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May 1990

Volume 87, Issue S1, pp. S1-S164

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back to top Session R. Structural Acoustics and Vibration II: “Smart” Materials and Structures II
Contributed Papers
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A method for reducing radiation from plates by actively controlling the restoring forces applied to the plate (A)

Koorosh Naghshineh, Gary H. Koopmann, and John S. Lamancusa

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S43-S43 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A novel method of active control is proposed that focuses on controlling the acoustic radiation of a plate by directly altering forces applied to the plate. By integrating active elements into the surface of the plate, its vibration characteristics can be made time dependent and thus, given a fluctuating load, the natural frequencies and their corresponding mode shapes can be actively adjusted to produce a minimum radiation condition. In this study, a model of an epoxy plate with shape memory alloy actuators is used in conjunction with an acoustic radiation model to predict the effectiveness of this concept. To demonstrate this behavior on a physical model, a cantilever beam is constructed of epoxy and shape memory alloy actuators. The actuators are attached to the surface of the beam in such a manner to put the beam under a state of localized axial compression that results in a shift of beam natural frequencies. Comparisons of the experimental results versus finite element analysis and an analytical model are presented.
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Optimization of location and amount of viscous damping to minimize random vibration (A)

Vernon H. Neubert

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S43-S43 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The random vibrations of a truss were minimized analytically by placing viscous dashpots where they would be most effective. It was assumed that there was a limited amount of damping available and the question was, where can the damping be used most effectively? Dashpots were placed parallel to the truss members, which deform only axially. The problem was developed as a nonlinear optimization problem. The response of the structure is represented in terms of its complex normal modes. The sensitivities of the individual modal natural frequencies, loss factors, and complex eigenvectors were obtained for nonproportional damping. The excitation was white noise applied as concentrated forces at nodal points. The mean‐square displacement was minimized using a modification of the method of steepest descent. An example is given for a 10‐bar truss. The problem was also formulated for the optimization program CONMIN and the results are summarized for several constraint situations. The work is related to a previous paper dealing with the minimization of settling time of free vibrations by maximizing the energy dissipation rate [V. H. Neubert, Proc. DAMPING89, Int. Symp., W. Palm Beach. FL, February 1989].
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Temperature‐induced, low‐frequency acoustic emission of quartz related to formation of microcracks, fluid inclusion decrepitation, and phase transitions in the crystal structure (A)

Andreas Schmidt‐Mumm

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Quartz, as the most ubiquitously occurring mineral, was chosen for investigations of the acoustic emission upon heating. Various samples from a wide range of conditions of formation were analyzed in the frequency range of 400–10 000 Hz with a high‐resolution decrepitometer. The acoustic emission patterns showed a variation of 5000–80 000 total signals over the entire temperature range of 90 °C‐610 °C (1‐cm3 sample, grainsize fraction 250–500 μm). Microthermometric investigations of the sound‐emitting processes and correlation with microcrack patterns determined by scanning electron microscopy (SEM) showed that various sources contribute to the sometimes vehement noise production. Stress built up because of thermal anisotropy causes the formation of microcracks along inter‐ and intragranular grain boundaries over most of the observed temperature range. Accumulation of signals from the decrepitation of fluid inclusions reveals information about the conditions of formation and subsequent alterations of the investigated sample. Acoustic emission caused by phase transitions in the crystal structure (α〈 − 〉β‐transition at 573 °C) is related to changes in the twinning state of the sample. Measurement of the complex patterns of acoustic emission produces a “fingerprint” of the conditions of formation and subsequent alterations the sample underwent in its geological history. They can be used to determine the extension of mineralizations and alterations or detection of geological structures even when covered by soil. Real‐time frequency analysis of the signals shall reveal further information about the origin of single acoustic signals. [Work supported by the DFG.]
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Control of flow‐induced vibrations in Nitinol‐reinforced composite beams (A)

