Metamaterials are man-made materials that behave uniquely and possess exclusively desired properties that are not found in natural materials. Usually, it is the combination of two or more materials and can be engineered to perform tasks that are not possible with traditional materials. These were initially discovered while working with electromagnetic radiation. Apart from electromagnetic radiation, metamaterials are also capable of affecting the wave propagation characteristics through any fluid such as air. These metamaterials are called acoustic metamaterials. Many acoustic metamaterials have gone beyond its definition but still, characterize the waveguiding properties. Incorporation of smart materials while constructing acoustic metamaterial, can achieve multifunctionality of the design. A prospective application field for such acoustic metamaterials is energy harvesting from low-frequency vibration. It is conceptualized that acoustic metamaterials can be used as noise barrier materials to filter roadside and industrial noise. This application can get extended to the aerospace application where engine noise mitigation inside the cabin is a challenge. In this article, a spiral-shaped acoustic metamaterial is modeled which has a dual function of noise filtering and energy harvesting. This acoustic metamaterial has a comparatively high reflection coefficient closer to the anti-resonance frequencies, resulting in high sound transmission loss. The filtered noise is trapped inside the cell in the form of strain energy. Hence, we claim that if the trapped energy which is any way wasted in the material could be harvested to power the local electronic devices, the new solution could make transformative for the 21st century’s green energy solution. Calculated placement of smart materials in the cell-matrix can help to extract the strain energy in the form of power. The acoustic metamaterial cell presented in this work has the capability of isolating noise and reducing diffraction by trapping sound in low frequencies and at the same time recover the trapped abundant energy in the form of electrical potential using piezoelectric materials. The spiral design is sensitive to vibration due to trampoline shaped attachments inside the cell. This makes it capable of harvesting energy using vibration also. This is a promising acoustoelastic metamaterial with multifunctionality properties for future applications.

This manuscript investigates one way sound propagation in Magnetorheological fluids (MRF) using spatio-temporal modulation of the applied magnetic field. One-way propagation of waves in a structure can have potential technological applications such as sound isolation, filtering and echo suppression. Several experimental works in the literature have shown that elastic properties of MRF’s (local speed of sound, in particular) are dependent on the applied magnetic field. Therefore, several fascinating possibilities regarding the manipulation of sound waves in MRF, by tailoring the applied magnetic field, exist. A effective medium approximation (previously used in literature) is used to analyze sound propagation in a MRF composed of hydrogen-reduced Iron particles suspended in pure glycerine. Floquet-Bloch theory is used to obtain a quadratic eigenvalue problem that gives the band structure as a function of the material and modulation parameters. When the applied magnetic field is allowed to vary only in space, regular bandgaps are obtained as a result of Bragg scattering. In contrast, the temporal variation of the magnetic field to induce a traveling wave like variation of the modulated parameters, breaks the symmetry of the Brilloouin zones and we obtain directional bandgaps. The theoretical band structure is validated by numerical band diagrams obtained using a Finite Element code. This research has important applications in active sound manipulation.

Periodic structures are the repetition of unit cells in space, that provide a filtering behavior for wave propagation. In particular, it is possible to tailor the geometrical, physical and elastic properties of the unit cells, in order to attenuate certain frequency bands, called band-gaps or stop-bands. Having each element characterized with the same parameters, the filtering behavior of the system can be described through the wave propagation properties of the unit cell. This is technologically impossible to obtain, therefore the Lyapunov factor is used, in order to define the mean attenuation of a quasi-periodic structure. Tailoring Gaussian unit cell properties potentially allows to extend the stop-bands width in the frequency domain. A drawback is that some unexpected resonance peaks may lie in the neighborhood of the extended regions. However, the correspondent mode-shapes are localized in a particular region of the structure, and they partially decrease the global attenuating behavior. In this paper, the aperiodicity introduced in the otherwise perfect repetition is investigated, providing an explanation for the mode-localization problem and for the stop-bands extension. Then, the proposed approach is applied to a passive quasi-periodic beam, characterized from a localized peak within a designed band-gap. The geometrical properties of its aperiodic parts are changed in order to deterministically move the localization peak in the frequency response. Numerical and experimental results are compared.

Periodic structures provide filtering behavior for vibrations, as a result of the repetition in space of unit blocks, or unit cells. In general, they are characterized by an internal mechanical impedance mismatch, so that waves are reflected and transmitted every time a discontinuity is present. The global behavior given by waves superposition is their cancellation, only for specific frequency ranges, generally called stop-bands or band-gaps. The variation of non-dimensional parameters shows how these attenuation regions move in the frequency domain: the correspondent diagrams are the main tools for the design problem and are known as band-maps. The selection of the geometrical, physical and elastic properties of the unit cell is therefore dependent on the designer experience and nothing can be said about the optimality of the proposed solution. Numerical methods are used for the selection of the best cell geometry, in order to get optimal attenuation. Generally, this is a time consuming approach. In this paper, an new method is presented, based on how the waves are reflected and transmitted at cells interface. Both beam and rod case studies are investigated. The algorithm allows matching between band-gap central frequency and the desired value, while the designed attenuation is optimal there, under certain physical and geometrical constraints. Moreover, the design of the bandgap location has been decoupled from the design of the magnitude of attenuation. This approach is purely analytic, therefore the computational efforts required are minimum. In order to validate the analytical model, a passive periodic beam has been manufactured. Its real frequency response is therefore compared to the expected one.

