Search

Capacitive devices that are able to efficiently generate and detect ultrasonic fields in air have already been designed and manufactured. They consist of metallized insulating films resting upon rigid conducting backplates. In order to increase efficiency, the backplates must be roughened, providing a resonant air cushion behind the thin insulating films. Unfortunately, roughening by conventional mechanical means [D. W. Schindel and D. A. Hutchins, Proc. IEEE Symp. 1, 301–304 (1991)] will be shown to produce a transducer whose beam profile is highly nonuniform above several hundred kilohertz. Ways of overcoming this are being investigated, including the manufacture of backplates through irradiation of metals by high‐intensity ultrasound and micromachining of silicon. The result of the characterization of such new transducers in terms of bandwidth, sensitivity and beam profiles will be given, along with a discussion of the versatility of air coupled transducers manufactured completely from integrated circu...

Miniaturized tuning forks, whose resonant frequencies are temperature dependent, have been manufactured from various materials using a precision milling machine under computer control. Two noncontact measurement systems that rely upon capacitive transducers as well as a pulsed ruby laser have allowed the generation and detection of fork vibrations at temperatures up to 900 °C. The usefulness of such a system for materials characterization will be demonstrated through the successful detection of the precipitation and dissociation of hydride platelets in a fork consisting of a Zr–Nb alloy. Also to be discussed are studies of single‐crystal silicon tuning forks and the resultant program currently underway for the development of micromachined thermometers for corrosive and high‐temperature environments.

The transient pressure fields of ultrasonic transducers may be described in terms of plane and edge wave contributions. Based on this concept, a technique is developed which allows characterization of transducers via a single measurement close to the transducer face. This method is examined experimentally for PVDF transducers.

A comparison is presented between the pressure fields of piezoelectric transducers measured experimentally, and those predicted theoretically at a single frequency. For PZT transducers, it will be shown that a good correlation results only when non-uniform vibrational characteristics across the transducer face are input to the theory. Conversely, PVDF transducers constructed by the authors seem to follow plane piston-type behaviour.

The directivity patterns of a wide range of electromagnetic acoustic transducers (EMATs) have been determined experimentally using a second, spiral coil EMAT as the detector around a cylindrical surface. A qualitative comparison to theory is presented, together with a discussion of the generation mechanisms involved. In addition, the use of a dipolar shear wave EMAT detector with a pulsed laser source is presented and some applications are discussed.

(1976). Learning Job Interview Techniques through Structured Group Interaction. Together: Vol. 1, No. 2, pp. 87-91.

A scale model study is presented of the performance of thin, perfectly reflecting semi‐infinite barriers in the presence of both asphalt and grass‐covered surfaces. The barrier insertion loss is shown to be strongly dependent on the type of ground on either side of the barrier. Observed behavior is explained by a study of the interference phenomena occurring in both the barrier’s presence and its absence.

The intensity fields of disk, bowl, and conical transducers have been evaluated theoretically throughout their nearfield regions at a single frequency. The results have been compared to the fields resulting from the square of the pressure (P2). It is shown that the two types of fields differ most markedly on axis and close to the transducer face. The direction of energy flow has also been analyzed, and it is demonstrated that parallel flow lines occur in regions where P2 is the closest to intensity.

An investigation is presented into the performance of parallel barrier configurations, using acoustical scale modeling. A realistic geometry is investigated, with the source being positioned over a paved roadway and the receiver over grass-covered ground. The grass-covered ground surface was properly modeled in terms of its impedance. Results were obtained for a range of barrier types, and demonstrate that frequency dependent effects are evident in barrier insertion loss data. In most cases, the barrier on the far side of the source did not significantly affect sound levels at the receiver. The most effective barrier design was found to be that of a gradual grass-covered slope up to an upright, thin barrier.

A design is presented for a capacitance transducer for use at metal surfaces. Provision is made for alignment of the electrode relative to the sample. An advantage over previous designs is the device's resistance to damage following arcing or accidental contact of the electrode to the metal surface. The device has been tested by use of standard acoustic sources, and found to perform satisfactorily over the required bandwidth.

A method is described by which transient ultrasonic Lamb waves may be generated and detected within thin materials, using laser techniques. The material is thin enough that only the two lowest‐order modes of Lamb waves need be considered. These are the a0 mode, which is dispersive in the thin plate limit, and the s0 mode, which is not. Signal processing techniques were used to determine the dispersion characteristics of the a0 mode, and the velocity of the s0 mode, in both aluminum and metallic glass samples. From these measurements, it was possible to estimate both the thickness of the samples and their elastic constants. The errors involved in such a measurement are discussed.

