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This paper presents a standardized process for fabricating multifunctional energy storage composites that involves using activated carbon fiber (CF) electrodes and a poly(vinylene fluoride) (PVDF)-...

In this study, we designed a novel hybrid underwater sound-absorbing material of the metastructure that contains a viscoelastic substrate with a microperforated panel. Two types of sound-absorbing metastructures were combined to achieve satisfactory sound absorption performance in the low-frequency range. A homogenized equivalent layer and the integrated transfer matrix method were used to theoretically evaluate the sound absorption performance of the designed nonhomogeneous hybrid metastructure. The theoretical results were then compared with the results obtained using the finite-element method. The designed hybrid sound-absorbing metastructure exhibited two absorption peaks because of its different sound-absorbing mechanisms. The acoustic performance of the developed metastructure is considerably better than that of a traditional sound absorber, and the sound absorption coefficient of the developed metastructure is 0.8 in the frequency range of 3–10 kHz. In addition, an adjustment method for the practical underwater application of the designed metastructure is described in this research. Further studies show that the sound absorption coefficient of the adjusted metastructure still has 0.75 in the frequency range of 3–10 kHz, which indicates that this metastructure has the potential to be used as an underwater sound-absorbing structure. The results of this study can be used as a reference in the design of other novel hybrid underwater sound-absorbing structures.

In this study, infrared thermography is used for vibration-based structural health monitoring (SHM). Heat sources are employed as sensors. An acrylic frame structure was experimentally investigated using the heat sources as structural marker points to record the vibration response. The effectiveness of the infrared thermography measurement system was verified by comparing the results obtained using an infrared thermal imager with those obtained using accelerometers. The average error in natural frequency was between only 0.64% and 3.84%. To guarantee the applicability of the system, this study employed the mode shape curvature method to locate damage on a structure under harsh environments, for instance, in dark, hindered, and hazy conditions. Moreover, we propose the mode shape recombination method (MSRM) to realize large-scale structural measurement. The partial mode shapes of the 3D frame structure are combined using the MSRM to obtain the entire mode shape with a satisfactory model assurance criterion. Experimental results confirmed the feasibility of using heat sources as sensors and indicated that the proposed methods are suitable for overcoming the numerous inherent limitations associated with SHM in harsh or remote environments as well as the limitations associated with the SHM of large-scale structures.

This paper presents a metamaterial plate (metaplate) consisting of a periodic array of holes on a homogeneous thin plate with slot-embedded resonators. The study numerically proves that the proposed model can generate a complete vibrational bandgap in the low-frequency range. A simplified analytical model was proposed for feasibly and accurately capturing the dispersion behavior and first bandgap characteristics in the low-frequency range, which can be used for initial design and bandgap study of the metaplate. A realistic and practical unit metaplate was subsequently designed to verify the analytical model through finite element simulations. The metaplate not only generated a complete vibrational bandgap but also exhibited excellent agreement in both analytical and finite element models for predicting the bandgap characteristics. This study facilitates the design of opening and tuning bandgaps for potential applications such as low-frequency vibration isolation and stress wave mitigation.

This paper presents a tunable planar auxetic metamaterial (PAM) for controlling and filtering acoustic waves and provides guidelines for bandgap design of the proposed PAMs. Numerical results for deformed and undeformed PAMs were obtained from several finite element analyses based on Bloch–Floquet theory. The acoustic band structures of the PAMs were calculated with periodic boundaries. Tunable bandgaps in certain frequency ranges were generated by various deformations applied to the PAMs. Wave attenuation in experimental transmission loss at specific frequencies was demonstrated, showing favorable agreement with the bandgaps obtained from numerical calculations. Both the numerical and experimental results indicate that the proposed structure demonstrates great tunability and offers significant advantages over the regular materials for controlling sound wave propagation and filtering sound waves within specific frequency ranges.

