Existence Of Stoneley Waves Research Articles

Analysis of Surface Waves in Saturated Layered Poroelastic Half-Spaces Using the Thin Layer Method

There are multiple modes of surface waves in saturated layered poroelastic half-spaces. The phase velocity and the attenuation of the modes are frequency dependent. The frequency behaviour of the modes can be studied using the layer transfer, stiffness and the transmission/reflection matrix methods. However, it is very difficult to find the complex roots of the determinants because the entries of the matrices involve the complex exponential functions of the wavenumber and the thickness of layer. To overcome this difficulty, the entries in the matrix are expressed in the form of algebraic functions using the thin layer method. Thus, the eigenvalues and eigenvectors can be easily solved using the matrix decomposition techniques instead of the root-searching ones. Some of the eigenvalues correspond to the wavenumbers of the surface waves, and can be picked out based on the characteristics of the surface waves. The frequency behaviour, variations of the pore pressure and the skeleton’s displacements with the depth can be then investigated from the corresponding eigenvalues and eigenvectors, respectively. The method is verified by comparing the analytical and the discrete results in the saturated poroelastic half-space with the permeable surface. The method is applied to appreciate the effects of an impermeable surface on Rayleigh waves (R-waves) and the existence of Stoneley waves in the poroelastic half-space. The frequency behaviour of Rayleigh waves in three typical layered poroelastic half-spaces is also analyzed.

Inspectability of interfaces between composite and metallic layers using ultrasonic interface waves

The interface between an anisotropic composite material and a metallic material is inspected non-destructively for disbonded regions using ultrasonic guided waves. The material properties of the composite and metal have been tailored to demonstrate their effect on inspectability. The material properties have been designed to be either favorable or unfavorable to the existence of propagating Stoneley waves. Stoneley waves can exist because the layer thicknesses are large enough compared to the wavelength to be considered half-spaces. The existence of Stoneley waves between generally anisotropic materials depends on the elastic constants and densities in a complicated way. The range of material properties that allow Stoneley waves is small; however, when the vertically polarized shear wave speeds are similar in the two materials, the existence of Stoneley waves is generally possible. If the conditions do not strictly allow Stoneley waves, other interface waves can still exist such as leaky waves. Disbonds are inserted into the materials before bonding and are inspected using interface waves. Sensitivity to disbonds is determined and thus inspectability is demonstrated for cases that are favorable and unfavorable to Stoneley waves. Both numerical and physical experimental results are shown.

Transition radiation of elastic waves at the interface of two elastic half-planes

Radiation of elastic waves is studied that is emitted by a point load that crosses the interface of two elastic half-planes. It is assumed that the load has a constant magnitude, moves along a straight line normal to the interface, and has a constant speed that is smaller than the minimum shear wave speed in the half-planes. In this case the mechanism of excitation of elastic waves is conventionally referred to as transition radiation. The adopted model allows to obtain an analytical expression for the elastic field excited by the load in the frequency–wavenumber domain. Using this expression, the energy of transition radiation is derived in a closed form. It is shown that transition radiation of the body waves occurs at any non-zero velocity of the load. Additionally, transition radiation of interface waves may occur provided that parameters of the half-planes allow existence of Stoneley waves. A parametric analysis of the directivity diagram of radiated body waves is accomplished focusing on dependence of the diagram on the load speed, load direction, and parameters of the half-planes. Using parameters that allow radiation of interface waves, the energy of this radiation is compared to that of the body waves. It is shown that the energy of the interface waves is greater unless the load velocity is close to the lowest body wave velocity.

Existence of Stoneley Waves at an Unbonded Interface Between Two Micropolar Elastic Half-Spaces

Following Eringen’s theory, the question whether a Stoneley mode can exist at an unbonded interface between two micropolar elastic half-spaces is examined analytically. Explicit conditions for its existence are derived when the two half-spaces are incompressible or Poisson solids whose material properties are close to each other.

Free surface (Rayleigh) waves in anisotropic elastic half­-spaces: the surface impedance method

The questions of uniqueness and existence of free surface waves in anisotropic linear elastic half-spaces have been settled in previous investigations by appeal to the theory of uniformly moving dislocations. An alternative framework relying on the surface impedance tensor is capable of settling the same issues; that framework is developed and fully exploited in the present work. The same framework may be used to study thoroughly the existence of Stoneley waves in bonded anisotropic half-spaces, which is the subject of an accompanying paper.

Existence of Stoneley waves on boundaries between acoustically similar solids

Stoneley waves exist on the boundaries between a limited range of isotropic and anisotropic solids and have been directly and indirectly detected on several pairs at ultrasonic frequencies [D. A. Lee and D. M. Corbly, IEEE Trans. Sonics Ultrason. SU-24, 206 (1977); R. O. Claus and C. H. Palmer, Appl. Phys. Lett. 31, 547 (1977)]. Of particular importance to current optical measurements are those waves that exist between pairs of transparent glass substrates for which Weichert's condition, μ1/μ2 ≈ ρ1/ρ2 ≈ 1, is nearly satisfied. We study the range of possible existence for these pairs and indicate that the existence region in the (μ1/μ2), (ρ1/ρ2) plane defined by Scholte [Proc. Ned. Akad. Weten. 45, 20, 159, 380, 439, 516, (1942)] degenerates to a single point when the media obey Weichert's equality. Experimental implication of the theoretical results is discussed. [Work supported by NSF under Grants ENG-78-05773 and ENG-78-11040.]

The existence of Stoneley waves as a loss mechanism in plane wave reflection problems

In a previous paper [’’Influence of Stoneley waves on plane-wave reflection coefficients: Characteristics of bottom reflection loss,’’ K. E. Hawker, J. Acoust. Soc. Am. 64, 548 (1978)] it was shown that for certain configurations of layered fluids overlying a rigid (solid) substrate, the reflection loss would display narrow peaks of significant amplitude. It was suggested in that paper that these peaks were due to excitation of Stoneley waves at the fluid–solid interface. In the present paper this association is made more precise and definite. Through an extension of the classical theory of Stoneley waves to the case of inhomogeneous media, it is shown that the angles at which the reflection-loss peaks occur are precisely those angles for which the horizontal component of phase velocity in the fluid equals the Stoneley wave phase velocity. In addition, the near independence of the reflection loss peaks on frequency is explained quantitatively.