Showing papers on "Lamb waves published in 1978"

A model for the electromagnetic generation of ultrasonic guided waves in ferromagnetic metal polycrystals

Abstract: A model is presented which predicts the efficiency of the electromagnetic generation of ultrasonic Rayleigh or Lamb waves in ferromagnetic metal polycrystals. The practically important case in which the static magnetic field, the dynamic magnetic field, and the propagation direction are parallel is considered. Two distinct forcing mechanisms are explicitly included, magnetostrictive stresses, and Lorentz forces on induced eddy currents. The ability of the resulting formulas, which depend only upon the applied magnetic field and independently known elastic and magnetic properties of the material, to correctly predict such important quantities as the field dependence of generation efficiency (both phase and amplitude), the relative importance of magnetic permeability, magnetostrictive coefficients, and electrical resistivity in determining this efficiency, and the decrease in efficiency with distance of the transducer from the sample, is demonstrated by direct comparison to measurements in iron. Results are included in a form useful in predicting the efficiencieosf transducers in new applications.

Scattering matrix for elastic waves. II. Application to elliptic cylinders

Abstract: Using the scattering‐matrix approach to elastic wave scattering, numerical results are presented for the scattering of P, SV, and SH waves from a cylinder of elliptic cross section for ratios of minor to major axis ranging from 0.25 to 1.0 and for nondimensional wave numbers in the range 0.1–3.2. Calculations were made for a tungsten cylinder embedded in aluminum and also for a cylindrical cavity in aluminum. The incident waves are taken to be plane waves incident obliquely with respect to the major axis of the ellipse.

Piezoelectric acoustic boundary waves propagating along the interface between SiO 2 and LiTaO 3

Abstract: The propagation characteristics of the layered structure of thick Si02 film on 126° rotated Y-cut X-propagation LiTa03 are studied. The existence of the new piezoelectric acoustic boundary waves (Stoneley type waves) has been found. The waves are tightly bound to the thick Si02 film-LiTa03 substrate interface. Piezoelectric acoustic boundary waves appear beyond H/λ = 0.5, where H is the film thickness and λ is the acoustic boundary wave length. At H/λ = 1.0, the energy concentration coefficient, η = |Us|2 / l |Ui|2 is 0.08, where Us and Ui, are displacement at the surface and interface, respectively, and piezoelectric coupling coefficient K2 is 0.02. The experimental results agree with the calculated results.

Interaction of acoustic waves with shock waves in elastic solids

Abstract: Transmission and reflexion of plane acoustic waves through a longitudinal shock wave in elastic isotropic solids are investigated. As a result, the amplitude of the transmitted and reflected waves and the jump of the acceleration of the shock are explicitly determined.

Negative group velocities of Lamb waves

Abstract: Negative group velocities, which mean that the direction of propagation of wave energy and that of wave phase are opposite, were found experimentally in S2 mode of Lamb waves excited on an A 1 plate of 1 mm thick at frequencies near 3 MHz. A stroboscopic schlieren technique was used to observe an ultrasonic pulse reradiated in water from the Lamb mode, together with specular‐reflected and transmitted pulses. The phase velocity of the Lamb mode was derived from the angle of incidence, and the direction of group velocity was determined from the displacement of reradiated pulse relative to the specular reflection. Phase velocities observed at various frequencies were in good agreement with theoretical predictions, which also described negative group velocities appearing at phase velocities above 10 km/s. Such a high phase velocity is negligibly affected by water loading. Existence of negative group velocity seems inherent in S2 mode and independent of plate materials. Also in S4 mode, negative group velocities are predicted in a very narrow frequency range.

An inclined-plate wave generator

Abstract: A numerical method for determining the characteristics of waves generated by a hinged inclined-plate wave generator operating in a constant depth channel is discussed. The analysis is in reasonably good agreement with experimental results. The results indicate the sloping generator operating under certain conditions is completely inefficient, i.e., for a given stroke and depth-to-wave-length ratio, very small waves are produced; for other wave periods for the same conditions significantly larger waves are generated.

Evaluation of the Relative Importance of Circumferential or Creeping Waves in the Acoustic Scattering from Rigid and Elastic Solid Cylinders and from Cylindrical Shells.

Abstract: : The relationship is determined between predicted circumferential waves on cylinders and the normal-mode series solution for the acoustic scattering by submerged metal cylinders Specific circumferential waves are linked to the free modes of vibration of the cylinder, and this identification is used to experimentally detect the Rayleigh surface wave at low ka Circumferential waves on thin cylindrical shells are demonstrated to closely follow prediction based on Lamb theory for flat plates (Author)

