Shi Hoa Kuo

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.

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.

A new point contact surface acoustic wave transducer for measurement of acoustoelastic effect of polymethylmethacrylate

A new acoustic transducer and measurement method have been developed for precise measurement of surface wave velocity. This measurement method is used to investigate the acoustoelastic effects for waves propagating on the surface of a polymethylmethacrylate (PMMA) sample. The transducer uses two miniature conical PZT elements for acoustic wave transmitter and receiver on the sample surface; hence, it can be viewed as a point-source/point-receiver transducer. Acoustic waves are excited and detected with the PZT elements, and the wave velocity can be accurately determined with a cross-correlation waveform comparison method. The transducer and its measurement method are particularly sensitive and accurate in determining small changes in wave velocity; therefore, they are applied to the measurement of acoustoelastic effects in PMMA materials. Both the surface skimming longitudinal wave and Rayleigh surface wave can be simultaneously excited and measured. With a uniaxial-loaded PMMA sample, both acoustoelastic effects for surface skimming longitudinal wave and Rayleigh waves of PMMA are measured. The acoustoelastic coefficients for both types of surface wave motions are simultaneously determined. The transducer and its measurement method provide a practical way for measuring surface stresses nondestructively.

Fluid dielectric loading on leaky Lamb wave of a piezoelectric plate

This letter reports a high-precision wave measurement system for accurately determining the velocity of leaky Lamb wave. For an X-cut LiNbO3 plate, the change in leaky Lamb wave velocity induced by fluid’s dielectric loading effect has been measured. The measured correlation between fluid’s permittivity and induced wave velocity variation has been compared with theoretical results obtained form partial wave theory. Good agreements are observed. Potential applications of this wave measurement system are addressed.

Leaky lamb waves of a piezoelectric plate subjected to conductive fluid loading: an experimental study

This paper proposes a novel experimental method for measuring the propagating characteristics of leaky Lamb waves in a piezoelectric plate surrounded by a fluid. It is a differential type of measurement and is very sensitive to the velocity change and wave attenuation of leaky Lamb waves induced by fluid-loading effects. Experimental measurements on an X-cut LiNbO3 plate immersed in a dielectric and conductive fluid have been carried out. The velocity change and wave attenuation of the leaky Lamb waves caused by dielectric and conductive loadings of the fluid have been experimentally determined. The measured data have been compared with the theoretical ones that are calculated from a partial wave analysis. For the wave velocity, very good agreements between the experimental and theoretical results are observed. For the wave attenuation, there are some discrepancies, but an important characteristic in the relationship between wave attenuation and fluid conductivity as predicted by the theory have been verified experimentally

Depth Measurement of Surface-Breaking Cracks Using Point-Source/Point-Receiver Acoustic Transducer

A newly developed point-source/point-receiver (PS/PR) acoustic transducer is used for measuring the depth of surface-breaking cracks. The transducer consists of two miniature conical PZT elements which form a transmitter/receiver pair for generating and detecting surface waves. A tone-burst measurement system operates the PS/PR transducer at a fixed frequency. When a surface-breaking crack is present in between the transmitter and receiver, the change in time-of-flight of surface wave propagation caused by the crack is measured and used to estimate the crack depth. To verify the feasibility of this method, machined slots on a carbon steel sample are tested by the PS/PR transducer for crack depth determination. Good experimental results are obtained. Future applications and improvements on the PS/PR transducer system are addressed.

Miniature conical transducer for acoustic emission measurement

A miniature conical transducer with an aperture size of 0.2 mm has been successfully fabricated based on excimer-laser micro-machining technique. Specially-designed laser masks and a stage-rotation method have been used for directly machining a thin PZT ceramic sheet into a cone-shape piezoelectric element of the mini-conical transducer. The transducer thus fabricated has been tested experimentally to characterize its performance as an acoustic emission transducer. Good results are obtained.

Measurement of Acoustoelastic Effect of Surface Wave with an Improved Acoustic Transducer

A new acoustic transducer and measurement method have been developed for precise measurements of surface wave velocity. This technique is used to investigate the acoustoelastic effect of surface waves in polymethylmethacrylate (PMMA). The transducer utilizes two miniature conical PZT elements for surface wave transmitter and receiver, and hence can be viewed as a point-source/point-receiver (PS/PR) surface wave transducer. Surface waves are excited and detected with the PZT elements and the surface wave velocity can be accurately determined over a small measurement area. Improvement has been made so that the transducer can be used for both conductive and non-conductive materials. The transducer is then applied to measure the acoustoelastic effect of surface wave in a PMMA specimen. The acoustoelastic coefficients are experimental determined.