. In addition, three thermal load types, i.e., uniform temperature rise (UTR), nonlinear temperature rise (NLTR), and linear temperature rise (LTR), are researched to explore their effects on the vibration characteristics of porous FGM cylindrical shells. A modified power-law formulation is used to describe the material properties of FGM shells in the thickness direction. Love’s shell theory is used to formulate the strain-displacement equations, and the Rayleigh-Ritz method is utilized to calculate the natural frequencies of the system. The results show that the natural frequencies are affected by the porosity volume fraction, constituent volume fraction, and thermal load. Moreover, the natural frequencies obtained from the LTR have insignificant differences compared with those from the NLTR. Due to the calculation complexity of the NLTR, we propose that it is reasonable to replace it by its linear counterpart for the analysis of thin porous FGM cylindrical shells. The present results are verified in comparison with the published ones in the literature.

This paper presents a study on nonlinear vibration of inhomogeneous functional plates composed of sigmoid graded metal-ceramic materials. The material properties vary continuously along the thickness direction according to a sigmoid distribution rule, which is defined by piecewise functions to ensure smooth distribution of stress among all the interfaces. The geometric nonlinearity is considered by adopting the von Karman geometrical relations. Based on the d’Alembert’s principle, the nonlinear out-of-plane equation of motion of the plates is developed. The Galerkin method is employed to discretize the motion equation to a series of ordinary differential ones, which are subsequently analyzed via the use of the method of harmonic balance. Then, the analytical results are validated by the comparison to numerical solutions, which are obtained by using the adaptive step-size fourth-order Runge-Kutta method. The stability of the steady-state response is examined by the perturbation technique. Results show the first and third modes are both activated while the second mode is not activated for the plates under harmonic point excitation. The frequency response relationships of activated modes exhibit very complicated curves due to the nonlinear modal interaction. In addition, influences of key system parameters on nonlinear vibrational characteristics of the present inhomogeneous plates are illustrated.

Since their debut in 2012, triboelectric nanogenerators (TENGs) have attained high performance in terms of both energy density and instantaneous conversion, reaching up to 500 W m−2 and 85%, respectively, synchronous with multiple energy sources and hybridized designs. Here, a comprehensive review of the design guidelines of TENGs, their performance, and their designs in the context of Internet of Things (IoT) applications is presented. The development stages of TENGs in large‐scale self‐powered systems and technological applications enabled by harvesting energy from water waves or wind energy sources are also reviewed. This self‐powered capability is essential considering that IoT applications should be capable of operation anywhere and anytime, supported by a network of energy harvesting systems in arbitrary environments. In addition, this review paper investigates the development of self‐charging power units (SCPUs), which can be realized by pairing TENGs with energy storage devices, such as batteries and capacitors. Consequently, different designs of power management circuits, supercapacitors, and batteries that can be integrated with TENG devices are also reviewed. Finally, the significant factors that need to be addressed when designing and optimizing TENG‐based systems for energy harvesting and self‐powered sensing applications are discussed., Self‐powered and integrated triboelectric nanogenerators are comprehensively reviewed based on blue energy, their performance, and hybrid designs under the context of large‐scale systems and Internet of Things applications.

This study investigates the free vibration of metal foam circular cylindrical shells under various boundary conditions. The elasticity modulus and mass density of the shells vary gradually and continually in the thickness direction. Two types of porosity distribution are taken into account including symmetrical and unsymmetrical distributions. Love’s shell theory is employed to formulate the governing equations and then the Rayleigh–Ritz method is utilized to solve natural frequencies of the system. The results show that the porosity coefficient has important effect on the natural frequencies of metal foam shells. Its effect also relates to the boundary conditions of the shells. Moreover, different porosity distributions make the metal foam shells possess different vibration characteristics, which is quite obvious at large porosity coefficient. As the circumferential wave number increases, the natural frequencies of the metal foam shells tend to the same under various boundary conditions. Additionally, the present results are verified by the comparison with the published ones in the literature.

This paper reports a novel hybridized flexible electromagnetic–triboelectric generator for vibration/deflection monitoring as it is implemented in a cantilever or clamped-clamped configuration. The proposed self-powered sensor operates based on the concepts of electromagnetism and triboelectricity. The fabricated device consists of a stack of magnets and coils, a flexible tube as the main body, and also, highly flexible, mechanically and thermally durable, and cost-effective polymeric materials. The configuration of the electromagnetic component is optimized based on the magnetization direction of the utilized magnets. The device can effectively convert the shear force and bending moment to electrical voltage through the hybridized system with exerting an external force. The performance of the self-powered sensor is investigated for different cases including a single stack and also a double stack of magnetic components. The design of the triboelectric component of the device is based on the vertical contact separation mode. Results of the paper show how the change of configuration of the magnetic components alters the electrical output of the sensor. A detailed experimental analysis is provided to show the capability of the device under different excitation conditions for both triboelectric nanogenerator (TENG) and electromagnetic generator (EMG) components of the sensor. As the experimental analysis shows, the proposed self-powered system has the potential to be utilized for knee rehabilitation, as it shows explicit results under periodical bending load with different frequencies and amplitudes of excitation.

Thin cylindrical shells are commonly employed to connect adjacent disks in large rotating machinery. The vibration characteristics of coupled cylindrical shell and annular plate components have a significant influence on the rotor dynamics. This paper provides a general approach for the vibration analysis of a rotating cylindrical shell coupled with an annular plate. The Sanders shell theory and Mindlin plate theory are employed to calculate the strain energy of the shell and plate, respectively. The Coriolis and centrifugal effects due to the rotation are taken into account. The artificial spring technique is adopted to simulate the coupling and boundary conditions. By taking Chebyshev polynomials as the admissible functions, the Rayleigh–Ritz method is employed to derive the motion equations for the rotating shell-plate combination. The approach proposed is validated by comparing with the available results in literature and finite element analysis results. The traveling wave motion of the coupled shell and plate structure is investigated. The effects of the geometric parameters and the boundary and coupling conditions on the vibration behavior of the coupled structure are evaluated. The present approach can not only evaluate free vibrations of coupled shell and plate structures, but also be extended to vibration analysis of disk–drum rotors involving cylindrical shells.

Energy harvesting holds great potential to achieve long-lifespan self-powered operations of wireless sensor networks, wearable devices, and medical implants, and thus has attracted substantial interest from both academia and industry. This paper presents a comprehensive review of piezoelectric energy-harvesting techniques developed in the last decade. The piezoelectric effect has been widely adopted to convert mechanical energy to electricity, due to its high energy conversion efficiency, ease of implementation, and miniaturization. From the viewpoint of applications, we are most concerned about whether an energy harvester can generate sufficient power under a variable excitation. Therefore, here we concentrate on methodologies leading to high power output and broad operational bandwidth. Different designs, nonlinear methods, optimization techniques, and harvesting materials are reviewed and discussed in depth. Furthermore, we identify four promising applications: shoes, pacemakers, tire pressure monitoring systems, and bridge and building monitoring. We review new high-performance energy harvesters proposed for each application.

This paper presents a novel hybridized flexible electromagnetic-triboelectric generator that consists of a round/square shaped coil and magnet, and also, highly flexible, mechanically and thermally durable, and cost-effective polymeric materials. The reported hybridized nano generator is capable of converting external mechanical load to electricity. Using a systematic optimization approach results in an optimal configuration and size for the electromagnetic components of the self-powered sensor. Combination of the electromagnetic and triboelectric components provides several advantages for the proposed self-powered device including high resolution and power density even in low frequency and small amplitude of the excitations. We probe the sensitivity of the fabricated self-powered sensor considering different amplitude and frequency of excitations as well as external resistors. After providing a general performance analysis for the proposed self powered sensor, we show its potential for different specific applications including human motion based energy harvesting and sensing, tire condition monitoring, and pressure sensing. The utilization of the proposed self-powered sensor can provide a sustainable energy source for wireless sensor nodes, and also overcomes the battery capacity limitation that restricts the life time durability of mobile electrical devices.

