TPSAS-NF1676L-17553-DND
Investigate the potential of using an efficient damage imaging technique for damage identification by using a commercial finite element code to simulate the Lamb wave propagation in stiffened composite structures.
J T Wang↗
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Application of the Pseudo Wigner-Ville Distribution to the Measurement of the Dispersion of Lamb Modes in Graphite/Epoxy Plates
Acoustic waves propagate in thin plates as guided or Lamb modes. The velocities of these modes are dispersive in that they depend not only on the material elastic properties and density, but also on the frequency. Accurate characterization of Lamb wave dispersion is important in many acoustic based nondestructive evaluation techniques. It is necessary for ultrasonic measurements in thin plates to determine elastic properties and for flaw detection and localization. In acoustic emission (AE) testing, if not taken into account, highly dispersive Lamb mode propagation can lead to large errors in source location. In this study, the pseudo Wigner-Ville distribution (PWVD) was used for measurement of group velocity dispersion of Lamb waves in a unidirectional graphite/epoxy (AS4/3502) laminate. The PWVD is one of a number of transforms which provide a time-frequency representation of a digitized time series. Broad band acoustic waves were generated by a pencil lead fracture (Hsu-Neilsen source) and were detected with broad band ultrasonic transducers. The arrival times for the lowest order symmetric (S(sub 0)) and antisymmetric (A(sub 0)) Lamb modes were determined from measurements of the time at which the respective peak amplitudes occurred in the PWVD. Measurements were made at several source-to-detector distances and a least squares fit used to calculate the velocity. Results are presented for propagation along, and perpendicular to, the fiber direction. Theoretical dispersion curves were also calculated and a comparison between theory and experiment demonstrates good agreement.
Prosser, W. H.↗
Low Frequency Guided Plate Waves Propagation in Fiber Reinforced Composites
Composite materials are used in aerospace, automotive, and underwater applications.
Composite materials elastic properties batch varia↗
Progress and Challenges to NDE of Composites Using Obliquely Insonified Ultrasonic Waves
The use of obliquely insonified ultrasonic waves has shown great promise in providing information about flaw characteristics and material properties of composite materials.
Leaky↗
Lamb shift in
Lamb shift in lithium isotope measured by determining lifetime from photon emission and employing analog formula of Bethe and Salpeter
PHOTON DENSITY↗
Measuring Thickness and Elastic Properties of Electroactive Thin-Film Polymers Using Platewave Dispersion Data
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electroactive thin-film polymers sensors actuators↗
Real-time LLW Spectrum Measurement and Its Application to NDE of Composites
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Real-time Leaky Lamb Wave NDE↗
A Semi-Numerical Method for Elastic Wave Scattering Calculations
This paper is concerned with the calculation of elastic wavefields in strongly heterogeneous media containing scatters of arbitrary shape and size.
NDE↗
Electroactive Polymers as Artificial Muscles-Capabilities, Potentials and Challenges
For many years, electroactive polymers (EAP) received relatively little attention due to the small number of available materials and their limited actuation capability.
Leaky↗
Modeling and Design of an Electro-Rheological Fluid Based Haptic System for Tele-Operation of Space Robots
In this paper a novel haptic interface is presented to enable human-operators to feel and intuitively mirror the stiffness/forces at remote/virtual sites enabling control of robots as human-surrogates.
Leaky↗
Ultrasonic stress wave characterization of composite materials
The work reported covers three simultaneous projects. The first project was concerned with: (1) establishing the sensitivity of the acousto-ultrasonic method for evaluating subtle forms of damage development in cyclically loaded composite materials, (2) establishing the ability of the acousto-ultrasonic method for detecting initial material imperfections that lead to localized damage growth and final specimen failure, and (3) characteristics of the NBS/Proctor sensor/receiver for acousto-ultrasonic evaluation of laminated composite materials. The second project was concerned with examining the nature of the wave propagation that occurs during acoustic-ultrasonic evaluation of composite laminates and demonstrating the role of Lamb or plate wave modes and their utilization for characterizing composite laminates. The third project was concerned with the replacement of contact-type receiving piezotransducers with noncontacting laser-optical sensors for acousto-ultrasonic signal acquisition.