Studies of mode conversion phenomena, have been undertaken, using ultrasonic mode conversion techniques. The approach was first investigated using laser techniques to identify the likely mechanisms. This was followed by immersion experiments using either conventional piezoelectric transducers or pulsed laser generation. It will be shown that mode conversion of an incident longitudinal wave to either shear or Rayleigh waves at a surface defect may have practical application to materials evaluation.

A method for computing the transient radiation from a wide circular source, acting at a solid surface, is given for two different normal force apodizations. This method, based on a double Laplace–Hankel transform and a Cagniard inversion, allows an exact formulation of the impulse response to be obtained for two components of the displacement at any point inside the solid and is amenable to computing the field for an arbitrary time excitation. Numerical results are presented and are compared with experimental results for generation by a piezoelectric transducer.

Summary. Measurements using standard contacting piezoelectric transducers and non-contacting laser sources and detectors, have been investigated for the study of ultrasonic anisotropy in rocks. An ultrasonic polariscope has been constructed in order to obtain reproducible travel-time and amplitude nieasurements. Three case studies are described to demonstrate the apparatus. namely isotropic halite, anisotropic calcite and transversely anisotropic mudstune. A novel technique has been developed in order to construct pseudo-particle motion diagrams. to highlight shear-wave birefringence in rock samples using 2.25 MHz transducers. A pulsed laser has been used to generate compressional and shear waves for comparison with piezoelectric transducer results. The pulses generated by laser irradiation have many advantages for the study of velocity and attenuation anisotropy because of their known characteristics. broad bandwidth and high level of reproducibility. The use of a non-contacting laser source and detector eliminates the need for elaborate coupling agents. stress bonding or immersion techniques. Point-source and line-focusing of the laser beam provides ail indirect method of studying shear-wave polarization phenomena. Results from rotation of the line-focused laser beam and rotation of piezoelectric shear-wave transducers with respect to anisotropy, are compared for both velocity and amplitude phenomena in an anisotropic rock sample.

The various methods for non-contact ultrasonic inspection are reviewed. Electromagnetic, electrostatic and optical systems are described with comments on their application to testing situations outside the laboratory.

Longitudinal, shear, and surface acoustic pulses have been generated by irradiation of free metal surfaces with a Q‐switched Nd:YAG laser. Laser energies as low as 3 mJ are sufficient to generate ultrasonic pulses, in various metals, that are readily detected by conventional piezoelectric transducers, without the need for laser focusing. The generation efficiency of longitudinal, shear, and Rayleigh modes has been studied in both the presence and absence of a plasma. Experimental data in graphical form highlight several features of the acoustic source, from which a qualitative model is proposed.

The mechanisms by which ultrasonic transients may be generated by pulsed laser irradiation are reviewed, with particular reference to the wave forms resulting in both fluid and solid media. Specific examples of experimental results are presented that demonstrate that a pulsed ultraviolet source is a convenient means for ultrasonic generation in both liquids and solids.

The radiated pressure fields of axisymmetric ultrasonic transducers have been measured experimentally at a single frequency to a high resolution. Results in both the nearfield and farfield have been compared to the prediction of theory in three dimensions, developed to examine transducers vibrating with an arbitrary amplitude and phase distribution across their front face. Good correlation was obtained for polyvinylidene fluoride (PVDF) radiators, assuming plane‐piston‐type behavior. For lead zirconate titanate (PZT) transducers, marked amplitude variations with radius across the disk face were encountered, and these needed to be modeled theoretically to obtain good agreement with experiment. Other predictions of theory which are of interest to applications involving acoustical imaging are presented.

Directivity patterns of laser‐generated longitudinal and shear waves in aluminum have been determined experimentally, for laser power densities both above and below the threshold for plasma formation on the target’s surface. Comparison of experimental results with theory has allowed generation mechanisms to be identified. Thermoelastic mechanisms predominate in the absence of a plasma, where stresses parallel to the surface are a dominant source characteristic. At higher power densities, momentum transfer from ablating material becomes more important, leading to a source with normal drive components.

Experiments have been undertaken into the generation and detection of ultrasonic transients in metals over a 40-MHz bandwidth, using pulsed-laser generation and interferometric detection. Longitudinal, shear, and surface waves have been investigated, and their form compared favorably to theory. The techique has been used to study the interaction of surface and bulk waves with slots in the surface of a plate. It has been shown that significant mode conversion occurred from surface to bulk waves and vice versa. The significance of this result to nondestructive testing is discussed.

Modeling experiments are reported which have investigated the frequency dependence of barrier insertion loss for various noise barrier designs. The effect of ground surfaces has been studied, treating both grass‐covered ground and asphalt. Results show that interference effects are an important feature of observed behavior. Use is made of this knowledge in the design and testing of new barrier designs, which deliberately introduce a beneficial destructive interference phenomenon to increase insertion losses over well‐defined ranges of frequency.

Geotomographic imaging is a technique which allows seismic waves to be used to gain information about the internal structure of rock masses, in a way conceptually similar to medical CAT scanning. Traditional approaches to the study of mining-induced seismic phenomena have concentrated on using passive monitoring methods. This paper gives an overview of the developments in the acquisition, processing and interpretation of geotomographic data and outlines how images can be used in conjunction with passive techniques to study mining-induced seismicity.

The generation of acoustic waves in metals by pulsed laser irradiation over a wide range of material conditions has been studied. Capacitance transducers have been used to obtain quantitative measurements of the amplitude of bulk acoustic waveforms where the laser beam was directed onto free metal surfaces in the presence and absence of surface plasmas, and onto modified metal surfaces. The application of acoustic wave propagation theory has allowed theoretical waveforms to be determined. By combining data for thermoelastic and normal force sources, waveforms have been produced that follow closely those measured experimentally.

A study has been made of the ultrasonic pulses generated at modified metal surfaces by laser irradiation. Both longitudinal and shear acoustic modes have been investigated, generation at modified and free surfaces being compared. The use of a Q -switched Nd: YAG laser enabled experiments to be carried out at optical power densities above and below the threshold for plasma formation. Qualitative models of the acoustic sources existing in each case are proposed, which explain the experimental effects of surface modification; these include the observed changes in generation efficiency and longitudinal directivity.

A Q-switched Nd:YAG laser has been used to generate reproducible acoustic displacements in metallic samples. The acoustic waveforms were initially detected by an absolute displacement-sensitive, capacitance transducer. The capacitance transducer was then replaced in turn by two piezoelectric ceramic transducers of different thickness and diameter. Comparison of waveforms from the two types of detector indicate that thick piezoelectric transducers are useful in the detection of fast rise-time displacement waveforms at metal surfaces. Their performance is optimized by making them as thick as possible, with a small area of contact with the surface.

A theoretical investigation is presented which examines the effect of introducing a thin layer into a previously established ultrasonic field, present within a liquid. The wave system induced within the layer is investigated and the distrubance to the field estimated. Various conclusions derived from the theory are discussed in relation to relevant ultrasonic visualization techniques, which ultilize thin layers.

The radiated pressure fields of pulsed polyvinylindene difluoride (PVDF) transducers have been determined experimentally, and compared to theory in three dimensions. The tranducers were in the form of disks, cones, or bowls, excited with a range of known transients in the form of gated sinusoidal voltages. It is demonstrated that good agreement is possible between theory and experiment, and it is shown that more marked interference effects occur as the duration of excitation tone burst is increased. It is also shown that PVDF transducers may be manufactured so as to behave as plane piston radiators.

A method for computing the radiation from a thermal expansion source, generated at a solid surface by a laser pulse of finite width, is evaluated for two radial beam energy distributions. A theoretical expression for the impulse response is obtained for two components of the displacement at any point within the solid, and may be used to compute the field for an arbitrary time excitation. Numerical results are presented, and are compared to experimental results for generation by a pulsed laser.

Quantitative experimental measurements have been made in the study of thermoelastic generation of elastic waves in a metal by unfocused laser radiation. A calibrated wide‐band detection system, incorporating a capacitance transducer, has enabled acoustic waveforms to be recorded with a minimum of distortion. From these measurements, a theoretical model has been developed. The transfer function of the metal block has been deconvoluted to give the acoustic source function, which was modeled as a rapidly expanding point volume of material. The thermoelastic source generated longitudinal (L) and (S) waves, but the latter predominated at the epicenter, where, in experiments presented here, both wave amplitudes L and S were proportional to the total absorbed energy in the laser pulse.

Rockbursts and mining-induced seismic events have serious socio-economic consequences for the Canadian mining industry, as their mines are extended to greater depths. Automatic multichannel monitoring systems (Electro-Lab MP250s) are routinely, used to detect the arrival times of seismic waves radiated by mining-induced events and sensed on an array of single component transducers installed throughout a mine. These arrival times are then used to locate the events and produce maps of areas of high activity for use in mine planning and design. This approach has limitations in that, it does not allow a detailed analysis of source mechanisms, which could be extracted if whole waveform signals are recorded and analyzed.

A technique is presented for the optical display of three-dimensional ultrasonic fields, involving the visualization of thin sections of the field within a layer of suitable photoelastic solid, such as polyurethane rubber. A valid display is produced with a negligible disturbance to the ultrasonic field. The technique has a low sensitivity, but has been successfully applied to the visualization of 200 kHz, continuous-wave fields in water.

