Leaky Rayleigh Wave Research Articles

Boundary enhanced coupling processes for rotated horizontal solid aluminum cylinders: Helical rays, synthetic aperture sonar images, and coupling conditions

Experiments with solid aluminum cylinders placed near a flat free surface provide insight into scattering processes relevant to other flat reflecting boundaries [J. R. La Follett, K. L. Williams, and P. L. Marston, J. Acoust. Soc. Am. 130, 669–672 (2011); J. R. La Follett, Ph.D. thesis, WSU (2010)]. This presentation concerns the coupling to surface guided leaky Rayleigh waves that have been shown to contribute significantly to backscattering by solid metallic cylinders [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 106, 1673–1689 (1999)]. The emphasis here is on horizontal cylinders rotated about a vertical axis away from broadside viewed at grazing incidence. The range of rotation angles for which helical rays can contribute is limited in the free field by the cylinder’s length [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. Some examples of surface enhanced backscattering may be summarized as follows. In agreement with geometrical considerations, the angular range for coupling to helical rays may be significantly extended when a short cylinder is adjacent to a flat surface. In addition, the presence of a flat surface splits synthetic aperture sonar (SAS) image features from various guided wave mechanisms on rotated cylinders. [Work supported by ONR.]

Energy leakage rate of Rayleigh wave at fluid-solid interface

The energy leakage rate of Rayleigh wave generated by a laser pulse applied on a fluid-solid interface was analyzed. Data were presented for different fluid-solid interfaces based on the overall solution of wave motion, which was obtained by using finite element method. The results show that the energy leakage rate increases linearly with the increase of acoustic impedance ratio of the fluid and solid material, and that the increase in acoustic impedance matching of those two materials will be able to increase the acoustic leakage of Rayleigh wave and the excitation efficiency of leaky Rayleigh wave.

Numerical simulation of ultrasonic minimum reflection for residual stress evaluation in 2D case

A number of interesting phenomena at fluid-solid interfaces can be observed when the incident angle approaches the Rayleigh angle, including Schoch displacement and leaky Rayleigh waves. Besides the experimental and theoretical research on these problems, numerical tools have been more and more widely used for these complex problems. Based on previous experimental and numerical researches, a 2D finite element model has been built to reproduce the Schoch effects. With the same model, the minimum reflection profile is investigated for the feasibility of material characterization, especially for residual stress evaluation. Residual stress is one of the important properties for structures, and its measurement is a popular research topic in nondestructive evaluation. However, it is not possible to put the residual stress into the numerical model directly. According to the relation of residual stress with mechanical properties, the material damping and wave speed have been alternatively adopted in this work. The influence of minimum reflection profile by residual stress has been shown by the change of wave speed and damping factor. Simulation results show that the minimum reflection profile is a potential method for residual stress evaluation.

Rayleigh wave scattering from sessile droplets

Radiation of energy by large-amplitude leaky Rayleigh waves is regarded as one of the key physical mechanisms regulating the actuation and manipulation of droplets in surface acoustic wave (SAW) microfluidic devices. The interaction between a SAW and a droplet is highly complex and is presently the subject of extensive research. This paper investigates the existence of an additional interaction mechanism based on the propagation of quasi-Stoneley waves inside sessile droplets deposited on a solid substrate. In contrast with the leaky Rayleigh wave, the energy of the Stoneley wave is confined within a thin fluid layer in contact with the substrate. The hypothesis is confirmed by three-dimensional finite element simulations and ultrasonic scattering experiments measuring the reflection of Rayleigh waves from droplets of different diameters. Moreover, real-time monitoring of the droplet evaporation process reveals a clear correlation between the droplet contact angle and the spectral information of the reflected Rayleigh signal, thus paving the way for ultrasonic measurements of surface tension.

Leaky Interface Waves Optical Detection at Solid-Solid Interface

The experimental investigation on transparent solid/solid (Aluminum and Plexiglas) interface leaky waves generated by a pulse laser and detected with a photo elastic effect technique is reported. Three waves Lateral wave, Leaky Rayleigh (LR) wave and Leaky Interface wave (IW) are detected successfully; The velocity of the detected interface wave is in good agreement with theoretical calculation and the attenuation characteristic of the two Leaky waves is also in accordance with the theoretical prediction. The Leaky waves propagate along the weak bonding interface is also measured, it was found with the continue Epoxy solidifying, the wave amplitude gradually decrease and the two Leaky waves are more difficult to distinguish. The successful measurement should improve the scientific and technological potential for the research of solid/solid interface waves.

