摘要:

The boundary conditions for an interface between two solids are analyzed to model a thin viscoelastic interface layer. Boundary conditions that relate stresses and displacements on both sides of the interface are obtained as an asymptotic representation of three‐dimensional solutions for an interface layer in the limit of small wavelength to thickness ratio. The interface boundary conditions obtained include interface stiffnesses and inertia and terms involving coupling between normal and tangential stresses and displacements. The applicability of such boundary conditions is analyzed by comparison with exact solutions for ultrasonic wave reflection. Fundamental boundary conditions are introduced where only one transverse or normal mass or stiffness is included. It is shown that the solution for more exact interface boundary conditions which include two inertia elements and two stiffness elements can be decomposed into a sum of fundamental solutions. The transition between welded and slip boundary conditions on an interface with a thin viscous layer is also analyzed as a function of interface thickness, viscous skin depth, and frequency.

ISSN:0001-4966    

DOI:10.1121/1.400374

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An attempt is made to develop a simple analytical model to study the influence of voids in the interface zones—leading to a weak adhesion—between the constituent laminas of a composite plate treating the thin adhesive layer as a linear elastic material with voids (LEMV) [S. C. Cowin and J. W. Nunziato, J. Elasticity13, 125–147 (1983)]. The frequency equation for free harmonic wave motion in a sandwich plate with an LEMV adhesive layer at the core is derived and the dispersion curves for Al/adhesive/Al sandwich are compared for a thin adhesive layer core with those obtained from other available imperfect bond model studies.

ISSN:0001-4966    

DOI:10.1121/1.400375

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Available diffraction tomography algorithms are based on Fourier transform techniques and require either plane‐wave illumination in a uniform background medium or far‐field illumination combined with paraxial approximations. In this paper a generalized diffraction tomography algorithm is introduced that can handle both irregularly spaced measurement data, nonuniform background models, and general aquisition geometries. Using data from water tank experiments, the method’s ability to yield high‐quality reconstructions of geometry and velocity, as long as the weak‐scattering assumption is satisfied, is demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.400376

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The scattered field and its normal gradient obey a mutual linear relation at the scattering surface that is distinct from the physical boundary condition connecting either quantity to the incident radiation. On a moderately rough surface this relation can be represented by an operator whose series expansion in surface slope converges nicely even for large values of the Rayleigh parameter. This allows the Helmholtz integral for scattering amplitude to be written as a series of readily computable terms, one or two of which provide good approximations for surfaces too irregular for the usual Bragg expansion. This formulation reproduces the Bragg series for small surface elevations, and in the limit of low roughness wavenumber gives the Kirchhoff approximation with an explicit correction term in surface curvature. The usual results for a composite surface also emerge naturally. On several test profiles the method produces better overall accuracy than other multiscale approximations, at comparable efficiency.

ISSN:0001-4966    

DOI:10.1121/1.400377

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Theoretical investigations supported by extensive experimental comparisons are carried out on the interaction of ultrasonic waves with multilayered media. It is assumed that each constituent of the plate can possess up to as low as monoclinic symmetry. The plate is assumed to be immersed in a fluid and subjected to incident acoustic waves at arbitrary angles from the normal as well as at arbitrary azimuthal angles. Reflection and transmission coefficients are derived from which all characteristic behavior of the system is identified. Solutions are obtained for the individual layers that relate the field variables at the upper and lower layer surfaces. The response of the total plate proceeds by satisfying appropriate interfacial conditions across the layers.

