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Nov 1989

Volume 86, Issue S1, pp. S1-S125

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back to top Session LL. Physical Acoustics VI: Ultrasound and Nondestructive Evaluation
Contributed Papers
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Elastic constant reconstruction using resonances of coupled modes in thin anisotropic plates (A)

S. I. Rokhlin, C. Y. Wu, and L. Wang

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S93-S93 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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At oblique incidence of ultrasonic waves on a thin anisotropic composite plate immersed in fluid, the quasilongitudinal, flexural, and quasi‐SH vibrations are excited. At some angles of incidence, coherent interactions of these waves lead to minima of the normal displacement on the back surface of the plate and therefore to minima of the transmission coefficient. At off‐axis orientations of the thin composite plate, two such minima may be observed. Their positions are only slightly dependent on frequency and are determined by plate properties and orientation. This effect was studied both theoretically and experimentally. The elastic constants have been reconstructed for a unidirectional graphite‐epoxy composite plate from experimental data that have been taken at different plate orientations and frequencies.
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On the transition between slip and rigid boundary conditions between two solid media (A)

Y. J. Wang and S. I. Rokhlin

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S93-S93 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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In order to describe the transition between slip and rigid boundary conditions, a model with a thin interphasial viscoelastic layer is introduced. By changing the relaxation time (ωτ), the transition from a viscous liquid to a solid layer is observed. When the interface layer is in the liquid state with very small shear nodulus the shear stress components on the boundary vanish, resulting in a slip boundary condition. On the other hand, after the layer is solidified, the two semispaces become welded with the transition of shear and longitudinal stresses to one another. It is shown that the ratio of a hydrodynamic boundary layer thickness to the interface layer thickness plays a major role in this transition. A simple approximate equation for the reflection coefficient from the interface is derived and compared to the exact solutions for different transitional stages of the boundary condition. Theoretical predictions are compared to experimental results.
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Method for measurement of elastic constants of thin anisotropic membranes (A)

W. Wang and S. I. Rokhlin

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S93-S94 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

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In this work the elastic properties of a thin layer of porous aluminum oxide obtained by phosphorus anodization have been studied by a novel ultrasonic technique. The thickness of the porous oxide samples is about 50 μm with through vertical pores of about 2000 Å diameter and porosity of about 70%. The ultrasonic technique is based on measurements of the angle of the minimum of the transmitted energy at oblique incidence on the immeresed sample. The effective elastic constants that have been theoretically calculated from the experimental data are in good agreement with that found from estimations of bulk properties of anisotropic porous media. The knowledge of the properties of porous anisotropic anodized Al2O3 layers has major importance for adhesive joint characterization since such layers play an important part in interphasial strength of the joint.
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Focusing of ultrasound in solids with applications to QNDE (A)

John G. Harris

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Ultrasound can be focused by the curved fluid‐solid interface of a solid part being interrogated for cracks or by using a lens as part of an imaging system such as the acoustic microscope. In the latter case it is usually difficult to form a diffraction‐limited spot when the focus lies in the solid because the focused beam field must cross a fluid‐solid interface. This paper summarizes recent investigations of these two situations. A calculation of the wave fields near the caustics formed when a two‐dimensional (unfocused) beam scatters from a concave fluid‐solid interface is discussed. Also discussed is a calculation of the wave fields within the focal region of a three‐dimensional, wide aperture source when the focused beam is directed across a plane fluid‐solid interface and the focal region lies in the solid. In both cases attention is paid to the interplay between the diffraction effects of the source and of the focal region, and also to the excitation of leaky surface waves. The implications of these considerations for the quantitative interpretation of ultrasonic measurements are explored.
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Guided wave generation by direct excitation (A)

