Line-focusing electromagnetic acoustic transducers for the detection of slit defects

This paper describes the design principles of a line-focusing electromagnetic acoustic transducer (LF-EMAT) and the results of a feasibility test for detecting slit-type defects in metals. The LF-EMAT excites shear vertical (SV) elastic waves and focuses them to a line in a metal body. It consists of a permanent magnet block and a meanderline coil, whose spacing is continuously varied so that the excited SV waves become coherent on a focal line after traveling oblique paths. The measured directivity of generation and reception show a sharp peak at the designed focal line. The LF-EMATs are then applied to detecting slit defects in the bottom surface of steel blocks, on which the focal lines are located. Portions of the scattered defect signals are received by the same EMAT. When operated at 4 MHz, the LF-EMATs are capable of detecting slits deeper than 0.05 mm. The sensitivity decreases with liftoff and the LF-EMATs are usable with liftoff up to 0.6 mm.     

The excitation of Lamb waves in pipes using dry-coupled piezoelectric transducers

The development of a dry-coupled piezoelectric transducer system for the detection of corrosion in chemical plant pipework using cylindrical Lamb waves is described. It is shown that the axisymmetricL(0,2) mode at a frequency of about 70 kHz is an attractive mode to use for longdistance propagation. The results show that a ring of piezoelectric length-expander elements can be used to excite theL(0,2) mode and to suppress all the nonaxisymmetric modes. Tests have been carried out both with the piezoelectric elements bonded to the pipe and fabricated into a simple transducer which was clamped against the pipe. The performance of the dry-coupled system was very similar to that of the bonded elements. In pulse echo tests, the noise floor obtained with the dry-coupled system was less than 1% of the amplitude of the propagatingL(0,2) mode. The drycoupled transducers provide a simple, light, readily detachable system for the long-range inspection of pipework.     

Minimizing influence of multi-modes and dispersion of electromagnetic ultrasonic lamb waves

Electromagnetic ultrasonic (EMU) Lamb waves excited by electromagnetic acoustic transducers (EMATs) possess many advantages in NDT. However, their characteristic multi-modes and dispersion are disadvantageous for inspection and restrict further improvements in their real applications. By deducing the excitation equation of EMU Lamb waves, the primary design parameters of EMATs and the characteristic equation of Lamb waves are combined, and excitation curves based on the excitation equation are plotted to aid the design of EMATs. The excitation characteristic of EMU Lamb waves on different thickness of plates is analyzed according to the excitation curves. The influence of multi-modes of EMU Lamb waves is minimized by choosing reasonable operating points and operating zones to excite a single-mode Lamb wave or multi-mode Lamb waves with identical or approximate propagation velocities. The influence of dispersion is minimized by searching corresponding points whose slope of group velocity tends to zero. The validity of the proposed method is verified by experiments.     

The velocity and attenuation of outgoing surface acoustic waves measured using an ultrasonic microscope with two focusing transducers

A new method for measuring the local parameters of outgoing surface acoustic waves (OSAWs) is proposed. The technique employs a system of two focusing transducers, with the output signal taken from one of these scanning over the sample surface. In comparison with the conventional method with a single transducer moving in the direction normal to the sample surface, the proposed technique is capable of conducting measurements in a greater range of OSAW velocities and is less sensitive to parameters of the immersion liquid. The new method was experimentally verified by measuring systems with known properties.     

Generation and detection of guided waves using PZT wafer transducers

We report here the use of finite element simulation and experiments to further explore the operation of the wafer transducer. We have separately modeled the emission and detection processes. In particular, we have calculated the wave velocities and the received voltage signals due to A0 and S0 modes at an output transducer as a function of pulse center frequency. These calculations include the effects of finite pulse width, pulse dispersion, and the detailed interaction between the piezoelectric element and the transmitting medium. We show that the received signals for A0 and S0 modes have maxima near the frequencies predicted from the previously published point-force model.     