This paper presents a novel buckled compressive-mode piezoelectric energy harvesting device (BC-PEH) to improve the energy harvesting capability. The BC-PEH synthesizes the merits of the force amplification effect of the flexural motion and the broad bandwidth of buckled-beam structures. In this paper, both theoretical and experimental investigations are performed on the BC-PEH, under both harmonic and random excitations. Taking negative stiffness, nonlinear damping and nonlinear piezoelectric coupling into account, we build an analytical model that provides a comprehensive insight into the nonlinear electromechanical coupling system. The influence of parameters on the performance is thoroughly discussed via the harmonic balance method and numerical technique. The analytical results closely render the experimental data and demonstrate a considerable performance enhancement. Compared with state-of-the-art energy harvesters, the BC-PEH prototype not only exhibits its superiority in terms of power density, normalized power density and power normalized by mass and acceleration, but also indicates a 194% broader working bandwidth.

We proposed a nonlinear multi-mode tri-leg compliant orthoplanar spring piezoelectric vibration-based energy harvester (COPS-PVEH), which provided the improved performance using a pure geometric nonlinearity, in terms of a wide bandwidth, and a high-voltage output in forward and reverse sweeps. This paper presents an application of magnetic force interaction to further enhance the performance of the COPS-PVEH. A repulsive magnetic force is added to the spring to change the harvester system’s stiffness. A lumped parameter model is derived and analytically solved for the designed harvester with a magnetic coupling. The experiments are carried out by adding a moving magnet at the center of the spring and a pair of fixed magnets at the top and bottom sides of the harvester. Both experimental data and simulation results show the benefits brought through the dynamics of a magnetic oscillator. In an experiment by adding magnetic force, a slight increase is observed in the operating bandwidth and a 10% increase in the magnitude of voltage output is achieved at the same excitation level. The magnetic interaction on the COPS-PVEH with an addition of multiple masses has also achieved an increase in voltage output and bandwidth. In addition, the gap between the three vibration modes has been reduced.

This paper presents the modeling and experimental parametric study of a nonlinear multi-frequency broad bandwidth piezoelectric vibration-based energy harvester. The proposed harvester consists of a tri-leg compliant orthoplanar spring (COPS) and multiple masses with piezoelectric plates attached at three different locations. The vibration modes, resonant frequencies, and strain distributions are studied using the finite element analysis. The prototype is manufactured and experimentally investigated to study the effect of single as well as multiple light-weight masses on the bandwidth. The dynamic behavior of the harvester with a mass at the center is modeled numerically and characterized experimentally. The simulation and experimental results are in good agreement. A wide bandwidth with three close nonlinear vibration modes is observed during the experiments when four masses are added to the proposed harvester. The current generator with four masses shows a significant performance improvement with multiple nonlinear peaks under both forward and reverse frequency sweeps.

Bio-inspired technologies have remarkable potential for energy harvesting from clean and sustainable energy sources. Inspired by the hummingbird-wing structure, we propose a shape-adaptive, lightweight triboelectric nanogenerator (TENG) designed to exploit the unique flutter mechanics of the hummingbird for small-scale wind energy harvesting. The flutter is confined between two surfaces for contact electrification upon oscillation. We investigate the flutter mechanics on multiple contact surfaces with several free-standing and lightweight electrification designs. The flutter driven-TENGs are deposited on simplified wing designs to match the electrical performance with variations in wind speed. The hummingbird TENG (H-TENG) device weighed 10 g, making it one of the lightest TENG harvesters in the literature. With a six TENG network, the hybrid design attained a 1.5 W m−2 peak electrical output at 7.5 m/s wind speed with an approximately linear increase in charge rate with the increased number of TENG harvesters. We demonstrate the ability of the H-TENG networks to operate Internet of Things (IoT) devices from sustainable and renewable energy sources.

