摘要:

In this paper source localization results obtained by matched‐field (MF) processing of vertical line array (VLA) data from the PACIFIC ECHO III sea trial are presented. The experimental site was located in a region of significant bathymetric variation. An harmonic source was towed on a series of arcs, providing data for a sequence of different ranges and azimuths. In spite of the significant bathymetric variation, good MF localization results were obtained for a large fraction of the trial time. In the first part of this analysis it is assumed that the azimuth of the source is known. Substantial improvements in MF localization were obtained by using an adiabatic normal mode model to calculate replica acoustic fields for the range‐dependent bathymetry. The improvement over the results obtained with flat‐bottom waveguide replicas is demonstrated for data from the longer range arcs. In the second part of the analysis, the concept of environmental signal processing, an enhancement to MF processing that is obtained by making use of the complexity of the oceanic waveguide environment, is investigated. The azimuthal variation of bathymetry for the source tracks along the arcs was used to obtain estimates of the source bearing from the VLA data. In many instances, the bearing estimates were in reasonable agreement with the known source bearings.

ISSN:0001-4966    

DOI:10.1121/1.414538

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A method for estimating the orientation distribution function (ODF) of short‐fiber composites using measured ultrasonic velocities is presented. The method is based on the coupling of (i) a forward micromechanics model of the effective ultrasonic velocities of the textured composite in terms of the coefficients in an expansion of the ODF in generalized spherical harmonics, (ii) ultrasonic measurements of the velocities, and (iii) an inversion algorithm to obtain the coefficients of the ODF from the measured velocities and the forward model. The approach is used to determine the ODF coefficients for a transversely isotropic short‐fiber SiC/Al metal‐matrix composite. The ODF coefficients obtained by inversion of ultrasonic velocities agree reasonably with those obtained from neutron‐diffraction pole figures. Corresponding elastic constants predicted with these coefficients also agree well with independently measured elastic constants.

ISSN:0001-4966    

DOI:10.1121/1.414539

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This paper describes experimental results on velocity measurements of bulk and Rayleigh waves in a highly anisotropic natural medium (slate). Bulk waves velocities are measured using a standard ultrasonic pulse transmission setup. The Rayleigh waves are excited using the wedge technique and recorded by an interferometer which detects both the in‐plane and out‐of‐plane components of the displacement they produce in a point of a sample surface. The experiments constitute part of a study of the propagation of surface seismic waves in anisotropic geological structures using scaled‐down models. The elastic behavior of a block of slate whose faces are traversed by these surface waves is studied, and it is verified that this rock shows a good fit to a physical model of a solid with an axis of symmetry of revolution.

ISSN:0001-4966    

DOI:10.1121/1.414540

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Hertzian, dry, pointlike contacts are used to couple ultrasonic energy efficiently to plate wave modes in anisotropic solid plates. A new transducer configuration using a piezoelectric PZT‐5H transducer and a quartz buffer rod is realized for this purpose. In particular, the lowest order antisymmetric Lamb wave (A0) mode is excited and detected in anisotropic plates at a frequency of 200 kHz and with signal‐to‐noise levels exceeding 65 dB. Time delay measurements and FFT techniques are used to obtain dispersion and anisotropy curves of phase velocity in anisotropic plates. The accuracy of the method is limited mainly by the distance measurements. In (001) silicon plates the agreement between theory and experiments is obtained with an accuracy of ±0.08%. Anisotropic phase velocity measurements are also performed for uniaxial graphite/epoxy composite plates and the results are compared with the theoretical model to derive the elastic constants of the composite. Through accurate phase velocity measurements, it is possible to detect delamination defects in composite plates with high resolution operating at 200 kHz. Using the surface impedance approach, a model is also developed to predict the effects of delaminations on phase velocity and the results are verified by experiments.

ISSN:0001-4966    

DOI:10.1121/1.414541

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The free vibration of a laminated hollow circular cylinder with traction‐free surfaces is studied by taking care of continuity conditions at the interfaces. The frequency equations are obtained for the composite cylinder composed of two different piezoelectric materials belonging to the 6mmclass and a hypothetical linear elastic material with voids (LEMV) as bonding layers. Numerical results for complex‐valued frequency are carried out for the composite cylinder ceramic‐1/LEMV/ceramic‐2. The dispersion curves for the flexural vibration of a composite hollow cylinder with voids and without voids are obtained and are compared with that of a hollow cylinder of piezoelectric material.

