Showing papers on "Lamb waves published in 1986"

Method and apparatus for measuring fluid characteristics using surface generated volumetric interrogation signals

Abstract: A method and apparatus for measuring fluid characteristics use a non-invasive ultrasonic system for generating spatially extended signals into a volume of a fluid and detecting said signals for measuring the characteristic of the fluid. The generated waves are Rayleigh-like surface waves creating, in effect, an extended aperture transducer (10) from which the waves leak into the fluid. The Rayleigh-like surface wave operates in an environment wherein the plate or other structure (14) on which the surface wave is generated has a thickness, normal to the primary direction of propagation of the wave, of less than four Rayleigh wavelengths and greater than approximately one-half of a Rayleigh wavelength. The extended aperture has a length of at least about ten Rayleigh wavelengths. The excitation for the system is generally a short pulse interrogation in order to avoid those interferences which may cause Lamb waves to be set up in the solid material. Several different configurations employing the Rayleigh-like surface wave are illustrated and discussed.

A fourth-order accurate finite-difference scheme for the computation of elastic waves

Abstract: A finite difference for elastic waves is introduced. The model is based on the first order system of equations for the velocities and stresses. The differencing is fourth order accurate on the spatial derivatives and second order accurate in time. The model is tested on a series of examples including the Lamb problem, scattering from plane interf aces and scattering from a fluid-elastic interface. The scheme is shown to be effective for these problems. The accuracy and stability is insensitive to the Poisson ratio. For the class of problems considered here it is found that the fourth order scheme requires for two-thirds to one-half the resolution of a typical second order scheme to give comparable accuracy.

Analytical Studies of Body Wave Propagation and Attenuation

Abstract: : In situ crosshole and downhole seismic methods are becoming widely used as a means of nondestructively evaluating the elastic properties of geotechnical systems. The elastic constants are calculated from the records of body waves (longitudinal and transverse waves) traveling through the media. Measurements are made by generating a seismic disturbance at one point and measuring the time required for the disturbance to travel to one or more seismic receivers. Several simplifying assumptions are made in traditional analysis of seismic measurements for engineering purposes. These include assuming plane wave propagation, measurement of only far-field waves, and independence of the measurements on the source-receiver configuration and on the amount of material damping. An analytical study of the effects and the validity of the different assumptions is presented. It is found that, for the range of distances and frequencies typically used in engineering applications: body wave fronts generated by point source cannot be considered plane; and near-field effects associated with spherical wave fronts can be very important. The near-field effects are caused by coupling between waves which exhibit the same particle motion but which propagate at different velocities and attenuate at different rates. To minimize the detrimental effects of near-field waves in those methods based on spectral analysis techniques, it is recommended that, in the field setup, the ratio of distances from the source to the second and first receivers be of the order of two or greater.

A plane-wave decomposition for elastic wave fields applied to the separation of P-waves and S-waves In vector seismic data

Abstract: We describe a method to decompose a two‐dimensional (2-D) elastic wave field recorded along a line into its longitudinal and transverse parts, that is, into compressional (P) waves and shear (S) waves. Separation of the data into P-waves and S-waves is useful when analyzing vector seismic measurements along surface lines or in boreholes. The method described is based on a plane‐wave expansion for elastic wave fields and is illustrated with a synthetic example of an offset vertical seismic profile (VSP) in a layered elastic medium.

Split mode traveling wave ring-resonator

Abstract: This invention is an improved traveling wave ring resonator utilizing only one source of waves. The standard traveling wave ring resonator uses two wave sources phased 90 degrees apart and physically separated by a quarter wavelength to separately excite two equal-frequency standing wave modes which make up a traveling wave. The present invention uses perturbations of the ring resonator to shift the frequency of the standing wave modes, by specified amounts, and to correctly fix their position, to allow a single wave source to properly excite the standing wave modes which comprise a traveling wave. This invention has application to traveling wave ring resonators in acoustics, mechanical devices, and electromagnetic devices, as well as to simply-connected resonators to be excited in rotating wave modes. Specific applications include improved thermoacoustic traveling wave heat engines and pumps, simplified surface wave motors, and energy efficient wave pools for recreational purposes.

Nonlinear surface acoustic waves on an elastic solid of general anisotropy

Abstract: The effect of elastic nonlinearity on the propagation of Rayleigh waves in an anisotropic elastic solid is considered. A nonlinear integro-differential equation is derived for a quantity which is related to the Fourier transform of the displacement component on the surface. The variation of this quantity along the surface accounts for the slow modulation of the wave through formation and depletion of the different harmonics. Explicit results are given for harmonic generation in an initially sinusoidal wave and for parametric amplification of a weak signal by a pump wave of twice its frequency.

