The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform

Charles A. Langston - One of the best experts on this subject based on the ideXlab platform.

  • Wave Gradiometry in the Time Domain
    Bulletin of the Seismological Society of America, 2007
    Co-Authors: Charles A. Langston
    Abstract:

    A time-domain approach for solving for the change in geometrical spreading and horizontal wave slowness in wave gradiometry is presented based on the use of the analytic signal. The horizontal Displacement Gradient of a wave is linearly related to the Displacement and its time derivative. The coefficients of this relationship give the change of geometrical spreading, the change in radiation pattern, and horizontal slowness. The new time-domain technique incorporates estimates of the instantaneous amplitude and frequency of the three time series to solve uniquely for the wave-field coefficients. The analysis is simpler and more suited to fast array processing of Displacement Gradient data sets compared with a spectral ratio method.

  • Wave Gradiometry in Two Dimensions
    Bulletin of the Seismological Society of America, 2007
    Co-Authors: Charles A. Langston
    Abstract:

    The spatial Displacement Gradient of a seismic wave is related to dis- placement and velocity through two spatial coefficients for any one dimension. One coefficient gives the relative change of wave geometrical spreading with distance and the other gives the horizontal slowness and its change with distance. The essential feature of spatial Gradient analysis is a time-domain relation between three seismo- grams that yields information on the amplitude and phase behavior of a seismic wave. Filter theory is used to find these coefficients for data from 2D areal arrays of seismometers, termed gradiometers. A finite-difference star is used to compute the Displacement Gradient for irregularly shaped gradiometers, and a relation for the frequency-dependent error in the Displacement Gradient is obtained and applied to ensure accurate estimates. This kind of array analysis is useful for gradiometers at any distance from a source and yields a variety of time-domain and frequency-domain views of wave-amplitude changes and horizontal phase velocity estimates across the gradiometer. For example, time-dependent horizontal slowness and wave-azimuth plots are natural results of the analysis. These time-domain maps may be used in conjunction with time-distance and horizontal slowness-distance models to locate seismic sources or may be used directly to study earth structure. These methods are demonstrated by using data from a small-aperture (40 m) seismic gradiometer.

David Mora - One of the best experts on this subject based on the ideXlab platform.

  • a virtual element method for elastic and inelastic problems on polytope meshes
    Computer Methods in Applied Mechanics and Engineering, 2015
    Co-Authors: Beirao L Da Veiga, C Lovadina, David Mora
    Abstract:

    Abstract We present a Virtual Element Method (VEM) for possibly nonlinear elastic and inelastic problems, mainly focusing on a small deformation regime. The numerical scheme is based on a low-order approximation of the Displacement field, as well as a suitable treatment of the Displacement Gradient. The proposed method allows for general polygonal and polyhedral meshes, it is efficient in terms of number of applications of the constitutive law, and it can make use of any standard black-box constitutive law algorithm. Some theoretical results have been developed for the elastic case. Several numerical results within the 2D setting are presented, and a brief discussion on the extension to large deformation problems is included.

Wolf Altman - One of the best experts on this subject based on the ideXlab platform.

  • Small Strain and Moderate Rotation Theory in Elasticity and Stability
    Applied Mechanics Reviews, 1991
    Co-Authors: Wolf Altman, Luiz Bevilacqua
    Abstract:

    This paper is concerned with a small strain and moderate rotation theory in elasticity and stability. Approximate kinematic relations and associated equations of motion are obtained, based on estimates of the Displacement Gradient components. Stability equations are derived with several levels of approximation from the equations of motion and boundary conditions. A version of the basic equations referred to a local orthogonal coordinate system is written in terms of physical components.

  • A refined theory for anisotropic shells in the presence of small strains and moderate rotations
    International Journal of Non-linear Mechanics, 1991
    Co-Authors: Wolf Altman, A.f. Palmerio
    Abstract:

    Abstract This paper is concerned with a geometrically non-linear theory of anisotropic shells in the presence of small strains accompanied by moderate rotations. The theory accounts for the effects of transverse shear and transverse normal deformations. Approximate kinematic relations and consequent equations of motion are obtained, based on estimates of the relative Displacement Gradient components. The contribution of the non-linear terms to these equations is discussed and compared to other theories.

Ralph P Tatam - One of the best experts on this subject based on the ideXlab platform.

  • surface strain measurement of rotating objects using pulsed laser shearography with coherent fibre optic imaging bundles
    Measurement Science and Technology, 2008
    Co-Authors: Daniel Francis, Stephen W. James, Ralph P Tatam
    Abstract:

    In this paper, surface strain measurements made using a pulsed laser shearography system from a thermally loaded test object, rotating at 610 rpm, are presented. The shearography instrument described here has four measurement channels consisting of four observation directions and a single illumination direction. The use of multiple channels, combined with orthogonal shear directions, enables the measurement of the six orthogonal components of Displacement Gradient required to determine the surface strain. Fibre-optic imaging bundles are used to transfer images from the observation positions to the shearing interferometer and CCD camera. Spatial multiplexing of the images onto the CCD camera allows for simultaneous acquisition of data from the four views. The repetition rate of a pulsed Nd:YAG laser and the framing rate of the CCD camera were synchronized with the rotation rate of the test object, which enabled the capture of images recorded on subsequent revolutions of the object. Analysis of the deformation-induced phase change between recordings was performed using the spatial carrier technique. The orthogonal Displacement Gradient components were obtained from the unwrapped phase measurements using a matrix transformation based on the sensitivity vector of each of the four measurement channels.

