摘要:

The response of a structure may be stated in terms of an impulse response function integrated over the external drive or equivalently in terms of an impulse response operator acting on the external drive. A complex structure usually consists of either several elemental structural components that are interconnected (e.g., two welded plates) each admitting to a single response type (e.g., either longitudinal or flexural), an elemental structural component that admits to several response types (e.g., longitudinal and flexural), or a combination of both. To derive the impulse response operator directly is not a simple task, even in these primitive examples. The boundaries between elemental structural components and/or the various response types in each component render the response of the complex structure orderly but multifaceted. Therefore, it is suggested that one may advantageously analyze the response, at a given spatial position in the structure and at a specific instant of time, by superposing the contributions to the response of a number of paths. A path defines a unique type of an impulse response operator. The impulse response operator of a path may be simpler to derive than that derived directly for the complex structure as a whole. In this multipaths analysis the complex structure is described by an impulse response vector operator, each element is an impulse response operator that is associated with a unique path. Correspondingly, the external drive is a vector, and the response is the scalar product of the impulse response vector operator and the external drive vector. A sequential procedure is introduced in which the complex structure and the external drive are further decomposed in the hope of achieving further simplifications. In this procedure one recognizes that a dynamic system may be associated with each type of response in an elemental structural component. With this recognition the complex structure is modeled by a set of dynamic systems. The formalism is then stated in terms of matrices and vectors, e.g., the response is a vector (each element represents the response of a specific dynamic system), the impulse response operator is a matrix (the off-diagonal elements describe the couplings between the dynamic systems), etc. If the dynamic systems are chosen so that each, in isolation, can be described in terms of an eigen-impedance operator, then, in addition, a modal analysis can be applied to the multiple dynamic systems that compose the model of the structure. In the modal analysis, however, the ranks of the impulse response matrix, the response vector, and the drive vector are swollen by the modal count, usually rendering the matrix equation unwieldy. In the parallel wave analysis, the propagations in the dynamic systems are described by impulse response operators that are commensurate with those pertaining to boundlessly extrapolated dynamic systems. The finiteness of the dynamic systems is accounted for by junction matrices; a junction defines the boundaries through which dynamic systems interact either with each other (transmissions) or with self (reflections). As in the modal approach, in this wave approach, the resulting formalism is, again, rather unwieldy. It is shown that considerable reductions and simplifications are attained if the structure can be modeled by spatially one-dimensional dynamic systems.

ISSN:0001-4966    

DOI:10.1121/1.400987

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Exact analytical treatment of the interaction of harmonic elastic waves withn-layered anisotropic plates is presented. Each layer of the plate can possess up to as low as monoclinic symmetry and thus allowing results for higher symmetry materials such as orthotropic, transversely isotropic, cubic, and isotropic to be obtained as special cases. The wave is allowed to propagate along an arbitrary angle from the normal to the plate as well as along any azimuthal angle. Solutions are obtained by using the transfer matrix method. According to this method formal solutions for each layer are derived and expressed in terms of wave amplitudes. By eliminating these amplitudes the stresses and displacements on one side of the layer are related to those of the other side. By satisfying appropriate continuity conditions at interlayer interfaces a global transfer matrix can be constructed which relates the displacements and stresses on one side of the plate to those on the other. Invoking appropriate boundary conditions on the plates outer boundaries a large variety of important problems can be solved. Of these mention is made of the propagation of free waves on the plate and the propagation of waves in a periodic media consisting of a periodic repetition of the plate. Confidence is the approach and results are confirmed by comparisons with whatever is available from specialized solutions. A variety of numerical illustrations are included.

ISSN:0001-4966    

DOI:10.1121/1.400988

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The poroviscoelastic model, which is asynthesis of the well-known viscoelastic model and Biot’s poroelastic model, is presented in the context of the propagation of plane acoustic waves in linear viscoelastic porous media. Except for the propagation of a second compressionalP2wave in such media, results concerning particle displacement, maximum attenuation, and direction of maximum energy flow are very similar to that obtained by Borcherdt [J. Geophys. Res.78, 2442–2453 (1973)] in simple viscoelastic media. The energy conservation relation is given explicitly in the case of time harmonic radiation field. From this, expressions of the energy flux, energy densities, dissipated energy, andQ−1are derived. Furthermore it is demonstrated that the total attenuationQtotal−1is equal to the sum of the viscoelastic attenuationQvisco−1and Biot’s poroelastic attenuationQporo−1.This allows a direct comparison between energy dissipated by viscoelasticity and that dissipated by Biot mechanism. For propagation of acoustic waves in infinite natural porous media the latter is always negligible compared to the former. Computed curves and physical interpretations are proposed to illustrate the theoretical derivations.