Duke, J. C., Jr.↗
Precise Laboratory Measurement of Line Frequencies Useful to Studies of Star and Planet Formation
In March 2002, we began a program in laboratory spectroscopy to provide accurate molecular line frequencies essential to studies of the motions and abundance in star-forming dense cores and planet-forming circumstellar disks. Summarized here is the progress that has been made in Year 3 of this grant. Work included measurement of 10 successive rotational lines in the ground vibrational state of SiO between 86 and 500 GHz, and two lines near 800 GHz to an accuracy of a few kHz; conducting pilot experiments on molecular ions in collision-free supersonic beams, including HCO+, N2H+, and H2D+; measurement of 22 lines of CN between 113 and 340 GHz; and setting up an experiment that would allow us to refine earlier measurements of the neutral species such as C3H2, CCS, H2CS, and SO by observing the very narrow sub-Doppler (Lamb dip) features in the millimeter-wave spectra of these species.
Myers, Philip C.↗
Image formation in microwave holography
Microwave holograms are made without offset reference beam, but it has been found that Van der Lugt filter can be used to produce image offset. Also, filter permits "decoding" of holograms in contrast with usual practice of reconstructing visible-light analogs of original micro-wave wave fronts.
Cribbs, R. W.↗
Hydrostatic Adjustment in Vertically Stratified Atmospheres
Hydrostatic adjustment due to diabatic heat in two nonisothermal atmospheres is examined. In the first case the temperature stratification is continuous; in the second case the atmosphere is composed of a warm, isothermal troposphere and a colder, isothermal semi-infinitely deep stratosphere.In both cases hydrostatic adjustment, to a good approximation, follows the pattern found in the Lamb problem (semi-infinitely deep. isothermal atmosphere): Initially we have acoustic waves with the kinetic energy increasing or decreasing at the expense of available elastic energy. After this initial period the acoustic waves evolve into acoustic-gravity waves with the kinetic, available potential and available elastic energies interacting with each other. Relaxation to hydrostatic balance occurs within a few oscillations. Stratification in an atmosphere with a continuous temperature profile affects primarily the shape and amplitude of the disturbances. In the two-layer atmosphere, a certain amount of energy is trapped in the tropospheric waveguide as disturbances reflect off the tropopause and back into the troposphere. With each internal reflection a portion of this trapped energy escapes and radiates to infinity.
Duffy, Dean G.↗
Laser-induced line-narrowing effects in coupled Doppler-broadened transitions. II - Standing-wave features.
Previous theoretical results on the influence of a laser on the line shape of a coupled transition (laser-induced line narrowing) have been restricted to the case where the laser is detuned from the center of its atomic gain profile or is in the form of a traveling wave. The present paper extends these results to the case where the laser is an intense standing-wave field tunable to the center of its atomic gain profile (conditions for Lamb dip). A theoretical solution of the problem is developed, and a detailed discussion of line shapes and physical processes involved is included.
Feldman, B. J.↗
Diode laser saturation spectroscopy of NH3
Saturation of molecular transitions using a tuneable diode laser was demonstrated for the first time using a standing-wave cavity configuration with an f/8 beam focussed at the sample. Observed saturation effects in NH3 transitions near 888/cm include sub-Doppler (Lamb-dip) resonances at line center.
Jennings, D. E.↗
The influence of a vertical magnetic field on oscillations in an isothermal stratified atmosphere
The effect of a uniform vertical magnetic field on the modes of an isothermal stratified atmosphere is examined. The general solutions of the wave equation for an isothermal magnetized medium are given. Asymptotic expansions of these solutions are presented in the strong- and weak-field limits. For a weak field, it is found that, to lowest order of the present perturbation expansion, the oscillation spectrum can be analyzed in terms of p- and g-like modes, magnetic Lamb modes, and magnetic or slow modes. The frequency corrections for each of the modes due to coupling with the remaining modes are calculated. It is demonstrated that when the frequencies of different modes are almost identical, strong mode coupling occurs and the waves acquire a mixed character. An analysis of the solutions in the vicinity of the degenerate frequencies is carried out. The solutions reveal the presence of avoided crossings, which occur at the otherwise degenerate frequencies. Examples of different types of avoided crossings are given.
Hasan, S. S.↗