Acousto-optic cell based on paratellurite crystal with surface excitation of acoustic waves

An acousto-optic cell based on a paratellurite (TeO₂) crystal, in which bulk acoustic waves are excited directly from the surface due to an intrinsic piezoelectric effect in the material, has been studied. The bulk shear acoustic waves with a frequency of 50 MHz propagate along the [001] and [110] axes with a polarization along the [ [`1]10 ]left[ {bar 110} right] axis. The ultrasound has been excited by a simple system of two electrodes formed on one face of the crystal. Characteristics of the acousto-optic cell have been determined and the parameters of acoustic waves have been measured at 633 nm by optical beam diffraction on the acoustic diffraction grating.     

Warped basis pursuit for damage detection using lamb waves

This paper presents a novel time-frequency procedure based on the warped frequency transform (WFT) to process multi-mode and dispersive Lamb waves for structural health monitoring (SHM) applications. The proposed signal processing technique is applied to time waveforms recorded at an array of scan points after waveguide excitation. The WFT is combined with a basis pursuit algorithm to extract the distance traveled by the ultrasonic waves even in the case of multi-modal dispersive propagation associated with broadband excitation of the waveguide. This is obtained through a decomposition of the acquired signals using dictionaries composed by optimized atomic functions which are designed to match the spectro-temporal structure of the various propagating modes. The warped basis pursuit (W-BP) analysis of several acquired waveforms results in distance signals that can be combined through classical beamforming techniques for acoustical source imaging purposes. A masking procedure is also proposed to suppress imaging noise. This approach is tested on experimental data obtained by broadband guided wave excitation in a 1-mm-thick aluminum plate with an artificially introduced through crack and tiny holes, followed by multiple waveguide displacement recording through a scanning laser Doppler vibrometer. Dispersion compensation, high-resolution source, and defect imaging are demonstrated even in domain regions that are not directly accessible for measurement.     

AFM observation of surface acoustic waves emitted from single symmetric SAW transducers

We report the first experimental observation of surface acoustic waves (SAWs) launched from a single symmetric SAW transducer, employing scanning acoustic force microscopy (SAFM). SAFM is a simple technique for the imaging of complex interdigital transducer (IDT) radiation patterns with nanometer lateral resolution. We demonstrate submicron lateral resolution and high sensitivity by investigating a single excitation element on a weakly coupling substrate (GaAs), visualizing the launched wave and second-order effects     

Quantitative modeling of the transduction of electromagnetic acoustic transducers operating on ferromagnetic media

The noncontact nature of electromagnetic acoustic transducers (EMATs) offers a series of advantages over traditional piezoelectric transducers, but these features are counter-balanced by their relatively low signal-to-noise ratio and their strong dependence on material properties such as electric conductivity, magnetic permeability, and magnetostriction. The implication is that full exploitation of EMATs needs detailed modeling of their operation. A finite element model, accounting for the main transduction mechanisms, has been developed to allow the optimization of the transducers. Magnetostriction is included and described through an analogy with piezoelectricity. The model is used to predict the performance of a simple EMAT: a single current-carrying wire, parallel to a bias magnetic field generating shear horizontal waves in a nickel plate close to it. The results are validated against experiments. The model is able to successfully predict the wave amplitude dependence on significant parameters: the static bias field, the driving current amplitude, and the excitation frequency. The comparison does not employ any arbitrary adjustable parameter; for the first time an absolute validation of a magnetostrictive EMAT model has been achieved. The results are satisfactory: the discrepancy between the numerical predictions and the measured values of wave amplitude per unit current is less than 20% over a 200 kHz frequency range. The study has also shown that magnetostrictive EMAT sensitivity is not only a function of the magnetostrictive properties, because the magnetic permeability also plays a significant role in the transduction mechanism, partly counterbalancing the magnetostrictive effects.     

Transmitting electric energy through a closed elastic wall by acoustic waves and piezoelectric transducers

Transmission of electric energy through a closed elastic wall by piezoelectric transducers and acoustic waves is studied based on the linear theory of piezoelectricity and elasticity. A theoretical analysis is performed. For the structure and motion considered, the 3-D equations of linear piezoelectricity reduce to a 1-D mathematical problem. An exact solution is obtained. Transmitted voltage, current, power, efficiency and stress distribution are obtained. Their dependence on various parameters is examined. The model and results of this paper are closer to real situations compared with those in a previous analysis.     

