565 research outputs found
HighâSensitivity AirâCoupled Ultrasonic Imaging with the FirstâOrder Symmetric Lamb Mode at Zero Group Velocity
A new method for highâsensitivity nonâcontact, throughâtransmission, airâcoupled imaging of small material property changes or discontinuities in plates is demonstrated. Our approach exploits the excitation of the firstâorder symmetric Lamb wave mode at its minimum frequency point of zero group velocity. Because this Lamb wave resonance couples energy extremely efficiently with the air and does not propagate energy in the plane of the plate, it is the dominant mode of transmission of an airborne focussedâbeam broadband impulse through the plate. We take advantage of the sensitivity of this mode by performing Câscans at the frequency of the groupâvelocity zero to image spatial discontinuities and property changes. Our results show that images measured at this frequency are more sensitive and more consistent than those measured elsewhere in the plateâwave spectrum
High contrast air-coupled acoustic imaging with zero group velocity Lamb modes
The well known zero in the group velocity of the first-order symmetric (S1) plate wave mode has been exploited in air-coupled ultrasonic imaging to obtain significantly higher sensitivity than can be achieved in conventional air-coupled scanning. At the zero group velocity point at the frequency minimum of the S1mode, a broad range of wavenumbers couple into the first-order symmetric mode at nearly a constant frequency, greatly enhancing transmission at that frequency. Coupled energy remains localized near the coupling point because the group velocity is zero. We excite the mode with a broadband, focussing, air-coupled transducer at the frequency of the zero group velocity point in the S1 mode. By exploiting the efficient coupling at the zero group velocity frequency, we have easily imaged a single layer of Scotch tape attached to a 6.4-mm thick Plexiglas plate and 3.2-mm Teflon inserts in a composite laminate
Air-coupled acoustic imaging with zero-group-velocity Lamb modes
A Lamb wave resonance has been found that allows unusually efïŹcient transmission of airborne sound waves through plates. This occurs at the zero-group-velocity point at the frequency minimum of the ïŹrst-order symmetric (S1 ) Lamb mode. At this frequency, plane waves with a range of incident angles can couple between the air and the Lamb mode in the solid plate, dominating the spectrum of transmitted focused sound beams by 10 dB or more. We use this frequency for C-scan imaging, and demonstrate the detection of both a 3.2-mm-diameter buried ïŹaw and a subwavelength thickness changes of .005l ~1%!
Spherically focused capacitive-film, air-coupled ultrasonic transducer
A spherically focused no mirrors capacitive-film, air-coupled ultrasonic transducer, constructed using a spherically deformed backplate and metalized polymer film, has been designed, fabricated, and its performance characterized. A 1 cm diameter device has a center frequency of 805 kHz and a 6 dB bandwidth of 760 kHz. Comparisons of field strength in the focal zone with theoretical calculations for a spherically focused piston show that the device achieves diffraction-limited focusing. The nominal focal point of 25 mm lies within 0.01 mm of the calculated value for this device
Residual Bias Phenomenon in AirâCoupled Ultrasonic Capacitive Film Transducers
We discuss in this paper the underlying physics of a residual bias phenomenon, whereby the metalized Mylar films of airâcoupled film transducers accept and retain a residual electrostatic charge. Experimental measurements to demonstrate and quantify this effect are reported here, along with a hypothesis of the mechanism of charge transfer and embedding. The measurements show the amplitude performance of the capacitive film transducers as a function of applied bias voltage and frequency. Factors such as humidity and decay time also play roles in the acquisition and holding of charge on a film. We hypothesize that charge transfers from the conductive backplate and collects on the nonâmetalized side of the film. The charged films therefore are electrostatically attracted to the transducer backplate even with no applied voltage bias. Typically, an externally applied bias voltage is needed to charge the capacitor. With a persistent residual bias effect, these airâcoupled capacitive film transducers could be used like conventional piezoelectric transducers with no biasing required. This effect has substantial implications for the operation of airâcoupled film transducers
Towards Intelligent Databases
This article is a presentation of the objectives and techniques
of deductive databases. The deductive approach to databases aims at extending
with intensional definitions other database paradigms that describe
applications extensionaUy. We first show how constructive specifications can
be expressed with deduction rules, and how normative conditions can be defined
using integrity constraints. We outline the principles of bottom-up and
top-down query answering procedures and present the techniques used for
integrity checking. We then argue that it is often desirable to manage with
a database system not only database applications, but also specifications of
system components. We present such meta-level specifications and discuss
their advantages over conventional approaches
Leak detection using structure-borne noise
A method for detection and location of air leaks in a pressure vessel, such as a spacecraft, includes sensing structure-borne ultrasound waveforms associated with turbulence caused by a leak from a plurality of sensors and cross correlating the waveforms to determine existence and location of the leak. Different configurations of sensors and corresponding methods can be used. An apparatus for performing the methods is also provided
Wednesday Evening Session XIX âTown Meeting on a Working Group in NDEâ
The following record of the Wednesday evening problem session at th effort Magruder Inn, Williamsburg was transcribed from audio tapes made during the presentations and discussion
Air-coupled, focused ultrasonic dispersion spectrum reconstruction in plates
This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50â700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t â x domain to the v â k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Luciteâą, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory
The Influence of Thin Bonding Layers on the Leaky Waves at Liquid-Solid Interfaces
This paper presents theoretical and experimental results on the problem of bounded acoustic beam reflection at the Rayleigh angle from a fluid-solid interface which is loaded by a thin solid layer. The theoretical development exploits the framework of existing theory to yield a simple, analytic model which is reasonably accurate for thin layers. It is shown that the influence of the layer is contained entirely in the dispersive Rayleigh wavespeed and the thickness-dependent displacement parameter Îs. Measurements of the reflected acoustic field amplitude have been performed on several samples of stainless steel loaded with a thin copper layer. We have found reasonably good agreement between the theoretical model calculations and experimental measurements for ratios of the layer thickness to the Rayleigh wavelength as large as 0.3. Beyond this value, some disparity is observed, particularly in the calculation of the thickness-dependent Rayleigh wavespeed
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