This research focuses on the large-amplitude vibration of sigmoid functionally graded material (S-FGM) thin plates with porosities. Porosities in S-FGM plates can happen due to technical issues during the preparation of S-FGMs. Two types of porosity distribution, i.e., even and uneven distribution, are taken into account. The material properties of S-FGM plates with porosities change smoothly along the thickness direction based on the sigmoid distribution law, which is described by modified piecewise functions. The geometrical nonlinearity is considered by applying the von Karman non-linear plate theory. The nonlinear governing equation of S-FGM plates with porosities is derived using the D′Alembert's principle. By applying the Galerkin method with the first three modes, the governing equation is discretized to three ordinary differential equations. Then, the method of harmonic balance is used to solve these discretized equations. Analytical results are verified numerically with the adaptive step-size fourth-order Runge-Kutta method. The stability of the steady-state response is examined by means of the perturbation technique. Furthermore, the maximum amplitudes of each mode during the vibration period are obtained and shown in the neighborhood of the fundamental mode. Study demonstrates that the S-FGM plates with porosities possess hardening spring characteristics in nonlinear frequency response. Moreover, a complex multi-solution phenomenon occurs in the present dynamic system which is rooted from the nonlinear mode interaction. Finally, investigation is made on the effects of porosity along with other key parameters on large-amplitude vibration response of porous S-FGM plates.

A first known study is conducted on the vibrations of functionally graded material (FGM) rectangular plates with porosities and moving in thermal environment. The FGM plates contain porosities owing to the technical issues during the preparation of FGMs. Two types of porosity distribution, namely, even and uneven distribution, are considered. The geometric nonlinearity is taken into account by using von Karman nonlinear plate theory. The out-of-plane equation of motion of the system is derived based on the D'Alembert's principle with the consideration of the thermal effect and longitudinal speed. Then the Galerkin method is employed to discretize the partial differential equation of motion to a set of ordinary differential equations. These time-varying ordinary differential equations are solved analytically by means of the method of harmonic balance. The accuracy of approximately analytical solutions is verified by the adaptive step-size fourth-order Runge–Kutta technique. Additionally, the stability of steady-state solutions is analyzed for the analytical solutions. Vibration characteristics such as natural frequency and nonlinear frequency response are shown. The present model is a hardening-spring system based on the nonlinear frequency response results. Effects of some key parameters are investigated on the vibration of rectangular FGM plates with porosities and moving in thermal environment.

Piezoelectric energy harvesting technology is regarded as a remedy to the short-life battery issue in low-power electronic devices, and has been extensively studied in the past two decades. Although a variety of new structures and mechanisms have been proposed for piezoelectric energy harvesters (PEHs), there is not much progress made on the piezoelectric materials that play a determinant role in PEHs performance. Most energy harvesters are still constructed with lead zirconate titanate (PZT). Recently, the new-generation piezoelectric materials PMN-PT and PZN-PT single crystals are proposed and start to be used in transducers. Their electromechanical coupling factor ( k 33 ) is over 90%, and the piezoelectric coefficient ( d 33 ) can reach three times of that of PZT. This study characterizes the PMN-PT and PZN-PT single crystals used in energy harvesters, compared with the polycrystalline PZT, in terms of charging capacitors. Systematic discussions are presented on the voltage, power and energy responses, as well as the reverse coupling effect on the mechanical response and the self-discharge effect of capacitors. Furthermore, a self-powered synchronized switch harvesting on inductor (SSHI) circuit is constructed and tested with the new single-crystal based energy harvesters. Experiments indicate that PMN-PT and PZN-PT can significantly boost the charging capability of energy harvesters, and that performance can be further enhanced by the SSHI circuit.

