David K. Hsu

Bessel beam ultrasonic transducer: Fabrication method and experimental results

We report experimental results from a first‐of‐a‐kind ultrasonic transducer that generates a beam with a Bessel function profile. Using a technique of nonuniform poling, an axially symmetric Bessel function pattern is ‘‘polarized into’’ a piezoelectric ceramic element. The resulting circular‐disk transducer has the usual full‐plating electrode configuration, but produces an ultrasonic beam with a radial displacement profile approximating that of the Bessel function J0 (r), both in amplitude and in phase. The radiation field of a 1‐in.‐diam, 2.25 MHz Bessel transducer mapped out with a point probe shows good agreement with calculated results using a Gauss‐Hermite model. Bessel transducers are of particular interest in attempts to achieve ‘‘diffractionless’’ beams.

Experimental analysis of porosity-induced ultrasonic attenuation and velocity change in carbon composites

Abstract In this paper, ultrasonic measurement results are presented for analysing the wave propagation in carbon fibre reinforced plastics (CFRP) containing voids. The composite samples studied include laminates fabricated from unidirectional and woven fabric prepregs. The ultrasonic methods stress the utilization of spectral analysis and frequency dependence of the attenuation and phase velocity due to porosity. Morphological parameters of pores are found to play an important role in the proper interpretation of measured data. The measured attenuation shows approximately linear behaviour over the frequency range used. The corresponding phase velocity decreases substantially with increasing void content. In addition, the velocity in porous CFRP is found to be more dispersive than that in void-free composites. The relationship between the ultrasonic attenuation and dispersion is subsequently tested using the local form of the Kramers-Kronig relation. The linear relationship between the void content and the attenuation slope ( d α d f ) is found to hold, but the constant of proportionality is quite different for samples with different pore morphology. This morphology has a large effect on the dispersion of phase velocity. The fractional velocity decrease ( ΔV V 0 ) is correlated with the void content, and its dependence on the frequency and void shape is discussed. The present analysis will aid in the model development of wave propagation in porous composites for detecting and characterizing the porosity in these materials.

Quantitative estimation of material properties of porous ceramics by means of composite micromechanics and ultrasonic velocity

Abstract This paper describes a nondestructive method for the quantitative estimation of property variations due to porosity in advanced ceramics. The method employs a composite micromechanics which accounts for the effective density and elastic stiffness of a porous composite medium with a measurement of ultrasonic velocity. When the measured velocity is coupled with the theoretically predicted velocity, the unknown pore volume fraction is solved, from which other material properties are determined. The micromechanics model based on the Mori-Tanaka theory can handle ellipsoidal pores with a certain orientation distribution. Given the zero-porosity matrix moduli and the pore aspect ratio, the oblate spheroidal theory is first applied to hot pressed silicon carbide (SiC) samples in the range of about 85–100% of theoretical density and then extended to sintered samples in the density range of 93.6–97.6%. It is shown that the bulk density and elastic modulus of porous ceramics can be estimated accurately by the proposed method.

Reconstruction of inclusions in solids using ultrasonic Born inversion

Voids and inclusions in elastic solids are characterized experimentally using scattered ultrasonic waves. The flaws are reconstructed using a one‐dimensional elastic wave inverse scattering algorithm based on the Born–Neuman expansion. This method emphasizes the role of low and intermediate frequency longitudinal waves. The utility of the inverse Born approximation is tested for several new circumstances. First the algorithm is tested for pitch‐catch (bistatic) geometries. Secondly the effects of resonant excitation of the scatterer on flaw characterization are measured for several spherical flaws. The third and major result shows that the one‐dimensional algorithm can be used to determine the size, shape and orientation of nearly ellipsoidal flaws when access angle is limited. The effects of varying access aperture on the reconstruction are reported. Another common experimental limitation in flaw characterization arises from interferences of the flaw signal with nearby surfaces. We briefly report that th...

Apparatus and method for detection of icing onset and ice thickness

An apparatus and method for detection of icing onset and ice thickness upon an accretion surface utilizing ultrasonic echo ranging techniques, including propagation of ultrasonic waves through a buffer block. A portion of the wave energy is reflected by reference reflection means and another portion of the wave energy is propagated to the ice accretion surface and to a reflecting interface. The reflecting interface is represented either by the accretion surface in absence of icing, or by a thin ice layer at the icing onset, or by the ice/air interface of an ice layer accreted upon the accretion surface. Reflected waves are transduced to electrical signals. Relative signal amplitudes and time delays provide measures of particular icing conditions upon the accretion surface, and are appropriately resolved into calibrated signals indicating icing onset, ice thickness, and ice accretion rate.

Characterization of anisotropie elastic constants of silicon-carbide participate reinforced aluminum metal matrix composites: Part I. Experiment

The anisotropic elastic properties of silicon-carbide particulate (SiCp) reinforced Al metal matrix composites were characterized using ultrasonic techniques and microstructural analysis. The composite materials, fabricated by a powder metallurgy extrusion process, included 2124, 6061, and 7091 Al alloys reinforced by 10 to 30 pct ofα-SiCp by volume. Results were presented for the assumed orthotropic elastic constants obtained from ultrasonic velocities and for the microstructural data on particulate shape, aspect ratio, and orientation distribution. All of the composite samples exhibited a systematic anisotropy: the stiffness in the extrusion direction was the highest, and the stiffness in the out-of-plane direction was the lowest. Microstructural analysis suggested that the observed anisotropy could be attributed to the preferred orientation of SiCp. The ultrasonic velocity was found to be sensitive to internal defects such as porosity and intermetallic compounds. It has been observed that ultrasonics may be a useful, nondestructive technique for detecting small directional differences in the overall elastic constants of the composites since a good correlation has been noted between the velocity and microstructure and the mechanical test. By incorporating the observed microstructural characteristics, a theoretical model for predicting the anisotropic stiffnesses of the composites has been developed and is presented in a companion article (Part II).

Porosity estimation using the frequency dependence of the ultrasonic attenuation

A new technique is reported for estimating the volume fraction of porosity in structural materials. The estimate for the volume fraction is proportional to the slope of the ultrasonic attenuation when plotted as a function of frequency. Both theory and experiment are considered. The theory, appropriate for dilute porosity, uses the uncorrelated, single-scatter approximation. An “attenuation slope” algorithm is derived within this approximation and its limits of validity are tested by computer simulation. Experimental tests consist of three parts. First, the method is compared with other existing techniques through estimates from the published data on gas porosity in aluminum casts. For the second test, cylindrical porosity is simulated by parallel through-holes drilled in aluminum blocks. Finally, the attenuation in porous graphite-epoxy samples is measured and compared with results predicted from theory.

Apparatus for acoustically inspecting a workpiece

An apparatus for acoustically inspecting a workpiece has a tubular member with a membrane sealing an end of the tube to form a first chamber. An acoustic transducer assembly is mounted in the tubular member. An adapter secured to the tubular member is adapted to contact the workpiece and space the membrane from the workpiece and form a second chamber. In operation, the transducer generates an acoustic wave that travels through coupling fluid disposed in the chambers to strike the workpiece. The acoustic wave is reflected from the workpiece, received by the transducer assembly, and a corresponding electrical signal is sent to a processor for evaluation. Additional features include a fluid conduit providing the independent coupling fluid, a spacer that accommodates workpiece surface variations while maintaining acoustic coupling with the workpiece, and a vacuum housing that removes excess coupling fluid from around the inspection apparatus and workpiece mating surface.

Investigation of metal and ceramic-matrix composites moduli : Experiment and theory

Abstract The elastic behavior of metal and ceramic-matrix composites were characterized by ultrasonic techniques. While an immersion ultrasonic technique was used to measure the stiffness moduli of silicon carbide (SiC) particulate reinforced aluminium metal-matrix composites, a dry-coupling method was used to determine the elastic constants of woven Nicalon ™ fiber reinforced SiC ceramic-matrix composites. A unified micromechanics model was developed to predict the elastic moduli of these composites from the knowledge of their constituent elastic constants. The model quantitatively predicted the effects of microstructural characteristics, such as the reinforcement content and porosity in the material, on the elastic moduli of the composite systems studied. The predicted moduli were in good agreement with the experimental results for both the particulate reinforced metal-matrix composites and woven fiber reinforced ceramic-matrix composites.

Fabrication and comparison of PMN-PT single crystal, PZT and PZT-based 1-3 composite ultrasonic transducers for NDE applications.

This paper describes fabrication and comparison of PMN-PT single crystal, PZT, and PZT-based 1-3 composite ultrasonic transducers for NDE applications. As a front matching layer between test material (Austenite stainless steel, SUS316) and piezoelectric materials, alumina ceramics was selected. The appropriate acoustic impedance of the backing materials for each transducer was determined based on the results of KLM model simulation. Prototype ultrasonic transducers with the center frequencies of approximately 2.25 and 5MHz for contact measurement were fabricated and compared to each other. The PMN-PT single crystal ultrasonic transducer shows considerably improved performance in sensitivity over the PZT and PZT-based 1-3 composite ultrasonic transducers.