ISSN:0001-4966    

DOI:10.1121/1.400989

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A curvilinear grid is constructed for the numerical integration of a wave characteristics formulation, where the independent variables are the two-spatial polar coordinates and the time dimension. The grid is formed by the bicharacteristic curves which represent the space-time location of the wave surfaces. This scheme of integration paths allows the solution to propagate along all existing waves that traverse the solution domain. Without detriment to generality, the wavegrid construction method is developed by considering longitudinal and shear waves with constant speeds of which the speed of the shear waves is the smallest.

ISSN:0001-4966    

DOI:10.1121/1.400990

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The response of an ideal elastic half-space to an impulsive load is obtained by a computational method based on the theory of characteristics. The impulsive load is distributede unevenly but radially over the entire boundary of a semicircular infinite canyon embedded in the surface of the half-space. For this configuration, a wave characteristics formulation is presented where its differential equations are extended to accommodate strong discontinuities which occur in the material motion of the half-space. A step-by-step numerical integration of these extended differential equations is then carried out in a two-dimensional curvilinear wavegrid, formed by the bicharacteristic curves of the wave characteristics formulation. The resultant transient deformation of the half-space is shown by plots which give the time history behavior of the dependent variables at a specific spatial location. In these plots, the various wave fronts traversing the half-space explicitly reveal themselves.

ISSN:0001-4966    

DOI:10.1121/1.400991

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Scattered sound field from three concentric shells ensonified by a plane wave is treated. Formulation of the problem is carried out using impedance expressions for the shells and the acoustic media interior and exterior to the shells. Results of exact equations of elasticity have been used to obtain the shell impedances, making the results applicable to arbitrary thicknesses. The results show that in a target made up of multiple shells and layers, the first elastic continuum that the incoming waves interact, which most closely resembles an impenetrable target dominates scattering. For example, a very thick shell or a very light-density fluid layer effectively insulates scattering from the layers or shells in its interior. When locally resistive compliant coatings are placed on the surfaces of the shell, they alter the scattering response. A very compliant layer on the exterior of the target changes it to a pressure-release type of boundary. ©1991 Acoustical Society of America.

ISSN:0001-4966    

DOI:10.1121/1.4794490

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper describes a matrix method for computing the exact reflection and transmission coefficients for harmonic plane waves within a stratified medium of homogeneous, isotropic, and absorbing plane layers. The new feature is that each layer can be either liquid or solid, whatever their successive order. Furthermore, this algorithm takes into account evanescent waves, but also applies whatever the thickness of each layer. A numerical example is shown.

ISSN:0001-4966    

DOI:10.1121/1.400993

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper is concerned with the use of leaky Lamb waves for the nondestructive evaluation (NDE) of damage in anisotropic materials such as fiber-reinforced composites. Two fundamental acoustic properties of the material, namely, the wave speed and attenuation have been measured. Stiffness is deduced from the wave speed. The damage mode selected for this study is matrix cracking. As expected, the in-plane stiffness decreases and the attenuation increases with an increase in the linear crack density.

ISSN:0001-4966    

DOI:10.1121/1.401017

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The Fourier diffraction theorem is the basis of diffraction tomography. The theorem states that the field scattered by a semitransparent object maps onto arcs in the Fourier space of the object. In this paper, the stochastic analog is derived to the theorem for a general anisotropic, statistically homogeneous random continuum. By acoustically probing the random medium from various angles, the second-order statistics of the medium can be recovered from the second-order statistics of the perturbed field. The robust nature of the result is confirmed by numerical experiments using finite arrays and autocorrelation estimation theory.

ISSN:0001-4966    

DOI:10.1121/1.400994

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The time harmonic elastodynamic response of two semi-infinite elastic plates of dissimilar material properties perfectly bonded along their lateral faces is studied. The wave field in either half-plate can be written as a superposition of the so-called Rayleigh–Lamb eigenmodes of an infinite plate. The interaction of a time harmonic incident wave with the interface results in reflected and transmitted fields that contain contributions from all of the real, imaginary, and complex eigenmodes of an infinite plate. Attention is focused on the distribution of energy among the various reflected and transmitted eigenmodes over a range of frequencies. The fundamental symmetric and the fundamental antisymmetric Lamb modes are each used as input excitations. Such excitations can be approximately realized in experiments. It is assumed that the solution of such a canonical problem will facilitate the solution of problems with complicated time-dependent sources.

ISSN:0001-4966    

DOI:10.1121/1.400995

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP