Nicolas Perez

Identification of elastic, dielectric, and piezoelectric constants in piezoceramic disks

Three-dimensional modeling of piezoelectric devices requires a precise knowledge of piezoelectric material parameters. The commonly used piezoelectric materials belong to the 6mm symmetry class, which have ten independent constants. In this work, a methodology to obtain precise material constants over a wide frequency band through finite element analysis of a piezoceramic disk is presented. Given an experimental electrical impedance curve and a first estimate for the piezoelectric material properties, the objective is to find the material properties that minimize the difference between the electrical impedance calculated by the finite element method and that obtained experimentally by an electrical impedance analyzer. The methodology consists of four basic steps: experimental measurement, identification of vibration modes and their sensitivity to material constants, a preliminary identification algorithm, and final refinement of the material constants using an optimization algorithm. The application of the methodology is exemplified using a hard lead zirconate titanate piezoceramic. The same methodology is applied to a soft piezoceramic. The errors in the identification of each parameter are statistically estimated in both cases, and are less than 0.6% for elastic constants, and less than 6.3% for dielectric and piezoelectric constants.

Matrix method for acoustic levitation simulation

A matrix method is presented for simulating acoustic levitators. A typical acoustic levitator consists of an ultrasonic transducer and a reflector. The matrix method is used to determine the potential for acoustic radiation force that acts on a small sphere in the standing wave field produced by the levitator. The method is based on the Rayleigh integral and it takes into account the multiple reflections that occur between the transducer and the reflector. The potential for acoustic radiation force obtained by the matrix method is validated by comparing the matrix method results with those obtained by the finite element method when using an axisymmetric model of a single-axis acoustic levitator. After validation, the method is applied in the simulation of a noncontact manipulation system consisting of two 37.9-kHz Langevin-type transducers and a plane reflector. The manipulation system allows control of the horizontal position of a small levitated sphere from -6 mm to 6 mm, which is done by changing the phase difference between the two transducers. The horizontal position of the sphere predicted by the matrix method agrees with the horizontal positions measured experimentally with a charge-coupled device camera. The main advantage of the matrix method is that it allows simulation of non-symmetric acoustic levitators without requiring much computational effort.

The spatial focusing of a leaky time reversal chaotic cavity

Elastic waves propagating inside a solid chaotic cavity create a diffusive random field that contains both longitudinal and shear waves. In the current paper, we are interested in the field radiated in a fluid in contact with such cavity. The goal of this paper is to predict the spatial focusing properties that can be obtained in the fluid using a time-reversal piezoelectric transducer in contact with the cavity. We present a statistical approach that supposes a fully diffused wavefield inside the cavity with an equipartition of energy between longitudinal and shear waves. We show that the critical angles of transmission in the solid–fluid interface generate a cut-off of the spatial frequencies and then a degradation in the spatial focusing. This limitation can be overcome using a rough surface. A set of experiments conducted in the MHz range confirm the theoretical model.

A large aperture ultrasonic receiver for through-transmission determination of elastic constants of composite materials

This paper describes the use of a large aperture PVDF receiver in angle beam through-transmission method of velocity measurement in fiber reinforced composites. This technique avoids the beam diffraction effect that occurs when using limited size ultrasonic transducers. This effect increases as the frequency decreases for the same size of transducer. On the other hand, the velocity dispersion effect, present in composite materials, increases with frequency. Analyzing the diffraction effect, it was observed that the longitudinal velocity in an aluminum plate increases more than 1% when using a pair of 1 MHz transducers. That effect disappears when using the large aperture receiver and it is negligible when using a pair of 10 MHz transducers. On the other hand, it was observed that in the acrylic plate the longitudinal velocity increases 0.7%, and increases more than 0.8% in a 2.115 mm thick CFRP (carbon fiber reinforced plastic) plate, from 1 to 10 MHz. As a compromise between axial resolution and velocity dispersion, the elastic constants of the unidirectional CFRP plate were determined in the frequency of 2.25 MHz, showing good agreement with tensile test results.

Strain sensitivity model for guided waves in plates using the time-reversal technique

The use of the time-reversal technique to detect variations in the external applied traction in a strip of aluminum plate is presented. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the plate. Two different parameters of the time-reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. The inverse filter technique was used in the transmitted signal to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by 5 times. The external traction sensitivity was experimentally verified by using an aluminum plate (800 × 100 × 3 mm) and two 5-MHz ultrasonic transducers spaced 700 mm apart. At the maximum strain state (180 μstrain), the peak value was reduced by about 10% in the conventional process and by 50% using the inverse filter. To evaluate the effects of the strain in the time-reversal signal, a theoretical model was constructed. This model is successful in predicting the changes in the group delay and, consequently, the focusing time using a linear equation. This relationship can also be used to determine the strain level quantitatively. The experimental results show that the time-reversal signal technique can be used in practical monitoring of changes at the strain level in mechanical structures.

