KEYWORDS: Transducers, Electrodes, Ferroelectric materials, Microsoft Foundation Class Library, Manufacturing, Composites, Structural health monitoring, Semiconducting wafers, Epoxies, Actuators
This work is focused on design and testing of a novel class of transducers for Structural Health Monitoring (SHM), able to perform directional interrogation of plate-like structures. These transducers leverage guided waves (GWs), and in particular Lamb waves, that have emerged as a very prominent option for assessing the state of a structure during operation. GW-SHM approaches greatly benefit from the use of transducers with controllable directional characteristics, so that selective scanning of a surface can be performed to locate damage, impacts, or cracks. In the concepts that we propose, continuous beam steering and directional actuation are achieved through proper selection of the excitation frequency. The design procedure takes advantage of the wavenumber representation of the device, and formulates the problem using a Fourier-based approach. The active layer of the transducer is made of piezoelectric fibers embedded into an epoxy matrix, allowing the device to be flexible, and thus suitable for application on non{ at surfaces. Proper shaping of the electrodes pattern through a compensation function allows taking into account the anisotropy level introduced by the active layer. The resulting spiral frequency steerable acoustic actuator is a configuration that features (i) enhanced performance, (ii) reduced complexity, and (iii) reduced hardware requirements of such devices.
In this work we explore efficient transformation of broadband wave energy into low-power electricity using patterned polymer piezoelectrics integrated with an Elliptical Acoustic Mirror (EAM) configuration. The mirror under consideration features a semi-elliptical continuous mirror with a rectangular arrangement of harvesting material overlapping the geometrical focus of the mirror. Spatial and temporal transformation of the wave propagation field into the frequency-wavenumber domain is performed in order to identify the wavenumber content inside the mirror region. A frequency-domain Root-Mean-Square (RMS) evaluation is then applied in order to guarantee broadband harvesting characteristics to the resulting Distributed Harvester (DH). Computational modeling and experimental testing are employed to quantify performance enhancement of the presented approach in the 20-120 kHz range, where broadband focusing characteristics of the continuous EAM are confirmed experimentally. Additionally the patterned configuration with proper wiring results in substantial power enhancement over 20-60 kHz, i.e. the neighborhood of the center frequency used in its Fourier transform-based design.
Recent work has demonstrated efficient transformation of structure-borne propagating waves into low-power electricity using metamaterial-inspired mirror configurations. Elastoacoustic waves (i) originating from a point source and (ii) arriving as plane waves have been successfully focused on a piezoelectric energy harvester using elliptical and parabolic mirror concepts, respectively. Our present work investigates the spatial optimization of a piezoelectric energy harvester domain weakly coupled to a thin plate housing an elastoacoustic mirror (or lens). Mirrors considered include elliptical arrangements of periodic stubs, and an elliptical arrangement of continuous material. Spatial and temporal transformation of the wave propagation field into the frequency- wavenumber domain is performed in order to identify the wavenumber content inside the mirror. A frequency- domain root-mean-square (RMS) evaluation is then applied to the transformed field in order to extract the preferred propagation directions. Computational modeling and experimental testing are employed to quantify performance enhancement of the presented approach. Specifically, dramatic enhancement of the harvested power output is reported by patterned electroding of a rectangular PVDF harvester in the elliptical mirror domain.
The paper reports on the design of surface acoustic wave strain sensing devices based on a Fourier approach. A patterned sensing surface acts as a narrow band filter for surface acoustic waves. The center frequencies vary as functions of the amount of strain present in the sensing area, and can be thus used to estimate the surface strain. The design of the sensing pattern in the Fourier space allows the selection of multiple sensing frequencies and for their shifting characteristics to be related to the three components of strain, i.e. two normal and one shear. The result is a surface acoustic wave strain gauge that acts as a strain rosette. The design procedure formulates the problem in the wavenumber domain, whereby the radiation characteristics of the sensing surface in response to an incident broadband surface wave are selected to ensure sensitivity to all three strain components. The concept is first illustrated for a one-dimensional pattern, whose radiation characteristics are governed by a simple, scalar, grating equation. The design approach is then extended to a two-dimensional pattern to demonstrate the ability to simultaneously measure all three surface strain components. The rosette-like operation of the considered strain sensor is demonstrated through numerical simulations conducted on an elastic surface subjected to a pre-imposed strain state. Eventually, the multi-band filtering properties of the proposed sensor patterning are evaluated experimentally by means of a Scanning Laser Doppler Vibrometer (SLDV).
Stiffness and damping are conflicting requirements in many material systems. High stiffness is required in a wide range of structural components to provide sufficient robustness under demanding loading conditions. Simultaneously, a structure should be able to effectively mitigate shock and vibrations dynamically transmitted to it by the environment. While most conventional structures currently exhibit limited adaptability and damping capabilities, design strategies to simultaneously endow structural assemblies with high stiffness and high damping performance are proposed in this work. To this aim, a backbone structure suitable to meet stiffness requirements is combined with metamaterial inclusions able to provide fully-passive shock and vibration absorption. Viscoelastic resonant lattices with chiral topology are employed as inclusions, whose aim is to confine vibrational energy, pump it away from the backbone structure, and dissipate it through viscoelastic damping. The lattices are composed by an elastomeric matrix with the desired chiral shape, and stiff resonating inclusions are inserted at nodal locations. Both finite element simulations and experimental tests demonstrate that periodic chiral assemblies give rise to wide frequency bandgaps. Low-frequency tuning of the assembly for effective suppression of the first resonant mode of a backbone structure represented by an aluminum box-beam is demonstrated both numerically and experimentally. The considered lightweight inclusion is a chiral matrix realized with castable rubber, featuring graded cylinder mass insertions. The proposed design methodology can be flexibly tailored to various frequency ranges and is applicable to both existing and novel structural components at different scales.
In this paper, we investigate the scattering behavior of defects in composite plates. Scattering coefficients are of great importance for the estimation of wave-damage interaction, for the interpretation of recorded Lamb wave signals, and for the development of novel signal processing strategies. The anisotropy of composite laminates makes modeling of wave propagation and the numerical estimation of scattering coefficients particularly challenging. The paper studies numerical models of wave propagation in composites, and evaluates their predictive ability. These evaluations rely of full wave field measurements on selected composite plate specimens, through which information on dispersion and directionality of propagation are conveniently extracted. In addition, filtering in the frequency-wavenumber domain allows the extraction of scattered wave fields and the estimation of the scattering coefficients. Comparisons between numerical and experimental data highlight modeling challenges, illustrate mesh-driven directional propagation, and suggest an effective strategy for the estimation of the scattering coefficients through tests and FE modeling.
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