During the last decades, a growing interest has been devoted to periodic structures and metamaterials. One of the most interesting characteristics of this class of materials is that they present a transmission gap for given frequency ranges. This peculiar characteristic has many potential applications: from optics to seismic isolation, from filtering to wave guiding. In literature, different approaches were developed to study such kind of structures. In this paper, using an approach based on transfer matrices of a single unit cell and its invariants, a way to represent in compact form the behavior of a mono-coupled periodic structure is presented. As a result, the wave propagation properties are shown as being dependent both on the frequency range and on some chosen design parameters. Furthermore, the adding of multiphysics materials (in the case of this paper piezoelectric inserts with dedicated electric circuits) inside the structure allows, through the tuning of both the mechanical and the electrical parameters, to actively control the bandgap position. This approach also allows checking the robustness of parameter choices with respect to desired bandgap frequency ranges. Finally, some applications of this method for active control of wave propagation are presented.

The adoption of wireless sensing technology by the structural health monitoring community has shown advantages over traditional cable-based systems, such as convenient sensor installation and lower system cost in many applications. Recently, a new generation of wireless sensing platform, named Martlet, has been collaboratively developed by researchers at the University of Michigan, Georgia Tech, and Michigan Tech. Martlet adopts a Texas Instruments Piccolo microcontroller running up to 90 MHz clock frequency, which enables Martlet to support high-frequency data acquisition and high-speed onboard computation. The extensible design of the Martlet printed circuit boards allows convenient incorporation of various sensor boards. In order to obtain accurate acceleration data and meanwhile reduce the sensor cost, a new Martlet sensor board, named integrated accelerometer wing, is developed. The integrated accelerometer wing adopts a commercial-off-the-shelf MEMS (microelectromechanical systems) accelerometer and contains an onboard signal conditioner performing three basic functions, including mean shifting, anti-aliasing filtering and signal amplification. One distinct feature of the signal conditioner is the on-the-fly programmable cut-off frequency and amplification gain factor. To validate the performance of Martlet and the integrated accelerometer wing, experiments are carried out on a laboratory four-story aluminum shear-frame structure. The laboratory experiment results demonstrate that the performance of the wireless sensing system is comparable to that of cabled reference sensors. In addition, using data collected by wireless sensors, vibration modal properties of the structure are identified and finite element (FE) model updating is performed.

Lamb wave based methods for structural health monitoring and non-destructive evaluation have shown promise in composite and laminated structure applications. Many methods have been considered, but exploitation of the phase of the propagating wave is a recent addition. Utilizing the phase alleviates the ambiguity of many amplitude and modal analysis approaches. This paper considered two-dimensional damage analogs in analytical simulations, numerical simulations and experimental studies. Additionally, this paper addresses the importance of proper filtering for successful spatial domain reconstruction. The phase gradient approach was successful in cases where modal filtering yielded good results. In the case where modal filtering did not successfully reconstruct the time-space domain wave, the phase gradient approach could not be employed, underscoring the need for robust modal filtering techniques.

Guided ultrasonic waves (GUW) have the potential to be an efficient and cost-effective method for rapid damage detection and quantification of large structures. Attractive features include sensitivity to a variety of damage types and the capability of traveling relatively long distances. They have proven to be an efficient approach for crack detection and localization in isotropic materials. However, techniques must be pushed beyond isotropic materials in order to be valid for composite aircraft components. This paper presents our study on GUW propagation and interaction with delamination damage in composite structures using wavenumber array data processing, together with advanced wave propagation simulations. Parallel elastodynamic finite integration technique (EFIT) is used for the example simulations. Multi-dimensional Fourier transform is used to convert time-space wavefield data into frequency-wavenumber domain. Wave propagation in the wavenumber-frequency domain shows clear distinction among the guided wave modes that are present. This allows for extracting a guided wave mode through filtering and reconstruction techniques. Presence of delamination causes spectral change accordingly. Results from 3D CFRP guided wave simulations with delamination damage in flat-plate specimens are used for wave interaction with structural defect study.