ISSN:0001-4966    

DOI:10.1121/1.400378

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The general Kirchhoff theory of sound propagation in a circular tube is shown to take a considerably simpler form in a regime that includes both narrow and wide tubes. For tube radii greater thanrw=10−3cm and sound frequenciesfsuch thatrwf3/2<106cm s−3/2, the Kirchhoff solution reduces to the approximate solution suggested by Zwikker and Kosten. In this regime, viscosity and thermal conductivity effects are treated separately, within complex density and complex compressibility functions. The sound pressure is essentially constant through each cross section, and the excess density and sound pressure (when scaled by the equilibrium density and pressure of air, respectively) are comparable in magnitude. These last two observations are assumed to apply to uniform tubes havingarbitrarycross‐sectional shape, and a generalized theory of sound propagation in narrow and wide tubes is derived. The two‐dimensional wave equation that results can be used to describe the variation of either particle velocity or excess temperature over a cross section. Complex density and compressibility functions, propagation constants, and characteristic impedances may then be calculated. As an example, this procedure has been used to determine the propagation characteristics for a tube of rectangular cross section.

ISSN:0001-4966    

DOI:10.1121/1.400379

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An investigation is made of the signal reception of a hydrophone placed in front of an elastic plate backed by a compliant baffle layer. The baffle layer is the compliant tube array modeled by Junger [J. Acoust. Soc. Am.78, 1010 (1985)] to represent a homogeneous, dispersive fluid layer. Noise reduction baffles such as compliant tubes are acoustically soft and thus tend to degrade the signal received. This paper describes the development of a theoretical model for evaluating the signal reception, and a comparison of theoretical results with experimental data is presented.

ISSN:0001-4966    

DOI:10.1121/1.400380

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A general time domain approach is presented to evaluate the vibratory response of cylindrical shells in time‐invariant axial flows to broadband time‐dependent excitations. The approach is based on the use of aninvacuoeigenvector expansion with time‐dependent coefficients for the velocity field of the shell. A set of convolution integral equations are developed for the eigenvelocities that are coupled due to the fluid loading. These equations are similiar to those developed for planar vibrators and are readily solved by marching forward in time. The known terms in the equations are the ρcfluid‐loaded admittances of each eigenvector and the radition impulse responses. Since these are easily obtained, a large number of eigenvectors can be included in the analysis. Numerical results illustrating the responses of a simply supported cylindrical shell to an impulsive ring excitation are presented.

ISSN:0001-4966    

DOI:10.1121/1.400381

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A comparative analysis of recursive ray tracing strategies for tomographic reconstruction from projections with diffracting sources is presented. One algorithm employs ray tracing in reprojection toward a correction of the true projection values for subsequent straight‐ray reconstruction. The other algorithm performs curved‐ray reconstruction along the retraced rays by algebraic methods for each recursive step. The performance for reconstruction of objects exhibiting discontinuity boundaries is studied. Within the geometrical optics approximation, the forward process is also shown to lead to multiple linking refracted rays. In an object of low velocity, there may be ranges of receive positions over which no linking refracted rays exist. Geometrically diffracted rays are introduced to describe the signal actually received over the ‘‘forbidden’’ range.

ISSN:0001-4966    

DOI:10.1121/1.400382

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An experiment was performed to investigate the noncollinear interaction of a high‐frequency tone with low‐frequency noise. The nonlinear interaction took place in an air‐filled rectangular duct. A low‐frequency band of noise was transmitted in the (0,0) mode together with a high‐frequency tone in the (1,0) mode. The angle between the two interacting waves was 55°. Measurements of the frequency spectra were made at locations that were well within the shock formation distance of either primary wave. The spectral shapes of the sum and difference frequency sidebands of noise are scalloped in appearance, and the number of nulls in each sideband increases with range from the source. Theoretical predictions for the nonlinearly generated sidebands are in good overall agreement with experiment. The predictions are based on a quasilinear analysis developed previously to describe the noncollinear interaction of two tones in a rectangular duct [M. F. Hamilton and J. A. TenCate, J. Acoust. Soc. Am.81, 1703–1712 (1987)]. Comparisons are made with the corresponding case of collinear interaction, in which both primary waves propagate in the (0,0) mode.

ISSN:0001-4966    

DOI:10.1121/1.400383

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP