Matteo Senesi

Broadband plate-type acoustic metamaterial for low-frequency sound attenuation

We show experimentally that plate-type acoustic metamaterials can serve to totally prohibit low frequency structure-borne sound at selective resonance frequencies ranging from 650 to 3500 Hz. Our metamaterial structures are consisting of a periodic arrangement of composite stubs (tungsten/silicone rubber) deposited on a thin aluminium plate. We report that these metamaterials present a broadband gap of out-of-plane modes at frequencies where the relevant sound wavelength in air is about three orders of magnitude larger than the plate thickness. Confinement and waveguiding of structure-borne sound in this sub-wavelength resonant regime is also experimentally evidenced and discussed.

Experimental characterization of periodic frequency-steerable arrays for structural health monitoring

Beam steering through phased arrays is a well-established technique, used extensively in ultrasonic imaging for medical, NDE and SHM applications. Phased arrays typically need individual control of their elements, which involves hardware and software complexity. This paper presents the characterization of a novel frequency-steerable array for structural health monitoring. In the considered configuration, beam steering is achieved by exploiting interference phenomena generated by the spatial lay-out of the array elements, and their simultaneous activation at specific frequencies. Such frequencies correspond to wavenumbers which are associated with radiation in determined spatial directions. In essence, the array acts as a spatial filter, which preferentially radiates at wavenumbers defined by the spatial arrangement of the elements. As such, the array is also effective at tuning its radiation to specific wave modes. In this paper, a simple quadrilateral periodic topology illustrates the directional properties of the array and shows its tuning capabilities. The investigations are supported by a preliminary numerical analysis, which is used to design an experimental prototype. Tests successfully validate the numerical predictions and demonstrate the directional and tuning capabilities of the proposed array design.

Double-channel, frequency-steered acoustic transducer with 2-D imaging capabilities

A frequency-steerable acoustic transducer (FSAT) is employed for imaging of damage in plates through guided wave inspection. The FSAT is a shaped array with a spatial distribution that defines a spiral in wavenumber space. Its resulting frequency-dependent directional properties allow beam steering to be performed by a single two-channel device, which can be used for the imaging of a two-dimensional half-plane. Ad hoc signal processing algorithms are developed and applied to the localization of acoustic sources and scatterers when FSAT arrays are used as part of pitch-catch and pulse-echo configurations. Localization schemes rely on the spectrogram analysis of received signals upon dispersion compensation through frequency warping and the application of the frequency-angle map characteristic of FSAT. The effectiveness of FSAT designs and associated imaging schemes are demonstrated through numerical simulations and experiments. Preliminary experimental validation is performed by forming a discrete array through the points of the measurement grid of a scanning laser Doppler vibrometer. The presented results demonstrate the frequency-dependent directionality of the spiral FSAT and suggest its application for frequency-selective acoustic sensors, for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health.

A frequency selective acoustic transducer for directional Lamb wave sensing.

A frequency selective acoustic transducer (FSAT) is proposed for directional sensing of guided waves. The considered FSAT design is characterized by a spiral configuration in wavenumber domain, which leads to a spatial arrangement of the sensing material producing output signals whose dominant frequency component is uniquely associated with the direction of incoming waves. The resulting spiral FSAT can be employed both for directional sensing and generation of guided waves, without relying on phasing and control of a large number of channels. The analytical expression of the shape of the spiral FSAT is obtained through the theoretical formulation for continuously distributed active material as part of a shaped piezoelectric device. Testing is performed by forming a discrete array through the points of the measurement grid of a scanning laser Doppler vibrometer. The discrete array approximates the continuous spiral FSAT geometry, and provides the flexibility to test several configurations. The experimental results demonstrate the strong frequency dependent directionality of the spiral FSAT and suggest its application for frequency selective acoustic sensors, to be employed for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health.

Piezoelectric superlattices as multi-field internally resonating metamaterials

Piezoelectric superlattices are investigated as examples of internally resonating metamaterials. The multi-field coupling characteristics of the considered configuration is identified as the mechanism enabling the generation of the internal resonances, and the related achievement of unusual wave properties. Numerical studies on two-dimensional piezoelectric superlattices illustrate the coupled behavior of this class of periodic systems. In addition, analytical studies developed on the basis of the long wavelength approximation support the interpretation of the coupling as an internally resonant mechanism, and allow the analysis of the influence of lattice topology on the frequencies of internal resonance.

