Wave Propagation In Dissipative Materials

Common experience reveals two basic aspects of wave propagation. First, while preserving their identity and travelling at definite speeds, sounds finally die out. Second, weak sounds may combine to form strong noises. Theories of acoustic propagation have succeeded in representing these aspects of experience separately, but never combined as in nature. The classical theories of sound in perfect fluids and elastic solids easily yield common speeds of propagation for plane infinitesimal disturbances, but no damping. Moreover, within EULER'S theory of the perfect fluid, or its generalization, the GREEN-KIRCHHOFF-KELVIN theory of finite elasticity, weak waves may grow stronger and become shock waves, which propagate according to more complicated but equally definite principles. Effects of internal damping are easily added for theories of infinitesimal deformation, but for finite motions a dead end was reached about sixty years ago. Indeed, in 1901 DUHEM proved that according to the NAVIER-STOKES theory of fluids acceleration waves and waves of higher order cannot exist, and for shock waves he claimed a similar result, which has since been shown to be valid subject to certain qualifications. So as to save the phenomena of sound and noise, as was necessary if the NAVIER-STOKES theory was to deserve the place proposed for it as a refinement upon EULER'S theory, DUHEM introduced the concept of "quasi-wave", a region of rapid but continuous transition.

This report discusses two analytical approaches to be used in future studies of stress-wave propagation in rock. The first of the discussions is the development of a method of Fourier analysis of waveforms, the Impulse Train technique. This technique makes use of a discontinuous derivative and the properties of the Dirac delta function for the numerical evaluation of the Fourier transform of a measured time history. The second describes a method whereby a possible stress-strain curve can be deduced from particle velocity histories. Incremental steps in stress are assumed to travel at a velocity determined by the slope of the material stress-strain curve at the stress level of the increment. Equations are derived for the relation of particle velocity to strain and stress. Fortran computer programs for each of the analytical discussions are included as appendixes. (Author).

Theories of surface waves develop since the end of XIX century and many fundamental problems like existence, phase and group velocities, attenuation (quality factor), mode conversion, etc. have been, in part successfully, solved within the framework of such simple models as ideal fluids^ or linear elasticity. However, a sufficiently complete presentation of this subject, particularly for solids, is still missing in the literature. The sole exception is the book of I. A. Viktorov^ which contains an extensive discussion of fundamental properties of surface waves in homogeneous and stratified linear elastic solids with particular emphasis on contributions of Russian scientists. Unfortunately, the book has never been translated to English and its Russian version is also hardly available. Practical applications of surface waves develop intensively since a much shorter period of time than theories even though the motivation of discoverers of surface waves such as Lord Rayleigh stems from their appearance in geophysics and seismology. Nowadays the growing interest in practical applications of surface waves stem from the following two main factors: surface waves are ideal for developing relatively cheap and convenient methods of nondestructive testing of various systems spanning from nanomaterials (e.g.

Elastic Wave Propagation in Structures and Materials initiates with a brief introduction to wave propagation, different wave equations, integral transforms including fundamentals of Fourier Transform, Wavelet Transform, Laplace Transform and their numerical implementation. Concept of spectral analysis and procedure to compute the wave parameters, wave propagation in 1-D isotropic waveguides, wave dispersion in 2-D waveguides is explained. Wave propagation in different media such as laminated composites, functionally graded structures, granular soils including non-local elasticity models is addressed. The entire book is written in modular form and analysis is performed in frequency domain. Features: Brings out idea of wave dispersion and its utility in the dynamic responses. Introduces concepts as Negative Group Speeds, Einstein’s Causality and escape frequencies using solid mathematical framework. Discusses the propagation of waves in materials such as laminated composites and functionally graded materials. Proposes spectral finite element as analysis tool for wave propagation. Each concept/chapter supported by homework problems and MATLAB/FORTRAN codes. This book aims at Senior Undergraduates and Advanced Graduates in all streams of engineering especially Mechanical and Aerospace Engineering.