Showing papers on "Lamb waves published in 1991"

A two-dimensional Fourier transform method for the measurement of propagating multimode signals

Abstract: A technique for the analysis of propagating multimode signals is presented. The method involves a two-dimensional Fourier transformation of the time history of the waves received at a series of equally spaced positions along the propagation path. The technique has been used to measure the amplitudes and velocities of the Lamb waves propagating in a plate, the output of the transform being presented using an isometric projection which gives a three-dimensional view of the wave-number dispersion curves. The results of numerical and experimental studies to measure the dispersion curves of Lamb waves propagating in 0.5-, 2.0-, and 3.0-mm-thick steel plates are presented. The results are in good agreement with analytical predictions and show the effectiveness of using the two-dimensional Fourier transform (2-D FFT) method to identify and measure the amplitudes of individual Lamb modes.

Microtransport induced by ultrasonic Lamb waves

Abstract: We have observed pumping of water induced by 4.7 MHz ultrasonic Lamb waves traveling in a 4‐μm‐thick composite membrane of silicon nitride and piezoelectric zinc oxide. The observed pumping speed is proportional to the square of the wave amplitude; the speed was 100 μm/s for a rf drive voltage of 8 V and a 6.5 nm wave amplitude. A nonlinear model based on acoustic streaming theory predicts velocities in good agreement with experiment.

Lamb wave interaction with lap-shear adhesive joints : theory and experiment

Abstract: Lap-shear joints are made from two plates that overlap in the joint area. In this paper a theoretical and experimental analysis of Lamb wave interaction with such joints is given. In this joint evaluation technique Lamb waves are excited and received outside the joint area and a conclusion about joint quality is drawn based on the phase delay and transmission losses of the ultrasonic signals in the joint. The mode transformation at the joint edges is very important for such tests. This phenomenon is studied in this paper by considering mode conversion on the boundary between the region exterior to the joint area and the two adhesively joined plates inside the joint. The problem is addressed theoretically by the Wiener–Hopf technique for a perfect joint that is represented by a double thickness plate and for a weak joint that is approximately by slip boundary conditions between the two plates in the joint area. This work shows, both theoretically and experimentally, how to select the mode type and frequenc...

Plate-mode ultrasonic structure including a gel

Abstract: An ultrasonic structure which has a thin planar sheet of material forming a Lamb wave propagation medium The propagation medium is coated with a gel. The structure may also include a Lamb wave generator for generating Lamb waves in the propagation medium and an output device for producing an electrical signal representative of the propagation characteristics of the Lamb waves propagating along the propagation medium. A measuring device can be included to measure selected characteristics of the output electrical signal. The propagation medium has some physical characteristics that are determined by the value of a measurand acting on the medium and the determined physical characteristics determine the propagation characteristics of the Lamb waves which are propagated along the medium. When the sensor is acted on by a measurand to determine the physical characteristics of the propagation medium, the characteristics of the electrical signal are also determined. The measuring device measures the electrical signal and provides an indication of the measurand value.

Ultrasonically induced microtransport

Abstract: Fluid motion induced by traveling flexural waves in 4 mu m thick membranes was observed using 2.5 mu m diameter polystyrene spheres in water to make the fluid motion visible. Visual observation of the spheres indicates that they move in the direction of wave propagation with a speed proportional to the square of the acoustic amplitude. The maximum speed is 130 mu m/s for an RF drive voltage of 7.1 Vrms at 3.5 MHz; the wavelength is 100 mu m. Standing Lamb waves, which can be seen visually with a phase-contrast microscope, are found to trap particles, including bacteria located in a drop of water that contacts the membrane. A first order model of Lamb-wave micropumping is presented, based on acoustic streaming theory. Important parameters for device design and operation are discussed, along with options for integrating the device into microflow systems. Possible applications include temperature redistribution in ICs and miniature chemical processing systems. >

Propagation of ultrasonic Lamb waves in piezoelectric plates

Abstract: The propagation of the lowest‐order symmetric (S0) and antisymmetric (A0) Lamb wave modes in a piezoelectric plate is investigated both theoretically as well as experimentally. The characteristics of the modes and their relation to the surface acoustic wave is modified by electrical boundary conditions on the plate surfaces. It is found that when both surfaces are either metallized or unmetallized, the surface acoustic wave is obtained by a linear superposition of the A0 and S0 modes. On the other hand, if only one surface is metallized, then the surface acoustic wave is equivalent to just one of modes (either the A0 or S0 mode, depending on which surface is metallized). The beating phenomenon, whereby wave energy launched on one surface transfers periodically back and forth between opposite plate surfaces, is present when both surfaces are either metallized or unmetallized, but is absent if only one surface is metallized. Experimental measurements performed on Lamb wave devices fabricated on Y‐cut, Z‐pro...