Fabrication and Characterization of a Wavenumber-Spiral Frequency-Steerable Acoustic Transducer for Source Localization in Plate Structures

This paper reports on the fabrication and the experimental characterization of a wavenumber frequency-steerable acoustic transducer (WS-FSAT). Here, the transducer is employed for the localization of broadband acoustic events corresponding to the propagation of guided elastic waves in an isotropic plate. The WS-FSAT records the plate response and defines the source location through a time-frequency analysis of the received signal. This is achieved by exploiting the frequency selective response of the transducer which directly maps the dominant component of the received signal to the direction of arrival of the incoming wave. This feature is the result of the spatial filtering effect produced by the characteristic shape of the sensing surface, which is designed in the wavenumber domain. Experiments are performed on a prototype fabricated on a polyvinylidene fluoride substrate mounted on an aluminum test plate. Tests are conducted for various source locations, and with multiple sources activated simultaneously. The results highlight the robustness of the proposed device, its good sensitivity and angular resolution, as well as the low complexity of hardware and signal processing. This paper suggests the WS-FSAT as an attractive solution for the detection of broadband acoustic events, such as impacts on structural substrates, and its potential use as part of active structural health monitoring systems based on pitch-catch or pulse-echo operations.

Frequency-Steered Acoustic Arrays: Application to Structural Health Monitoring of Composite Plates

This paper presents the design, characterization, and application of periodic piezoelectric actuator arrays for structural health monitoring. In the proposed array configuration, all elements are activated simultaneously to achieve strong frequency-dependent directional actuation, which allows beam steering through a sweep of the excitation frequency and limited hardware requirements. The array enables in situ monitoring of critical components through strongly focused actuation and directional scanning capabilities. The concept is illustrated for a selected design with quadrilateral topology on an isotropic plate. The application to composite panels is also specifically discussed by illustrating the complexity of the corresponding dispersion properties, which suggest design challenges for mode tuning. A quadrilateral design is tested for excitation of the A 0 mode on a selected lay-up sequence to illustrate the generality of the proposed concept and its applicability to composites.

A spiral frequency steerable acoustic transducer for SHM

A Frequency Steerable Acoustic Transducer (FSAT) is proposed for directional sensing of guided waves. The considered FSAT design is characterized by a spiral configuration in wavenumber domain, which leads to a spatial arrangement of the sensing material producing output signals whose dominant frequency component is uniquely associated with the direction of incoming waves. The resulting spiral FSAT can be employed both for directional sensing and generation of guided waves, without relying on phasing and control of a large number of channels. The analytical expression of the shape of the spiral FSAT is obtained through the theoretical formulation for continuously distributed active material as part of a shaped piezoelectric device. Testing is performed by forming a discrete array through the points of the measurement grid of a Scanning Laser Doppler Vibrometer. The discrete array approximates the continuous spiral FSAT geometry, and provides the flexibility to test several configurations. The experimental results demonstrate the strong frequency dependent directionality of the spiral FSAT and suggest its application for frequency selective acoustic sensors, to be employed for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health.

Piezoelectric Superlattices and Shunted Periodic Arrays as Tunable Periodic Structures and Metamaterials

Two examples of internally resonating metamaterials with behavior based on multi-field coupling are illustrated. The first example consists in a 1D waveguide with a periodic array of shunted piezoelectric patches. Each patch is shunted through a passive circuit which induces resonance in the equivalent mechanical impedance of the waveguide. Analytical, numerical and experimental studies illustrate the characteristics of the system and quantify such resonant mechanical properties due to electro-mechanical coupling. Piezoelectric superlattices are presented as additional examples of internally resonant metamaterials. Multi-field coupling is identified as the enabler mechanism for the generation of the internal resonance. Numerical studies for 1D and 2D piezoelectric superlattices and analytical studies developed on the basis of the long wavelength approximation support the interpretation of the coupling as an internally resonant mechanism.

Experimental demonstration of directional GW generation through wavenumber-spiral Frequency Steerable Acoustic Actuators

Directional inspection using guided waves (GWs) is a convenient approach for Structural Health Monitoring (SHM) of large 2D regions. While beam steering is conventionally achieved through phased arrays, often at the cost of a considerable hardware and software complexity, a single, differential-channel actuator is employed in this work to send GWs in arbitrary directions by properly choosing the excitation frequency. This frequency-steerable directional scanning is achieved through an angle-dependent wavelength tuning provided by the peculiar shape of the piezoelectric patch which forms the transducer. Although the underlying theoretical framework of Frequency-Steerable Acoustic Transducers (FSATs) encompasses both directional generation and sensing of GWs, directional actuation has only been predicted by simulations so far. This paper presents for the first time an experimental study of directional GW generation on an aluminum plate through an FSAT fabricated by inkjet printing of the electrodes on a PVDF substrate and previously operated in sensing mode.