Velocity Dispersion Of Acoustic Waves Research Articles

Influence of top-coat thickness of thermal barrier coating on time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave

Thermal barrier coating is commonly used to insulate the hot end components of aero engine and gas turbine in order to meet extreme high temperature service conditions. The performance of the thermal barrier coating will be significantly impacted by thinning or even spalling during the service process. Given the uniqueness of thermal barrier coating in structure and composition materials, and the fact that laser-induced surface acoustic waves are extremely sensitive to material surface characteristic parameters, laser-induced ultrasonic technology is used to detect the thermal barrier coating in this paper. To characterize the size and defect of thermal barrier coating, the excitation mechanism and propagation law of surface acoustic waves are very crucial. Firstly, the surface acoustic wave propagation law is investigated using spectral analysis and wavelet transform. Then, to study the influence of top-coat thickness on laser-induced surface acoustic wave, the surface acoustic wave velocity dispersion curves of thermal barrier coating with different top-coat thicknesses are extracted using wavelet analysis. The simulative and experimental results indicate that the time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave in multilayered heterostructure are strongly affected by the top-coat thickness. By the establishment of the relationship between phase velocity of laser-induced surface acoustic wave and top-coat thickness, a theoretical basis is provided to characterize the top-coat thickness of multilayered heterostructure.

Brillouin mirror with an inverted acoustic profile in the presence of strong acoustic dispersion

While usually negligible in standard optical fibers, the group velocity dispersion of acoustic waves may in some cases play a significant role in the dynamics of stimulated Brillouin scattering (SBS) in propagation media with more complex structures, such as microstructured fibers. The usual three-wave coherent model of SBS can be adapted to take perturbative acoustic dispersion into account, but the slowly varying envelope approximation does not hold for stronger values of the acoustic dispersion, which call for a more sophisticated inertial model of SBS. A new regime of SBS mirrors with a spatially inverted acoustic profile is predicted in this limit. In the presence of strong acoustic dispersion, this regime exhibits a higher conversion efficiency than the usual mirror in the dispersionless case, as well as nonlinear self-stabilization of the phase of the acoustic wave when the pump is strongly depleted. Formal calculations allow the identification of regions of strong dynamic dispersion.

Determination of elastic properties of a MnO2 coating by surface acoustic wave velocity dispersion analysis

MnO2 is a material of interest in the development of high energy-density batteries, specifically as a coating material for internal 3D structures, thus ensuring rapid energy deployment. Its electrochemical properties have been mapped extensively, but there are, to the best of the authors' knowledge, no records of the elastic properties of thin film MnO2. Impulsive stimulated thermal scattering (ISTS), also known as the heterodyne diffraction or transient grating technique, was used to determine the Young's modulus (E) and porosity (ψ) of a 500 nm thick MnO2 coating on a Si(001) substrate. ISTS is an all optical method that is able to excite and detect surface acoustic waves (SAWs) on opaque samples. From the measured SAW velocity dispersion, the Young's modulus and porosity were determined to be E = 25 ± 1 GPa and ψ=42±1%, respectively. These values were confirmed by independent techniques and determined by a most-squares analysis of the carefully fitted SAW velocity dispersion. This study demonstrates the ability of the presented technique to determine the elastic parameters of a thin, porous film on an anisotropic substrate.

Seismic time-frequency spectral decomposition by matching pursuit

A seismic trace may be decomposed into a series of wavelets that match their time-frequency signature by using a matching pursuit algorithm, an iterative procedure of wavelet selection among a large and redundant dictionary. For reflection seismic signals, the Morlet wavelet may be employed, because it can represent quantitatively the energy attenuation and velocity dispersion of acoustic waves propagating through porous media. The efficiency of an adaptive wavelet selection is improved by making first a preliminary estimate and then a localized refining search, whereas complex-trace attributes and derived analytical expressions are also used in various stages. For a constituent wavelet, the scale is an important adaptive parameter that controls the width of wavelet in time and the bandwidth of the frequency spectrum. After matching pursuit decomposition, deleting wavelets with either very small or very large scale values can suppress spikes and sinusoid functions effectively from the time-frequency spectrum. This time-frequency spectrum may be used in turn for lithological analysis—for instance, detection of a gas reservoir. Investigation shows that the low-frequency shadow associated with a carbonate gas reservoir still exists, even high-frequency amplitudes are compensated by inverse-[Formula: see text] filtering.

