Biomedical subjects

Gordon Hayward

Publications and source records attributed to Gordon Hayward.

10 recordsLinked to original sources

PubMed2006

Determination of lamb wave dispersion data in lossy anisotropic plates using time domain finite element analysis. Part I: theory and experimental verification.

A theoretical and experimental approach for extraction of guided wave dispersion data in plate structures is described. Finite element modeling is used to calculate the surface displacement data (in-plane and out-of-plane) when the plate is subject to either symmetrical or antisymmetrical impulsive force stimulation at one or both of the parallel faces. Fourier transformation of the resultant space-time displacement histories is then employed to obtain phase velocity as a function of frequency. Experimental verification in the case of antisymmetrical stimulation is provided by means of a high-power Q-switched laser source that is used to excite guided waves in the plate. The subsequent out-of-plane displacement data were then obtained by means of a scanning laser vibrometer, and good agreement between theory and experiment is demonstrated. Examples of dispersion data are provided for aluminum, and excellent correlation between the data sets and conventional Rayleigh-Lamb theory for plate structures was obtained. This was then extended to lossy polymeric plates, in addition to both unpolarized and polarized piezoelectric ceramic plates, again with good agreement between the finite element modeling and optical experiments. The last set of results prepares the way for a detailed investigation of the nonhomogeneous piezoelectric composite waveguides described in a companion paper (Part II).

Anisotropy↗

PubMed2006

Determination of lamb wave dispersion data in lossy anisotropic plates using time domain finite element analysis. Part II: application to 2-2 and 1-3 piezoelectric composite transducer arrays.

The use of finite element modeling, combined with optical generation and detection of Lamb waves in plate structures, was extended to encompass periodic ceramic-polymer materials typical of those encountered in 1-3 and 2-2 piezoelectric composite array transducers. The resultant dispersion data was employed to predict the occurrence of Lamb wave-induced cross talk in composite monolithic arrays. The finite element modeling method was then used to simulate the dispersion behavior of two array structures that were subsequently manufactured: a 1-D 45% volume fraction linear array coupon and a 2-D 35% volume fraction array coupon. Excellent agreement between theory and experiment was obtained using impedance measurements and laser scans of the surface displacement profile at selected frequencies. Regions of strong inter-element cross-coupling were identified and these are shown to correlate very well with the dispersion data obtained for the dual-phase plate material. This work is considered to provide a useful basis for the design of wideband monolithic composite arrays and minimization of guided wave propagation along the array substrate.

Anisotropy↗

PubMed2006

A block diagram model of the thickness mode piezoelectric transducer containing dual oppositely polarized piezoelectric zones.

A unidimensional, linear systems, block diagram model of a two-layer thickness mode piezoelectric transducer is presented. The layers are subject to opposing piezoelectric polarization and the device is assumed to be loaded by semi-infinite isotropic media at the two principal faces. Block diagram representations of the transducer acting as both a generator and a receiver of ultrasound are developed in conjunction with the equivalent model of the electrical admittance. When expressed in this manner, the underlying cause and effect relationships are identified, with the important contribution of the piezoelectric boundary highlighted. Comparisons with the conventional single-layer transducer are made throughout and the major physical differences in terms of transduction performance are discussed. The new model is compared with finite element analysis and good agreement is also demonstrated with experimental data. A key aspect of the methodology is the provision of a more intuitive understanding of such device behavior. Accordingly, emphasis has been placed on the physical relationships and this is considered a major contribution of the work.

Journal Article↗

PubMed2006

Comparison of the frequency and physical nature of the lowest order parasitic mode in single crystal and ceramic 2-2 and 1-3 piezoelectric composite transducers.

This work describes an investigation into the first order parasitic mode (i.e., that closest to the fundamental thickness mode) that can occur in 2-2 and 1-3 thickness drive piezoelectric composite transducers. Specifically, the authors compare the performance of piezoceramic and piezocrystal composites with a common passive phase. A local Lamb wave approach is used to describe the generation of such modes, and the validity of this theory is investigated over the entire volume fraction range. It is shown that, when the parasitic mode is primarily generated by Lamb wave activity in the passive phase, both active materials demonstrate similar behavior. However, at higher volume fractions, the first order mode is related to the lateral resonance of the active material, and quite different behavior may be observed between the two sets of devices. The phase velocity of the parasitic modes in each device configuration was investigated by a combination of experimental measurement on a number of transducers along with simulations using the finite-element code PZFlex. Both 2-2 and 1-3 composites made from the single crystal materials pzn-4.5%pt, pzn-8%pt, and pmn-30%pt were investigated along with composites made from pzt5h ceramic. The PZFlex results are compared with experimental impedance analysis and laser scanning of surface displacement, with good agreement demonstrated. By comparing two very different active materials, additional insight into parasitic resonant activity within composite devices is demonstrated.

Journal Article↗

PubMed2005

Factors affecting broadband acoustic emission measurements of a heterogeneous reaction.

