Yung Chun Lee

Acoustic microscopy measurement of elastic constants and mass density

A method is presented to determine the elastic constants and the mass density of isotropic and anisotropic solids and anisotropic thin films. The velocity and attenuation of leaky surface acoustic waves (SAWs) have been obtained for specified propagation directions from V(z) curves measured by line-focus acoustic microscopy (LFAM). The experimentally obtained velocities have been compared to velocities obtained from a measurement model for the V(z) curve which simulates the experiment. Since the measured and simulated V(z) curves have the same systemic errors, the material constants are free of such errors. For an isotropic solid, Youngs modulus E, the shear modulus G and the mass density /spl rho/ have been determined from the leaky Rayleigh wave velocity and attenuation, measured by LFAM, and a longitudinal wave velocity measured by a pulse-echo transit-time technique. For a cubic-crystalline solid, the ratios of the elastic constants to the mass density (c/sub 11///spl rho/, c/sub 12///spl rho/, c/sub 44///spl rho/) have been determined from the directional variation of measured SAW velocities, using a preliminary estimate of /spl rho/. The mass density /spl rho/ has subsequently been determined by additionally using the attenuation of leaky SAWs in crystal symmetry directions. For a cubic-crystalline thin film deposited on a substrate, the elastic constants and the mass density (c/sub 11/, c/sub 12/, c/sub 44/, /spl rho/) of the film have been determined from the directional variation of the measured SAW velocities, and a comparison of the corresponding attenuation coefficient with the measured attenuation coefficient has been used to verify the results.<<ETX>>

Self-calibrating ultrasonic technique for crack depth measurement

A configuration of transducers together with a self-calibrating measurement technique is proposed to investigate the reflection and transmission of surface waves by a surface-breaking or near surface defect. By means of this technique, the ratio of the reflection and transmission coefficients (R/T and/orT/R) can be obtained in a reliable and accurate manner. The reflection and transmission of surface waves for oblique incidence on a surface breaking crack is investigated in detail. Information onT/R for the latter case can be used to determine the depth of the crack. The experimental measurements ofT/R show excellent agreement with theoretical results.

Measurement of stresses by line-focus acoustic microscopy

Abstract In this paper, line-focus acoustic microscopy has been used to determine local near-surface stresses in isotropic materials. Two surface wave modes, namely a leaky Rayleigh wave and a leaky surface-skimming longitudinal wave, have been excited by the acoustic microscope. It has been observed that the changes of the wave velocities are linearly proportional to the applied stresses, as predicted by acoustoelastic theory. The non-uniform stress field in a loaded specimen has been determined from wave velocity measurements by the use of acoustoelastic constants obtained from a calibration test. The measured stresses are in good agreement with the results calculated by a finite element method. A self-calibrating method, which determines the stress profile directly from velocity measurements without a calibration test, is proposed and the results are compared with experimental data.

Measuring Lamb wave dispersion curves of a bi-layered plate and its application on material characterization of coating

This paper investigates the Lamb wave dispersion curves of a bi-layered plate and evaluates the feasibility of using the dispersion data to characterize the coatings material properties. The measurement of dispersion curves is based on a focusing PVDF transducer operating in a pulse/echo mode. An image displaying technique is used to determine the dispersion relation of Lamb waves from the measured data. Multiple dispersion curves of Lamb waves are accurately determined over a wide frequency range (4 to 20 MHz). Lamb wave dispersion curves for thin metal sheets electro-deposited with nickel coatings are measured. The elastic constants of the nickel coating are determined by comparing the experimental dispersion data with the theoretical ones calculated numerically. Potential applications of this measurement method are addressed.

Picosecond acoustic measurements of longitudinal wave velocity of submicron polymer films

We apply picosecond acoustic techniques to determine the longitudinal wave speed of submicron polymer films in the through‐thickness dimension. The time‐of‐flight of an elastic wave propagating through the polymer film is measured using an optical pump/probe technique. The longitudinal wave speed in poly(methyl methacrylate) films of thickness ranging from 20 to 130 nm has been determined. A pronounced increase in longitudinal wave speed is observed for films of thickness less than 40 nm.