Leaky Interface Wave Measurement at a Solid-Solid Interface with Laser Ultrasonics

The experimental investigation on transparent solid/solid (aluminum and plexiglas) interface leaky waves generated by a pulse laser and detected with a photoelastic effect technique is reported. Three waves, i.e., longitudinal head wave, leaky Rayleigh wave and leaky interface wave, are detected successfully. The leaky waves propagating along the ‘weak bonding’ interface are also measured. It is found that with the continuing epoxy solidification, the wave amplitude gradually decreases and the two leaky waves are more difficult to distinguish. The velocities of the detected interface wave are in good agreement with the theoretical calculation and the attenuation characteristics of the two leaky waves are also in accordance with the theoretical prediction.

Focusing and subwavelength imaging of surface acoustic waves in a solid-air phononic crystal

Focusing and subwavelength imaging of surface acoustic waves (SAWs) through a phononic crystal flat lens are discussed in the presented work. Experimental and numerical wave fields are obtained in the time-domain by an optical technique and by numerical simulations. Spatial distributions of the acoustic field are accessed using a temporal Fourier transform. The revealed focusing of the elastic waves in the first band of the crystal is governed by the concave equifrequency contour of the leaky-Rayleigh wave. The spatial distributions of the experimental and numerical acoustic fields also unfold subwavelength imaging of SAWs. Numerical simulations show that the imaging quality can be improved by embedding the flat lens into a medium with higher wave velocity.

Scaled behavior of interface waves at an imperfect solid-solid interface

Laser ultrasonic techniques allow the remote analysis of adhesion mechanisms at imperfect interfaces up to GHz frequencies. However, the sensitivity of interface waves to the properties of the contact is not very well known. In the present work, the mechanical boundary conditions are described considering that the contacting solid half-spaces are connected by tangential and normal springs. Such a modeling implies a discontinuity of the displacement field across the interface. To identify the relative amplitudes of the different types of interface waves—skimming, leaky Rayleigh (LR) and Stoneley (St) waves—a semi-analytical time domain model describing the thermoelastic laser generation is derived. The results illustrate the influence of the boundary conditions on the attenuation of the LR wave and on the existence of the St wave. In addition, a single compact and elegant dispersion equation is presented to investigate the behaviour of the interface waves propagating along a generalized imperfect boundary. Such analysis reveals the existence of a cutoff frequency fc close to which the St wave behaves like a skimming transverse wave. A scaled analysis demonstrates that two master curves suffice to describe the dispersion of LR and St waves and that fc is inversely proportional to the tangential interfacial spring constant.

Air-coupled sensing of leaky rayleigh waves and ZGV modes in concrete slabs

Rayleigh waves and zero-group velocity (ZGV) Lamb modes (the impact-echo test) are commonly used for non-destructive evaluation (NDE) of concrete structures to extract information about material property and interior delaminations. Traditional methods in civil engineering employ point impactors to generate the waves while accelerometers measure the resulting out-of-plane motion. Though well-established, this methodology does not lend itself to efficient scanning of large areas, which is critical for monitoring the safety of infrastructure. The ability to detect these two wave types through in-air sensing of Leaky Rayleigh or ZGV Lamb waves can greatly accelerate NDE for large structures. Unfortunately, air-coupled sensing suffers from significantly decreased signal amplitude. This paper presents theoretical analysis and experimental validation efforts to amplify in-air signals resulting from Leaky Rayleigh waves and ZGV Lamb modes using a parabolic focusing mirror. A time-domain impulse response to the Ki...

Noncontact MASW methods for near surface soil/infrastructure assessment

In near surface geophysics, a multi-channel analysis of surface wave (MASW) method has been increasingly applied for underground infrastructure assessments, in which conventional contact sensors like geophones and accelerometers were mostly employed to detect surface vibrations. In this study, noncontact sensors technology such as a laser Doppler vibrometer (LDV) and a microphone were used to measure Rayleigh wave and leaky Rayleigh waves respectively. These noncontact sensors were installed in a scanning platform that was driven by a stepper motor. The scanning MASW system consisted of two excitation sources: an electromechanical shaker and steel-balls to generate frequency sweeping (chirp) signals with frequency from 30 Hz to 500 Hz and high frequency (up to 40 kHz) impulsive signals respectively. The LDV-shaker-MASW was developed for near surface soil profile exploration and the microphone-steel ball-MASW was built for pavement assessment. The details of the system and several case studies will be addressed.