Influence of mass density and mechanical properties on the surface acoustic wave velocity dispersion

The surface acoustic wave technique is a non-contact, non-destructive opto-acoustic technique used to measure film thickness and material properties, such as elastic properties and density. Simulation of acoustic wave velocity dispersion revealed that the contribution density makes is larger in the first mode, whereas the modulus makes a larger contribution in the second mode. The effect of thickness does not depend on the wavelengths and oscillation modes as the density and modulus. In fact, the dispersion curves measured experimentally in the electro-deposited Cu films show more variation in second mode during self-anneal, while the difference between two samples, which contain different impurity level, is more significant in first mode. The velocity change during self-anneal is mainly due to the modulus change, which result from the texture change accompanied with grain growth. In contrast, a different impurity concentration causes density variation, which is considered to be related to the change of formation energy of voids or vacancy-type defect. These results coincide with the simulation. The understanding obtained in the present investigation suggests the possibility to improve the precision of thickness or material properties determination and the possibility to determine density and modulus simultaneously by analyzing the acoustic wave velocity dispersion.

Velocity dispersion in granular marine sediment

Acoustic wave velocity dispersion in granular marine sediment is important to the study of sediment acoustics. The reported dispersion results cannot be explained with Biot’s model using constant Biot parameters. In this study, the acoustic wave velocities in water-saturated glass beads and beach sands with different grain sizes are measured at different frequencies. The velocities in air-saturated same media and in same media in vacuum are also measured. Next the frame moduli for water-saturated media, air-saturated media, and media in vacuum are obtained from these measured velocities. From these results, it is seen that the frame bulk modulus has dependence on the grain size and the frequency. It is considered that one possibility of the velocity dispersion in water-saturated granular marine sediment is grain size and frequency-dependent frame bulk modulus due to the gap stiffness between the grains.

Velocity dispersion of acoustic waves in cancellous bone

Measurement of ultrasonic attenuation and velocity in cancellous bone are being applied to aid diagnosis of women with high fracture risk due to osteoporosis. However, velocity dispersion in cancellous bone has received little attention up to now. The overall goal of this research was to characterize the velocity dispersion of human cancellous bone based on a spectral analysis of ultrasound transmitted through the bone specimens. We have followed a systematic approach, beginning with the investigation of a test material, moving on to the investigation of bone specimens. Particular attention is given to diffraction effect, a potential source of artifacts. Parametric images of phase velocity (measured at the center frequency of the pulse spectrum), slope of attenuation coefficient (dB/cm/MHz) and velocity dispersion were obtained by scanning 15 bone specimens. We have demonstrated that the diffraction effect is negligible in the useful frequency bandwidth, and that the ultrasonic parameters reflect intrinsic acoustic properties of bone tissue. The measured attenuation showed approximately linear behavior over the frequency range 200 to 600 kHz. Velocity dispersion of cancellous bone in the frequency range 200 to 600 kHz was unexpectedly found to be either negative or positive and not correlated with the slope of attenuation coefficient. There was a highly significant correlation between the slope of attenuation coefficient and phase velocity at the center frequency of the spectrum. This behavior contrasts with other biological or nonbiological materials where the local form of the Kramers-Kronig relationship provides accurate prediction of velocity dispersion from the experimental frequency dependent-attenuation for unbounded waves.

Local Study of Thin-Film Adhesion by Surface Acoustic Waves and Subsurface Acoustic Microscopy Imaging

Four thin-film adhesion characterization methods are proposed. Two of them are surface acoustic wave based and the two others emphasize the complementary nature between acoustic microscopy, scratch test and tensile experiments. A continuous wave scanning acoustic microscope was used to propagate surface modes in the specimen and to measure the surface acoustic wave velocity as a function of the frequency. Characteristic equations have been calculated for generalized Lamb wave and generalized surface skimming compressional wave modes as a function of interface boundary conditions and frequency. Experimental surface acoustic wave velocity dispersion curves and numerically evaluated ones have been compared in shape and absolute position. It has been shown that this comparison allows one to characterize the interface preparation quality or adhesion property of thin films deposited by Physical Vapour Deposition (PVD). Two original coupling techniques were developed to increase the comprehension of intermediate adhesion states and thin-film delamination initiation and propagation characteristics. The first applies tensile stress to thin-film specimens directly under the microscope and the other creates a typical scratch at the top of the layered sample before surface and interface acoustic microscopy observations.