Volume 103, Issue 3, March 1998
Index of content:
- TRANSDUCTION 
103(1998); http://dx.doi.org/10.1121/1.421281View Description Hide Description
Absorption of ultrasound in adhesive materials is significant in forming ultrasonic signals obtained in nondestructive evaluation(NDE) procedures, and may also have diagnostic value in the evaluation of adhesive materials themselves. This paper investigates the effects of filler particles in adhesivepolymers on ultrasonic compression wave absorption and phase velocity dispersion as functions of frequency. Wave propagation is affected by relaxations in the continuous polymer phase as well as by scattering at filler particles. The complex compression wave number for the composite material is derived on the basis of conventional theories of scattering in randomly distributed fields of particles, together with a formulation of the particle diffraction problem based on a combination of the cases for solid particles in solid continua and solid particles in liquid continua first proposed by Ying and Truell, and Allegra and Hawley, respectively. Experiments are described which demonstrate the effects of relaxations and scattering on absorption and phase velocity as functions of frequency in an epoxy material containing a mineral filler. Good agreement is obtained between theory and experiment.
103(1998); http://dx.doi.org/10.1121/1.421282View Description Hide Description
A two-sided laser Doppler vibrometer (LDV) was designed to measure the electromechanical coupling coefficient of piezoelectric and electrostrictive thin films.Optical fibers and optical fiber couplers were used to split the light into multiple beams and to simultaneously illuminate the same spot on both sides of the film. This probe measured the normal displacement on each side of the sample and allowed computation of the change in thickness by summing the two LDV signals. Data for PVDF films are presented to illustrate problems associated with this type of measurement. A large bending motion of the sample occurred which was responsible for a significant error in the measured thickness change. Reducing the amplitude of this motion by stacking films was an efficient way to increase the accuracy of the data. An error analysis was carried out to qualitatively explain these experimental observations. It was also discovered that the measurement accuracy improved when the ac driving voltage frequency was less than the bending resonance frequency of the sample.
103(1998); http://dx.doi.org/10.1121/1.421283View Description Hide Description
In order to produce high-amplitude, low-frequency signals, an underwater transducer must generate a relatively large volume displacement. Since water exerts a large reaction force back on the transducer, “conventional wisdom” dictates that such a transducer would have to be a high Q resonant device and thus not be broadband. However, a transducer does not have to be broadband in the conventional sense to meet the requirements of communication and sonar systems. A transducer that is capable of instantaneously switching between two discrete frequencies is adequate for communication and transmission of coded signals; one that is capable of switching among several frequencies could produce the chirp signals commonly used in active sonars. Ordinarily, a broadband transducer is needed to accomplish the frequency switching rapidly. A way around this difficulty is the “state-switched” source concept originally proposed by Walter Munk in 1980 which permits instantaneous frequency switching of a high Q resonant transducer while always maintaining the resonance state. The objective of this research has been to investigate this novel approach to the design of high-power, low-frequency, broadband transducers for use in long-range underwater communication, active sonar, and underwater research applications. This paper presents a practical realization of a “state-switched” source.
Theoretical and experimental study of transducers aimed at low-frequency ultrasonic atomization of liquids103(1998); http://dx.doi.org/10.1121/1.421300View Description Hide Description
The fine atomization of liquids by means of low-frequency ultrasonic atomizers (about 50 kHz) results from unstable surface waves generated on the free surface of a thin liquid film. This thin liquid film develops as the liquid spreads fast over the atomizing surface of the atomizer. The displacement amplitude of the atomizing surface must be greater than 2 μm to initiate the atomization process. This may be achieved using a displacement amplitude transformer. The present study focuses on an analytical analysis of the longitudinal oscillations stimulated by the piezoelectric elements in a stepped horn which operates as an amplitude transformer. A sizing method of the stepped horn is established and experimentally tested. The influence of the materials’ mechanical damping on the displacement amplitude of the atomizing surface is investigated. The comparison between theoretical and experimental results allows the determination of the internal damping coefficient.
Calculating the performance of 1–3 piezoelectric composites for hydrophone applications: An effective medium approach103(1998); http://dx.doi.org/10.1121/1.421306View Description Hide Description
A new method is presented for evaluating the performance of 1–3 polymer/piezoelectric ceramic composites for hydrophone applications. The Poisson’s ratio effect, i.e., the enhancement of the hydrostatic performance which can be achieved by mixingpiezoelectricceramics with polymers, is studied in detail. Using an “effective medium” approach, algebraic expressions are derived for the compositehydrostatic charge coefficient the hydrostatic figure of merit and the hydrostatic electromechanical coupling coefficient in terms of the properties of the constituent materials, the ceramic volume fraction, and a microstructural parameter The high contrast in stiffness and dielectric constants existing between the two phases can be exploited to derive simple, geometry-independent approximations which explain quantitatively the Poisson’s ratio effect. It is demonstrated that the stiffness and the Poisson’s ratio of the polymer matrix play a crucial role in enhancing hydrophone performance. Using a differential scheme to model the parameter we evaluate and for polymer/piezoelectric ceramic systems at varying compositions. Several examples involving and piezoelectricceramics are given to illustrate the theory.