Noncontact MASW Methods for near Surface Soil/Pavement Shear Wave Velocities Measurements

In near surface geophysics, a multi-channel analysis of surface wave (MASW) method has been increasingly applied for underground infrastructure assessments by measuring shear wave velocity profile. The conventional MASW mostly employs contact sensors like geophones and accelerometers to detect surface vibrations. In this study, noncontact sensors technology such as a laser Doppler vibrometer (LDV) and a microphone were used to measure Rayleigh wave and leaky Rayleigh waves respectively. These noncontact sensors were installed in a scanning platform that was driven by a stepper motor. The scanning MASW system consisted of two excitation sources: an electromechanical shaker and steel-balls to generate frequency sweeping (chirp) signals with frequency from 30 Hz to 500 Hz and high frequency (up to 40 kHz) impulsive signals respectively. The LDV-shaker-MASW was developed for near surface soil shear wave velocity profile exploration and the microphone-steel ball-MASW was built for pavement assessment to extract the dispersion curve. The details of the system and several case studies will be addressed.

Viscosity and Diffusion Effects at the Boundary Surface of Viscous Fluid and Thermoelastic Diffusive Solid Medium

AbstractThis paper concentrates on the wave motion at the interface of viscous compressible fluid half-space and homogeneous isotropic, generalized thermoelastic diffusive half-space. The wave solutions in both the fluid and thermoelastic diffusive half-spaces have been investigated; and the complex dispersion equation of leaky Rayleigh wave motion have been derived. The phase velocity and attenuation coefficient of leaky Rayleigh waves have been computed from the complex dispersion equation by using the Muller’s method. The amplitudes of displacements, temperature change and concentration have been obtained. The effects of viscosity and diffusion on phase velocity and attenuation coefficient of leaky Rayleigh waves motion for different theories of thermoelastic diffusion have been depicted graphically. The magnitude of heat and mass diffusion flux vectors for different theories of thermoelastic diffusion have also been computed and represented graphically.

Boundary and elastic effects on backscattering by aluminum cylinders: Experiments and modeling for tilted cylinders viewed at grazing incidence.

Close proximity to a boundary or partial exposure to the incident sound modifies the frequency response of elastic objects. Understanding these changes in high-frequency scattering is important for detection and classification of metallic objects located near a boundary. Laboratory experiments, ray theory, synthetic aperture sonar (SAS) imaging, and backscattering spectroscopy were used to explore the effect of partial exposure and close proximity to a planar boundary for a solid aluminum cylinder. To investigate these boundary effects, targets were suspended through the air-water interface of a water tank and insonified from below at grazing incidence. Backscattering measurements were made as the target was lowered through the air-water interface, increasing the portion of the target exposed to sound. Backscattering measurements were also made as a function of aspect angle for varying levels of exposure. Line scan measurements were also obtained to construct SAS images. Some of the structure in the frequency response could be explained using a previously developed ray-based theory [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 106, 1673–1680 (1999); J. Acoust. Soc. Am. 107, 112–117 (2000)] involving leaky Rayleigh waves. The coupling to rays crossing the flat end of the cylinder strongly depends on the amount of exposure. [Work supported by ONR.]

Coating Evaluation Using Ultrasonic Wave Technology

The capability of Rayleigh wave to be used as a tool for the evaluation of CVD DLC coating layer has been presented in this paper. One uncoated specimen and two CVD DLC coated specimen has been used. The dispersion curves of group velocity of leaky Rayleigh wave were measured. The dispersion curve of uncoated specimen shows that the group velocities were same at different frequencies, as there is no coating layer affecting the group velocities at different frequencies. There are distinct different between ‘sound’ specimen and ‘bad bonding’ specimen as the ‘bad bonding’ specimen show unevenness on its dispersion curve.

Boundary enhanced helical ray coupling to high‐frequency modes of solid aluminum cylinders.

