Comparison of acoustoelastic Lamb wave propagation in stressed plates for different measurement orientations.

High order Lamb waves are investigated for the effects of stress on both symmetrical and anti-symmetrical modes in an aluminum plate for wave propagation and load parallel. Data are compared with those for the case of load and measurement axis perpendicular. It is the S1 mode which exhibits significantly higher sensitivity to stress than other Lamb modes. For aluminum the use of the S1 mode for stress measurement is found to be about six times more sensitive, than bulk waves, for the load-measurement axes parallel case and this compares with about ten times for the case of load-measurement axes perpendicular.

Higher order acoustoelastic Lamb wave propagation in stressed plates.

Modeling and experiments are used to investigate Lamb wave propagation in the direction perpendicular to an applied stress. Sensitivity, in terms of changes in velocity, for both symmetrical and anti-symmetrical modes was determined. Codes were developed based on analytical expressions for waves in loaded plates and they were used to give wave dispersion curves. The experimental system used a pair of compression wave transducers on variable angle wedges, with set separation, and variable frequency tone burst excitation, on an aluminum plate 0.16 cm thick with uniaxial applied loads. The loads, which were up to 600 µÎµ, were measured using strain gages. Model results and experimental data are in good agreement. It was found that the change in Lamb wave velocity, due to the acoustoelastic effect, for the S1 mode exhibits about ten times more sensitive, in terms of velocity change, than the traditional bulk wave measurements, and those performed using the fundamental Lamb modes. The data presented demonstrate the potential for the use of higher order Lamb modes for online industrial stress measurement in plate, and that the higher sensitivity seen offers potential for improved measurement systems.

Analysis of the directivity of longitudinal waves based on double-fold coil phased EMAT.

Longitudinal critically refracted (LCR) waves have already been widely applied for residual stress characterization. Such waves are usually generated using mode-conversion at the first critical angle of the incident longitudinal wave, which gives waves that then propagate at a dip-angle, and this places energy close to the surface of the specimen. The dip-angle needs to be minimized to improve both velocity measurement and residual stress characterization sensitivity. This paper reports a novel double-fold coil phased EMAT that can decrease the dip-angle. The performance of this new EMAT was investigated using both a COMSOL model and experiments. Initial model validation was provided through a comparison with experimental data. The EMAT design also enables scanning of samples, and operation in harsh environments where use of a PZT based transducer and couplants can complicate and limit inspection. The use of the EMAT has the potential to give more accurate time of flight (TOF) data and enhances the reliability and accuracy for residual stress measurement.

Analysis of the directivity of Longitudinal Critically Refracted (LCR) waves.

The use of ultrasonic longitudinal critically refracted (LCR) waves is one approach used for near surface material characterization. It has been shown to be sensitive to stress and, in general, less sensitive to the effects of the texture of the material. Although the LCR wave is increasingly widely applied, in experiments the factors that influence the formation of the LCR beam are seldom discussed. This paper reports a new numerical model used to investigate the transducers' parameters that can contribute to the directionality of the LCR wave and hence enable performance optimization when used for industrial applications. An orthogonal experimental method is used to study the sensitivity to the transducer parameters which influence the LCR wave beam characteristics. This method provides a design tool used to study and optimize multiple parameter experiments and it can identify which parameter or parameters are of most significance. The effects of incident angle, the aperture and the center frequency of the transducer were all studied. It is shown that the aperture of the transducer, the center frequency and the incident angle are the most important factors in controlling the directivity of the resulting LCR wave fields. The model was validated by comparision of data to those obtained with a finite element model. Experiments were also performed to confirm the numerical results. The model and experimental data provided improve understanding of the transducer selection and positioning in the optimization of LCR wave fields in experiments, which can be used to give signals which exhibit higher sensitivity for near-surface stress characterization.

Resonance analysis of a high temperature piezoelectric disc for sensitivity characterization.

Ultrasonic transducers for high temperature (200 °C+) applications are a key enabling technology for advanced nuclear power systems and in a range of chemical and petro-chemical industries. Design, fabrication and optimization of such transducers using piezoelectric materials remains a challenge. In this work, experimental data-based analysis is performed to investigate the fundamental causal factors for the resonance characteristics of a piezoelectric disc at elevated temperatures. The effect of all ten temperature-dependent piezoelectric constants (ε33, ε11, d33, d31, d15, s11, s12, s13, s33, s44) is studied numerically on both the radial and thickness mode resonances of a piezoelectric disc. A sensitivity index is defined to quantify the effect of each of the temperature-dependent coefficients on the resonance modes of the modified lead zirconium titanate disc. The temperature dependence of s33 showed highest sensitivity towards the thickness resonance mode followed by ε33, s11, s13, s12, d31, d33, s44, ε11, and d15 in the decreasing order of the sensitivity index. For radial resonance modes, the temperature dependence of ε33 showed highest sensitivity index followed by s11, s12 and d31 coefficient. This numerical study demonstrates that the magnitude of d33 is not the sole factor that affects the resonance characteristics of the piezoelectric disc at high temperatures. It appears that there exists a complex interplay between various temperature dependent piezoelectric coefficients that causes reduction in the thickness mode resonance frequencies which is found to be agreement in with the experimental data at an elevated temperature.

An estimate of biofilm properties using an acoustic microscope.

Noninvasive measurements over a biofilm, a three-dimensional (3-D) community of microorganisms immobilized at a substratum, were made using an acoustic microscope operating at frequencies up to 70 MHz. The microscope scanned a 2.5-mm by 2.5-mm region of a living biofilm having a nominal thickness of 100 microm. Spatial variation of surface heterogeneity, thickness, interior structure, and biomass were estimated. Thickness was estimated as the product of the speed of sound of the medium and the interim between the highest signal peak and that of the substratum plane without biofilm. The thickest portions of biofilm were 145 microm; however, slender structures attributed as streamers extended above, with one obtaining a 274-microm height above the substratum. Three-dimensional iso-contours of amplitude were used to estimate the internal structure of the biofilm. Backscatter amplitude was examined at five zones of increasing height from the substratum to examine biomass distribution. Ultrasound-based estimates of thickness were corroborated with optical microscopy. The experimental acoustic and optical systems, methods used to estimate biofilm properties, and potential applications for the resulting data are discussed.

Survey of advanced nuclear technologies for potential applications of sonoprocessing.

Ultrasonics has been used in many industrial applications for both sensing at low power and processing at higher power. Generally, the high power applications fall within the categories of liquid stream degassing, impurity separation, and sonochemical enhancement of chemical processes. Examples of such industrial applications include metal production, food processing, chemical production, and pharmaceutical production. There are many nuclear process streams that have similar physical and chemical processes to those applications listed above. These nuclear processes could potentially benefit from the use of high-power ultrasonics. There are also potential benefits to applying these techniques in advanced nuclear fuel cycle processes, and these benefits have not been fully investigated. Currently the dominant use of ultrasonic technology in the nuclear industry has been using low power ultrasonics for non-destructive testing/evaluation (NDT/NDE), where it is primarily used for inspections and for characterizing material degradation. Because there has been very little consideration given to how sonoprocessing can potentially improve efficiency and add value to important process streams throughout the nuclear fuel cycle, there are numerous opportunities for improvement in current and future nuclear technologies. In this paper, the relevant fundamental theory underlying sonoprocessing is highlighted, and some potential applications to advanced nuclear technologies throughout the nuclear fuel cycle are discussed.

Ultrasonic diffraction grating spectroscopy and characterization of fluids and slurries.

The ultrasonic diffraction grating is formed by machining triangular grooves, 300 microns apart, on a stainless steel surface. The grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m=1 beam in the liquid. The angle of this beam in the liquid increases with decreasing frequency and the critical frequency FCR occurs when the angle is 90 degrees. At frequencies below FCR, this m=1 wave does not exist and its energy is shared with other types of waves. The signal of the reflected m=0 wave is observed and an increase is observed at FCR. This information yields the velocity of sound in the liquid and particle size.

Measurement of the viscosity-density product using multiple reflections of ultrasonic shear horizontal waves.

We have developed an on-line computer-controlled sensor, based on ultrasound reflection measurements, to determine the product of the viscosity and density of a liquid or slurry for Newtonian fluids and the shear impedance of the liquid for non-Newtonian fluids. A 14 MHz shear wave transducer is bonded to one side of a 45-90 degrees fused silica wedge and the base is in contract with the liquid. Twenty-eight echoes were observed due to the multiple reflections of an ultrasonic shear horizontal (SH) wave within the wedge. The fast Fourier transform of each echo was obtained for a liquid and for water, which serves as the calibration fluid, and the reflection coefficient at the solid-liquid interface was obtained. Data were obtained for 11 sugar water solutions ranging in concentration from 10% to 66% by weight. The viscosity values are shown to be in good agreement with those obtained independently using a laboratory viscometer. The data acquisition time is 14s and this can be reduced by judicious selection of the echoes for determining the reflection coefficient. The measurement of the density results in a determination of the viscosity for Newtonian fluids or the shear wave velocity for non-Newtonian fluids. The sensor can be deployed for process control in a pipeline, with the base of the wedge as part of the pipeline wall, or immersed in a tank.