Life prediction based on the damage mechanics of aero-engine turbine blades is crucial for performing their strength design and ensuring the operational reliability under combined high and low cycle fatigue (CCF) loadings. In view of this, a simple and efficient life prediction method is developed to consider the interaction of high cycle fatigue (HCF) and low cycle fatigue (LCF) without any additional material constants, and as a basis of that a new nonlinear damage accumulation model is proposed by introducing threshold damage of the component related to material type and current HCF damage. In order to validate the presented methodology, model predictions were compared with the experimental data sets of turbine blades and its alloy materials. The predicted results indicate that the simple life prediction model is capable of estimating the fatigue life quickly with an acceptable accuracy in contrast to other four existing ones. Moreover, the threshold damage-based method provides the higher prediction accuracy than others due to improved HCF damage determined by the damage threshold.

Geometrically nonlinear oscillations are investigated on sigmoid functionally graded material (S-FGM) plates with a longitudinal speed. The material properties of the plates obey a sigmoid distribution rule along the thickness direction. Based on the D’Alembert’s principle, a nonlinear equation of motion is derived for the moving S-FGM plates, where the von K´arm´an nonlinear plate theory is adopted. Utilizing the Galerkin method, the equation of motion is discretized and solved via the method of harmonic bal-ance. The approximate analytical solutions are validated through the adaptive step-size fourth-order Runge-Kutta method. Besides, the stability of the steady-state solutions is examined. The results reveal that the mode interaction behavior can happen between the first two modes of the moving S-FGM plates, leading to a complex nonlinear frequency response. It is further found that the power-law index, the longitudinal speed, the exci-tation amplitude, and the in-plane pretension force can significantly affect the nonlinear frequency-response characteristics of longitudinally traveling S-FGM plates.

Piezoelectric energy harvesting is envisioned as an ideal complement to batteries for long-life operation of wireless or remote electronic devices. Most piezoelectric energy harvesters (PEHs) utilize piezoelectric plates or discs as the energy transducing elements, and they cannot generate enough power for most potential applications. In this study, we explore a new way of using arc-shaped piezoelectric patches as the core transducing elements in energy harvesters to improve their performance. An analytical model is developed via the Castigliano’s theorem to elucidate the low stiffness characteristic of the arc-shaped structure. A finite element analysis is conducted, and the results indicate that high and evenly-distributed stress can be easily induced in the arc-shaped PEHs. Prototypes are fabricated with arc-shaped piezoelectric elements and flat piezoelectric plates, respectively, and tested under the same conditions. The experimental data demonstrate that the arc-shaped PEHs are capable of generating 2.55–4.25 times as much power as the equivalent flat-plate PEHs. This study lays the foundation to enhance energy harvester performance by utilizing arc-shaped piezoelectric structures.

Energy harvesting is an essential technology for enabling low-power, maintenance-free electronic devices, and thus has attracted a great deal of attention in recent years. A variety of designs and approaches have been proposed to harvest ambient vibration energy, but crucial questions remain regarding figures of merit characterizing the performance of energy harvesters. Of primary importance is the energy conversion efficiency. There are large discrepancies in the definition and tested values of efficiency in the literature. This study is intended to answer the fundamental question for energy harvesters: how to define and calculate the energy conversion efficiency. We first review studies on efficiency and analyze the energy flow in an energy harvesting system. Based on the analysis, we derive an efficiency expression for linear cantilever energy harvesters. The developed efficiency expression transparently and quantitatively reveals the relationship between efficiency and key parameters. Experiments are performed to validate the efficiency expression. Furthermore nonlinear energy harvesters are tested in both on-resonance and off-resonance conditions. Both experimental and theoretical studies manifest that the energy conversion efficiency tends to decrease as the excitation frequency rises and its value is related to the phase difference between excitations and responses. Around resonance states where the phase difference of both linear and nonlinear energy harvesters is about 90 degrees, the efficiency calculation is much simplified.

