G. Hayward

Finite-element analysis of 1-3 composite transducers

The vibrational and electromechanical characteristics of a wide range of 1-3 composite structures, comprising ceramic pillars aligned within a polymer phase, are considered using finite-element analysis. The influence of pillar geometry, ceramic volume fraction, and pillar orientation is described in terms of overall transduction efficiency. It is shown that the finite-element method provides a versatile means of analysis and the results obtained permit a set of useful design guidelines to be developed. In general, a small pillar aspect ratio and a relatively high volume fraction provides the most satisfactory performance, in terms of electromechanical efficiency and uniformity of thickness dilation.<<ETX>>

Unidimensional modeling of 1‐3 composite transducers

Composite transducers, utilizing the complementary properties of a piezoelectric ceramic and a polymer, often possess acoustic matching and electromechanical efficiency that are superior to conventional isotropic materials. The extent of the performance enhancement is a complex function involving composite design in conjunction with external electrical and mechanical loading conditions. This work uses an established modeling technique, supported by finite element analysis and experimental observation, to predict the mechanical, electrical, and piezoelectric properties of 1‐3 composite structures. These parameters are then combined in a modified thickness‐drive model to predict the performance of composite probe assemblies under realistic operating conditions. Accurate correlation between simulation and experimental results is demonstrated over an extensive range of ceramic‐polymer volume fraction. Significantly, the results suggest that composites that possess relatively high‐volume fractions of ceramic a...

Single Sided Inspection of Composite Materials Using Air Coupled Ultrasound

A single sided, air coupled ultrasonic NDT system based on the generation and reception of the a0 Lamb mode is described. 1–3 composite ultrasonic transducers are employed, transmitting and receiving transducers being oriented at the appropriate coincidence angle for the generation and detection of the mode; they are placed close together and scanned over the surface of the plate to produce a ‘C-scan’ image. Tests have been carried out on carbon fiber composite plates with delamination defects, the damaged areas being readily detected. The measurements have also been compared with numerical predictions based on a finite element model, good agreement being obtained.

Applications of through-air ultrasound for rapid NDE scanning in the aerospace industry

This work describes two separate applications of air-coupled ultrasonic nondestructive evaluation (NDE) for rapid inspection of aerospace components. First, air-coupled transducer arrays are used for through transmission scanning of relatively large preproduction test samples. In this case a focused array system was designed to facilitate faster inspection rates and at the same time match the resolution and performance of available water jet apparatus. The second approach is geared toward in situ inspection and involves single-sided scanning using a dual transducer configuration for generation of surface, shear, and Lamb waves within the test specimen. In both cases, the difficulties of practical air-coupled inspection are discussed and the design solutions presented for each application. Piezocomposite transducer technology, in conjunction with narrow-band low-noise electronics, constitutes the basis for achieving the required signal to noise ratio (SNR) to ensure robust operation within the industrial environment. A number of scan images, performed on realistic samples, are shown and the results compared with those obtained using alternative methods. Excellent image quality is demonstrated from real time scanning and without recourse to any off-line data processing.

Characterization of air-coupled transducers

This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively.

An evaluation of 1-3 connectivity composite transducers for air coupled ultrasonic applications

A combination of theoretical modeling and experimental analysis is used to predict and assess the performance of 1–3 composite transducers for operation in an air‐coupled environment. Specifically, finite element analysis, supported by linear systems modeling, is employed to evaluate transmission and reception characteristics over the complete volume fraction range, for operating frequencies in the region of 500 kHz, and the theory confirmed using novel experimental techniques. The theoretical approach is then extended to assess the influence of matching and backing on transducer sensitivity and bandwidth. Throughout, the influence of air propagation is included in the simulation approach and highlighted where this factor is significant. The work is expected to provide useful design guidelines for the practical ultrasonic engineer.

Assessing the influence of pillar aspect ratio on the behavior of 1-3 connectivity composite transducers

This paper describes a theoretical study, using finite element analysis, into the influence of the ceramic pillar aspect ratio on the behavior of 1-3 connectivity composite transducers. The main objective is to provide working design guidelines for the transducer engineer, with a view toward the cost-effective manufacture of thickness drive and hydrostatic devices. Modal and harmonic analyses are performed to ascertain the conditions under which the composite behaves as a homogenous material, under different values of volume fractions, passive filler material, pillar shape, and distribution. Consequently, a set of criteria is generated to determine analytically the range of aspect ratios for which the composite material behaves homogeneously in the thickness dimension. The influence of polymer loss on these criteria is discussed, along with effects of practical encapsulation and protective layers. Where possible, real data are provided to supplement theoretical predictions, with reasonable correlation between theory and experiment.

Comparison of some non-adaptive deconvolution techniques for resolution enhancement of ultrasonic data

Abstract A selection of well established deconvolution techniques are assessed for operation in the ultrasonic environment. Both time and frequency domain methods are compared under varying conditions of data format, design wavelet profile and signal-to-noise ratio, thereby providing insight into some of the difficulties associated with typical ultrasonic signals. Those algorithms which are best suited to particular ultrasonic applications and conditions are identified via a simulation approach, based around a linear systems model.

Design of 1-3 piezocomposite hydrophones using finite element analysis

The 1-3 piezocomposite material was originally developed because of its perceived good performance under hydrostatic operating conditions. Several constrained-dimensional models for piezocomposite hydrophones have been proposed but were found to lack accuracy when compared with experimental data. In addition, they could not be easily extended to include the effect of ancillary components such as cover plates, on the transducer behavior. In this work a finite element model is used for modelling of 1-3 piezocomposite hydrophones to help overcome these two shortfalls. A finite element model initially developed for modelling of thickness mode operation has been extended to include lateral pressures typical of the hydrostatic environment. The response of the new model has been compared with experiment with satisfactory results, allowing an extensive set of simulations to be presented for comprehensive evaluation of 1-3 piezocomposite design as an actuator or a hydrophone. The best hydrostatic performance was obtained by using a low volume fraction composite of PZT-5H and a soft, compressible polymer, with potential enhancements by the incorporation of stiff cover plates covering the ceramic pillars. It is shown that the aspect ratio of the ceramic pillars should be minimized to maximize stress transfer. Additionally, ceramic pillar shape and distribution do not exert a major influence on the hydrostatic behavior.

Surface-bonded and embedded optical fibers as ultrasonic sensors

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.