  • Single-axis combined shearography and digital speckle photography instrument for full surface strain characterization
    Optical Engineering, 2005
    Co-Authors: Roger M. Groves, Stephen W. James, Ralph P Tatam
    Abstract:

    Full characterization of the surface strain requires the mea- surement of six Displacement Gradient components of the surface strain tensor. The out-of-plane Displacement Gradient component may be di- rectly measured using the full-field speckle interferometry technique of shearography, but to fully characterize the surface strain using shearog- raphy, a minimum of three illumination, or viewing, directions are re- quired. The image processing technique of digital speckle photography (DSP) is sensitive to in-plane Displacement for normal collinear illumina- tion and viewing, with the Displacement Gradient components obtained by differentiation. A combination of shearography and digital speckle photography is used to perform full characterization of the surface strain using a single illumination and viewing direction. The increase in com- plexity compared with a standard single-channel shearography system lies predominantly in the additional image processing requirements. Digi- tal speckle photography image processing is performed using the optical flow field technique and the advantages of this technique compared with correlation are discussed. The design of the instrument is described and full surface strain measurements made with the system are presented.

  • Multicomponent laser shearography for the investigation of defects in rotating machinery
    Optical Metrology in Production Engineering, 2004
    Co-Authors: Roger M. Groves, Stephen W. James, Stuart E. Barnes, Domenico Furfari, Philip E. Irving, Shan Fu, Ralph P Tatam
    Abstract:

    Shearography is a full-field interferometric optical technique that is usually used for the qualitative investigation of defects in non-destructive testing applications. The optical configuration is sensitive directly to Displacement Gradient, a parameter closely related to the surface strain. The component of the Displacement Gradient that is measured is determined by the illumination and viewing directions and by the direction of the applied shear. The sensitivity is governed by the magnitude of applied shear and by the optical wavelength. Full characterisation of the surface strain requires a measurement of six-components of Displacement Gradient; this is achieved in shearography by forming a number of distinct measurement channels using multiple illumination, or viewing, directions. In this paper the authors discuss the quantitative measurement of the strain field around a fatigue crack, using a time-division-multiplexed diode laser shearography instrument. To investigate moving objects, a pulsed laser provides a method of freezing the object position at two points in the loading cycle. A shearography instrument incorporating two frequency doubled pulsed Nd:YAG lasers, with a common injection seeder is described. The measurement channels are spatially-multiplexed by viewing from four directions using an optical fibre imaging bundle, with optical processing at a remotely located interferometer head. Preliminary experimental measurements are presented.

  • Multi-component pulsed-laser shearography using optical fiber imaging-bundles
    Optical Diagnostics for Fluids Solids and Combustion II, 2003
    Co-Authors: Roger M. Groves, Stephen W. James, Ralph P Tatam
    Abstract:

    Shearography is a full-field non-contact optical technique usually used for the identification of the location of defects in non-destructive testing. Shearography is sensitive to Displacement Gradient, a parameter closely related to the surface strain. To fully characterise the surface strain requires the determination of six orthogonal components of Displacement Gradient and this is achieved in shearography by measuring from three, or more, illumination, or viewing, directions and from a minimum of two directions of applied shear, followed by a coodinate transformation to the orthogonal components. In this paper the authors use illumination from a single direction using dual pulsed injection-seeded Nd:YAG lasers, and view the object from four directions. The images from the four viewing directions are ported to a single interferometer head using a four-leg optical fiber imaging bundle. At the interferometer head the four views are spatially-multiplexed into a single image, pass through the interferometer head and are recorded by a single high resolution dual-framing camera. The direction of applied shear in the interferometer head is adjustable to allow measurements from the two shear directions. Experimental results are presented of Displacement Gradient from this pulsed laser shearography instrument.

  • Pipe Weld Investigation using Shearography
    Strain, 2003
    Co-Authors: Roger M. Groves, Stephen W. James, Ralph P Tatam
    Abstract:

    In this paper the full-field optical technique of shearography is used to fully charac- terise the surface strain of the region around a welded joint in a pipe. Shearography is an inter- ferometric optical technique sensitive to Displacement Gradient, a parameter that is closely related to the surface strain. The surface of the material at a welded joint is uneven, but measurements may still be performed using full-field non-contact optical methods. In this paper the authors present a multi- component shearography system, which measures the in- and out-of-plane Displacement Gradient components by determining three non-orthogonal Displacement Gradient components and per- forming a coordinate transformation. To compensate for non-planar object surfaces the coordinate transformation procedure incorporates a correction for the object shape, measured using the same apparatus. The instrument was used to determine the six components of Displacement Gradient, thereby fully characterising the surface strain. In comparison with resistance strain gauges, an accuracy of ±11 le was achieved over a limited measurement range.

Luiz Bevilacqua - One of the best experts on this subject based on the ideXlab platform.

  • Small Strain and Moderate Rotation Theory in Elasticity and Stability
    Applied Mechanics Reviews, 1991
    Co-Authors: Wolf Altman, Luiz Bevilacqua
    Abstract:

    This paper is concerned with a small strain and moderate rotation theory in elasticity and stability. Approximate kinematic relations and associated equations of motion are obtained, based on estimates of the Displacement Gradient components. Stability equations are derived with several levels of approximation from the equations of motion and boundary conditions. A version of the basic equations referred to a local orthogonal coordinate system is written in terms of physical components.