This study investigates vibration characteristics of longitudinally moving sigmoid functionally graded material (S-FGM) plates containing porosities. Two types of porosity distribution, i.e., the even and uneven distributions, are taken into account. In accordance with the sigmoid distribution rule, the material properties of porous S-FGM plates vary smoothly along the plate thickness direction. The nonlinear geometrical relations are adopted by using the von Karman non-linear plate theory. Based on the d’Alembert’s principle, the nonlinear governing equation of the system is derived. Then, the governing equation is discretized to a set of ordinary differential equations via the Galerkin method. These discretized equations are subsequently solved by using the method of harmonic balance. Analytical solutions are verified with the aid of the adaptive step-size fourth-order Runge–Kutta method. By using the perturbation technique, the stability of the steady-state response is highlighted. Finally, both natural frequencies and nonlinear forced responses of moving porous S-FGM plates are examined. Results demonstrate that the moving porous S-FGM plates exhibit hardening spring characteristics in the nonlinear frequency response. Moreover, it is shown that the type of porosity distribution, moving speed, porosity volume fraction, constituent volume fraction and in-plane pretension all have significant influence on the nonlinear forced responses of moving porous S-FGM plates.

Summary Triboelectric nanogenerator (TENG) is a newly proposed technology for effectively converting mechanical energy into electricity. Triboelectric nanogenerator has shown a great potential for harvesting the clean and abundant energy of ocean waves. Recently, a duck-shaped TENG device has been proposed as a lightweight, cost-effective, highly stable, and efficient system for scavenging the existing energy in water waves. In this paper, a detailed investigation on the performance of the duck-shaped TENG is presented. Then, a comparative analysis between the TENG device and an equivalent electromagnetic generator (EMG) for wave energy harvesting is performed. The electric output characteristics of both techniques under various mechanical and electrical conditions are obtained. The analysis demonstrates that at a low operating frequency of 2.5 Hz, the TENG and EMG achieve the peak power density of 213.1 and 144.4 W/m3, respectively. The present paper provides guidance for design and optimization of hybrid TENG and EMG technology toward scavenging the blue energy.

A miniature-positioning device with a large stroke motion has attracted more and more attentions in these years because of the intensive development in precision engineering. In this paper, we have achieved the large stroke actuating and the high precision positioning, as well as realized a multi-degree-of-freedom in-plane motion using the developed Galfenol impact drive mechanism (IDM) actuator. In order to enhance the system robustness, two pieces of U-shape Galfenol (iron–gallium alloy) have been employed as the driving elements with a bias magnetic field contributed by a permanent magnet to generate the swing motion that amplifies the propelling inertia force. The current amplitude modulation has been applied in the precision positioning of the actuator under the quasi-static condition because of the motion step-size fineness. The results show that the actuator is able to achieve a sub-micrometer positioning accuracy that has reached the measurement limit of our setup. Meanwhile, the frequency modulation method has been explored in the large stroke actuation with a high motion speed. We have found out that this design is capable of achieving an accurate positioning without the frequency modulation because of the intrinsic fine step-size of the actuator. In addition, a rectangular in-plane motion has been realized with the image-based control for the multi-degree-of-freedom positioning. The actuator has an inductive impedance with a resistance of 3.796 Ω and an inductance of 0.4697 mH. Under the present driving ratings, the power consumption is smaller than 1.97 W while the reactive power can be ignored. Moreover, the experimental load analysis indicates that the design can achieve a maximum carry-load-to-weight ratio of 6.5.

Vibration energy harvesting has been a research subject of growing interest over the past few years, and is envisaged as a remedy to the unsatisfactory battery issue in low-power electronic devices. In this study, we propose a new magnetostrictive transducer to harvest energy from finger tapping. Galfenol is selected as the transducing material due to its high piezomagnetic coefficient and excellent machinability. To effectively harness energy in low-frequency conditions, we develop a frequency up-conversion mechanism that succeeds in converting vibration below 10 Hz from finger tapping up to the system's resonance of a few hundred Hz. Furthermore, multiple impacts are induced in each working cycle deliberately to boost the efficiency. A comprehensive model is built and solved to analyze the mechanical-magnetic-electrical coupling system. Based on the model, we elucidate the design criteria for high-performance magnetostrictive transducers. A prototype is fabricated with a Galfenol beam of 0.5 × 5 × 25 mm 3 and, under finger tapping, it generates 5.3 mW power and instantaneously lights up 10 commercial LEDs and a numeric LCD.