A. Baz, S. Poh, and J. Gilheany

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Flow‐induced vibrations of flexible fiberglas composite beams are attenuated by activating optimal sets of NIckel and TItanium alloy wire, called NITInol, which are embedded inside these beams. The dynamic and thermal characteristic of the Nitinol‐reinforced composite beams are determined by monitoring the vibration's modes, as well as the temperature of the wires and beam during various activation and de‐activation strategies of the Nitinol wires. The resulting shift of the vibration modes, which accompanies the activation of the Nitinol wires, is utilized to influence the fluid‐structure interaction in such a way that results in attenuating the flow‐induced vibrations. Experiments are conducted on 30‐cm‐long and 0.156‐cm‐thick prototypes of these beams that are mounted in a clamped‐clamped fashion inside a wind tunnel. The effect of varying the flow speed on the induced vibrations is determined with and without the activation of the Nitinol wires. The results obtained indicate that there is an optimal number of Nitinol wires that should be activated to attain the maximum frequency shift. This number ensures a balance between the amount of strain energy added to the composite beam and the softening effect imparted to the resin used due to the heating of the Nitinol wires. With such optimal configuration, it was possible to shift the first mode of vibration from 42 to 61 Hz in about 3 min during the activation phase. About 6 min was required to bring the beam back to its original frequency during the de‐activation phase. At steady‐state conditions, vibration attenuations in excess of 60% were attained near and at the resonance. [Work supported under a grant from ARC.]
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SAW sensors and their application to sense stresses in turbulent flows (A)

Yongrae Rob, B. Shanker, Vasundara V. Varadan, and Vijay K. Varadan

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The potential of the SAW devices as sensors has not been fully exploited. New SAW sensors are developed to sense the shear stress and the normal stress in turbulent flows. These devices are extremely sensitive to changes in the environment. The surface wave velocity is hence changed when forces are applied on the sensor. The direction of the surface wave can be changed and by suitable manipulation, the effect of each stress can be isolated. Experiments are conducted with a low‐frequency SAW oscillator sensor and the frequency shift is measured. The values obtained agree with the expected values for the experimental setup. As the frequency of operation is increased, the sensors become smaller and the values can be determined more accurately eliminating the effect of spatial variation of the stresses.
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The design of an intelligent vibration absorber using numerical optimization techniques (A)

Patricia L. Walsh and John S. Lamancusa

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The tuned absorber is an effective method of vibration reduction for systems that operate at constant frequency. However, the tuned absorber adds an additional low‐frequency resonance through which the system must pass during startup or shutdown conditions, thereby creating potentially excessive noise or vibration. An intelligent vibration absorber that will be effective during these periods has been designed. The mechanical system is modeled as a lumped mass, two‐degree‐of‐freedom system driven by a rotating unbalance that accelerates from rest to a constant driving frequency. The absorber has constant mass and damping but variable stiffness. The optimal value of absorber spring stiffness as a function of time was found using the optimization routine CONMIN. The time response was discretized and the optimization was performed on each successive time interval. The cost function and design variable used were the root‐mean‐squared displacement of the main mass and the absorber stiffness during each time interval. The process was repeated for several mass ratios, damping ratios, and accelerations. The intelligent absorber is shown to be superior to the tuned absorber for all cases.
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Fiber optic sound and strain measurements employing newly developed digital demodulators (A)

Brian H. Houston, J. A. Bucaro, and Larry A. Kraus

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Fiber optic interferometric sensor systems for “smart” materials and structures applications offer a number of advantages including minimal electrical interference, geometric flexibility, and multisensor capability. Demodulation of these interferometric sensor signals typically is carried out by analog hardware. However, accuracy, bandwidth, and linearity are hampered by limitations in available electronic components and low‐performance analog filters. A high‐precision off‐line digital demodulator has been developed and the techniques with fiber optic strain gauges and hydrophones are demonstrated. The measurement system is unique and represents an advancement in the state‐of‐the‐art. A real‐time implementation for control applications based on the Texas Instruments Inc. TMS 320C25 signal processing chip set is also proposed.
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Wall pressure fluctuations in the transition region (A)

Thomas A. Galib

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S44-S45 (1990); (2 pages)

Online Publication Date: 13 Aug 2005

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Wall pressure fluctuations on an axisymmetric body of revolution were measured with piezoelectric pressure transducers. Spectral analysis of the data showed discrete frequency bands of the pressure fluctuations corresponding to predicted Tollmien‐Schlichting disturbance frequencies, in the transitional boundary layer. Nondimensional power spectra showed that the peaks of the disturbance frequencies (for three free‐stream velocities) collapsed to a single Strouhal number. Further manipulation of this result yielded an expression for the Tollmien‐Schlichting frequencies in terms of a constant and the displacement thickness Reynolds number. These results were for a near‐zero pressure gradient. They were later reproduced in an adverse pressure gradient for a variety of Reynold's numbers.
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Application of surface acoustic waves for marine fouling prevention (A)

