K. W. Wang

A review of the recent research on vibration energy harvesting via bistable systems

The investigation of the conversion of vibrational energy into electrical power has become a major field of research. In recent years, bistable energy harvesting devices have attracted significant attention due to some of their unique features. Through a snap-through action, bistable systems transition from one stable state to the other, which could cause large amplitude motion and dramatically increase power generation. Due to their nonlinear characteristics, such devices may be effective across a broad-frequency bandwidth. Consequently, a rapid engagement of research has been undertaken to understand bistable electromechanical dynamics and to utilize the insight for the development of improved designs. This paper reviews, consolidates, and reports on the major efforts and findings documented in the literature. A common analytical framework for bistable electromechanical dynamics is presented, the principal results are provided, the wide variety of bistable energy harvesters are described, and some remaining challenges and proposed solutions are summarized.

Active-passive hybrid piezoelectric networks for vibration control: Comparisons and improvement

In this research, the passive damping and active control authority of several basic active-passive hybrid piezoelectric networks are analysed and compared. The comparison is performed in a nondimensionalized manner, throughout which the importance of the generalized electro-mechanical coupling coefficient is highlighted. It is concluded that these configurations yield very similar open-loop performance for the same electro-mechanical coupling. It is shown that larger electro-mechanical coupling leads to higher passive network damping and, depending on the design and configuration, could also derive better active authority and overall performance. A method of increasing the electro-mechanical coupling coefficient by using a negative capacitance circuit is proposed, analysed and experimentally verified.

Variable Stiffness Structures Utilizing Fluidic Flexible Matrix Composites

In this research, the capability of utilizing fluidic flexible matrix composites (F2MC) for autonomous structural tailoring is investigated. By taking advantage of the high anisotropy of flexible matrix composite (FMC) tubes and the high bulk modulus of the pressurizing fluid, significant changes in the effective modulus of elasticity can be achieved by controlling the inlet valve to the fluid-filled F2MC structure. The variable modulus F2MC structure has the flexibility to easily deform when desired (open-valve), possesses the high modulus required during loading conditions when deformation is not desired (closed-valve — locked state), and has the adaptability to vary the modulus between the flexible/stiff states through control of the valve. In the current study, a 3D analytical model is developed to characterize the axial stiffness behavior of a single F 2MC tube. Experiments are conducted to validate the proposed model, and the test results show good agreement with the model predictions. A closed/open modulus ratio as high as 56 times is achieved experimentally. With the validated model, an F2MC design space study is performed. It is found that by tailoring the properties of the FMC tube and inner liner, a wide range of moduli and modulus ratios can be attained. By embedding multiple F 2MC tubes side by side in a soft matrix, a multi-cellular F2MC sheet with a variable stiffness in one direction is constructed. The stiffness ratio of the multi-cellular F2MC sheet obtained experimentally shows good agreement with a model developed for this type of structure. A case study has been conducted to investigate the behavior of laminated [+60/0/-60] s multi-cellular F2MC sheets. It is shown that the laminate can achieve tunable, steerable, anisotropy by selective valve control.

The interfacial strength of carbon nanofiber epoxy composite using single fiber pullout experiments.

Carbon nanotubes and nanofibers are extensively researched as reinforcing agents in nanocomposites for their multifunctionality, light weight and high strength. However, it is the interface between the nanofiber and the matrix that dictates the overall properties of the nanocomposite. The current trend is to measure elastic properties of the bulk nanocomposite and then compare them with theoretical models to extract the information on the interfacial strength. The ideal experiment is single fiber pullout from the matrix because it directly measures the interfacial strength. However, the technique is difficult to apply to nanocomposites because of the small size of the fibers and the requirement for high resolution force and displacement sensing. We present an experimental technique for measuring the interfacial strength of nanofiber-reinforced composites using the single fiber pullout technique and demonstrate the technique for a carbon nanofiber-reinforced epoxy composite. The experiment is performed in situ in a scanning electron microscope and the interfacial strength for the epoxy composite was measured to be 170 MPa.

Concise and high-fidelity predictive criteria for maximizing performance and robustness of bistable energy harvesters

We employ an analytical model of a harmonically excited bistable vibration energy harvester to determine criteria governing continuous high-energy orbit (HEO) dynamics that maximize harvesting performance. Derivation of the criteria stems from previously unexplored dynamic relationships predicted by the model indicating critical conditions for HEO; experimental evidence of the phenomenon is provided as validation. The criteria are vastly more concise than existing HEO prediction methodology and can more accurately delineate HEO boundaries. This research addresses an essential need to create effective tools for high performance and robust bistable harvester design.

Vibration Analysis of Composite Beams With End Effects via the Formal Asymptotic Method

Vibration analysis of composite beams is carried out by using a finite element-based formal asymptotic expansioh method. The formulation begins with three-dimensional (3D) equilibrium equations in which cross-sectional coordinates are scaled by the characteristic length of the beam. Microscopic two-dimensional and macroscopic one-dimensional (ID) equations obtained via the asymptotic expansion method are discretized by applying a conventional finite element method. Boundary conditions associated with macroscopic ID equations are considered to investigate the end effect. It is then described how one could form and solve the eigenvalue problems derived from the asymptotic method beyond the classical approximation. The results obtained are compared with those of 3D finite element method and those available in the literature for composite beams with solid cross section and thin-walled cross section.

Damage detection and conductivity evolution in carbon nanofiber epoxy via electrical impedance tomography

Utilizing electrically conductive nanocomposites for integrated self-sensing and health monitoring is a promising area of structural health monitoring (SHM) research wherein local changes in conductivity coincide with damage. In this research we conduct proof of concept investigations using electrical impedance tomography (EIT) for damage detection by identifying conductivity changes and by imaging conductivity evolution in a carbon nanofiber (CNF) filled epoxy composite. CNF/epoxy is examined because fibrous composites can be manufactured with a CNF/epoxy matrix thereby enabling the entire matrix to become self-sensing. We also study the mechanisms of conductivity evolution in CNF/epoxy through electrical impedance spectroscopy (EIS) testing. The results of these tests indicate that thermal expansion is responsible for conductivity evolution in a CNF/epoxy composite.

Damage detection via electrical impedance tomography in glass fiber/epoxy laminates with carbon black filler

The conductivity of glass fiber reinforced polymers with nanocomposite matrices can be leveraged for structural health monitoring. Since nanocomposite matrices depend on well-connected networks of conductive nanofillers for electrical conductivity, matrix damage will sever the connection between fillers and result in a local conductivity loss. Monitoring composite conductivity changes can therefore give insight into the state of the matrix. Existing conductivity-based structural health monitoring methods are either insensitive to matrix damage or employ large electrode arrays. This research advances the state of the art by combining the superior imaging capabilities of electrical impedance tomography with conductive networks of nanofillers in the composite matrix. Electrical impedance tomography for damage detection in glass fiber/epoxy laminates with carbon black nanocomposite matrices is characterized by identifying a lower threshold of through-hole detection, demonstrating the capability of electrical impedance tomography to accurately resolve multiple through holes, and locating impact damage. It is found that through holes as small as 3.18 mm in diameter can be detected, and electrical impedance tomography can detect multiple through holes. However, sensitivity to new through holes is diminished in the presence of existing through holes unless a damaged baseline is used. Finally, it is shown that electrical impedance tomography is also able to accurately locate impact damage. These research findings demonstrate the considerable potential of conductivity-based health monitoring for glass fiber reinforced polymer laminates with conductive networks of nanoparticles in the matrix.

An arbitrary strains carbon nanotube composite piezoresistivity model for finite element integration

Piezoresistive carbon nanotube (CNT) composites can radically enhance structural identification and health monitoring through continuous self-sensing. However, prevailing piezoresistivity models examine only uniaxial strain and are too computationally burdensome to be implemented on a structural scale. This research circumvents these limitations by developing an analytical piezoresistivity model for CNT composites that is adaptable to the finite element formulation enabling the analysis of complicated structures subjected to arbitrary strain. The accuracy of the model is verified by comparison to uniaxial piezoresistivity experiments in existing literature.

Energy harvester synthesis via coupled linear-bistable system with multistable dynamics

In this research we study the dynamics of a coupled linear oscillator-bistable energy harvester system. The method of harmonic balance and perturbation analysis are used to predict the existence and stability of the bistable device interwell vibration. The influences of important parameters on tailoring the coupled system response are investigated to determine strategies for improved energy harvesting performance. We demonstrate analytically that for excitation frequencies in a bandwidth less than the natural frequency of the uncoupled linear oscillator having net mass that is the combination of the bistable and linear bodies, the bistable harvester dynamics may be substantially intensified as compared to a single (individual) bistable harvester. In addition, the linear-bistable coupled system may introduce a stable out-of-phase dynamic around the natural frequency of the uncoupled linear oscillator, enhancing the performance of the harvester by providing a second interwell response not possible when using a single bistable harvester. Key analytical findings are confirmed through numerical simulations and experiments, validating the predicted trends and demonstrating the advantages of the coupled system for energy harvesting. [DOI: 10.1115/1.4026555]