A. Alippi

Stopband edges in the dispersion curves of Lamb waves propagating in piezoelectric periodical structures

The fractional volume dependence of stopbands in piezoelectric periodical composite plates has been studied experimentally by exciting the band‐edge resonances. The frequency and symmetry of such modes below the thickness resonance of the plate are successfully interpreted in terms of a theoretical model which provides approximate dispersion curves of the lowest Lamb waves propagating in the composite plate.

Finite‐size effects in the frequency response of piezoelectric composite plates

The resonance spectrum of composite plates made of alternating piezoelectric ceramic and epoxy elements is studied both theoretically and experimentally in the frequency range below the thickness resonance. The transmissivity of the finite structure to plate modes is calculated, taking into account the effective plate velocities of the two constituent materials and the finite number of elements in the structure. The results are in excellent agreement with the electrically excited acoustical resonances of samples with different geometry and number of elements. In particular, the effects of the finite size on the number and frequency of modes, as well as their selection rules in the given experimental setup, are successfully interpreted. New ‘‘surface’’ resonances, attributed to the end elements, are observed in the stopbands when the surface elements have lower impedance.

Piezoelectric plate resonances due to first Lamb symmetrical mode

The resonances of the first Lamb s0 mode in parallelepiped piezoelectric plates excited by surface electrodes have been measured. Below the first thickness mode, the resonances are found to reproduce the dispersion of s0 in an infinite plate even for plates of small length. Moreover, a significant difference is observed between the behavior of the fundamental and its harmonics, reflecting the different effect of metallization in the two cases.

Threshold lowering for subharmonic generation in Cantor-like composite structures

We show evidence of extremely low thresholds for subharmonic generation in one-dimensional artificial composites with hierarchical structure, as compared to the corresponding homogeneous and periodic structures. The displacement profiles of the anharmonically coupled normal modes are probed experimentally, indicating that the enhanced non-linear interaction is due to the fact that, for a given fundamental mode, a second mode with much more favourable frequency and spatial matching is found in the hierarchical composite than in the non-fractal structures.

S0 lamb mode pattern investigation in piezoelectric composite plates through a heterodyne laser probe

The S0 Lamb mode pattern on the surface of a piezoelectric composite plate is experimentally determined by a heterodyne laser probe at the resonance frequencies corresponding to the symmetrical stop‐band edges. A transmission matrix model is used to calculate the surface vibration pattern and a good agreement is obtained between the theoretical and experimental results.

Stopband-edge frequencies in the resonance spectra of piezoelectric periodic composite plates

The stopband-edge frequencies of periodic composite plates made of alternating elements of piezoelectric ceramic and epoxy resin are studied both theoretically and experimentally. A model is proposed for the transmission function of plates with a finite number of elements below their thickness resonance, and the results are compared with the electrically excited acoustical resonances of samples with different structural parameters (size and number of elements, and volumic fraction). The excellent agreement between theory and experiments supports possibility of using the present approach to tailor the resonance spectra of composites in the below-thickness regime by varying their structural parameters in a controlled way.<<ETX>>

Aperiodic piezoelectric composites with Cantor-like code

Experimental results on acoustic localization in a Cantor-like piezoelectric composite are presented. Fracton modes are directly visualized and evidence about their strongly localized character is obtained. The results are compared with calculations made in terms of a transfer matrix model, and good agreement is obtained.<<ETX>>

Lifetime of localized and extended modes in complex composite structures

We have measured the lifetime of extended and localized modes electrically excited in periodic and self-similar composite piezoelectric plates. Using two different experimental methods, electrical measurement of the vibrational frequency spectrum and heterodyne optical probing of surface vibration in the time domain, evidence was found that the localized modes have on average a longer lifetime than the extended modes

Localization of acoustic waves in complex structures

The origin of the localization of acoustic waves in slightly disordered structures and the distribution of modes in the curve of the density of states are discussed. The experimental evidence of localized modes in one- and two-dimensional plates is briefly reported, together with two characteristics they present which are different from the ones of the distributed modes: an enhanced nonlinear behaviour in the generation of subharmonic waves and different lifetimes

Low threshold subharmonic generation in fractal piezoelectric composites

Subharmonic Lamb modes were observed in composite piezoelectric plates with self-similar structure. The thresholds for their generation are very low as compared to those of homogeneous or periodical plates with comparable acoustical and electrical parameters. In the frame of a first-order perturbative theory, this can be explained in terms of the unusually favorable frequency and spatial matching of coupled localized and extended modes in these structures.<<ETX>>