Two- and three-dimensional parametric instabilities in finite-amplitude internal gravity waves

Abstract: The linearized stability equations of a monochromatic finite-amplitude gravity wave in a Boussinesq fluid are solved for two-dimensional perturbations and three-dimensional perturbations propagating in the plane normal to the primary wave plane and the primary wave fronts. In statically stable primary waves, two-dimensional instability modes grow fastest. They can be divided into two classes: a continuous spectrum of small-scale modes and an isolated mode. In the limit of vanishing primary wave amplitudes, the small-scale modes reduce to second-order resonant wave interactions whereas the underlying wave interaction mechanism of the isolated mode remains uncertain. In statically unstable primary waves, three-dimensional instability modes grow fastest and are either fundamental or subharmonic. They reduce to resonant wave interactions of orders higher than three in the limit of zero primary wave amplitudes. The underlying interaction mechanisms of the fundamental modes also include vortical modes ...

Acoustic touch position sensor with first order lamb wave reflective arrays

Abstract: An acoustic touch position sensor is shown wherein a Lamb wave is imparted into a substrate by a transducer mounted on the substrate, the Lamb wave propagating along a first axis. Top and bottom reflecting arrays are disposed along the first axis to reflect portions of one mode of the Lamb wave along a plurality of parallel paths extending across a touch surface of the substrate to a second pair of top and bottom reflect arrays. The second pair of top and bottom reflecting arrays reflect the Lamb waves incident thereto along an axis parallel to the first axis to a receiving transducer that provides a signal representative of the received Lamb waves. A touch on the substrate results in a partial absorption of the energy in the Lamb wave propagating along a path intersection the touch position so as to produce a perturbation therein which is sensed to determine the axial position of the touch on the substrate. In a second embodiment a single transducer and reflective array is provided for each axis, a coordinate on which is to be determined. In a third embodiment of the present invention, a single transducer is provided for two axes.

Wave propagation in anisotropic liquid‐saturated porous solids

Abstract: Biot’s theory is employed to study the propagation of plane‐harmonic seismic waves in a transversely isotropic liquid‐saturated porous solid. Along with SH wave, the existence of three quasiwaves is discussed and analytical expressions for their velocities of propagation have been obtained. It has been observed that velocities of existing waves vary with the direction of propagation. Frequency equation for the propagation of Rayleigh‐type surface waves at the free surface of transversely isotropic liquid‐saturated porous solids has been obtained. Possibilities of existence of body waves and surface waves have been discussed numerically by assuming different sets of values of elastic constants. Dependence of the velocities of propagation on the direction of propagation has been exhibited.

傾斜機能材料平板におけるラム波伝播とその衝撃応答 : 第一報,解析手法

Abstract: The numerical methods which have been proposed by the present authors for Lamb waves in an anisotropic laminated plate and its transient responses are expanded for the wave propagation analysis of functionally gradient material (FGM) plates. The material properties of the plate change gradually in the thickness direction, and are anisotropic in the plane of the plate. The plate is divided into N plate elements. In the element, we assume that the material properties change linearly in the thickness direction, and that the displacement field is expressed by second-order polynomials. The principle of virtual work is used to develop approximate dynamic equilibrium equations. The dispersion relation and the mode shape of the Lamb waves are obtained by using the free boundary conditions. The energy velocities of the Lamb waves are formulated with the aid of the Rayleigh quotient. The method of Fourier transforms in conjunction with the modal analysis is used to determine the response of displacements. The formulation of the theory is described in this paper.

Leaky Lamb waves in an anisotropic plate. II: Nondestructive evaluation of matrix cracks in fiber-reinforced composites

Abstract: 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.

Selective excitation of single‐mode acoustic waves by phase velocity scanning of a laser beam

Abstract: A novel method for selective generation of single‐mode acoustic waves in multimode media has been developed using a laser beam scanned at the phase velocity of a specified mode. In dispersive media, the acoustic frequency can be varied by changing the scanning velocity. The number of carriers in the generated wave packet is proportional to the difference between the phase and the group velocities. These features were experimentally verified in the fundamental symmetric and asymmetric Lamb waves on an aluminum plate generated by a long‐pulse Nd:YAG laser. Applications to anisotropy and thickness measurements are discussed.

Acoustic touch position sensor with shear to lamb wave conversion

Abstract: An acoustic touch position sensor is shown wherein a transducer coupled to a side of a substrate imparts a shear wave into the substrate that propagates along a first axis. A shear to Lamb wave converting array disposed along the first axis converts the shear wave to Lamb waves propagating along a plurality of parallel paths where each path is associated with a different axial position on the substrate relative ot the first axis. The Lamb waves are converted back to shear waves by a Lamb to shear wave converting array disposed along an axis parallel to the first axis, the second converting array being spaced across a touch surface of the substrate from the first array. The shear wave from the second converting propagates to a receiving transducer that provides a signal representative of the waves. A touch on the substrate results in a partial absorption of the energy in the Lamb wave propagating along a path intersecting the touch position so as to produce a perturbation therein which is sensed to determine the axial position of the touch on the substrate. In a second embodiment a single transducer and reflective array is provided for each axis, a coordinate on which is to be determined. In a third embodiment of the present invention, a single transducer is provided for two axes.

Wave propagation in a multilayered laminated cross-ply composite plate

Abstract: Dispersion of guided waves in a cross-ply laminated plate has been studied here using a stiffness method and an exact method It is shown that the number of laminae strongly influences the dispersion behavior Further, it is found that when the number of laminae is sufficiently large, then the dispersion behavior can be predicted by treating the plate as homogeneous with six stiffness constants obtained by using an effective modulus method

Scattering of Short-Wave Groups by Submerged Horizontal Plate

Abstract: A fully submerged horizontal plate has been suggested as a breakwater. With a proper design in the length, thickness, and submerged depth of the plate, most of the incoming short waves can indeed be reflected. The method of eigenfunction expansion is used to solve for the reflection and transmission of the short waves. The present results agreed with earlier experimental data and in the long-wave limit. If the incident wave is bichromatic, a second-order long wave train accompanies the wave groups. This long wave train is also scattered by the horizontal plate. It is shown there that the scattered long waves exist on both sides of the horizontal plate even when the short waves are completely reflected. The long waves may prove to be a bigger hazard than the short waves, because the wave period of the long wave (1–3 min) is closer to the natural frequency of coastal structures.

Decomposition and interception of long waves by a submerged horizontal plate

Abstract: The results of two-dimensional hydraulic experiments are presented on the decomposition of regular and irregular surface waves by a fixed, submerged horizontal plate with a finite length. Nonlinear interactions, containing the near-resonant interaction as well as the self- and mutual-interactions, among the waves affected by the submerged plate are found to induce the generation of higher harmonic components which propagate independently in the lee side, and to result in appreciable disintegration of wave form. This wave decomposition may produce considerable modification of the spectrum of a transmitted-wave train through the nonlinear transfer of energy from primary wave components to higher frequency components and consequently reduce its significant wave period. With the above mechanism, thus, the submerged horizontal plate may serve as an effective interception device against waves, especially of long wave length.

Transient scattering of Rayleigh-Lamb waves by a surface-breaking crack: Comparison of numerical simulation and experiment

Abstract: The transient scattering of Rayleigh-Lamb waves by a surface-breaking crack in a plate is investigated in both time and frequency domains by using the hybrid numerical method which combines the finite element discretization of the vicinity of the crack with the Green's function integral representation of the exterior scattered field. The frequency domain response is obtained by solving a set of large sparse unsymmetric complex matrix equations, elements of which are stored in a compacted data structure, by the biconjugate gradient method. The time domain solution is then obtained by using FFT. The source function generated by a steel ball impact is extracted by a direct integration technique. It is then used to simulate some available experimental results. Good agreement has been obtained. Numerical and experimental results show the effect of the crack more in the near field than far away.

Determination of the flexural modulus of thin films from measurement of the first arrival of the symmetric Lamb wave

Abstract: A technique is presented to measure the flexural modulus of free‐standing thin metallic films from 1 to 25 μm thick. Lamb waves are excited in the film with a pulsed Nd:YAG laser, and detected using heterodyne interferometry. Variability in wave‐form structure is observed as a function of experimental parameters and film thickness. The first arrival of the symmetric Lamb mode can be unambiguously identified regardless of wave‐form shape, and is used to calculate the flexural modulus.