The near-field love waves by the exact ray method

Abstract: We have synthesized near-field Love waves by the exact ray method. The near-field Love waves are defined, in the present paper, as long-period seismic SH waves, which consist of waves reflected and/or refracted a small number of times at shallow boundaries in the crust such as the Mohorovicic and Conrad discontinuities. The long-period records obtained at the Abuyama Seismological Observatory of Kyoto University (φ-N85°W, Δ-301km) at the time of the Off-Izu-Peninsula earthquake of May 9, 1974 (M=6.8 after JMA) have been analyzed. As the Abuyama station is located almost west of the epicenter, the NS component seismogram of the records scarcely includes any seismic waves other than Love waves (SH waves). By comparing the NS record with the synthesized near-field Love waves, we have identified the first three longperiod (15-20 seconds) predominant phases on the record with head S waves, direct S waves and the S waves totally reflected once at the Moho. A method is proposed for determining an average S wave velocity structure by fitting peak-arrivals of some of the characterized phases of the synthesized near-field Love waves to those of the record. By applying this method, we have obtained the structure along the south coast of the Tokai district in central Japan with a satisfactory accuracy: the thickness of the crust is 30±5km, the S wave velocity in the crust and upper mantle are 3.5±0.05km/sec and 4.6-4.7km/sec, respectively. The lowest crustal layer with P wave velocities of 6.6-6.8km/sec may not exist or may be thin if it exists. We have also synthesized the normal mode Love waves and compared them with the near-field Love waves synthesized by the exact ray method. The agreement is excellent except for the head wave portion.

Propagation and decay of waves in porous media

Abstract: A study is made of the propagation of wave fronts in two types of fluid‐filled porous media, anisotropic and isotropic. The characteristic equation governing wave propagation velocities in anisotropic media is obtained by using the notion of surfaces of discontinuity. It is found that the wave velocities in such media vary with the direction of propagation and that for a specified direction there are four different wave velocities with which the disturbances can propagate. When the material is isotropic, its frequency equation shows the existence of three distinct waves, two longitudinal waves and one shear wave with velocities independent of the direction of propagation. The decay equations derived for these waves indicate that there are two factors which influence the decay of waves in isotropic porous media. The first is the geometry of the wave front, and the second is the friction between the solid and fluid phases.

Surface to bulk mode conversion at interfaces on y‐z LiNbO3

Abstract: The angular spectra of bulk waves mode converted from surface wave reflections at a free‐metallized, mass‐loaded, and 90° crystal corner interface were measured on y‐z lithium niobate. The fraction of surface acoustic wave energy converted into shear bulk wave energy was dependent on the strength of the surface perturbation at the interface.

Wave forces on piles

Abstract: A state of the art review of wave forces on rigid piles is presented. J.P. Morison equation and diffraction theory are presented with recent research on their applications.

A unidirectional transducer with three electrode groups for Lamb- wave devices

Abstract: The theory and design of a new type of unidirectional transducer for Lamb‐wave devices are described The transducer consists of three electrode groups One of them is used as a common earth electrode and is on the bottom surface The others are interdigital electrode groups on the top surface The unidirectionality of this transducer can be realized under the phase condition of x+y=1/2, where 2πx is the phase difference arising from the geometry and 2πy is the phase difference of two electrical signals applied to the transducer

Some properties of internal waves

Abstract: General properties of linearized internal waves are developed in preparation for a treatment of nonlinear waves. Two sets of results are obtained. The first pertain to propagation in an arbitrary stability stratified medium. The second set refer to the case when the Boussinesq approximation is made. Special attention is paid to the completeness of the normal modes and their use in solving initial value problems. Some novel sum rules are obtained. The principal tools of the investigations are two variational principles.

Propagation of plane waves in granular media

Abstract: Some recent theoretical results obtained on the propagation of plane longitudinal waves in granular materials are discussed. Using a one-dimensional formulation of the continuum theory of granular materials proposed by Goodman and Cowin, the longitudinal response of two classes of materials was modeled. The two classes are materials with compressible granules, such as pressed powders and sands at high confining pressures, and flowing materials with incompressible granules, such as sands at low confining pressures. The speed and the amplitude of the various types of acceleration waves possible in each class were considered. In materials with compressible granules, two waves were found to exist, an elastic wave followed by a compaction wave. However, in flowing materials with incompressible granules, the only wave possible is a wave of dilatancy. In each case, the influence of the initial material non-uniformity on the wave behavior is discussed, and some experimental observations which correlate with the results of the analysis are cited.

Nonlinear wave amplification in a beam-plasma system

Abstract: Nonlinear wave amplification is investigated in an electron beam-plasma system under conditions such that the values of the wave numbers of the interacting high frequency waves are in the resonant region (k approximately= omega pe/ nu 0). For weak wave coupling the amplification is shown to have an explosive nature and to be enhanced as compared with explosive amplification occurring in the non-resonant region. Self-similar solutions demonstrate that the wave amplitudes grow more rapidly in space.

The diffraction of elastic waves by small cylindrical cavities

Abstract: Abstract In order to use the method of asymptotic matching for low frequencies, the equations of plane elastostatics are reformulated in terms of the two scalar potentials commonly used in plane elastodynamics. It is shown that the resulting equations of plane elastostatics can be reduced to those first obtained by Muskhelishvili. The use of the formulation is illustrated by considering the case of the plane diffraction of a P wave by a circular, cylindrical cavity of small radius. The results agree with those obtained from the exact solution of the problem.

The attenuation of elastic waves by surface inhomogeneities: Rayleigh waves

Abstract: We present a theoretical study of the attenuation of Rayleigh waves as a consequence of their interaction with surface inhomogeneities. The frequency and correlation‐length dependence of the attenuation rate is calculated explicitly based on a Gaussian‐distributed fluctuation for the mass density and for the Lame coefficients in terms of the Green’s function method. All the surface modes of elastic waves are considered in the attenuation processes.