Functionally graded piezoelectric materials for modal transducers for exciting bulk and surface acoustic waves

We show that functionally graded piezoelectric materials can be used to make modal actuators through theoretical analyses of the excitation of extensional motion in an elastic rod and Rayleigh surface waves over an elastic half-plane. The results suggest alternatives with certain advantages for the excitation of bulk and surface acoustic waves.     

Transmitting electric energy through a metal wall by acoustic waves using piezoelectric transducers

The feasibility of transmitting electric energy through a metal wall by propagating acoustic waves using piezoelectric transducers is examined by studying the efficiency of power transmission and its dependence upon the relevant system parameters for a simplified system consisting of an elastic plate sandwiched by two piezoelectric layers. One of these layers models the driving transducer for generating acoustic wave, and the other layer models the receiving transducer for converting the acoustic energy into electric energy to power a load circuit. The output voltage, the output power, and the efficiency of this system are expressed as explicit functions of the system parameters. A numerical example is included to illustrate the dependence of the system performance upon the physical and geometrical parameters.     

A diffraction-based optical method for the detection of in-plane motion of lamb waves

This paper describes a laser optical technique that allows the detection of in-plane motion of Lamb waves. This interference-based laser optical technique includes a tiny square indentation with a width of about 30 micron on the sample surface and a relatively simple optical arrangement. The current technique is applied for the detection of in-plane motions of Lamb waves propagating in a 70-micron thick brass plate. Measurement of So mode dominated by in-plane motion in the low fd (frequency times thickness) regime is successfully demonstrated with the current technique. With the indentation replaced by a microreflector in a microelectromechanical (MEMS) structure, this technique is applicable for the detection of in-plane motion in MEMS structures.     

Mode-selective excitation and detection of ultrasonic guided waves for delamination detection in laminated aluminum plates

Selective modes of guided Lamb waves are generated in a laminated aluminum plate for damage detection using a broadband piezoelectric transducer structured with a rigid electrode. Appropriate excitation frequencies and modes for inspection are selected from theoretical and experimental dispersion curves. Dispersion curves are obtained experimentally by short time Fourier transform of the transient signals. Sensitivity of antisymmetric and symmetric modes for delamination detection are investigated. The antisymmetric mode is found to be more reliable for delamination detection. Unlike other studies, in which the attenuation of the propagating waves is related to the extent of the internal damage, in this investigation, the changes in the time-of-flight (TOF) of guided Lamb waves are related to the damage progression. The mode conversion phenomenon of Lamb waves during progressive delamination is investigated. Close matching between the theoretical and experimentally derived dispersion curves and TOF assures the reliability of the results presented here.     

Adaptive optical array receivers for detection of surface acoustic waves

Adaptive interferometric detection systems based on two-wave mixing in photorefractive crystals have been configured as distributed optical receivers. The spatial distribution of the detection laser power on the sample surface is controlled by use of phase gratings and amplitude masks. The responses of point, line, array, and chirped optical receivers to propagating surface acoustic waves (SAW's) are discussed theoretically and demonstrated experimentally. It is shown that by use of different object beam footprints it is possible to configure adaptive holographic SAW receivers that are either broadband or narrow band and that are preferentially sensitive to SAW's propagating in given directions. The receivers also allow for the distribution of laser power over the sample, eliminating the excessive heating or surface damage that can occur in some materials when high power, point-focused, detection lasers are used.     

Selective excitation of eigenmodes in a multilayer thin film resonator on bulk acoustic waves

We consider a method of control over the operating frequency of a resonator on bulk acoustic waves, which is based on the selective excitation of eigenmodes. The frequency switching is achieved by using several layers of a ferroelectric in the paraelectric state and applying a control voltage of appropriate magnitude and polarity to each layer. The principle of selectivity is formulated and the criterion function is defined, which ensure the most effective excitation of a selected eigenmode with the possible suppression of parasitic modes. An example of using this function for a resonator switched between four eigenmodes is presented.