Peter B. Nagy and Laszlo Adler

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Guided acoustic waves along interfaces and more complex layered structures are often used in ultrasonic NDE. Their outstanding sensitivity to specific properties associated with interface conditions and bond quality is due to the fact that their energy is effectively confined to the region of interest. On the other hand, this advantage turns out to be a great drawback for generation and detection of such guided waves. Certain modes with higher phase velocity than at least one of the bulk velocities of the surrounding media are called leaky modes that can be coupled to via phase matching bulk waves. True guided modes of lower phase velocity than any of the bulk velocities of the surrounding media are much more difficult to generate and detect. A simple experimental technique based on longitudinal or shear wave excitation along the interface is shown to be very effective in generating both symmetric and antisymmetric guided modes of Rayleigh (SV) or Love (SH) type of polarization. Experimental results from different plates and adhesive and solid‐state interfaces are shown to be in good quantitative agreement with theoretical calculations. This technique might find numerous applications in basic acoustic studies as well as ultrasonic nondestructive evaluation. [This work was supported by the Office of Naval Research under Contract N0014‐C‐2129.]
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Radiated fields determined from moments of surface velocity (A)

David D. Bennink and Anna L. Pate

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A quick and efficient method is derived for calculating the field radiated by a finite but otherwise arbitrary velocity profile. The method calculates the pressure field radiated into the half‐space (z > 0) from the normal component of velocity on the boundary plane (z = 0). The derivation starts from the Rayleigh integral and utilizes a power series expansion of the kernel in the radial coordinate of the source point. As a result, a formulation is obtained that contains separate terms for the source and for propagation. The source terms represent the moments of the velocity profile. The propagation terms can be obtained from a recurrence relation starting from certain special cases for which analytical expressions are known. Convergence and stability of the method will be discussed, and examples of field calculations for a few simple profiles will be given. [Work supported by Center for NDE at Iowa State University.]
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Surface motion reconstruction based on half‐space field data (A)

David D. Bennink and Anna L. Pate

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A finite but otherwise arbitrary normal velocity profile at the boundary of a half‐space is expanded in a set of orthonormal functions. The radiation problem is then formulated in terms of the moments of this profile. (Details of this formulation are given in the preceding abstract.) The radiation problem is discretized to obtain a matrix formulation and a singular value decomposition method is used to estimate the velocity profile from measurements of the radiated field. Additional constraints are imposed upon the derivatives of the profile in order to overcome the ill‐posed nature of the inverse source problem. The accuracy of the method will be discussed in terms of both the positioning of the field locations (nearfield versus farfield) and the weighting of constraints. Examples of velocity profile reconstruction will be given for a few simple radiators. [Work supported by Center for NDE at Iowa State University.]
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A prior knowledge based optimal Wiener filtering approach to ultrasonic scattering amplitude estimation (A)

Steven P. Neal and Donald O. Thompson

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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In order to interpret measurements of the scattered wave field resulting from sonification of an object, the effects of the measurement system must be removed from the measured data. In ultrasonic nondestructive evaluation, estimation of the farfield scattering amplitude for a flaw in a material, e.g., a crack, void, or inclusion, involves removing the measurement system effects in the presence of electronic noise and noise due to the scattering of sound from inherent material features such as grain boundaries. The purpose of the work reported here was to evaluate an optimal Wiener filtering approach to scattering amplitude estimation. The filter was shown to determine an optimal estimate as the weighted average of the information derived from measurement of the scattered wave field and prior information about the flaw distribution. The optimal Wiener filter was evaluated and compared to a nonoptimal form of the Wiener filter. The optimal Wiener filter was shown to provide improved scattering amplitude estimates by filtering out the noise and by utilizing prior flaw information. [Work supported by the Director of Energy Research, Office of Basic Energy Sciences.]
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The measurement and analysis of acoustic noise as a random variable in ultrasonic nondestructive evaluation (A)

Steven P. Neal and Donald O. Thompson

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S94-S94 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Sound scattered from inherent material features such as grain boundaries becomes a source of acoustic noise when attempting to detect or characterize a flaw in a material, e.g., a crack, void, or inclusion. In formulating a stochastic model to describe a flaw detection or flaw characterization experiment, acoustic noise is often assumed to be an uncorrelated, Gaussian random variable with zero mean and known average power spectrum. The work reported here focuses on the measurement of acoustic noise and on the evaluation of these assumptions for the cases of scattering from grains in stainless steel and scattering from porosity in aluminum, respectively. An estimate of the average power spectrum is determined for each case. It is shown that in both the time domain and in the frequency domain the acoustic noise considered has zero mean and is reasonably uncorrelated and Gaussian. [Work supported by the Director of Energy Research, Office of Basic Energy Sciences.]
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