A device for measuring the velocity of ultrasonic waves: An application to stress analysis

In this paper we present a device for the practical application of an ultrasonic critical-angle refractometry (UCRfr) technique. UCRfr is a technique for measuring the velocity of longitudinal, shear and Rayleight waves, developed to improve the traditional ultrasonic methods for measuring the stress level in materials by means of acousto-elasticity. The technique consists of relating the variations in wave propagation velocity to variations in the angle of refraction at the interface with a second medium. Variations in the angle of refraction are determined on the basis of delay in receiving of the same wave at two different points. The study deals with the measurements of velocity changes of longitudinal wave due to uniaxial stress. In the present work the effects of stress on aluminum and steel specimens have been studied. Experimentation has show the potential of the technique for stress measurement; on the other hand, when the applied stress is known, it allows the measurement of the acoustoelastic constants of longitudinal waves. As regards measuring variations in velocity induced by stress, using this method it is possible, with a suitable choice of the material the device is made of, to isolate the effects of stress on velocity from the possible effects of temperature.     

Effect of measurement volume size on turbulent flow measurement using ultrasonic Doppler method

This paper presents the results of an investigation on the effects of measurement volume size on the mean velocity profile and the Reynolds stress for fully developed turbulent pipe flows. The study employs the ultrasonic velocity profile method, which is based on the ultrasonic Doppler method. The ultrasonic Doppler method offers many advantages over conventional methods for flow rate measurement in the nuclear power plant piping system. This method is capable of measuring the instantaneous velocity profile along the measuring line and is applicable for opaque liquids and opaque pipe wall materials. Furthermore, the method has the characteristic of being non-intrusive. Although it is applicable to various flow conditions, it requires a relatively large measurement volume. The measurement volume of the present method has a disk-shape determined by the effective diameter of the piezoelectric element and the number of the wave cycles of the ultrasonic pulse. Considering this disk-shaped measurement volume and expressing the time-averaged velocity in a truncated Taylor series expansion around the value at the center of the measuring control volume, the value of the velocity can be obtained. The results are then compared with the data obtained from DNS and LDA measurements. The result shows that the effect of the measurement volume size appears in the buffer region and viscous sublayer.     

On the relationship between ultrasonic and micromechanical properties of contacting rough surfaces

In this work, it is suggested that a unique set of the interfacial stiffness constants, K_N and K_T, is sufficient to characterize the macroscopic elastic response of an interface between two rough contacting surfaces regardless of the direction of incidence of the ultrasonic wave. It is also shown that by combining ultrasonic spectroscopy with the theoretical procedures developed for a single imperfect interface, the stiffness constants of a double interface can be successfully recovered. The values of the stiffness constants determined from ultrasonic measurements are related to the micromechanical interaction and topography of the contacting surfaces using a micromechanical model of two rough surfaces in contact.     

Combined two-dimensional velocity and temperature measurements of natural convection using a high-speed camera and temperature-sensitive particles

This paper proposes a combined method for two-dimensional temperature and velocity measurements in liquid and gas flows using temperature-sensitive particles (TSPs), a pulsed ultraviolet laser, and a high-speed camera. TSPs respond to temperature changes in the flow and can also serve as tracers for the velocity field. The luminescence from the TSPs was recorded at 15,000 frames per second as sequential images for a lifetime-based temperature analysis. These images were also used for the particle image velocimetry calculations. The temperature field was estimated using several images, based on the lifetime method. The decay curves for various temperature conditions fit well to exponential functions, and from these the decay constants at each temperature were obtained. The proposed technique was applied to measure the temperature and velocity fields in natural convection driven by a Marangoni force and buoyancy in a rectangular tank. The accuracy of the temperature measurement of the proposed technique was ±0.35–0.40°C.     

Acoustical birefringence and the use of ultrasonic waves for experimental stress analysis

Stress-induced optical birefringence in transparent materials has long been a common technique of stress analysis. Although stress-induced acoustic birefringence was discovered more than 20 years ago, its development and actual applications are still limited. This paper will look at the similarities and differences between the propagation of light waves in photoelastic materials and the propagation of ultrasonic waves in deformed solids. Critical comparisons of the experimental methods employed in photoelasticity with those available in modern ultrasonic measuring technique show why previous studies on ultrasonic measurement of stresses were not very successful. A new experimental technique is devised for using ultrasonic waves for stress analysis. The technique employs a single rotatable 10-MHz shear transducer as the transmitter and receiver of ultrasonic pulses. The enlarged display of the 10-MHz modulated-pulse pattern of reflected echoes provides a convenient way to determine the directions of principal axis of the stress within ±3 deg. The pulse-echo-overlap method is used to measure the absolute velocities of the two principal shear waves. The difference in principal stresses is then calculated from the velocity measurements. Test results of common structural-aluminum and steel specimens under uniaxial compression show a linear relation between the velocity changes and the applied stress. Ultrasonic measurements of stress distribution in a 6.35-cm diameter, 1.9-cm-thick aluminum disk under diametric compression are also reported. Paper was presented at Third SESA International Congress on Experimental Mechanics held in Los Angeles, CA on May 13–18, 1973.     

Ultrasonic nondestructive material evaluation method and study on texture and cross slip effects under simple and pure shear states

Ultrasonic wave velocities propagating in a plastically deformed medium are known to depend upon its microstructural material properties. Therefore, the authors have proposed the theoretical modeling of an ultrasonic nondestructive method to evaluate plastically deformed states. In the present paper, we verify the proposed theoretical modeling of an ultrasonic nondestructive method and examine its accuracy by comparing the experimental results with the simulated subsequent yield surfaces, the longitudinal and transverse wave velocities under combined stress states of an aluminum alloy using internal state variables of an anisotropic distortional yield model which were determined to achieve a good fit for the experimental results of the longitudinal and transverse wave velocity changes under uniaxial tension test. As a special case, the velocity changes of longitudinal wave under pure shear state subjected to the combinations of tension and compression are also studied, it shows a different result compared with that of longitudinal wave velocity under torsional tests of thin thickness cylinders, i.e., simple shear state. The effects on ultrasonic wave velocity changes due to texture and cross slip under simple and pure shear states are studied via a finite element polycrystal model (FEPM).     

Influence of Anisotropy Generated by Rolling on the Stress Measurement by Ultrasound in 7050 T7451 Aluminum

Stress applied to a material can be evaluated using ultrasonic waves. This practice is based on acoustoelastic theory, which relates the stress to the velocity of a wave traveling through the body. How the stress affects the wave velocity is determined by the material’s acoustoelastic constant. This constant can be experimentally measured or calculated from the material’s elastic constants. However, ultrasonic techniques have yet to be adopted as an inspection tool in the field. A factor contributing to this fact is the non-uniformity of materials, mostly associated with grain alignment or texture. As researchers consider this factor, they should take into account the anisotropy generated by rolling. The common practice, however, when relating strain and wave velocity is to ignore anisotropy and to simply utilize isotropic models. No studies have been performed to evaluate the effect of anisotropy on the stress measurement by ultrasound, especially for methods using critically refracted longitudinal waves. The aim of this study is to evaluate how the anisotropy generated by rolling affects the acoustoelastic effect for 7050 T7451 aluminum alloy. We compare the value of the acoustoelastic constant obtained experimentally for rolled samples to the constant calculated with measured elastic constants when the material is assumed to be isotropic. The results show that the methods yield different results, suggesting that the simplified isotropic model should be applied with caution. Since no true known value for elastic constants exists, the results can be used to approach the uncertainty when employing the isotropic model to evaluate stresses in aluminum alloys.     

Acoustic evaluation of the endurance of steel specimens and recovery of their serviceability

A study is made of the possibility for evaluation of damage accumulation during fatigue loading by measuring the ultrasound velocity. It is shown that the dependence of the ultrasonic velocity on the number of loading cycles is represented by a three-stage curve and that the ultrasonic velocity decreases at each stage. The most rapid decrease occurs during the final stage of the test, immediately before fracture. It is also shown that the transition to the critical state can be observed on the basis of ultrasonic-velocity measurements. A method of increasing the service life of a specimen that has neared the critical state is proposed. The method involves exposure of the specimen to a series of high-energy pulses of electric current. The treatment increases the service life by 30–40% compared to the initial value. Institute of the Physics of Strength and Materials Science, Siberian Division, Russian Academy of Sciences, Tomsk 634021. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 4, pp. 180–183, July–August, 1998.     

Measurement of droplet size,velocity, and temperature distributions in rayleigh flow

 Experiments were performed on the droplets formed from a heated, unforced, cylindrical column of water (Rayleigh flow). Temperature distributions, velocity distributions and size distributions were obtained. A thermocouple was used for the temperature measurements. Two time constants were determined – one for the drop impinging on the thermocouple and another for the drop leaving the thermocouple. Received: 21 September 1995 / Accepted: 10 May 1996     

Ultrasonic scattering in polycrystals with orientation clusters of orthorhombic crystallites

Scattering-induced ultrasonic attenuation and backscattering in a polycrystalline medium with orientation clusters composed of orthotropic crystallites are studied theoretically, aiming to improve understanding of ultrasonic wave interaction with such clustered microstructures for application to the modeling of titanium alloys. Both orthorhombic crystallites and their arrangements in orientation clusters (also termed microtexture regions, MTRs) are of general ellipsoidal shape. The preferred orientation of orthotropic crystallites in the clusters is represented by a generalized Gaussian orientation distribution function with three independent texture parameters. The effective elastic properties of the clusters, which have in general orthorhombic symmetry, are determined by a volume average of elastic constants weighted by orientation distribution functions and then used to obtain the cluster-scattering-induced attenuation and backscattering in the polycrystalline medium. In the model the wave propagation direction is arbitrary relative to the ellipsoidal axes of the clusters. The contribution of crystallite-scattering-induced attenuation is estimated by the untextured attenuation coefficient factored by a texture transition function. The total attenuation and backscattering are determined by combining scattering by the clusters and crystallites. Drastic effects of clustering and the transition to unclustered polycrystals are demonstrated. Reasonable agreement is observed between the model’s prediction and measurements on two Ti-alloy samples with different crystallite clustering.     

Ultrasonic Doppler velocimetry in liquid gallium

For the first time, flow velocity is measured in a vortex of liquid gallium, using the pulsed Doppler shift ultrasonic method. At the top of a copper cylinder filled with liquid gallium, we spin a disk and create a turbulent vortex with a dominant nearly axisymmetric velocity field with little variation in the axial direction. The velocity profiles are shown to be well resolved and in quantitative agreement with earlier observations. Reliable velocity measurements in liquid gallium could be obtained only after serious problems due to the formation of oxides were solved. This work opens the way to performing accurate velocity measurements in other liquid metals; preliminary results for liquid sodium are shown. Received: 14 January 2000 / Accepted: 12 January 2001     

NDE of metal damage: ultrasonics with a damage mechanics model

This research describes a nondestructive method for the quantitative estimation of property variations due to damage in metal materials. The method employs a damage mechanics model, which accounts for stiffness degradation and damage evolution of a metal medium with a measurement of ultrasonic velocity. In order to describe the progressive deterioration of materials prior to the initiation of macrocracks, we have developed a new damage mechanics model. Thereafter, a finite element model valid for numerically describing such damage process has been developed by ABAQUS/Standard code, and correlations between damage state, elastic stiffness and plastic strain could be found by the results of the finite element simulation. The property variations due to damage evolution are calculated based on the Mori–Tanaka theory, and then the ultrasonic velocity can be predicted by Christoffel’s equation. When the measured velocity is coupled with the theoretically predicted velocity, the unknown damage variable is solved, from which other residual properties are determined by the predictions of damage model. The proposed technique is performed on type 304 stainless steel bars. The numerical results obtained by the simulation were compared with experimental ones in order to verify the validity of the proposed finite element model and good agreement was found. It is shown that the damaged properties of metals can be estimated accurately by the proposed method.     

In-situ swinging arm calibration for hot-film anemometers

A method of calibration for hot-film anemometers is presented. A swinging arm that moves under the influence of gravity serves as both a calibration mechanism and a probe support. The velocity of the probe is found by differentiating the angular position history of the arm and multiplying it with the arm length. Limitations on the quality of calibration data while the arm is accelerating are discussed. The hot film voltage output is then matched to the velocity to find the two constants in King's law. The calibration was tested by taking velocity profile measurements in a laminar boundary layer. The results of these compared well to the Blasius profile.     

Perpendicular ultrasound velocity measurement by 2D cross correlation of RF data. Part B: volume flow estimation in curved vessels

A novel axial velocity profile integration method, obtained from ultrasonic perpendicular velocimetry, for flow estimation in curved tubes was validated. In an experimental set-up, physiologically relevant curved geometries and flows were considered. Axial velocity profile measurements were taken by applying particle imaging velocimetry-based methods to ultrasound data acquired by means of a linear array transducer positioned perpendicular to the axial velocity component. Comparison of the assessed asymmetric velocity profiles to computational fluid dynamics calculations showed excellent agreement. Subsequently, the recently introduced cos θ-integration method for flow estimation was compared to the presently applied Poiseuille and Womersley models. The average deviation between the cos θ-integration-based unsteady flow estimate and the reference flow was about 5%, compared to an average deviation of 20% for both the Poiseuille and Womersley approximation. Additionally, the effect of off-centre measurement was analysed for the three models. It was found that only for the cos θ-integration method, an accurate flow estimation is feasible, even when it is measured off centre.     

Ultrasonic measurement of the bending of a bolt in a shear joint

An ultrasonic pulse/echo technique is used to measure preload in bolts used in structural joints. In this paper, the same instrument is used in a different way to measure change in the ultrasonic measurements due to bending in the bolts. A theory that explains the ultrasonic measurements is developed. The bending loads result in a rotation and a translation of the ultrasonic pulse reflecting face. It also creates a stress gradient in the bolt. This results in a phase variation (or gradient) in the received ultrasonic beam across the face of the transducer. It also results in a physical shift in the received beam relative to the ultrasonic transducer. The phase gradient and the shift in the beam results in change in the pulse travel time. A number of experiments were performed on the bolt to study the effect of the bending on the ultrasonic measurements. The experiments and the theory validate a sensitive new method for measuring the bending loads in the bolts.     

Elastic constant measurement by using line-focus-beam acoustic microscope

The purpose of this study is to develop a method for measuring elastic constants using a LFB acoustic microscope. For this purpose, a theoretical procedure for the estimation of Young's modulus and Poisson's ratio is introduced based on elastic theory. According to this procedure, experimental velocity measurement and elastic constants estimation are made by use of a LFB acoustic microscope. For the confirmation of the availability of this method, the estimated elastic constant is compared to measurements of elastic constants by other methods. The resulting estimated values of Young's modulus and Poisson's ratio obtained by the LFB acoustic microscope are highly accurate which confirms the usefulness of the elastic constant measurement system. T. Mihara is Research Associate, and M. Obata (former SEM Member), deceased, was Professor, Department of Materials Processing, Faculty of Engineering, Tohoku University, Aramaki Aoba, Sendai, 980, Japan.     

Direct measurement of circulation using ultrasound

 Ultrasound time-of-flight methods employing counter-propagating ultrasonic pulses are utilized for the direct measurement of circulation in vortical flows. Two schemes are described here which involve either a single straight path or a closed path. Both techniques are shown to result in time differences, between the counter-propagating pulses around the path, linearly proportional to the circulation enclosed by the ultrasound path. The ultrasound methods of circulation measurement do not require calibration constants and can be non-invasive. The reliability of the closed path ultrasound method was assessed by comparing the measured circulation values with those calculated from the lift measurements of a NACA 0012 airfoil. Two examples are also presented where the closed path ultrasound method has been applied to the flow over a delta wing and a free-surface vortex in a cylindrical tank. Received: 8 October 1997/Accepted: 23 April 1998     

Influence of temperature on the slip velocity of semidilute xanthan gum solutions

We obtain experimental evidence of the influence of temperature in the range 12-32°C on the slip phenomena of two different 0.3% xanthan solutions in a glass capillary rheometer. Enhancement of the slip velocity was observed for both samples around the corresponding thermally induced order-disorder transition temperature. Intrinsic viscosity measurements were performed to find the conformation change of both samples. Concentrations of 0.15% and 0.2% were analyzed for one sample, showing absence of slip at 0.15%.Slip velocity measurements were determined with the traditional Mooney method for a L/D ratio of the capillaries (640) enough to neglect entry head losses. Comparisons were done with the method developed by Piau et al. (1990) and with the one developed by Hatzikiriakos and Dealy (1992). The resulting behavior of the slip velocity with the capillary diameters, calculated with the method of Hatzikiriakos and Dealy, was contrary to the behavior experimentally found by other authors. The observed differences in the slip velocity, measured with the other two methods, were proportional and nearly independent of temperature and diameter of the capillaries.     

Green water void fraction due to breaking wave impinging and overtopping

The present study uses laboratory measurements to investigate the void fraction of an overtopping flow on a structure. The overtopping flow, also called green water, was generated by the impingement of a plunging breaking wave on the structure following the Froude similarity of an extreme hurricane wave and a simplified offshore structure. The flow is multi-phased and turbulent with significant aeration. A fiber optic reflectometer (FOR) and bubble image velocimetry (BIV) were employed to measure the void fraction and velocity in the flow, respectively, and to determine the water level on the deck. Mean properties of void fraction and velocity were obtained by ensemble-averaging and time-averaging the repeated instantaneous measurements. The temporal and spatial distributions of void fraction reveal that the flow is very highly aerated near the front of green water and has relatively low aeration near the deck surface. The mean void fraction and velocity distributions were also depth-averaged for simplicity and potential use in engineering applications. Using the measured data, similarity profiles for depth-averaged void fraction, depth-averaged velocity, and water level were found. The study suggests that using only the velocity data is insufficient if the flow momentum or the flow rate is to be determined. The accuracy of the void fraction measurements was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume based on the measured velocity and void fraction.     

Determination of hydrodynamic behavior of gas–solid fluidized beds using statistical analysis of acoustic emissions

In the processes involving the movement of solid particles, acoustic emissions are caused by particle friction, collision and fluid turbulence. Particle behavior can therefore be monitored and characterized by assessing the acoustic emission signals. Herein, extensive measurements were carried out by microphone at different superficial gas velocities with different particle sizes. Acoustic emission signals were processed using statistical analysis from which the minimum fluidization velocity was determined from the variation of standard deviation, skewness and kurtosis of acoustic emission signals against superficial gas velocity. Initial minimum fluidization velocity, corresponding to onset of fluidization of finer particles in the solids mixture, at which isolated bubbles occur, was also detected by this method. It was shown that the acoustic emission measurement is highly feasible as a practical method for monitoring the hydrodynamics of gas–solid fluidized beds.