A direct determination of acoustical impedance of a thin layer on a thick elastic substrate by the ultrasound

A direct determination method of acoustical impedance of a linear-viscoelastic thin layer on a thick elastic substrate is proposed without the prior knowledge of other layer parameters. With the rectangular windows that start at the time zero and are rightward expanded, the successive windowing operations on the total ultrasonic reflected signals are performed. Using the transfer functions of the windowed signals and the analytical formula, the distribution characteristic of the calculated results of acoustical impedance with the different frequency intervals and the behavior of the corresponding estimated values are numerically investigated as the lengths of the sampling windows change. Acoustical impedance of the thin layer can be directly determined from the distribution characteristic of the calculated results and the behavior of the estimated values. The proposed direct determination method is independent of the parameters of the next substrates.     

Lens multielement acoustic microscope in the mode for measuring the parameters of layered objects

An acoustic microscope with a cylindrical lens and ultrasound transducer have been considered, as well as the method based on it for the measuring of longitudinal and transverse wave velocities, the thickness and density of the investigated layer. A theoretical model of the microscope has been constructed, and the relation between the spatiotemporal output signal of the transducer and the angular dependence of the sample reflection coefficient has been found. It has been shown that the velocities of body waves and the thickness can be determined by the delays of ultrasound responses reflected from the layer boundaries measured by the transducer elements, and the density, by the amplitudes of these responses. The method was tested experimentally using a 20-element transducer with a central frequency of 15 MHz and a period of 0.8 mm. The example of a duralumin plate has shown that the error in measuring the thickness and velocity of longitudinal waves error does not exceed 1%; the velocity of transverse waves, 2%; and the density can be estimated with an accuracy of about 5%.     

Parameter measurement of the cylindrically curved thin layer using low-frequency circumferential Lamb waves

The characteristic parameters of a cylindrically curved thin layer include its elastic constants, thickness and curved radius. A layer is considered thin if the echoes from the front and back surfaces of the layer cannot be separated in the time domain, and/or that the wave arrivals corresponding to longitudinal and shear wave part cannot be identified in the time or space domain. This paper describes a low-frequency circumferential Lamb wave method to characterize those parameters of a cylindrically curved thin layer. The technique is based on the measurement of circumferential Lamb wave phase velocity and the unknown parameter is estimated through minimizing the mean square error obtained by comparing theoretical and experimental phase velocities. The sensitivity and accuracy of the proposed technique to different parameters are analyzed. Using the present technique, a cylindrically curved thin layer with thickness down to ten percent of the longitudinal wavelength can be successfully measured with an average relative error less than two-percent in our experiment.     

Processing an acoustic microscope’s spatiotemporal signal to determine the parameters of an isotropic layer

This paper presents a method for measuring the thickness and velocities of body waves and the density of an isotropic layer by a pulse scanning acoustic microscope. The method is based on recording the microscope signal as a function of the displacement magnitude of the focused ultrasonic transducer along its axis perpendicular to the sample surface and on the decomposition of the recorded 2D spatiotemporal signal into the spectrum of plane pulse waves. The velocities of the longitudinal and transverse waves and the layer’s thickness are calculated from the relative delays of the components of the spectrum of plane waves reflected from the surfaces of the layer and the density is computed by the amplitudes of these components. An experimental investigation of a test sample in the form of a glass plate carried out in the 50-MHz range shows that the error in measuring the thickness and velocities of body waves does not exceed 1% and the density measurement error does not exceed 10%.     

Measurements of the longitudinal wave speed in thin materials using a wideband PVDF transducer

A flat transducer was constructed, using a 9-microm-thick PVDF (polyvinylidene fluoride) film for generation and detection of high-frequency ultrasonic waves, and used for measurements of the phase velocity of longitudinal waves traveling along the thickness direction in a very thin material. The transducer has a useful wideband frequency characteristic extending from 10 MHz to over 150 MHz. Measurements of the phase velocity of the longitudinal waves are carried out using a 0.212-mm-thick glass slide and a 0.102-mm-thick stainless-steel shim, using water as a coupling medium. The thickness limit for this measurement appears to be approximately 20 microm. The phase velocity of the longitudinal mode is obtained as a function of frequency in the frequency domain by using a modified sampled continuous wave (cw) technique. It can also be measured in the time domain by using a broadband pulse of short duration.     