Laser ultrasonic surface wave dispersion technique for non-destructive evaluation of human dental enamel

This paper describes a novel optical system for clinical diagnosis of dental enamel based on its elasticity. Current examination techniques are typically destructive, and frequently impractical for in-vivo inspection. This paper describes the first application of a laser ultrasonic non-destructive evaluation (NDE) method for clinical dental diagnosis. It performs remote elasticity evaluation on small dimension samples. A focused laser line-source generates broadband surface acoustic wave (SAW) impulses which are detected with a simplified optical fibre interferometer. The measured SAW velocity dispersion spectrum was in turn used to characterise the elasticity of the specimen. Different metal structures were measured to verify the system performance. The results agree well with theoretical values and confirm the reliability and accuracy of the laser NDE system. This technique was then applied to evaluate the surface of sound natural human dental enamel. The measured dispersion spectra match theoretical expectations and the influences of both the enamel and the underlying dentin on the surface wave propagation were observed. This is the first time, to the best of our knowledge, that a laser based SAW velocity dispersion technique has been successfully applied on human dental enamel. As a remote, non-destructive technique it is applicable in-vivo and opens the way for early diagnosis of dental caries.

Leaky Lamb wave of a piezoelectric plate subjected to conductive fluid loading: Theoretical analysis and numerical calculation

This paper examines in detail the characteristics of a leaky Lamb wave propagating in a piezoelectric plate immersed in a dielectric or conductive fluid. The analysis is based on the partial wave analysis and its numerical calculations. Some corrections have been made so that both the changes in phase velocity and the wave attenuation of leaky Lamb waves caused by fluid’s mechanical, dielectric, and conductive loadings are determined accurately. Influences of each material property of the loading fluid such as mass density, wave speed, permittivity, and conductivity on the leaky Lamb wave have been quantitatively characterized. Particular emphasis is placed on the velocity variations and excessive attenuation caused by the fluid’s conductivity. The results are important in designing piezoelectric acoustic wave sensors working in liquid phase.

Measurements of dispersion curves of leaky Lamb waves using a lens-less line-focus transducer

This paper presents a new method of measuring dispersion curves of leaky Lamb waves propagating in a thin plate. The measurement system is based on a lens-less line-focus transducer and its defocusing measurement technique. A new waveform processing method known as V(f, z) method is introduced for analyzing the measured waveforms and for extracting the dispersion relation. For two thin-plate samples, namely a 150 microm thick alumina plate and a 320 microm thick stainless steel plate, the dispersion curves of several lower order modes of leaky Lamb waves have been experimentally determined in the frequency range of few to 25 MHz. The experimental dispersion curves have then been compared with the theoretical ones. Excellent agreement is observed. It is then verify that this method indeed provides a convenient and accurate way for experimentally measuring dispersion curves of leaky Lamb waves of thin-plate samples.

V(z) curves of layered anisotropic materials for the line‐focus acoustic microscope

V(z) curves for a line‐focus acoustic microscope have been calculated in terms of the characteristic functions of the acoustic lens and the reflectance function of the fluid‐loaded specimen. More accurate expressions for the characteristic functions of the acoustic lens are presented by taking account of attenuation in the coupling fluid, of the angular dependence of transmission by the antireflection coating on the lens surface, and by making a better estimate of the focal length. The reflectance function has been calculated for anisotropic layers deposited on anisotropic substrates. The calculated V(z) curves have been compared with measurements for isotropic and anisotropic materials, and layered anisotropic materials. The surface acoustic wave velocities obtained from the theoretical and the measured V(z) curves have been compared for the full range of directions of wave propagation. The comparisons of V(z) curves and surface acoustic wave velocities show excellent agreement between theoretical and ex...

Miniature conical transducer realized by excimer laser micro-machining technique

This paper presents the fabrication and characterization of a miniature conical transducer that can be used as an acoustic emission sensor for localized measurements. The development of the miniature conical transducer is based on the commercial NBS conical transducer and the micro-fabrication capability of an excimer laser. The conical PZT element, which is the key component in the transducer, is fabricated by an excimer laser micro-machining workstation with a specially designed laser mask and a new work piece rotation machining technique. Small conical piezoelectric elements with end surface around 200 μm in diameter are directly fabricated out of a polarized PZT ceramic plate. The miniature conical transducer is then constructed. Standard acoustic emission measurements are carried out to characterize the performance of the transducer. Very good results are observed which indicate that the miniature conical transducer can be successfully used for measuring high-frequency dynamic surface motion.