This article describes the rules and procedures for designing a three-dimensional (3D) antitetrachiral structure that achieves a negative Poisson's ratio. Numerical analysis and experimental validation showed good agreement between the deformations and Poisson's ratios. Moreover, the proposed auxetic structure displayed anisotropic behavior in x- and y-directional compression as well as isotropic behavior in z-directional compression. The parametric study and design examples presented in this article demonstrate the relationships among the Poisson's ratio, effective Young's modulus, porosity, and geometry of the proposed structure, and provide insights into the potential applications for future auxetic designs.

By combining rotating squares with auxetic properties, we developed a metamaterial transformer capable of realizing metamaterials with tunable functionalities. We investigated the use of a metamaterial transformer-based thermal cloak–concentrator that can change from a cloak to a concentrator when the device configuration is transformed. We established that the proposed dual-functional metamaterial can either thermally protect a region (cloak) or focus heat flux in a small region (concentrator). The dual functionality was verified by finite element simulations and validated by experiments with a specimen composed of copper, epoxy, and rotating squares. This work provides an effective and efficient method for controlling the gradient of heat, in addition to providing a reference for other thermal metamaterials to possess such controllable functionalities by adapting the concept of a metamaterial transformer.

This study evaluated, by time-domain simulations, the fatigue life of the jacket support structure of a 3.6 MW wind turbine operating in Fuhai Offshore Wind Farm. The long-term statistical environment was based on a preliminary site survey that served as the basis for a convergence study for an accurate fatigue life evaluation. The wave loads were determined by the Morison equation, executed via the in-house HydroCRest code, and the wind loads on the wind turbine rotor were calculated by an unsteady BEM method. A Finite Element model of the wind turbine was built using Beam elements. However, to reduce the time of computation, the hot spot stress evaluation combined FE-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Counting scheme and appropriate SN curves. Based on a preliminary sensitivity study of the fatigue damage prediction, an optimal load setting of 60-min short-term environmental conditions with one-second time steps was selected. After analysis, a sufficient fatigue strength was identified, but further calculations involving more extensive long-term data measurements are required in order to confirm these results. Finally, this study highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as opposed to the mean wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind and wave conditions.

Structures created by incorporating resonating endo-structures within a load bearing exo-structure forbid dynamic disturbances within a specific frequency range from propagating into them without attenuations. Their dynamic behavior can be characterized using a negative effective mass (NM) density. The suitability of such negative effective mass structures (NMS) as infrastructural building-blocks implicitly less susceptible to both harmonic and broadband impact-type loadings is demonstrated. For harmonic loading, an apparent damping coefficient is derived to compare the degree of attenuation achieved in the wholly elastic NMS to an “equivalent” conventionally damped structure. Parametric studies were used to design and construct a low frequency vibration isolator with tip-loaded cantilever beam resonators that evinced 98% payload isolation at resonance. It was found that the higher the stiffness of the host structure, the narrower the isolation bandwidth is in the vicinity of the resonance frequency. Under impact loading, using a numerical optimization procedure, it was established that resonator frequencies toward the higher end of the incoming spectrum gave better reduction in transmitted peak stress. Compact and efficient resonators were constructed using plate springs with chemically etched reentrant patterns and machined titanium resonator masses. Tests performed using an impact pendulum on a resonator stack attached to a transmission bar, substantiated a peak stress reduction of about 60% and filtering of the resonator frequencies in the transmitted spectrum.

This paper presents the use of elastic metamaterials for impact attenuation and blast-wave mitigation. Metamaterials represent a novel and emerging research area where materials exhibit exceptional properties not commonly found in natural materials. These unique properties are enacted by specifically designed microstructures. In this study, a single-resonator model and a dual-resonator microstructural design are proposed to exhibit negative effective mass density. The effect of negative effective mass density is explicitly confirmed by analysis of wave propagation using numerical simulations. Results evidently show that impact stress wave attenuation occurs over a wider frequency spectrum for the dual-resonator model as compared to the narrow band gap of a single-resonator design. Parametric studies of blast-wave simulation reveal that the mass and number of internal resonators have significant influence over the frequency range of blast-wave attenuation. The effectiveness and performance of the single-resonator and dual-resonator models on blast-wave mitigation are examined and discussed. Finally, practical ways to design and manufacture elastic metamaterials with negative effective mass density are presented and explored.

