A combined genetic algorithm and finite element method for the determination of a practical elasto-electric set for 1-3 piezocomposite phases.

1-3 piezocomposites are widely used in ultrasonic transducers, particularly for imaging applications. The fabrication process is often based on the dice and fill method, leading to a periodic structure. This process can modify the initial properties of the two phases due to the machining of the piezoelectric bulk ceramic and setting of the polymer. A method is proposed to directly determine a practical set for 1-3 piezocomposite properties and all the elastic, dielectric and piezoelectric parameters of the two piezoelectric (11 constants) and inert phases (3 constants). This method is based on a fitting process of the electrical impedance as a function of frequency (one thickness and two lateral modes). For this purpose, a genetic algorithm coupled with a finite element method (GA/FEM) was used in an iterative process to deduce all these parameters. This method was first performed on a numerical phantom (Pz21/epoxy resin). Comparisons showed that the GA/FEM obtained a good set of the 14 parameters, and the accuracy of several parameters was discussed. Finally, the GA/FEM algorithm was applied to a fabricated 1-3 piezocomposite (dice and fill method). The results showed that the fabrication process introduced several changes in the properties of the two phases (in particular, the dielectric constants of the ceramic and one elastic constant of the polymer) compared to the initial commercial data, while keeping the identical thickness coupling factor at 64%.

Generation of transversal envelope soliton in polymeric and wooden rods.

This paper is concerned with the probing of the transversal envelope solitons propagation in circular waveguides when a set of requirements (non-linearity and dispersion) are fulfilled in the waveguide and balanced. The basic idea is to analyze the shape of an acoustic pulse after it has traveled one or few trips through samples constituted of a rod and two ended beads. The dispersive behavior is associated to the bounded medium (rod) and the contacts between the elements of the specimens are assumed being described by non-linear Hertz' law type. The experimental data are obviously material dependent and have pointed out the existence of common properties on the formation and propagation properties of the envelope solitons whatever is the material (polymers, carbon fibers and wood) of the rods and spheres. Peculiar behaviors were also observed for specific material (woods) probably caused by the anisotropy of this kind of rod material leading to a double envelope soliton.

Extension of the crosstalk cancellation method in ultrasonic transducer arrays from the harmonic regime to the transient one.

This paper describes a procedure to extend the crosstalk correction method presented in a previous paper [A. Bybi, S. Grondel, J. Assaad, A.-C. Hladky-Hennion, M. Rguiti, Reducing crosstalk in array structures by controlling the excitation voltage of individual elements: a feasibility study, Ultrasonics, 53 (6) (2013) 1135-1140] from the harmonic regime to the transient one. For this purpose a part of an ultrasonic transducer array radiating in water is modeled around the frequency 0.5 MHz using the finite element method. The study is carried out at low frequency in order to respect the same operating conditions than the previous paper. This choice facilitated the fabrication of the transducer arrays and the comparison of the numerical results with the experimental ones. The modeled array is composed of seventeen elements with the central element excited, while the others are grounded. The matching layers and the backing are not taken into account which limits the crosstalk only to the piezoelectric elements and fluid. This consideration reduces the structure density mesh and results in faster computation time (about 25 min for each configuration using a computer with a processor Intel Core i5-3210M, frequency 2.5 GHz and having 4 Go memory (RAM)). The novelty of this research work is to prove the efficiency of the crosstalk correction method in large frequency band as it is the case in medical imaging. The numerical results show the validity of the approach and demonstrate that crosstalk can be reduced by at least 13 dB in terms of displacement. Consequently, the directivity pattern of the individual element can be improved.

Reducing crosstalk in array structures by controlling the excitation voltage of individual elements: a feasibility study.