Thick film PZT arrays vibration modes

In this work we report some electroacoustic characteristics of miniature piezoelectric ultrasound transducers, manufactured by screen printing and using thick film technology. To make transducer samples, ink with PZT powder and a glass frit was employed. PZT films of 100-130 /spl mu/m thick were screen-printed between two gold electrodes over alumina. Electrical impedance and acoustic emission continuous-wave analysis at 1-10 MHz range shows two main vibrating modes, one of PZT layer itself and other with substrate and PZT vibrating as a whole and coupling characteristics of a two elements set and an array.

Development of a mechanical strain sensor based on time reversal of ultrasonic guided waves

The development of a strain sensor based on the time reversal focusing technique is presented. The sensor is composed by a strip of aluminum plate with two ultrasonic piezocomposite transducers bonded at the ends of the plate. The time reversal technique acts as a dispersion compensator of the guided waves propagating in the plate, allowing time recompression of the waves. When the plate is subjected to a longitudinal traction, a time reversal focusing is performed between the transducers in order to detect the change in the focus due to strain. The strain can be evaluated by measuring the change of the amplitude and shift in the time of flight, and comparing them with a reference signal obtained at zero strain state. In order to improve the systems sensitivity, 2-2 piezocomposite transducers designed to operate between 0.2 to 3.0 MHz are used. Experiments are conducted by applying strain up to 150 μ-strain. The results show an increase in sensitivity when compared with the results of the conventional mono-element transducer greater than 200%. Results presented here can be used in the project of stress monitoring transducers and structural health systems.

Ultrasonic measurement of micrometric wall-thickness loss due to corrosion inside pipes

Pipelines are subject to wall-thickness loss due to corrosion along time. Ultrasonic pulse-echo techniques are widely used for thickness measurement achieving a high resolution. However, the precision of measurement is highly dependent on temperature and on the ultrasonic system. This work presents the temperature correction strategy of an ultrasonic measurement system to obtain one micron accuracy, in a pipeline corrosion long-lasting monitoring. The proposed technique is based on the fact that the coupling layer has a constant thickness during a long period of time (year) and can be used in the same way as a thermometer to compensate the changes in the ultrasonic velocity. The variation of time of flight in the coupling layer can relate to the variation of time of flight in the pipe wall to construct a correction polynomial. This function compensates the variation on propagation velocity and thermal expansion. The results are experimentally evaluated using an array of eight ultrasonic transducers (5 MHz, 10-mm diameter) operating in pulse-echo mode with a water coupling layer. Long term corrosion tests were conducted using an electrolytic bath along ten months. Good agreement was found between the theoretical corrosion rate and the results of the ultrasonic measuring system.

Identification of piezoelectric complex parameters in rings for power ultrasound applications

Power ultrasonic devices frequently use Langevin type transducers. These types of transducers are essentially constructed using a sandwich of piezoelectric rings and two metal masses at the ends. The whole assembly is tuned to resonate in a desired main frequency and the total length corresponds to a half of the wavelength of that frequency. Finite element simulations (FEM) are used in the design of such complex structures; however the accuracy of the results is limited by the knowledge of the constitutive properties for the materials used in the transducer construction. Metals like aluminum or steel are well characterized, but the complete set of piezoelectric parameters for piezoceramics are difficult to find in the literature. In the few cases where the manufacturer gives the complete set of parameters, strong differences are observed between simulated and experimental data. In this work a novel methodology proposed by our research group is applied in the case of piezoelectric rings made with a hard piezoelectric material. The results are evaluated in rings with internal diameter 8 mm, external diameter 27 mm and thickness 5 mm. Finally the results are validated using an optical interferometer showing a good agreement.

Fabrication and characterization of piezoelectric thick film elements and arrays

PZT thick film discs have been produced in the range of 100-200 /spl mu/m using screen printing technology and suitable new ink prepared with piezoelectric powder as main active component. After poling, characterization of electrical impedance was made and an acoustic spectroscopy technique used to analyze the vibrating behavior of PZT discs over different substrates in order to know typical parameters of the film. Also a dark field Schlieren technique was employed to evaluate emission field in water. It has been found that thick film piezoelectric layer of this material has a lower piezoelectric charge constant d/sub 33/ and remanent polarization than the bulk one. Electrical impedance and acoustic emission analysis shows two main resonance modes, one of the PZT layer itself and other with the substrate and PZT vibrating as a whole. We also show some results evaluating a manufactured array structure with this technology and coupling modes through the substrate.