Study of the influence of thin layer loading on the propagation of Lamb waves using perturbation method

I.IntroductionSincethepublicationoftheclassicalpaper"Onthewavesinanelasticplate"byH.Lambin1917l1l,theterm"LaInbwave"hasbeenusedtorefertoanelasticdisturbancepropagatinginasolidplatewithfreeboundaries.Lambwavesarewidelyusedintheapplicationsofthedefectinspectionofthinwa.lledmaterial[2'8].InrecelltyearsLambwaveshavebeenwide1yusedinthefabricationofsensors.LamInwavesensorsdetectthechangesofenvironmentbymeasuringthechangeofphasevelocityofLambwaves.IncomparisonwithbuIkwavesandSAW's,Lambwavsprovi…     

Angular distribution of Cherenkov radiation from relativistic heavy ions taking into account deceleration in the radiator

Numerical methods are used to study the dependence of the structure and the width of the angular distribution of Vavilov-Cherenkov radiation with a fixed wavelength in the vicinity of the Cherenkov cone on the radiator parameters (thickness and refractive index), as well as on the parameters of the relativistic heavy ion beam (charge and initial energy). The deceleration of relativistic heavy ions in the radiator, which decreases the velocity of ions, modifies the condition of structural interference of the waves emitted from various segments of the trajectory; as a result, a complex distribution of Vavilov-Cherenkov radiation appears. The main quantity is the stopping power of a thin layer of the radiator (average loss of the ion energy), which is calculated by the Bethe-Bloch formula and using the SRIM code package. A simple formula is obtained to estimate the angular distribution width of Cherenkov radiation (with a fixed wavelength) from relativistic heavy ions taking into account the deceleration in the radiator. The measurement of this width can provide direct information on the charge of the ion that passes through the radiator, which extends the potentialities of Cherenkov detectors. The isotopic effect (dependence of the angular distribution of Vavilov-Cherenkov radiation on the ion mass) is also considered.     

Nondestructive testing in human teeth using a leaky Lamb wave device

A Lamb wave interdigital transducer mounted on a layered substrate composed of two plates, a thin piezoelectric ceramic plate and an acrylic plate, operating at a liquid-solid boundary, is investigated for ultrasonic nondestructive testing of the layer thickness in human teeth. A higher-order mode having a phase velocity higher than the longitudinal wave velocity in the human teeth can be used for nondestructive testing. In the combination of the two layers, the fourth mode of leaky Lamb wave is most favorable for a frequency-controllable radiation angle of an ultrasound beam into a water layer as an acoustic coupler. In the configuration of an acoustic delay line, the layer-thickness measurement in vivo, evaluated from the time interval between two reflected ultrasound echoes, is successfully realized under a thin water layer as the acoustic coupler.     

An ultrasonic method for determination of elastic moduli, density, attenuation and thickness of a polymer coating on a stiff plate

An ultrasonic method proposed by us for determination of the complete set of acoustical and geometrical properties of a thin isotropic layer between semispaces (J. Acoust. Soc. Am. 102 (1997) 3467) is extended to determination of the properties of a coating on a thin plate. The method allows simultaneous determination of the coating thickness, density, elastic moduli and attenuation (longitudinal and shear) from normal and oblique incidence reflection (transmission) frequency spectra. Reflection (transmission) from the coated plate is represented as a function of six nondimensional parameters of the coating which are determined from two experimentally measured spectra: one at normal and one at oblique incidence. The introduction of the set of nondimensional parameters allows one to transform the reconstruction process from one search in a six-dimensional space to two searches in three-dimensional spaces (one search for normal incidence and one for oblique). Thickness, density, and longitudinal and shear elastic moduli of the coating are calculated from the nondimensional parameters determined. The sensitivity of the method to individual properties and its stability against experimental noise are studied and the inversion algorithm is accordingly optimized. An example of the method and experimental measurement for comparison is given for a polypropylene coating on a steel foil.     

Acoustic method for obtaining the pressure reflection coefficient using a half-wave layer

This paper describes a method for obtaining the pressure reflection coefficient by using a half-wave layer. Two transducers were placed in reference liquid and test liquid respectively. The reference and test liquid were separated by the thin half-wave layer. Both transducers could operate in two modes. One acted as a transmitter and worked in pulse-echo mode, and the other operated in receiver mode. By adjusting the frequency of drive signal according to the thickness and material property of the half-wave layer, it was possible to generate the maximum interference signal of multiple waves reflected at the two interfaces of the layer. Therefore, the amplitude of reflection wave in steady-state depended only on the reflection coefficient at the interface between the half-wave layer and the test liquid. The effects of the signal-to-noise-ratio and the half-wave layer attenuation on the uncertainty characteristics of the pressure reflection coefficient were discussed. The experimental results showed high accuracy for measurement of reflection coefficient.     

