F.M. de Espinosa

High sensitive piezoelectric transducers for NDE air borne applications

We present work intended to help in the systematization of the manufacturing of piezoelectric transducers for NDT air coupling. Only the piezoelectric and passive materials, together with the construction techniques, are studied. A high driving signal method has been used to select the piezoelectric materials. Flat and spherically-shaped transducers from 500 kHz up to 2.5 MHz made of piezoceramics and 1-3 piezocomposites have been manufactured and tested. Highly porous materials have been used to match the transducers and the scanned structures.

A computational method to calculate the longitudinal wave evolution caused by interfaces between isotropic media

This paper presents a computational method to calculate the reflected and transmitted ultrasonic fields at interfaces of complex geometry. The method is performed in two steps. As first step, the velocity potential impulse response from an arbitrary aperture is determined at the interface using the Rayleigh integral and considering the reflection and transmission coefficients. In a second step, the simulated fields are calculated by applying the Rayleigh-Sommerfeld integral to the whole, extended interface. In order to validate the method, some experimental cases as, for instance, plane and cylindrical concave surfaces between two media (water-acrylic) were tested. The experimental ultrasonic fields are in good agreement with those provided by the model. Furthermore, in the work, the compromise between the accuracy of the method and the computation time is studied.

Energy method to calculate the density of liquids using ultrasonic reflection techniques

In this work it is introduced a new approach to calculate the density of liquids in terms of the energies of the acoustic signals. This method is compared to other methods in the time domain (peak-to-peak amplitudes) and frequency domain (magnitudes at a single frequency). It is used a measurement cell based on a multiple reflection technique, and it is developed an acoustic model for the cell. Simulations and experiments using several liquids are presented, showing that the energy method is less sensitive to noise than the other techniques. The relative errors in the density are smaller than 0.2% when compared to the values measured with a pycnometer.

P3R-3 Design and Characterization of Air Coupled Ultrasonic Transducers Based on MUMPs

This paper deals with the design and characterization of several capacitive micromachined ultrasonic transducer (cMUT) cells for the future design of an air-coupled transducer for non destructive testing (NDT). Each design was manufactured using the multi-user MEMS process (MUMPs). Special boundary conditions were used to obtain high efficiency: two opposite sides of the 150 mum square shaped membranes are free while the other sides are anchored to a fixed layer of the process. Using these designs a large displacement of the membrane and a higher surface contact is obtained in spite of their mechanical losses

Double frequency piezoelectric transducer design for harmonic imaging purposes in NDT

Harmonic imaging (HI) has emerged as a very promising tool for medical imaging, although there has been little published work using this technique in ultrasonic non-destructive testing (NDT). The core of the technique, which uses nonlinear propagation effects arising in the medium due to the microstructure or the existence of defects, is the ability to design transducers capable of emitting at one frequency and receiving at twice this frequency. The transducers that have been used so far are usually double crystal configurations with coaxial geometry, and commonly using a disc surrounded by a ring. Such a geometry permits the design of broadband transducers if each transducer element is adapted to the medium with its corresponding matching layers. Nevertheless, the different geometry of the emission and reception apertures creates difficulties when resolving the images. In this work, a new transducer design with different emission and reception apertures is presented. It makes use of the traditional construction procedures used to make piezocomposite transducers and the well-known theory of the mode coupling in piezoelectric resonators when the lateral dimensions are comparable with the thickness of the piezoceramic. In this work the design, construction, and characterization of a prototype to be used in NDT of metallic materials is presented. The acoustic field is calculated using water as a propagation medium, and these theoretical predictions then are compared with the experimental measurements. The predicted acoustic performances for the case of propagation in stainless steel are shown.

Ultrasonic Lamb wave NDE system using an air coupled concave array transducer

Rapid and non contact ultrasonic NDE techniques are of great industrial interest. This paper describes an air-coupled ultrasonic inspection system based on concave linear arrays working on pitch-catch configuration, The system has been designed for real-time characterisation of sheet and plate manufactured materials such as paper and resin-fibre composites. The proposed system is based on air-coupled Lamb wave excitation and reception that performs a rapid measurement of the optimum input angle of the incident beam impinging the material surface. No mechanical parts are used for tuning the plate wave excitation with the angle, doing that electronically by steering the acoustic beam. This solution increases the exploration velocity and the measurement repeatability and system reliability. The main contributions are related to the utilization of a 0.8 MHz ultrasonic air-coupled concave array transducer. This transducer, using only 32 elements, is able to generate a 2.5 square cm size flat wavefront, steering up to /spl plusmn/26 degrees with 1.6 degree resolution, keeping the distance of wave flight and the impact point constant. The angular resolution can be improved up to 0.2 degrees using a novel micro-deflection technique, without any increment of the system complexity.

Fabrication and characterization of silica aerogel films for air-coupled piezoelectric transducers in the megahertz range

New materials and designs are needed for efficient air-coupling of piezoelectric transducers in the megahertz range. In this sense, a new type of matching layer has been essayed and is presented here. They are made of two quarter-wavelength (/spl lambda//4) layers (both at 3 MHz): a silica aerogel film (0.015 Mrayl) and a polymer sheet. The aerogel film was directly produced on the polymer sheet and it was characterized by scanning electron microscopy. The matching layer was attached to a PZ-27 piezoceramic (3 MHz), the electrical impedance was both measured and theoretically calculated. The pitch-catch response of transducers (3 MHz) that could be produced with this technology was theoretically calculated.

Ultrasonic velocity measurements in the ternary mixtures water-lactose-lactate, for the purpose of monitoring the lactic acid fermentation of lactose

A preliminary study of the lactic acid fermentation of lactose using an ultrasonic velocity technique is presented. During this fermentation, lactose (milk sugar) is transformed into lactate (lactic acid) by the action of bacteria. It is shown that the changes occurring during the course of the process can be monitored on-line by measuring the changes experienced by ultrasonic waves propagating through the fermenting media. Measurements of density and ultrasonic velocity in the ternary mixtures water-lactose-lactate and during the fermentation of a lactose solution were carried out. The ultrasonic propagation velocity has been correlated with the mass concentration of the mixture components using a semi-empirical model. These relations may be used to achieve an on-line concentration control for microbial lactic acid production.

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.

Optimal design of piezocomposite materials using topology optimization techniques and homogenization theory

Piezocomposite materials require an improvement in their performance characteristics for hydrophone and ultrasonic transducer applications. We have proposed in this work a method that combines topology optimization techniques and homogenization theory for designing new topologies of piezocomposite unit cells with better performance. It consists of finding the distribution of material and void (or gas) phases in a periodic unit cell that optimizes piezocomposite electromechanical-efficiency. In order to calculate the effective properties of a unit cell with complex topology, a general homogenization method applied to piezoelectricity was implemented using the finite element method. The microstructures obtained show a large performance improvement compared to usual designs of piezocomposite unit cells.