Phased array approach to generate guided waves and extract phase velocities using the V(z) method.

A novel method using only the easily measured velocity of SAWs (surface acoustic waves) is demonstrated for determination of elastic constants of anisotropic layers on arbitrarily anisotropic and piezoelectric substrates. The elastic constants and propagation properties of any nonpiezoelectric isotropic, cubic, or even trigonal layer on an arbitrarily anisotropic substrate can be determined with this method. The method was applied to proton-exchanged (PE) lithium niobate in order to determine its as yet unknown elastic constants. Dispersion curves and elastic stiffness constants of PE Y-cut LiNbO/sub 3/ have been evaluated. The results are reliable for waveguides fabricated in diluted melts containing 0.25-2.00 mol.% lithium benzoate. Using the obtained elastic-stiffness constants, SAW propagation properties such as power flow, diffraction, and strain distribution can be analyzed in order to achieve sophisticated design of guided SAW and acoustooptic devices based on PE:LiNbO/sub 3/ waveguides. >

Damage localization in aluminum plate with compact rectangular phased piezoelectric transducer array

The acoustic hologram has made it possible to shape ultrasound waves and project complex pressure patterns in 2D and in 3D, as we have also recently shown. The hologram contains tens of thousands of elements which can be used to form sophisticated pressure patterns by setting the phase and/or amplitude across the wavefront of a sound wave. It thereby offers more degrees of freedom than conventional phased array transducers (PATs). However, the hologram is generally static, whereas PATs are dynamic. PATs do not only have fewer elements, but they are also more difficult to fabricate and require complex electronics. In this talk, I will describe our efforts to combine the best of both worlds. For this, we are exploring the use of light waves as a means to control ultrasound waves. I will describe direct and indirect approaches that can be used to control and modulate holographic ultrasound fields.

A series of ultrasonic elements arranged in a phased array transducer can provide the capability to activate each element separately but in a programmed sequence. This will help the acoustic signal to be generated at desired focusing distances and anticipated angles for specific materials and structures. In case of composite material inspection, this characteristic of the phased array method can improve the undesirable effects of the high attenuation and anisotropic structure of composite materials on response signals. In this study different phased array probes and wedges which are commercially available were evaluated for their response signals’ characteristics. First, the capability and resolution of bulk wave generation were studied for each set of probe and wedge, and the response signals were compared to that of the conventional single element ultrasonic transducers for different thicknesses composite plates. Then the resolution of the response signals and their sensitivity to defect size were evaluated and compared to the single element transducers as well. Next, each phased array probe and wedge set was used to generate plate waves in aluminum plates based on plate wave propagation theory, probe and wedge physical properties and the definition of delay law. Results show a general improvement in response signals’ strength and resolution for phased array method in comparison to the single element transducers. Also some plate wave modes could be generated with optimized signal generation parameters in phased array system.

Present state of the art sizing measurements of defects by ultrasonic techniques has been inadequate, especially in support of a quantitative fracture mechanical analysis of the strength of a material. Present capabilities of defect sizing in some cases do not allow an accept/reject decision to be made. Currently, ultrasonic imaging systems including phased array, synthetic aperture, and wavefront reconstruction systems are being developed to quantitatively measure defect sizes. Also, in the medical field phased array systems have been developed for imaging purposes. Ibis work describes an annular phased array system which is being developed for general NDE applications on metal and ceramic materials. The main usefulness of phased array techniques is the ability of electronically changing the focus of the acoustic beam radiating from a phased array transducer or reflecting from a defect in a solid material.

Previous work has led to the design, simulation, and development of a linear phased array transducer. The intention of the array is to be used as a non-destructive ultrasonic device to monitor and evaluate the health of a given specimen. The phased array has been manufactured and tested for the detection and characterization of defects on a target. The array was fabricated with a four-row stepped design with four wires to transfer data and one wire for grounding. The stepped design allows for the interrogation of a larger region using time delays and beam sweeping without the use of additional electrical channels. The array was designed to be utilized in a water immersion environment with about one inch between the array and the target specimen. An OmniScan MX system was used to operate the phased array and perform real-time linear and sectorial scans on a set of rectangular plates. S-scans allow for beam sweeping over an angle range as well as adjustments for time delays and a true-depth display. The array was operated with sixteen active elements and an angle range of 0 to 30 degrees. The phased array was tested with a variety of targets and was used to investigate and characterize different types of defects such as cracking, warping, and corrosion. The ability of the phased array to distinguish between defect types as well as resolve defect size was evaluated.

This paper describes the application of Impulse Stimulated Thermal Scattering (ISTS) for measurement of surface acoustic wave (SAW) velocity and thermal diffusion along a free surface of a strongly anisotropic material. The motivation for this work stems from the study of thermoelastic properties of individual phases of ferroelastics; experimental results were obtained on a single crystal in the austenitic phase a Cu-Al-Ni alloy (bcc single crystal having elastic anisotropy factor of about 12). The measured SAW velocities in specific directions are in a good agreement with the values calculated for the elastic constants obtained by other ultrasonic methods. Similarly, the evaluated thermal diffusivity coefficient (22 ± 2).10−6 m2/s of the austenite is consistent with the data in the literature. The proposed approach has also a potential for characterization of thin films grown on anisotropic substrates.