Effect of porous surface layer on wave propagation in elastic cylinder immersed in fluid

To study the damage to an elastic cylinder immersed in fluid, a model of an elastic cylinder wrapped with a porous medium immersed in fluid is designed. This structure can both identify the properties of guided waves in a more practical model and address the relationship between the cylinder damage degree and the surface and surrounding medium. The principal motivation is to perform a detailed quantitative analysis of the longitudinal mode and flexural mode in an elastic cylinder wrapped with a porous medium immersed in fluid. The frequency equations for the propagation of waves are derived each for a pervious surface and an impervious surface by employing Biot theory. The influences of the various parameters of the porous medium wrapping layer on the phase velocity and attenuation are discussed. The results show that the influences of porosity on the dispersion curves of guided waves are much more significant than those of thickness, whereas the phase velocity is independent of the static permeability. There is an apparent "mode switching" between the two low-order modes. The characteristics of attenuation are in good agreement with the results from the dispersion curves. This work can support future studies for optimizing the theory on detecting the damage to cylinder or pipeline.     

Acoustic dispersion and attenuation measurement using both transmitted and reflected pulses

Traditional broadband transmission method for measuring acoustic dispersion and attenuation requires the measurement of the thickness of the specimen, the transmission coefficient at the water–specimen interface, and the Fourier spectra of two transmitted pulses. A new method has recently been developed that can determine both the thickness and dispersion of the specimen by utilizing the phase spectra of two additional pulses reflected back from the front and back surfaces of the specimen. In this paper, the method is further extended to the measurement of attenuation. If the density of the specimen is known, the frequency-dependent transmission coefficient can be determined based on the measured phase velocity, and only the amplitude spectra of the two transmitted pulses are used to determine the attenuation. If the density of the specimen is unknown, the attenuation can be determined from the amplitude spectra of all the four pulses. In both cases, the thickness estimated from the phase spectra of the four pulses is utilized. Experimental results from two specimens are presented to demonstrate the application of the new method.     

Determination of isotropic layer parameters from spatiotemporal signals of an ultrasonic array

The paper discusses a method for measuring the velocities and attenuation of longitudinal and transverse ultrasonic waves and the density and thickness of the isotropic layer with an array placed in an immersion liquid parallel to the sample. The method is based on the recording of the total spatiotemporal signal of the array and its expansion into a spatial spectrum of pulse plane wave response. The ultrasonic velocity and sample thickness depend on the response delay of the plane wave in the layer from the transverse projection of the slowness vector. The density and attenuation are determined from the behavior of the amplitudes of spectral responses. To confirm this method in experiment, the parameters of a polystyrene plate have been measured using a linear 32-element array with a central frequency of 17 MHz.     

Interference in the ultracold neutron transmission through thin layers

At the time-of-flight spectrometer for ultracold neutrons (UCN) with energies below 10^?4 eV at the FRM, Munich, the perpendicular neutron transmission through carbon and gold films of thickness 450 to 800 Å was measured as a function of neutron velocity in the range 11≧v _z≧3.7 m/s. The transmission curves obtained show the expected interference patterns due to interference between the partial waves transmitted and reflected at the two surfaces. From these curves absolute values for the thickness and density could be obtained. From the UCN transmission through a 4.71 μm thick copper foil a value a_coh=7.5±0.15 F was derived for the nuclear scattering length of Cu.     

Theoretical relationship between sound velocity and the physical properties of submarine sediment

Density and elastic modulus change ratios are introduced to describe the sound velocity of submarine sediment.The density change ratio is a composite parameter describing the sound velocity.It is expressed by three physical parameters:porosity,solid phase density and seawater density.The elastic modulus change ratio is also a composite parameter of sound velocity.It is expressed by three physical parameters,including porosity,solid phase modulus and seawater bulk modulus.The sound velocity formula can be developed into a Taylor polynomial formula of these two composite parameters.The change in the two composite parameters constitutes the sound velocity surface,which contains the complete information regarding velocity properties and sediment characteristics.The one-parameter velocity formula is a curve on the velocity surface.Each porosity-velocity empirical formula,which represents various sea locations and conditions,is transformed to a standard form.This result is the product of a reference velocity and a modulation function.Comparisons of the numerical calculation and measurements show that the derived modulation functions yield similar results.The difference between the velocity formula derived in this paper and the Wood velocity formula is due to the elastic modulus models.     

双层光学薄膜参数的多入射角椭偏分析方法

本文讨论了多入射角椭偏测量中光学参数的误差因子以及最佳测量条件的选取.指出,当薄膜较厚时,多入射角椭偏测量可以精确确定膜系的光学常数和几何厚度,并用二例实测结果加以证实.本文的方法也适用于分析多层光学薄膜.     

Poroelastic modeling of seismic boundary conditions across a fracture

Permeability of a fracture can affect how the fracture interacts with seismic waves. To examine this effect, a simple mathematical model that describes the poroelastic nature of wave-fracture interaction is useful. In this paper, a set of boundary conditions is presented which relate wave-induced particle velocity (or displacement) and stress including fluid pressure across a compliant, fluid-bearing fracture. These conditions are derived by modeling a fracture as a thin porous layer with increased compliance and finite permeability. Assuming a small layer thickness, the boundary conditions can be derived by integrating the governing equations of poroelastic wave propagation. A finite jump in the stress and velocity across a fracture is expressed as a function of the stress and velocity at the boundaries. Further simplification for a thin fracture yields a set of characteristic parameters that control the seismic response of single fractures with a wide range of mechanical and hydraulic properties. These boundary conditions have potential applications in simplifying numerical models such as finite-difference and finite-element methods to compute seismic wave scattering off nonplanar (e.g., curved and intersecting) fractures.     

电弧增材成形中熔积层表面形貌对电弧形态影响的仿真

电弧增材成形常采用单道多层或多道搭接的熔积方式,不同的熔积方式下对应的熔积层表面形貌不同,从而影响电弧的形态及其传热传质过程.本文建立了纯氩保护电弧增材成形的电弧磁流体动力学三维数值模型,以及不同表面形貌的熔积层模型,并在保持阳极与阴极之间距离和熔积电流不变的条件下,通过模拟计算获得增材成形特有的单道和多道搭接熔积条件下的不同表面形貌对应的电弧形态以及相应的温度场、流场、电流密度、电磁力、电弧压力分布.数值模拟结果表明:平面基板上起弧情况下电弧中心具有较高的温度、速度、电流密度以及压强;单道多层熔积情况下熔积层数对电弧的各个参量影响较小;多道搭接熔积情况下电弧呈非对称分布,电弧中心温度较前两者低,电流密度、电磁力和电弧压强的分布偏向熔积层一侧.     

Bragg diffraction of light by ultrasonic shear waves in a plane-parallel layer

We have studied anisotropic Bragg diffraction of light by ultrasonic shear waves in an optically isotropic plane-parallel layer. We have established the analytical dependences of the relative intensities and polarization azimuths of reflected and transmitted diffracted waves on the intensity of the ultrasound, the layer thickness, the angle of incidence, and the polarization azimuth of the incident light. We show that rotation of the plane of polarization of the diffracted wave is determined by the different Fresnel reflection of the s and p polarized components of the incident light in the plane-parallel layer. We have determined that in mismatched acousto-optic structures, deep amplitude modulation of transmitted and reflected light is possible which is an order of magnitude greater than the usual modulation in matched structures.     

Measurement of acoustic dispersion using both transmitted and reflected pulses

Traditional broadband transmission method for measuring acoustic dispersion requires the measurements of the sound speed in water, the thickness of the specimen, and the phase spectra of two transmitted ultrasound pulses. When the sound speed in the specimen is significantly different from that in water, the overall uncertainty of the dispersion measurement is generally dominated by the uncertainty of the thickness measurement. In this paper, a new water immersion method for measuring dispersion is proposed which eliminates the need for thickness measurement and the associated uncertainty. In addition to recording the two transmitted pulses, the new method requires recording two reflected pulses, one from the front surface and one from the back surface of the specimen. The phase velocity as well as the thickness of the specimen can be determined from the phase spectra of the four pulses. Theoretical analysis and experimental results from three specimens demonstrate the advantages of this new method.