Modifying the geometry of a bonded area improves the strength of adhesively bonded joints. In this study, the joint performance of an adhesively bonded joint with nonflat interfaces was experimentally and numerically evaluated. Various parameters of embedded reinforcements were studied. A new fracture mechanism, termed a two-stage failure mode, was discovered. Moreover, two dominant factors were identified as influencing the strength of the bonded joints. The optimized nonflat-interface specimens exhibited improvements in the ultimate load and average shear stress of up to 45% and 52%, respectively, compared with typical flat-interface specimens. Applying the proposed nonflat joints together with artificial defects has little influence on the ultimate load of specimens and further reduces total weight, and joints can be fabricated without using an additional mold. Therefore, these joints are significantly advantageous for promoting this technique in the industry.

A mechanical model representing an acoustic metamaterial that exhibits simultaneously negative mass density and negative Young's modulus was proposed. Wave propagation was studied in the frequency range of double negativity. In view of positive energy flow, it was found that the phase velocity in this range is negative. This phenomenon was also observed using transient wave propagation finite-element analyses of a transient sinusoidal wave and a transient wave packet. In contrast to wave propagation in the region of positive mass and modulus, the peculiar backward wave motion in the region of double negativity was clearly displayed.

Two continuum methods were investigated for modeling the dynamic behavior of an acoustic metamaterial in the form of a composite material with internal resonators. First, an effective homogeneous classical continuum model was proposed. The effective elastic constants for this continuum are obtained by taking the static equivalence between the continuum and the composite, while the effective mass density adopts the form of a second order tensor. The second model is also a continuum model that is described by two displacement variables. In addition to the usual displacement vector, a displacement vector for the motion of the resonator mass is included, thus making this multi-displacement model quite different from the classical model for elastic solids. It was shown that the dispersion relations predicted by the proposed two approaches were practically the same. The accuracy of the dispersion curves was verified by finite element analyses. Simplicity is the main advantage of the first approach. However, it has to adopt an unusual frequency-dependent effective mass density which may become negative in certain frequency range. On the other hand, the multi-displacement model can be constructed based on the actual material properties of the composite and, in general, is more versatile for further extensions to complex microstructures.

Elastic metamaterials have unusual microstructures that can make them exhibit unusual dynamic behavior. For instance, if treated as classical elastic solids, these materials may have frequency-dependent effective mass densities which may become negative in certain frequency range. In this study, an approach for developing microstructure continuum models to represent elastic metamaterials was presented. Subsequently, this continuum model was used to study wave propagation and band gaps in elastic metamaterial with resonators. In contrast to the use of the conventional continuum theory with which the effective mass density would become frequency-dependent and negative, the main advantage of the microstructure continuum model is that the local microstructural deformation/motion is accounted for with the introduction of additional kinematic variables. Moreover, the material constants of the microstructure continuum model are explicitly expressed in term of the properties of the host medium and the resonator. The accuracy of the microstructure continuum model was evaluated by comparing dispersion curves of harmonic waves to those obtained by the finite element analysis based on the exact geometry of the elastic metamaterial.

A mechanical model with local resonators is proposed as an acoustic metamaterial that exhibits an unusual frequency-dependent effective stiffness. If treated as an equivalent elastic solid, its effective Young's modulus can become unbounded or vanishingly small at two respective frequencies. Moreover, in a certain frequency range, the effective Young's modulus would become negative, resulting in a band gap that coincides with this frequency range. The wave attenuation behavior and mechanism are studied through numerical simulations on the acoustic metamaterial model. The capability of the metamaterial to selectively block or filter unwanted waves is demonstrated by a numerical simulation example.

In this Letter, both in-plane and out-of-plane guided waves in a thin plate with local resonators are studied numerically and experimentally. Through the numerical simulation, a new metamaterial plate design is achieved for a low-frequency bandgap in both in-plane and out-of-plane guided waves. Experiments were conducted to validate the numerical design. In the experiment, piezoelectric transducers were used to generate and receive guided wave signals. The results show that the numerical predictions are in very good agreement with the experimental measurements. Specifically, the connection between the local resonance in the thin plate and its wave attenuation mechanism was discussed.