Surface guided leaky Rayleigh waves have been shown to provide a significant contribution to the backscattering by a solid metallic cylinder [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 106, 1673 (1999)]. Free field observations of the backscattering by solid aluminum cylinders confirm that both meridional and helical rays contribute to the backscattering for different angles of cylinder tilt [K. Baik, Ph.D. thesis, WSU (2008)]. Those studies show that a previously developed criteria, based on the length of the cylinder, limits the range of tilt angles for which helical rays can contribute [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 112, 528 (2002)]. Experiments with solid aluminum cylinders placed near a flat reflecting boundary suggest that the presence of a boundary can increase the number of helical ray paths that contribute to the backscattering; this increase can lead to increased backscattering amplitudes and an increased range of angles for which coupling to helical rays can occur. To investigate this effect, solid aluminum cylinders were suspended below the free surface of a water tank. Targets were illuminated from below at grazing incidence, and the backscattering was measured as a function of target rotation. [Work supported by ONR.]

Dynamic problem of generalized thermoelastic diffusive medium

The equations of generalized thermoelastic diffusion, based on the theory of Lord and Shulman with one relaxation time, are derived for anisotropic media with rotation. The variational principle and reciprocity theorem for the governing equations are derived. The propagation of leaky Rayleigh waves in a viscous fluid layer overlying a homogeneous isotropic, generalized thermoelastic diffusive halfspace with rotating frame of reference is studied.

Numerical study of Rayleigh wave transmission through an acoustic barrier

Recent experiments [N. F. Declercq and E. Lamkanfi, Appl. Phys. Lett. 93, 2 (2008)] showed evidence of the relationship between the generation of leaky Rayleigh waves and the formation of the two reflected lobes and a null strip for bounded beam incidence at the Rayleigh angle. The evidence was based on an experimental setup using a sound barrier on a liquid solid interface. The current paper presents a finite element investigation of the experiments and confirms the earlier conclusions. In addition, unexplained experimental observations are clarified. Furthermore, a detailed investigation of the influence of the bonding properties between the barrier and the solid on the observed radiation patterns is achieved, which is of broader importance to the nondestructive characterization of bonding in general.

Study by means of liquid side acoustic barrier of the influence of leaky Rayleigh waves on bounded beam reflection

The Schoch effect in ultrasonics, when sound is incident at the Rayleigh angle on a liquid-solid interface, consists of a forward beam displacement of the reflected beam, usually accompanied by a null strip in between the specular portion and the nonspecular reflected beam portion. It is a widely accepted idea that the effect is caused by reflected sound in combination with a sound field emitted by leaky Rayleigh waves. The current work presents an experimental technique to separate and investigate both fields separately by applying an acoustic barrier. The experiments are based on acousto-optic Schlieren photography.

Numerical computation of reflected and transmitted waves at a fluid/solid interface

In this paper, we propose a numerical computation of the different waves generated when a spherical incident pulse is reflected and transmitted by a fluid/solid interface. In addition to the standard reflected and transmitted waves that propagate inside the volume, various surface waves can also be found in both propagation media. In the fluid, we can observe the longitudinal and transverse head waves, and the so‐called leaky Rayleigh wave that generalizes the Rayleigh wave in a semi‐infinite free solid medium to the case of an immersed interface. Similar effects can also be observed on the transmitted displacement field inside the solid material. We compare different numerical approaches, including semianalytic methods (high‐frequency approximation coupled to ray modeling approach) and implicit methods (finite elements and/or finite differences scheme), each method having its own advantages and inconvenients, and domains of validity. These different methods are used to evaluate the field reflected by the interface ; the transmitted displacement field is also analyzed from the same point of view. The plane and curved geometries will be analyzed and the influence of the curvature of the interface will be pointed out.

V(z) oscillations in acoustic microscope at upward defocusing

Acoustic microscopy is widely used for imaging and study of elastic properties of transparent and opaque materials. As a rule, multiple periodic oscillations in the output signal V of a reflection acoustic microscope can be observed in the case of negative defocusing of the wide‐angle acoustic lens, i.e., when its focus is a distance z below the solid sample surface. The main well‐known mechanism for V(z) oscillations is the interference of radiation reflected perpendicularly from the sample surface and re‐radiation of leaky Rayleigh waves generated on the sample by the lens. This effect explains high contrast imaging in reflection scanning acoustic microscopy, and it is a popular method to study properties of solids by measuring Rayleigh wave speeds. As it is shown in this work, bulk acoustic waves in the sample can also give rise to V(z) oscillations. A new mechanism of such oscillations is predicted in the case of positive defocusing (focusing above the sample surface) in acoustic microscopy of anisotropic plates exhibiting negative acoustic refraction. The ray model of this effect shows a possibility to find a relationship between extrema of V(z) curve and separate points on the acoustic slowness surface of the sample.