As phased array ultrasonic transducers become more wide spread in underwater acoustic measurement applications, a better understanding of their basic performance characteristic is of benefit. The beam patterns of a phased array can be approximated by a steady state analysis. This approach indicates the directional response of an array in response to a sinusoidal excitation. However, a steady state examination does not properly describe transient behavior of the transmit waveform and also the received signal that is subsequently beamformed. We further investigate the transient performance of the beamformer using a closed-form solution to the acoustic wave equation for a multielement ultrasonic phased array transducer. The model produces transducer beam patterns and transient beamformer output for a series of reflective targets in the far-field of the transducer. The results show a close agreement with laboratory measurements and provide a tool for further investigation of phased array transducer design and performance.

A novel automatic ultrasonic system used for the inspection of welds in T nodes is developed, in which a linear phased array transducer using electronic scan is adopted. This paper introduces the testing method of welds in T nodes, and the structure of the whole testing system. Optimization of array transducer has been also pursued based on a mathematical model of acoustic field for linear phased array. The experimental results have proved that the phased array transducer system had the same detectability as that of conventional ultrasonic transducer system, but the component part of the system can be simplified greatly, and the testing flexibility and the testing speed can be improved greatly. It provides an excellent base for the further research and development of the defect inspection of welds in TKY nodes.

We adapt the angular spectrum of plane waves (ASPW) decomposition to numerical simulations of the diffraction of surface-acoustic waves (SAWs) on anisotropic model substrate, such as YZ lithium niobate. We utilize the thin-element decomposition (TED) method, appropriately modified for an anisotropic substrate; we also introduce a novel "average-wavenumber" variation of this scheme; these are numerically found to be mutually consistent. We apply the TED method to simulate wave propagation both in infinite periodic structures of metallized gratings and also in finite gratings. We demonstrate that the ASPW provides a convenient and numerically fast tool for precise diffraction calculations in practical SAW devices which also display structural variations in the lateral direction, perpendicular to the principal direction of wave propagation. Additionally, we compare the present TED method to waveguide theory in an analysis of the role of SAW reflections from the electrodes.

An adaptive dynamic focusing system used for the ultrasonic nondestructive testing of pipeline girth welds is developed. This system relies on ultrasonic linear phased array transducers connected to a multichannel acquisition system, which provides delay and amplitude laws, allowing one to control the ultrasonic beams dynamically. We introduce a testing method for welds and the system’s structure. Optimization of array transducers has also been pursued based on a mathematical model of the acoustic field for a linear phased array. The experimental results have proved that the phased array transducers system has the same capability to detect as a conventional ultrasonic transducers system, but the components of the system can be simplified greatly, and the testing flexibility and the testing speed can be improved greatly. It provides an excellent basis for further research and development of defect inspection of pipeline girth welds.

Performance of Two Dimensional Displacement and Strain Estimation Techniques Using a Phased Array Transducer

The high frequency, wideband acousto-optic light deflector is an excellent candidate in many EW systems such as radar warning receivers and communications signal processing. In order to provide such needs, broad bandwidth acousto-optic Bragg cells for the gigahertz frequency range are developed. To track Bragg angle for broadband operation, a phased array design is employed and therefore its performance needs to be well understood, especially at microwave frequencies. In this paper we propose a new acoustic phased array transducer model. A microwave model for conversion of an RF signal to an acoustic beam is presented. The model is based on calculating the phased array transducer interdigital capacitance from coupled transmission lines theory. We also discuss the impact of other transmission line effects at electromagnetic wavelengths. The microwave phased array transducer model is then used to predict the behavior of the electrode voltages and currents and of the overall capacitance.

Peculiarities of anisotropic light diffraction in a periodic acoustic field excited by a phased array transducer with the phase shift by π are studied theoretically. Possibilities of this system application for control of unpolarized optical radiation are discussed.

The authors present an approximate analysis of guided waves in a SAW (surface acoustic wave) Delta nu / nu waveguide with general anisotropy. The proposed analysis is performed under the assumption that fields in the guiding and surrounding regions can be expressed in terms of a linear combination of variable-crested surface wave solutions for the unbounded metallized or free surface having a common wave number in the propagation direction. Specifically, the expression is developed for an eigenvalue equation of the guided wave solution based on the scattering analysis of the variable-crested waves at the metal/free surfaces. Numerical results on dispersion curves and field distributions of the guided on various anisotropic substrates are shown. A number of peculiar properties due to the anisotropy are demonstrated. The results of the accurate analysis based on the integral equation method are also shown, to verify the effectiveness of the proposed analysis. >