A two-dimensional (2D) lattice model with anisotropic resonant microstructures is found to provide an anisotropic band gap structure. A 2D continuum with anisotropic effective mass density is introduced to represent this lattice system. Two methods are proposed to derive the equivalent continuum. In the first method, the effective mass density of the equivalent continuum is obtained by matching the dispersion relations for harmonic waves propagating in the principal directions. The second approach employs an approximate estimation of the effective mass density by volume-averaging an effective mass that represents the resonant microstructure. For both equivalent continuum models, the effective mass density is frequency-dependent and may become negative in certain frequency ranges. Subsequently, the effective mass density of the equivalent continuum assumes the form of a second-order tensor. Thus, it suffices to determine the effective mass density tensor with respect to the principle directions. It is show...

A first-order multi-displacement microstructure continuum model is introduced to represent a discrete diatomic lattice system. This model is developed based on a two-term Taylor series expansion of the local displacement of the lattice. It is found that the multi-displacement continuum model obtained by keeping two terms in the Taylor series yields, in general, a better representation of the lattice system than the effective modulus model. However, this microstructure continuum model cannot characterize the negative group velocity of an optical mode of harmonic wave motion in the diatomic lattice. To capture the negative group velocity, a higher-order multi-displacement continuum model is necessary.

The dynamic characteristics of two acoustic metamaterials were investigated using their representative mass-spring lattice models. Attention was focused on the band-gap behaviour of wave propagation in the two models. The study was carried out by performing finite element analyses. From the simulation results, it was found that both models have demonstrated the capability of filtering or blocking dynamic disturbances. However, in spite of having similar local resonance, the two models display different dynamic responses in the band gap. In addition, parametric studies of both metamaterials were carried out with the aim of providing a guide for the design of band gaps in acoustic metamaterials for desired functions.

In this paper, we demonstrate the consequence of using different equivalent models to represent a lattice system consisting of mass-in-mass units and why negative mass is needed in the equivalent model. Dispersive wave propagation in the lattice system is studied and compared to various equivalent models. It is found that, if the classical elastic continuum is used to represent the original mass-in-mass lattice system, the effective mass density becomes frequency dependent and may become negative for frequencies near the resonance frequency of the internal mass. In contrast, if a multi-displacement microstructure continuum model is used to represent the mass-in-mass lattice system, the dispersive behavior of wave propagation and the band gap structure can be adequately described. However, while the acoustic mode is accurately described by the microstructure continuum model, the description of the optical mode is accurate only for a limited frequency range.

For the recent surge of interests in Taiwan’s offshore wind farm program, the government takes the initiatives promoting pilot wind farm projects and developing strategy for 600 wind turbines off Taiwan’s west coast by 2030. Together with 450 land based turbines at the time, it anticipates a total power capacity of 4.2 GW. The announcement of this ambitious offshore energy program has stimulated significant interests among various industrial sectors, as well as academic and R&D institutions in Taiwan. The offshore wind turbines are exposed to extreme environmental conditions and high dynamic stresses. Incipient damage must be detected at the earliest stage possible in order to plan and take reasonable repair measures in due time. This can prevent the development of severe damage and thus lower the repair costs. Periodic inspections are not sufficient for an early detection of damage. Continuous monitoring ensures a higher level of safety. Wireless structural health monitoring (SHM) and risk-based reliability management (RRM) are cost-effective, infrastructural solutions to stable and increased energy yield. This paper presents an overview and prospect on the development of indigenous SHM/RRM for Taiwan offshore wind turbines to be erected in the coastal regions west of Taiwan. A strategic plan is discussed to use the demonstration wind turbines as a test platform for further technology enhancement for field applications, and to consolidate SHM/RRM into wind farm technology core R&D programs for academic research and industrial applications. doi: 10.12783/SHM2015/54

Monopile usually consists of a `grout joint' to connect the pile and the sleeve and supports the wind turbine. The grout corrects the alignment of the sleeve and the pile from imperfection when hammering it to the seabed. Nevertheless, the destruction of the concrete remains critical. This article focuses on the damage behavior of the grout joint, and proposes a few types of improved joints for the connection of the monopile. Numerical simulations are performed for the general type and the improved types of the joints. In the analysis, we construct both two- and three-dimensional models of the offshore wind turbine. The damage location and maximum stress from the static analysis are discussed. Models with improved types of the joint are also proposed, analyzed, and discussed.