This paper describes a procedure to minimize crosstalk between the individual elements of a piezoelectric transducer array. A two-dimensional finite elements model was developed and the excitation voltages predicted by the model were applied to the array prototypes made of PZT 27 ceramic. Symmetric and asymmetric linear phased arrays operating at approximately 450 kHz were tested in the feasibility study. The studies were carried out at low frequency to facilitate the fabrication of the transducer arrays and to check the feasibility in this case. The novelty of our approach is to offer active cancellation of crosstalk in transducer arrays generating continuous waves, even in the presence of fabrication defects. The experimental results showed the validity of the approach and demonstrated that crosstalk can be reduced by about 6-10 dB. In ultrasonic imaging systems, this method could be introduced by using a multichannel generator providing electrical signals containing both phased signals required to focalize and deflect the acoustic beam associated with the correction signals.

On the formation of envelope solitons with tube ended by spherical beads.

In this paper it is suggested the possibility of using a simple device to explore the propagation of short transverse perturbations in specimens of three linearly aligned elements (bead, tube and bead). By using time signal analysis and frequency spectroscopy some properties of the transmitted wave packets are experimentally analyzed in the acoustic domain. It is clearly demonstrated that the shape and the velocity of propagation of the waves do not change with distance. The amplitude may vary depending on the dissipation effect. These properties allow us to identify the wave packets as "envelope solitons". The influence of the elasticity of these materials composing the beads and the tube on the generation of envelope solitons is also investigated.

Acoustic wave transmission through piezoelectric structured materials.

This paper deals with the transmission of acoustic waves through multilayered piezoelectric materials. It is modeled in an octet formalism via the hybrid matrix of the structure. The theoretical evolution with the angle and frequency of the transmission coefficients of ultrasonic plane waves propagating through a partially depoled PZT plate is compared to finite element calculations showing that both methods are in very good agreement. The model is then used to study a periodic stack of 0.65 PMN-0.35 PT/0.90 PMN-0.10 PT layers. The transmission spectra are interpreted in terms of a dispersive behavior of the critical angles of longitudinal and transverse waves, and band gap structures are analysed. Transmission measurements confirm the theoretical calculations and deliver an experimental validation of the model.

On the splitting of aspherical structure submitted to an acoustic polarized excitation.

We experimentally show how the deviations from spherical symmetry such as the inhomogeneity, the removal mass or the asphericity affect the frequency spectrum of a single elastic sphere. The recorded spectrograms point out the great influence of the polarization of the source on the splitting and the shift of the peaks. Qualitatively, the results are consistent with the data reported in Geophysics although the present study is concerned with a low degree and low- [Formula: see text] modes in contrast to those usually encountered in seismic investigations. From a practical point of view, this study suggests that Geophysics phenomena could be analyzed through spherical beads reproducing the imperfections encountered in the mantle and in the core of the earth.

Modeling and underwater characterization of cymbal transducers and arrays.

The cymbal is a miniaturized class V flextensional transducer that was developed for potential use as a shallow water sound projector and receiver. Single elements are characterized by high Q, low efficiency, and medium power output capability. Its low cost and thin profile allow the transducer to be assembled into large flexible arrays. Efforts were made to model both single element and transducer arrays by coupling finite element analysis (ATILA) and the integral equation formulation (EQI). The pressure and velocity distributions on the surface elements were calculated by ATILA and later used with EQI to calculate the far field properties of the transducer element and arrays. It eliminates the mesh of the fluid domain and makes the 3-D model of a transducer possible. Three-dimensional models of a cymbal transducer and a 3 x 3 cymbal array were developed in the modeling. Very good agreement was obtained between modeling and measurement for single element transducers. By coupling finite element analysis with the integral equation method using boundary elements, acoustic interaction effects were taken into account. Reasonable agreement was obtained between calculation and measurement for a 3 x 3 array.

A miniature class V flextensional cymbal transducer with directional beam patterns: the double-driver.

The "double-driver" cymbal, a directional class V flextensional transducer, is described in this paper. Its basic structure is a bilaminar piezoelectric disk with metal caps as mechanical transformers and amplifiers. The directivity was accomplished by exciting the double-driver in a combined flexural and bending mode causing the sound pressure to add in one direction and cancel in the opposite direction. The cardioid beam pattern predicted by finite element modeling agreed well with the experimental measurements. A 3 x 3 double-driver array was constructed to demonstrate that under optimal conditions the array can provide a directional beam pattern with a front-to-back ratio of more than 20 dB.