Broadband acoustic emission signals were obtained by attaching a piezoelectric transducer, sensitive up to 750 kHz, to the external wall of a 1 L jacketed glass reactor. Measurements were acquired of itaconic acid particles mixing in toluene; the total area of the acoustic emission signal from 55-500 kHz increased when the particle concentration, particle size or stir rate were increased. Signals at frequencies above 200 kHz were less sensitive to changes in particle size than those at lower frequencies. From calculation of the area of the signal in the range 55-200 kHz as a percentage of the signal area over the range 55-500 kHz, for mixtures of different size ranges of itaconic acid, it was possible to obtain an estimate of the mean particle size of a mixture. The heterogeneous esterification reaction of itaconic acid and 1-butanol was monitored non-invasively. A decrease in the overall acoustic signal area between 60 and 500 kHz was observed as the reaction progressed. Particle size and concentration information were contained in the amplitude of the acoustic emission signal, while the emission frequency yielded information on changes in the mean particle size.

1-Butanol↗

PubMed2005

Improving the thermal stability of 1-3 piezoelectric composite transducers.

The effect of temperature on the behavior of 1-3 piezoelectric composites manufactured using various polymeric materials was assessed experimentally through electrical impedance analysis and laser vibrometry. Device behavior varied with temperature irrespective of the polymer filler. Most significant changes in the piezoelectric composites were recorded around the glass transition temperature (Tg) of the polymer; movement to lower fundamental resonant frequencies and higher values of electrical impedance minima were observed at higher temperatures. Decoupling of the pillars from the polymer matrix was observed by laser vibrometry at high temperatures. The use of high Tg polymer extended the operational temperature range of a piezoelectric composite, and a high Tg polymer with improved thermal conductivity also proved beneficial. For all devices, at temperatures very close to room temperature, subtle changes in device performance, linked to polymer softening, were observed. Particulate filled materials have been investigated, and it is recognized that the high viscosities and low mechanical damping of such materials could be problematic for piezoelectric composite manufacture. The thermal solver of the PZFlex finite element code has been used to predict the temporal and spatial temperature response of a selection of the devices presented. The simulated and experimental data compare favorably.

Journal Article↗

PubMed2005

A flexible piezoelectric transducer design for efficient generation and reception of ultrasonic Lamb waves.

This paper describes the development of a flexible piezoelectric transducer for the generation and detection of ultrasonic symmetrical Lamb waves in plate-like structures. This piezoplatelet transducer structure comprises an array of miniature piezoceramic plates embedded within a soft setting polymer filler material, combining the efficiency of the active piezoceramic phase with a degree of flexibility, which is a function of the platelet/polymer dimensions. For many condition-monitoring applications, the generation of ultrasonic Lamb waves is often appropriate, and this was achieved by incorporating interdigital design techniques via the transducer electrode pattern. The performance of the piezoplatelet transducer structure was evaluated using a combination of linear systems and finite-element modeling, substantiated by experimental results. Importantly, the transducer is shown to operate as an ensemble of platelets, each operating in the thickness mode and well decoupled from neighboring piezoelectric elements. Using this transducer configuration, an unimodal s1 Lamb wave, at 1.45 MHz, has been generated and detected in a 3-mm thick steel plate. Furthermore, a propagation distance of almost 1 m was recorded for s0 Lamb wave generation/detection in a fiber-reinforced composite plate.

Journal Article↗

PubMed2004

Monitoring of a heterogeneous reaction by acoustic emission.

The feasibility of monitoring the reaction of itaconic acid and 1-butanol by non-invasive acoustic emission measurements has been assessed. A piezoelectric transducer with a resonant mode at 90 kHz was attached to the external wall of a 1 L jacketed glass reactor. Acoustic emission from the oil jacket, stirrer and toluene was insignificant in comparison to that produced by the itaconic acid particles, which was transmitted through the glass walls and heating oil to the transducer. The transducer responded to acoustic emission from itaconic acid up to approximately 300 kHz, with the region around 90 kHz having the highest sensitivity. The effect of particle concentration and size on the acoustic emission generated has also been investigated, with higher concentrations and larger particles giving the greater signals. The detection limit for itaconic acid particles was 14 g dm(-3) of toluene. The effect of 1-butanol concentration and temperature on the progression of reactions was monitored using acoustic emission. It was possible to detect differences in the rate and extent of the reaction under different conditions, and also to identify when a combination of the concentration and/or size of itaconic acid particles had reached a steady state. However, it was not possible to differentiate between changes in particle size and concentration using the resonant transducer.

1-Butanol↗

PubMed2004

Characterization and assessment of an integrated matching layer for air-coupled ultrasonic applications.

A novel ultrasonic matching layer for improving coupling between piezoelectric transducers and an air load is presented and the results of a theoretical and experimental program of work are provided. A combination of a porous material that has very low acoustic impedance with a low-density rubber material forms the basis of the approach. These matching layers were first analyzed experimentally using scanning electron and optical microscopy to determine the microscopic structure. Air-coupled resonance measurements were then performed to reveal the acoustic parameters of the individual layers that were identified within this multilayered structure. These data were then incorporated into a conventional linear model, and this has been verified and used to study performance and produce designs. Close correlation between experiment and theory is demonstrated. The most efficient designs have been implemented in a pitch/catch air-coupled system, and an improvement in received signal amplitude of 30 dB was achieved when compared with the unmatched case.

Journal Article↗

PubMed2003

Fuzzy logic applications.

Fuzzy logic is a modeling method well suited for the control of complex and non-linear systems. This paper illustrates some of the power of fuzzy logic through a simple control example. For the analytical chemist, fuzzy logic incorporates imprecision from measurement noise as well as from linguistic process descriptions to produce operational control systems.

Journal Article↗