This study numerically and experimentally investigated the wave propagation and vibrational behavior of a metamaterial beam with lateral local resonators. A two-dimensional simplified analytical model was proposed for feasibly and accurately capturing the in-plane dispersion behavior, which can be used for the initial design. The out-of-plane wave motions, however, required advanced three-dimensional (3D) modeling. Through experimental validations, 3D finite element simulations were demonstrated to be suitable for advanced design and analysis. This study provided a basis for designing metabeams for transverse wave mitigation. The proposed concept can be further extended to 3D metamaterial plates for wave and vibrational mitigation applications.

A microstructure design of anisotropic resonant inclusions is investigated for the elastic metamaterial plate with the aid of the numerically based effective medium model. Experimental validation is then conducted in the anisotropic metamaterial plate through both harmonic and transient wave testing, from which the anisotropic effective dynamic mass density, group, and phase velocities are determined as functions of frequency. The strongly anisotropic mass density along two principal orientations is observed experimentally and the prediction from the experimental measurements agrees well with that from the numerical simulation. Finally, based on the numerically obtained effective dynamic properties, a continuum theory is developed to simulate different guided wave modes in the elastic metamaterial plate. Particularly, high-order guided wave coupling and repulsion as well as the preferential energy flow in the anisotropic elastic metamaterial plate are discussed.

A two-dimensional (2D) metamaterial possessing an effective anisotropic mass is investigated. This metamaterial is a composite material in the form of an internal mass connected in two directions to the host medium. A 2D mass-in-mass lattice model is used to characterize the dynamic behavior of the metamaterial. If modeled as an effective spring-mass lattice system, the metamaterial may possess a frequency-dependent effective mass. Moreover, if an equivalent homogenous elastic continuum is used to represent the metamaterial, an anisotropic mass density may result and may assume negative values for wave frequencies that are near the local resonance frequency of the internal mass. In fact, it was found that negative mass density occurs in the band-gap of the metamaterial. Unusual wave motion arises from the anisotropic band gap structure. In the present study, wave propagation in the representative continuum model for the metamaterial is studied in order to understand the unusual features of the dynamic behavior of the metamaterial.Copyright © 2009 by ASME

Metamaterials are materials with manmade microstructures. Recently, researchers have looked at a class of metamaterials whose microstructures contain internal degrees of freedom that are different from those of the macro-medium. These metamaterials exhibit unusual dynamic behavior and if modeled as homogeneous solids then their effective mass densities would become negative in certain frequency range. Specifically, a new stop band in the vicinity of the local resonance frequency of the internal mass in the microstructure would result. In this paper, a one dimensional metamaterial is employed to investigate the meaning of the negative mass density in the material and the energy flow in and out of the microstructure. In addition, numerical solutions are used to illustrate the phenomenon.Copyright © 2008 by ASME

A dual-resonator microstructure design is proposed for acoustic metamaterials to achieve broadband effective mass negativity. We demonstrate the advantage of acoustic wave attenuation over a wider frequency spectrum as compared to the narrow band gap of a single-resonator design. We explicitly confirm the effect of negative effective mass density by analysis of wave propagation using finite element simulations. Examples of practical application like vibration isolation and blast wave mitigation are presented and discussed.

The wave attenuation and energy transfer mechanisms of a metamaterial having a negative effective mass density are studied. The metamaterial considered is represented by a lattice system consisting of mass-in-mass units. The attenuation of wave amplitude for frequencies in the stop band is studied from the energy transfer point of view. It is found that most of the work done by the external force on the lattice system is stored by the internal mass if the forcing frequency is close to the local resonance frequency. However, the energy stored in the internal mass is only temporary; it is taken out by the external force in the form of negative work in a cyclic manner. This behavior is utilized to design metamaterials for preventing stress waves from passing them.