Properties of composite laminates using leaky lamb waves

Abstract: Method and system for detecting defects in the structure of a composite laminate material are described which comprise a first transducer for directing an ultrasonic beam of preselected frequency along a transmission axis onto a surface of the laminate at a preselected angle to generate Lamb waves in the laminate and leaky Lamb waves reflected therefrom; a second transducer for detecting the reflected leaky Lamb waves and for providing an output signal corresponding to the amplitude of the reflected waves; a fluid sonically coupling the laminate with the transducers; a positioning system for selectively positioning the transducers along the surface of the laminate to produce a set of output signals characterizing the laminate; and suitable electronics for receiving and processing the output signals and comparing them with signals characteristic of a defect free sample of the laminate.

Analysis of lamb wave propagation characteristics in rotated Y-cut X-propagation LiNbO3 plates

Abstract: Lamb wave propagation characteristics in rotated Y-cut X-propagation LiNbO3 plates are analyzed theoretically as a parameter of the rotation angle of the crystal to elucidate velocity dispersion characteristics, mechanical displacement and electric potential, and electromechanical coupling constants. Calculation results show Lamb wave propagation characteristics in the 90° rotated Z-cut plate and the 128° rotated Y-cut plate. The zero-order symmetrical mode at a 0° rotation angle (Y-cut) has a high coupling constant over 20 percent. The zero-order symmetrical and antisymmetrical modes degenerate with a plate thickness more than three wavelengths and become the Rayleigh wave range in substance. In measurement of the central frequency of the Lamb wave filter, the experimental results agree well with the theoretical analysis and the analytical results by this method are useful in constructing Lamb wave devices using interdigital transducers.

Thermoelastic Coupling to Lamb Waves

Abstract: We consider the thermoelastic generation of Lamb waves produced by incidence of a light beam on a solid plate. In addition to the variables relevant for thermoelastic SAW generation -- frequency and the optical, thermal, and acoustical properties of the materials used -- one has the plate thickness and mode type (symmetric, anti-symmetric). The characteristics of the problem depend on the ratios of plate thickness to thermal wavenumber and acoustic wavelength, and on the symmetry of optical beam incidence. Initial results indicate that the thermoelastic coupling efficiency for plate modes can be appreciably higher than for SAWs. Furthermore, if the plate is a suitable semiconductor structure with an applied electric field, one can increase the local heating over that produced by optical absorption alone, as shown by surface displacement measurements made in the photo-thermal-displacement microscope. These features may be useful in novel sensors that employ Lamb waves.

Determination of the spectral content of transient stress wave events

Abstract: The spectral content of a transient stress wave event occuring on a plate-like structure is determined by locating a transducer on the surface of the structure at a position remote from the location of the transient stress wave event. Lamb waves resulting from and characteristic of the transient stress wave event propogate through the plate-like structure in a dispersive manner to be intercepted by the transducer. Analysis of the transducer output enables the spectral content of the transient stress wave event to be realized.

Generation of Lamb Wave Using Linear Array Probe and Its Application for Flaw Detection

Abstract: The method with which a Lamb wave of the required mode is generated and detected by electronically adjusting the incident angle using a linear array probe, and its application to flaw detection is presented. To test the ability of this method, experiments were carried out using specimens with an artificial flaw. The method was shown to be successful. The two dimensional CRT display method of the detected flaws, in which the polar coordinates consist of the propagation time and the incident angle of the wave, and the brightness corresponds to the amplitude of echo is also presented.

Absorption of longitudinal and shear waves and generation of heat in soft tissues

Abstract: The author analysed the oblique incidence of a longitudinal plane wave and the generation of a shear wave at a boundary between a soft tissue and a gas, considering a viscoelastic model (Voigt solid) of the medium. From the data measured by Frizzell et al. (J. Acoust. Soc. Am. 60, 1409-1411) for shear waves, the average velocity 30 m/s and the average absorption coefficient 7.7 X 10(3) cm-1 at the frequency of 5 MHz were assumed for calculations. The amplitude and the intensity of the generated shear wave were obtained and hence the rate of heat production per unit volume was determined. At the boundary, this quantity was found to be of the same order of magnitude for the generated shear wave as for the incident longitudinal wave. In the case of the shear wave, which propagates almost perpendicularly to the boundary, it decreases rapidly with the distance. Therefore, the temperature increase caused by shear waves was negligible in respect to longitudinal waves in spite of the extremely high absorption coefficient. This conclusion could be confirmed by solving the inhomogeneous equation of heat conductivity for the case under consideration.

Analysis of Subsurface Cracks in Acoustic Microscopy

Abstract: Acoustic micrographs of samples containing subsurface cracks have been analyzed with a plate model which faces the coupling liquid above and a vacuum below. Minima have been found in the intensity of the reflected beam as a function of the thickness D of the plate when the normalized parameter FD (F is the frequency) is among multiples of the half velocities of either longitudinal or shear waves in the sample. A model is proposed to explain these minima as destructive interference due to the higher order modes of leaky Lamb waves with very large phase velocities.

An Instrument for Layer Thickness Measurement Using Pseudo-Sezawa Waves

Abstract: An instrument for layer thickness measurement has been developed to measure the layer thickness of lead frames for LSI's more rapidly and accurately than the conventional X-ray fluorescence method. The lead frame consists of 42-alloy substrate with gold layer, 2-5pm in thickness. The measurement is made by means of the excitation of pseudo-Sezawa waves. This paper discusses the performance of the instrument, in respect to the stability, accuracy and temperature-dependence of the measurement. Considering the measurement environment in the mass-production-line, effects of alignment and displacement of a sensor on the measurement are also discussed. Influence of Lamb waves, which are excited when the substrate is thin, is also discussed. The performance of the instrument is achieved as follows: Measurable thickness range; 1-2OPm, stability;O il%, measurement time: 1-sec.

On inverse scattering in an elastic medium with vertical inhomogeneities

Abstract: This paper treats the nonlinear inverse scattering problem for an elastic horizontally stratified medium with vertical inhomogeneities. It is shown that the vector wave equation can be presented as a set of two scalar wave equations; the first scalar equation describes the propagation of normally incident compressional waves (it is assumed that we deal with compressional incident plane waves impinging on an elastic half space). Using any inverse scattering technique (based, for example, on the Gelfand–Levitan theorem) applied to this equation, the acoustic impedance can be uniquely recovered from the recorded impulse response. The second equation is valid only in the region where incident compressional waves decay exponentially (evanescent waves) and propagate with complex angles, whereas mode‐converted shear waves propagate with real angles. The main contribution in this region in the high‐frequency approximation comes from mode‐converted shear waves. We derived a new equation which describes the propagation of mode‐converted shear waves in this region. Applying any inverse scattering technique to this equation, the rigidity modulus and the density can be removed separately. Knowing the acoustic impedance, the rigidity modulus, and the density, all Lame’s parameters and the density can be recovered separately.

Analysis of Broadband Lamb-Wave Delay Lines

Abstract: We present an equivalent circuit analy- sis method for Lamb-wave (LW) acoustic delay lines on piezoelectric ceramic plates. The transducer consists of a symmetric structure on the two plate surfaces in or- der to generate only the zeroth-order symmetric Lamb- wave. High efficiency and wideband unidirectionality are achieved by cutting a straight edge parallel to the fin- gers; placed in a distance of a fourth wavelength from the finger centre the edge behaves as a perfect acoustic re- flector. A minimum insertion loss of some 10 dB, caused by electrical mismatch, has to be accepted to keep the triple transit signal low. Our model for analysis is based on a modified Mason equivalent circuit combined with a P-matrix description in order to calculate reflections. Additionally, two detrimental effects, the electromag- netic coupling (crosstalk) between the input and output transducer caused by the high dielectric constant, and the bulk wave plate resonance, are taken into account. These are modelled by appropriate equivalent circuits. Furthermore, an extension of the aforementioned model is developed by calculating the charge distribution on the fingers. We compare the results of our model with measurements in the range between 0 and 4 MHz finding a fairly good agreement.

A simplified second-order solution for a spiral wave maker

Abstract: A second-order solution for the spiral wave field produced by a rotating circular cylinder in a basin of constant depth is presented. The analysis is limited to cases in which the wave amplitude is small compared with the wave maker radius, which is in turn small compared to the wavelength. Under these circumstances, we show by an order-of magnitude argument that one may consistently simplify the second-order analysis by neglecting the quadratic terms in the free surface boundary conditions while retaining the leading terms in the wave maker boundary condition. The resulting solution indicates the presence of free secondary waves which do not propagate at the same speed or in the same direction as the first-order waves. Laboratory measurements are presented which demonstrate the ability of the solution to reproduce measurements of the surface displacement with reasonable accuracy. At large distances from the wave maker, the free waves completely dominate the second-order waves forced by the nonlinear terms in the free surface boundary conditions. Consequently, a cylindrical spiral wave maker does not simulate correctly the second-order component of regular plane waves.

Adhesive joint evaluation by ultrasonic interface and lamb waves

Abstract: Some results on the application of interface and Lamb waves for the study of curing of thin adhesive layers were summarized. In the case of thick substrates (thickness much more than the wave length) the interface waves can be used. In this case the experimental data can be inverted and the shear modulus of the adhesive film may be explicitly found based on the measured interface wave velocity. It is shown that interface waves can be used for the study of curing of structural adhesives as a function of different temperatures and other experimental conditions. The kinetics of curing was studied. In the case of thin substrates the wave phenomena are much more complicated. It is shown that for successful measurements proper selection of experimental conditions is very important. This can be done based on theoretical estimations. For correctly selected experimental conditions the Lamb waves may be a sensitive probe of adhesive bond quality and may be used or cure monitoring.

Extensional‐wave and flexural‐wave contributions to the sound field radiated by a fluid‐loaded infinite plate

Abstract: A structural thick‐plate theory that accounts for the propagation of both extensional and flexural waves in an infinite elastic plate is used to calculate the farfield directional response of the sound radiated by a plate with fluid on one side. The plate is driven by a harmonic point force, and the radiation in the farfield of the driving point is considered. Computations are made for metal plates and plates composed of polymeric materials in water and in air. Anelastic behavior of the plates is considered. The computations, which are made in order to compare the radiated fields of plates in which only flexural waves or only extensional waves can propagate, or in which both wave types can propagate concurrently, show that the radiation caused by extensional waves can make an important contribution to the sound field of an elastic plate.

Propagation of normal waves in a layer lying on a rigid base

Abstract: An analysis of the processes behind the propagation of harmonic waves in an elastic layer has been the subject of numerous theoretical and experimental investigations over the past i00 years, beginning with the works of Rayleigh [12] and Lamb [i0, ii] Much interest has recently developed over data on the behavior of Lamb waves in a layer with symmetric or antisymmetric boundary conditions~ A collection of results on this problem can be found in [i, 3, 5, 7] The propagation of normal waves taking into account combined boundary conditions over various sides of the layer was studied in [2, 4], where the methods for solving combined problems for bands and layers differ This study presents new results for an analysis of wave mo~ion in an elastic layer assuming combined boundary conditions~ Data is given on cut-off frequencies and on the high-frequency values of the phase and group velocities Special attention is given to the phenomenon of scattered waves, which were first observed in [13] and later thoroughly analyzed in [5, 6, 8] for longitudinal and transverse Lamb waves in a layer with free boundaries

Finite‐element solution for scattering of lamb waves by asymmetrically located discontinuities in an elastic plate waveguide

Abstract: This paper considers the scattering of Lamb waves due to the discontinuity of elastic plate waveguide. A method of analysis is presented which is a combination of the finite-element method and analytical solution. The analytical solution is derived taking the cut-off mode into consideration. Consequently, the region of application of the finite-element method is reduced drastically compared with the traditional method of analysis by finite-element method ignoring the cut-off mode. In the past, the formulation has been made by restricting the discontinuities to symmetrical distribution in regard to the central plane of the plate. In this paper, this restriction is removed and discontinuities of any distribution in regard to the elastic plate can be handled. The effectiveness of taking the cut-off mode into consideration is verified by applying the proposed method to the scattering of Lamb waves by a free-edge of the elastic plate and by a slit which is symmetrical in regard to the central plane of the plate. As examples of discontinuities with asymmetrical distribution in regard to the central plane of elastic plate, a slit on one of the surfaces and wedge-shaped crack are considered. The result of analysis is presented for the scattering characteristics when a fundamental symmetric Lamb wave or fundamental antisymmetric Lamb wave is incident.

Interaction between weak and short waves in a one‐dimensional inhomogeneous nonlinear elastic material

Abstract: A wave composed of weak and short waves (WS waves) in a dynamically deformed elastic material is studied theoretically by using quasigeometrical acoustics. The material considered is one dimensional and inhomogeneous with a nonlinear stress–strain relation. A general theory for analyzing the composite wave is exploited. It is shown to the second‐order approximation that the analysis of the composite wave can essentially be reduced to the analysis of a single WS wave. As an example of the general theory, a head‐on collision between two WS waves is investigated.

Generation of Lamb Wave Using Linear Array Probe and Its Application for Flaw Detection

Abstract: The method with which a Lamb wave of the required mode is generated and detected by electronically adjusting the incident angle using a linear array probe, and its application to flaw detection is presented. To test the ability of this method, experiments were carried out using specimens with an artificial flaw. The method was shown to be successful. The two dimensional CRT display method of the detected flaws, in which the polar coordinates consist of the propagation time and the incident angle of the wave, and the brightness corresponds to the amplitude of echo is also presented.