In many scenarios where vibration energy harvesting can be utilized — particularly those involving bio-motions or environmental disturbances — energy sources are broadband and non-stationary. On the other hand, design procedures have been predominantly developed for harmonic or white noise excitation, specifically for single degree of freedom approximations of the transducer. In this paper, a general approach for design optimization of cantilevered, piezoelectric energy harvesters in the presence of band-limited, white-noise excitation is outlined. For this study, human and vehicular motions are considered; these complex waveforms are distilled into a small set of dominant features with regard to their impact on the power output of the device. Criteria based on modal participation factors, including pre-filtering of the disturbance, are used in guiding the reduction of the input and plant degrees of freedom in order to make the design optimization problem tractable. This process determines the error in assuming a low-order model for the transducer in the presence of broadband noise that may excite multiple modes of vibration. Furthermore, this study considers the quantitative impact of charge cancellation in higher modes and the benefits of inserting multiple electrodes along the length. To illustrate these methods, energy harvesters are designed for acceleration data collected from walking and car idling. It is shown that a simple method that is a generalization of naïve approaches that assume harmonic or white noise excitation and a single degree of freedom can determine which simplifications are appropriate and the inaccuracies that can be expected from them.

Lamb waves are dispersive and multi-modal. Various wave modes make the interpretation of Lamb wave signal very difficult. It is desired that different modes can be separated for individual analysis. In the this paper, we present our studies on the multimodal Lamb wave propagation and wave mode extraction using frequency-wavenumber analysis. Wave spectrum in the frequency-wavenumber domain shows clear distinction among Lamb wave modes being present. This allows separating them or extracting a desired Lamb wave mode through a novel filtering strategy. Thus a single mode Lamb can be identified and extracted for certain types of damage detection in structural health monitoring (SHM). These concepts are illustrated through experimental testing. A scanning laser Doppler vibrometer is used to acquiring the time-space wavefield regarding the multimodal Lamb wave propagation. Then the recorded wavefield was analyzed in frequency-wavenumber domain and decomposed into different wave modes.

Research interests in structural health monitoring have increased due to in-situ monitoring of structural components to detect damage. This can secure personal safety and reduce maintenance effort for mechanical systems. Conventional damage detection techniques known as nondestructive evaluation (NDE) have been conducted to detect and locate damaged area in structures. Ultrasonic testing, using ultrasonic transducers or electromagnetic acoustic transducers, is one of the most widespread NDE techniques, based on monitoring changes in acoustic impedance. Although the ultrasonic testing has advantages such as high sensitivity to discontinuities and evaluation accuracy, it requires testing surface accessibility, close location to the damaged area, and decent skill and training of technicians. In recent years, modal analysis techniques to capture changes of mode shapes and natural frequency of structures have been investigated. However, the technique is relatively insensitive to small amount of damage such as an initial crack which can rapidly grow in structures under cyclic loadings. In addition, structural health monitoring based on guided waves has become a preferred damage detection approach due to its quick examination of large area and simple inspection mechanisms. There are many techniques used to analyze sensor signals to bring out features related to damage. A phased array coupled with the guided wave approach has been introduced to effectively analyze complicated guided wave signals. Phased array theory as a directional filtering technique is usually used in antenna applications. By using phased array signal processing, virtually steering the array to find the largest response of source, the desired signal component can be enhanced while unwanted information is eliminated.

Cable tension force is one of the most important structural parameters to monitor in cable-stayed bridges. For example, cable tension needs to be monitored during construction and maintenance to ensure the bridge is not overloaded. To economically monitor tension forces, this study proposes the use of an automated wireless tension force estimation system (WFTES) developed solely for cable force estimation. The design of the WFTES system can be divided into two parts: low-cost hardware and automated software. The low-cost hardware consists of an integrated platform containing a wireless sensing unit constructed from commercial off-the-shelf components, a low-cost commercial MEMS accelerometer, and a signal conditioning board for signal amplification and filtering. With respect to the automated software, a vibration-based algorithm using estimated modal parameters and information on the cable sag and bending stiffness is embedded into the wireless sensing unit. Since modal parameters are inputs to the algorithm, additional algorithms are necessary to extract modal features from measured cable accelerations. To validate the proposed WFTES, a scaled-down cable model was constructed in the laboratory using steel rope wire. The wire was exposed to broad-band excitations while the WFTES recorded the cable response and embedded algorithms interrogated the measured acceleration to estimate tension force. The results reveal the embedded algorithms properly identify the lower natural frequencies of the cable and make accurate estimates of cable tension. This paper concludes with a summary of the salient research findings and suggestions for future work.

This paper introduces a wave propagation-based damage index (DI) which relies on the estimation of phase gradients of propagating waves for accurate damage localization and on the prediction of mode conversion coefficients for damage quantification. The undamaged, or unperturbed, reference response is derived directly from the damaged component, through the application of filtering procedures in the wavenumber/frequency domain. These procedures separate incident waves from reflections caused by structural discontinuities encountered along the wave path. The DI formulation is illustrated through a numerical model of a beam with a small notch, modeled as a localized thickness reduction. The beam’s wave propagation response is simulated through the combined application of perturbation techniques and the Spectral Finite Element Method (SFEM). The resulting numerical tool allows efficient computation of the wave propagation response and the analysis of the effects of localized damages of various extent and location. The dynamic behavior of damaged beams is described through a general higher order model which couples bending and axial behavior, thus allowing the prediction of mode conversion phenomena. Numerical examples are presented to illustrate the model capabilities.