Yongrae Roh, Vasundara V. Varadan, and Vijay K. Varadan

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S45-S45 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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For ships, the attachment of marine fouling species on the hull causes a lot of problems. Conventional methods for antifouling include toxic painting and hull vibration. But these used to be expensive and ineffective. A new antifouling technique is developed by employing surface acoustic waves. Of various types of surface waves, SH plate mode waves are chosen, which turn into Love waves when the thickness of a hull is much larger than the wavelength of the waves, because they are most effective in shaking off the attachments. All the energy of the waves are confined to the surface of the hull and leaks very little into water. PVDF thin film and interdigital transducers are used to launch the waves and occupy only a small fraction of the hull surface. The energy transfer of the waves across vertical discontinuities in their propagation path is found to be satisfactory for the application. Compared with conventional methods, this new technique is more practical, powerful, and cost effective.
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Algorithm‐based method for suppressing the transmission of sound in a fluid‐filled waveguide (A)

R. Homer, Pieter S. Dubbelday, and Ben A. Park

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S45-S45 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A 4‐in.‐diam open‐ended water‐filled tube is excited at its base by an electrodynamically driven piston which is 2 in. in diameter and suspended by two sheets of rubber. Directly above the piston, a thin disk of piezorubber encapsulated in rho‐c rubber occupies a full cross section of the tube. The piston is driven with constant force over the range of frequency from 1–2 kHz. The piezorubber is driven by a source that is synchronized to the piston's excitation frequency with a phase‐locked loop. A hydrophone located above the piezorubber disk is connected to a lock‐in amplifier and is used to detect the transmitted signal. Based on the inphase and quadrature components of the transmitted signal, an amplitude and phase for the piezorubber's source is algorithmically determined such that the transmitted signal will be minimized. The process is iterated until the transmitted signal is reduced to the noise level of the hydrophone. The algorithm chosen is well suited for a noisy environment and was successfully applied in our experiments.
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Wave propagation in submerged piezoelectric layered media (A)

B. Honein, A. M. B. Braga, P. Barbone, and G. Herrmann

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S45-S45 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A formulation for the systematic study of wave propagation in a layered piezoelectric solid interacting with an acoustic fluid is presented. The methodology employed is based on an eight‐dimensional vector formalism, due originally to Kraut [Phys. Rev. 57, 1450–1455 (1969)]. Within this framework, one may easily define “characteristic” impedances of a single material. With this as a building block, the invariant‐imbedding technique is employed to construct the “global” surface impedance of an arbitrarily layered piezoelectric solid. In some special cases, the overall problem can be simplified by exploiting material symmetries and so reducing the order of the eigenvalue problem (by a factor of 2). Results presented include calculations of dispersion curves for layered piezoelectric plates and interfacial waves between a fluid and a piezoelectric solid. [Work supported by ONR.]
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Fiber‐optic interferometric differential strain sensor: “The smart strutt.” (A)

D. A. Brown and S. L. Garrett

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S45-S45 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Two optical fibers, comprising the legs of a Michelson interferometer, are cast into a long rod (1.33‐cm diameter, 35. 1‐cm length) of epoxy at off‐axis coplanar distances of 5.3 _+ 0. I min. The free‐free bar is electodynamically excited in its gravest flexural, torsional, and longitudinal modes. The absolute displacement of the bar end for each mode was measured using the MTI fotonic vibration sensor and compared with the change in optical path length induced in the interferometer. At the fundamental flexural resonance of the bar (158.5 Hz), the correlation between measured transverse peak displacement (15.4 μm) and the measured number of strain‐induced optical fringes (14.5 at a wavelength of 817 nm) was within 7% of theoretical predictions. No fringes were observed for the torsional and longitudinal modes, as expected, since those modes produce equal strain in each leg of the interferometer. Experimental values and techniques used to obtain data for the complex Young's and shear modulus for the elastomeric casting material (Stycast 1266) will also be presented.
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