Transducers have been designed and tested based on a piezoelectric element that is 25 mm thick and conical in cross section. These devices do not require any backing, have low levels of radial mode vibration, and present a small area of contact to the surface of a solid. Theory and experiment are compared, and reasonable results are obtained.

A pulsed laser ultrasonic source has been used, in conjunction with finite-sized electromagnetic-acoustic transducers as detectors, to produce wide bandwidth waveforms in metals. These have been compared successfully to theory, and it has been shown that waveforms may also be obtained at elevated temperatures.

The polarization features of ultrasonic shear waves from a pulsed laser line source have been investigated, using detection by an electromagnetic acoustic transducer. The result is a means for undertaking polarized shear wave experiments over a wide bandwidth.

On presente une methode et un appareil a ultrasons permettant de controler l'epaisseur lors du coulage de la barbotine

Absolute acoustic waveforms, generated by laser irradiation of aluminium and mild steel samples, have been detected at epicenter with a wide‐band capacitance transducer. Comparison has been made with theoretical waveforms, utilizing theory for wave propagation within a plate and assuming certain source characteristics. Results have shown that laser generation is able to produce three types of standard acoustic source, namely a horizontal force dipole and a normal force monopole, each with step‐function time dependence, and a normal force monopole with δ‐function time dependence. Underlying generation mechanisms are discussed.

An acoustic source formed by pulsed laser irradiation has been used to investigate the piezoelectric and acoustoelectric effects within a cadmium sulphide detector. Analysis of detected waveforms has enabled the two effects to be identified. The resulting transducer combination has application to imaging systems, where phase insensitivity may be required.

Counselors in educational settings are usually not free to select the kinds of clients and problems with which they may work most effectively. Therefore, counselors must select counseling strategies that maximize the probability of helping all clients achieve personal goals. These strategies may focus primarily on changing how clients think, feel, or act (T-F-A). Combinations of thinking, feeling, and acting strategies are illustrated along with relationships to different counseling theories. Effective counselors need to systematically use varying combinations of T-F-A strategies that will be of maximum value in helping individual clients to change their behavior.

The use of a pulsed laser to generate well characterised acoustic sources has certain advantages over traditional methods in that it is reproducible and results in a wide bandwidth. Further, it is not necessary to couple the source transducer to the sample by immersion techniques or under pressure, as is frequently the case when the attenuation properties of porous solids are studied. An investigation into the propagation of ultrasonic transients within porous solids using the pulsed laser technique is described. The mechanisms leading to ultrasonic generation by irradiation of solids with pulsed lasers is reviewed in Section 1.1, followed by a brief outline of elastic wave propagation through porous media. A description of the apparatus used is given in section 2 and the results obtained by laser irradiation are compared to those obtained using PZT transducers in section 3.

Illumination of a specimen by a pulsed focused laser is known to induce elastic waves, and models are available for both cases of thermoelastic generation and liquid evaporation. In this article, the inverse problem of recovering the elastic constants of a specimen from experimental waveforms recorded on‐ and off‐epicenter is considered. An algorithm, based on nonlinear least‐squares fitting, is presented and has been used on experimental displacement signals recorded on an aluminum plate with a Michelson interferometer. The choice of the receiver position is discussed and experimental results are given.

An investigation is presented into the near-field radiation patterns of disc transducers whose amplitude of vibration across their front face varies with radius. Selected electrode configurations are examined, and theoretical predictions are presented of field patterns following the application of various voltage ratios to the electrodes.

The transient pressure fields of ideal planar ultrasonic transducers may be described in terms of plane and edge wave contributions. Based on this concept, a technique is developed which allows characterization of wideband unfocused transducers by a single measurement close to the transducer face. This method is examined experimentally for both polyvinylidene difluoride and commercial PZT transducers. Also investigated is the effective radiated frequency of these devices. >

The interaction of surface (Rayleigh) waves with simulated surface defects has been studied, using pulsed laser generation and interferometric detection. It is shown that mode conversion from Rayleigh to both longitudinal and shear waves at defect corners results, and that these phenomena can be used to estimate the depth of a surfacebreaking defect in the form of a rectangular slot.

The transient pressure fields of ultrasonic transducers may be described in terms of plane and edge wave contributions. Based on this concept, a technique is developed which allows characterization of transducers via a single measurement close to the transducer face. This method is examined experimentally for PVDF transducers.

A Q-switched Nd:YAG laser has been used by the authors to generate elastic waves at metal surfaces. As a result of this work, various applications have been highlighted, and are described. These include use as a remote method of acoustic generation for nondestructive evaluation, as a standard acoustic source with excellent repeatibility, and as a method for liquid depth estimation.