Surface-acoustic-wave (SAW) flow sensor

Abstract: The use of a surface-acoustic-wave (SAW) device to measure the rate of gas flow is described. A SAW oscillator heated to a suitable temperature above ambient is placed in the path of a flowing gas. Convective cooling caused by the gas flow results in a change in the oscillator frequency. A 73-MHz oscillator fabricated on 128 degrees rotated Y-cut lithium niobate substrate and heated to 55 degrees C above ambient shows a frequency variation greater than 142 kHz for flow-rate variation from 0 to 1000 cm/sup 3//min. The output of the sensor can be calibrated to provide a measurement of volume flow rate, pressure differential across channel ports, or mass flow rate. High sensitivity, wide dynamic range, and direct digital output are among the attractive features of this sensor. Theoretical expressions for the sensitivity and response time of the sensor are derived. It is shown that by using ultrasonic Lamb waves, propagating in thin membranes, a flow sensor with faster response than a SAW sensor can be realized. >

Excitation of ultrasonic Lamb waves in piezolectric plates

Abstract: The excitation of ultrasonic Lamb waves by an interdigital transducer (IDT) deposited on a piezoelectric plate is analyzed using the Green's-function method. The amplitudes of the generated Lamb waves are obtained in terms of the charge density on transducer electrodes. An electrostatic analysis that neglects piezoelectric coupling is used to relate this charge density to the voltage applied across the IDT. This is then used to calculate the radiation conductance of the transducer

Ray synthesis of the form function for backscattering from an elastic spherical shell : leaky lamb waves and longitudinal resonances

Abstract: An acoustic ray analysis is employed in synthesizing the form function for backscattering, f(θ=π,ka), from a fluid-loaded evacuated elastic spherical shell where k is the wave number of the incident plane wave and a is the outer radius of the shell The synthesis contains a component associated with a specular reflection, fsp, and contributions from leaky Lamb waves The contribution fl of the lth leaky Lamb wave is expressible in a Fabry–Perot resonator form [P L Marston, J Acoust Soc Am 83, 25–37 (1988)] A comparison of the ray synthesis for f(ka) with the exact partial-wave series representation for a 440c stainless-steel shell verifies the usefulness of the ray synthesis for the present case of a shell The present synthesis is also new in that it includes the effects of longitudinal resonances on fsp A novel ray synthesis of fsp indicates a significant resonance effect near the condition kLh=nπ (n=1,2,…) The thickness of the shell is h, and kL=ω/cL is the longitudinal wave number where cL is

傾斜機能材料平板におけるラム波伝播とその衝撃応答 第二報 計算結果

Abstract: The numerical mathods present in the previous paper for the wave propagation analysis of anisotropic functionally gradient material (FGM) plates are applicated to a SiC-C plate, which is a kind of functionally gradient material plate. The frequency spectum, energy velocities, and the mode shapes of the Lamb waves in the SiC-C plate are obtained. The transient responses of the SiC-C plate excited by time-step impact loads time-pulse ones are computed. The results obtained for the SiC-C plate and for an isotropic homogeneous C monolith plate are compared. It is found that the displacements are much more concentrated on the softer surface of the SiC-C plate than on the surfaces of the homogeneous C monolith plate, and that the displacement responses cooresponding to the surface waves of the SiC-C plate are larger than those of the C monolith plate.

Analysis of resonances related to Scholte–Stoneley waves around circular cylindrical shells

Abstract: It is shown experimentally and theoretically that waves propagate around circular cylindrical shells with a phase velocity that is less than the phase velocity of sound in water. These waves are called Scholte–Stoneley waves. First this paper presents results on air‐filled aluminum pipes of different ratios of the internal radius b to the external radius a. The frequency resonances and the orders n of the resonances increase with the b/a ratio. The frequency range of the observed resonances is fixed for a given pipe. This is related to the evolution of the resonance width with frequency. The frequency resonances and their width are calculated with the use of the resonance scattering theory. The so obtained results agree quite well with the experimental ones. The Scholte–Stoneley wave, which propagates around the shell, is compared to the antisymmetric Lamb wave, which appears on a thin plate when the plate is imbedded in water. Second, the influence of the internal fluid on the Scholte–Stoneley resonances...