ISSN:0001-4966    

DOI:10.1121/1.414542

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The phase of the transfer function between two points in an extended system can be easily measured if it is taken to be the accumulated phase obtained by smoothly raising the measurement frequency from zero to the reference frequency. Lyonetal., in an extended series of papers [most recently J. Acoust. Soc. Am.95, 286–296 (1994)], have examined the behavior of this accumulated phase in systems of two and three dimensions and have elucidated the concept of a reverberant phase which is independent of the separation between the two measurement points, provided they are far enough apart, but which rises sharply with increasing frequency. In some applications, for example, in nondestructive testing of extended structures, it is important to be able to observe simple wave‐propagation behavior and in particular to measure the propagant phase as a function of frequency and position. The conditions under which this is possible are investigated, and are shown to impose constraints on the ratio between the propagation distance and the size of the structure under test, and either the material damping coefficient or the reflection coefficient at the domain boundaries. These results, which represent an extension of those of Lyonetal., are discussed in terms of the distribution of zeros of the transfer function in the complex frequency plane. Many platelike structures of practical interest are found to satisfy these conditions, so that measurement of propagant phase behavior can provide the basis for a useful technique of nondestructive examination.

ISSN:0001-4966    

DOI:10.1121/1.414543

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Although nonlinear systems subject to combined parametric and external excitations have been examined in some depth, very few of these investigations have addressed the influence of a mean (time‐invariant) load on the system response. In particular, the importance of the mean load effect has been highlighted in the study of whirling asymmetric shafts, where significant changes in system response and stability occur under the influence of gravity. The present paper intends to characterize the specific effect of mean load on the dynamic behavior of a Hill’s oscillator with a clearance‐type nonlinearity while also being subjected to a periodic base displacement excitation. The parametric continuation technique and method of harmonic balance would be used for this purpose. Issues discussed would include the coupling between the mean load and the dynamic response amplitude, interaction between the parametric excitation effect and the clearance nonlinearity, and comparison between time‐invariant and time‐varying systems. Further, a geared system has been analyzed as a practical application of the nonlinear Hill’s equation examined. Earlier studies on gear dynamics have been reexamined to provide improved correlation with prior experimental data; new interpretations for some of the nonlinear phenomena in the experimental data has also been provided based on our study.

ISSN:0001-4966    

DOI:10.1121/1.414544

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The paper presents a simple deterministic derivation of the vibration damping induced in a large main structure by a multitude of small substructures attached more‐or‐less compliantly to the main structure, each dissipating vibratory power at its frequencies of resonance. A simple expression is obtained for the damping induced in the large structure indicating that the damping depends primarily on the total effective mass of substructures resonating within a band centered at the frequency of vibration, and is relatively independent of the damping factors of the individual substructures. The application of this simple expression for induced damping is illustrated by calculating the damping induced by substructures attached to an axially vibrating rod and to a flexurally vibrating beam. For the latter case, the calculated damping factors are compared with measured values.

ISSN:0001-4966    

DOI:10.1121/1.414545

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Identifying the sources of vibration in a structure and the energy exchange between structure elements is an important issue in vibration and noise control. Several formulations for the structural intensity have been presented in the past, but little is said about implementing the formulations using measured data. In this paper, two structural intensity formulas based on different assumptions are compared, and practical issues of implementing the calculations of the structural intensity usingk‐space analysis are discussed. A force distribution, calculated from Mindlin’s plate motion equations, is shown to be an additional tool to locate vibration sources. Since the calculation of both the structural intensity and the force distribution require the use of fast Fourier transforms (FFT), numerical difficulties arise that are solved by signal processing techniques such as windowing and filtering. The influences of the windowing and the filtering on the calculation of the structural intensity and the force distribution are described. Results show that windowing and filtering are needed to obtain reasonable results of the structural intensity and the force distribution from measurement data; however, they may remove useful information.

ISSN:0001-4966    

DOI:10.1121/1.414546

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The influence of attachments and substructure on the vibration of a plate is a very important problem in noise and vibration control. This paper presents several examples of using an energy viewpoint with experimental data. The normal velocity of each plate, measured with a laser vibrometer or in some cases obtained from analytic calculation, is then used to calculate the structural intensity and force distribution. The structural intensity and force distribution are then used to locate single and double forces applied to a plate with some limitations. In addition, a rib attached to a plate and a constrained layer damping patch attached to a plate is studied. The energy absorbed by the rib and the force exerted by the rib in response to the waves incident on the rib are discussed. Results also show that a constrained layer damping patch is more effective in absorbing shear and bending vibrational energy than in absorbing twisting energy and that the damping patch does not apply a normal reaction force to the plate.

ISSN:0001-4966    

DOI:10.1121/1.414547

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP