Showing papers on "Digital image correlation published in 1997"

Measurement of deformations on concrete subjected to compression using image correlation

Abstract: Because the nature of failure in concrete is complicated due to the material heterogeneity, a robust measuring method is essential to obtain reliable deformation data. A nondestructive displacement evaluation system using a digital image cross-correlation scheme, often called computer vision, is developed to make microscopic examinations of the fracture processes in concrete. This is a full-field measuring method that gives an accuracy within the micron range for a 100 mm × 75 mm viewing area. A feedback signal that combines the lateral and axial deformations provides a well-balanced imaging rate both before and after the peak load. Displacement vector diagrams or displacement contour maps of concrete reveal highly nonuniform deformations even in the elastic range. The processes of fracture in concrete are well defined at different deformation levels.

Detection of plastic deformation patterns in a binary aluminum alloy

Abstract: A whole-field, in-plane strain-mapping technique is evaluated for in situ monitoring of plastic deformation patterns in aluminum sheet metals. This technique is built on the recent developments in digital image correlation and improved data reduction procedures. The sensitivity and accuracy of the measured local strain variations are critically examined in terms of random and systematic experimental errors, free-surface roughening due to large plastic deformation and microscopic surface grain deformation. Tensile specimens made from an annealed Al−Mg alloy sheet metal are subjected to a large plastic and macroscopically uniform deformation, and no visible deformation patterns can be identified by direct surface observation. Using an incremental strain-mapping approach, the existence of nonstationary deformation bands in the annealed Al−Mg alloy is uncovered. The developed technique can be used to study the formation and evolution of plastic deformation patterns and their effect on tensile ductility, formability and surface finish of sheet metals.

Quantitative DIC microscopy using a geometric phase shifter

Abstract: In this paper we investigate the use of a geometric phase- shifting (GPS) technique which allows us to convert conventional transmission or reflection differential interference contrast (DIC) microscopy into a quantitative mode. A phase-shifting algorithm is employed to extract the specimen phase gradient from the mixture of phase and amplitude information which is common in DIC. Fourier techniques are then used to recover the exact phase (i.e. optical path length variations) throughout the biological specimen viewed. In addition to this quantitative 'phase map,' we demonstrate that the GPS process simultaneously yields an 'amplitude-only' representation in which various absorption and transmission properties of the specimen are displayed as intensity variations in the image, similar to brightfield microscopy. These two resulting images can then be analyzed or further processed in a number of ways that are not possible with conventional DIC and which improve the microscopist's ability to correctly identify, interpret and measure features in the specimen.

Surface deformation measurements of a cylindrical specimen by digital image correlation

Abstract: Planar digital image correlation has been extended to measure surface deformations of cylindrical specimens without physical contact for high-temperature situations. A single CCD camera acquires the surface image patterns of a section of a specimen in the undeformed and deformed states to determine two-dimensional displacements on a projection plane. Axial, circumferential and shear deformations are determined through curvature transformation on the two-dimensional projection displacement field. The resolution of this technique was determined for a cylinder of 22.23-mm diameter to be 3.5 μm for the axial displacement, 0.05 percent for the axial and shear strains and 0.08 percent for the circumferential strain when correlation computations are carried out over a field of 5 mm×5 mm.

Measurement of surface profile using digital image correlation

Abstract: Presented in this paper is the development of a system for measuring surface profile using digital image correlation All needed equations for camera calibration and system profile measurements are shown Equations included needed transformation to account for subset perspective distortions The system is capable of an accuracy of 1/20,000 of the camera-to-object distance Experimental results are shown for several cases, with results giving a maximum error of 005 mm with camera-to-object distance of 1000 mm

Sparse array image correlation

Abstract: A method of image correlation includes providing one or more image arrays of pixel values wherein each pixel value is associated with one of a number of pixels. Pixel values in the image array which are beyond a pixel threshold value are selected and a correlation process is performed on the selected pixel values. The correlation process is preferably performed according to an error correlation function. The sparse array correlation process provides several orders of magnitude in increased processing speed over known correlation processing techniques and is useful for Particle Image Velocimetry analysis.

Imaging apparatus having a spatial filter and image shifting mechanism controller based on an image mode

Abstract: A single imaging apparatus is adapted to pick up both a motion image and a still image. In an imaging apparatus 1, image light from an object is separated into multiple monochromatic image light components by an optical system 3 and then imaged by an imaging means 4 for generation of an original image signal. The imaging apparatus 1 switches an imaging mode depending upon an output mode. The optical system 3 switches the sate of a variable spatial filter 18 depending upon an imaging mode thereby changing an effect for limiting the spatial frequency. In an output mode for motion images, an interpolative synthesizer circuit 7 generates the output image signal from a single original image signal. In an output mode for still image, an image shift mechanism 19 parallely shifts the optical axis of the incoming image light while the imaging means captures the original image light multiple times so that an output image is produced from these original image signals. An amount of attenuation of the filter 18 in the output mode for motion images is greater than that for an equivalent spatial frequency component in the output mode for still image.

Image Correlation Analysis of Multiple-Bolt Wood Connections

Abstract: Displacement beneath the bolts in multiple-bolted wood connections was studied using digital image correlation. This method combines digital image analysis and image correlation to calculate surface displacements from a set of digitized video images of an object under an applied load. Double-shear connections constructed of clear, straight-grained yellow-popular were tested in compression parallel to grain. Five different bolt patterns were used to analyze the effect of number of bolts in a vertical row and number of bolts in a horizontal column on displacement distribution among bolts. It was discovered that for multi-bolt patterns in a vertical row, parallel to load displacements below the outer bolts, are higher than those below the center bolt(s) but not equal in magnitude as previously assumed and the surface displacements perpendicular to the load beneath the bolts split along the centerline of the bolts. Variation in material properties, rigid body motion, eccentric loading and/or wood failure beneath a bolt are detectable with digital image correlation and may influence the results if not carefully considered in experimental design.

Compact, low power polymer-based optical correlator

Abstract: A low cost, low power compact optical correlator is demonstrated. It operates interchangeably in the matched-filter or joint-transform configurations. The active medium is a high diffraction efficiency photorefractive host–guest polymer system. Successful image correlation has been achieved.

Continuous-field Image-Correlation Velocimetry and its application to unsteady flow over an airfoil

Abstract: Continuous-field Image Correlation Velocimetry (ICV) is an extension to the ICV technique of Tokumaru & Dimotakis (1995). The method determines the optical flow in sequences of images, and relies on a convected Lagrangian marker, e.g., a conserved scalar field, or particles, etc. The method has been applied to several simulated-flow test cases and results are presented for the error of the method, with and without noise added to the correlated test-images. The results of further tests are reported, for two laboratory flows; a NACA-0012 airfoil at high angle of attack, and a transverse jet in a coflowing stream.

High-resolution ultrafast eye tracking system

Abstract: This paper presents an eye tracking system that combines optical and electronic image processing for the precise measurement of eye movement parameters, including direction, speed, and acceleration. The system applies optical joint Fourier transform correlator and novelty filtering to measure the cross-correlation peak displacement caused by eye movements using magnified images of iris landmark patterns. This permits a 0.026 degree(s) accuracy of eye displacement measurements within a 6 degree(s) tracking range, or a 0.1 degree(s) accuracy within a 20 degree(s) range. Using digital micromirror device SLMs in the Fourier transform optical system, an inverse Fourier transform DSP module, and a high-speed digital camera, the eye tracking system will make it possible to detect and characterize saccadic intrusions in smooth pursuit eye movement.

Deformation behaviour of the cross-section of a heat-sealed area in plastic film used for liquid package bags

Abstract: Displacements on a cross-section of a heat-sealed area in laminated plastic film used for liquid package bags are measured by a digital image correlation. All the specimens used for obtaining the images necessary to measure the displacements are cut into the required shape from heat-sealed package bags. The images are recorded by an 8mm video camera via a microscope, and digitized by an image-processing device. Twenty-six images are used for obtaining the displacements between the no-load condition and failure of the specimens. As a result, a characteristic displacement is found in the heat-sealed area.

In vivo Doppler flow imaging of picoliter blood volumes using optical coherence tomography

Abstract: CWD6 Fig. 2 ODT images of in vivo blood flow in the vasculature of rat mesentery. Three circular structures are evident in the structural image (A). Gray scale velocity images of arterial flow (B) and venous flow (C) indicate that there are one arteriole and two veins in the region scanned. The horizontal cross-sectional velocity profile (D) through the arteriole and large vein resembles parabolic functions and suggests laminar flow.

Optical Measurement of Biaxial Strain in Thin Film Polymers

Abstract: A non-contacting optical method for the measurement of strain simultaneously in two directions has been developed in the process of characterizing the behavior of thin film polymers. The thin film specimen geometry is a cylinder created by heat sealing two layers of the material together. The technique utilizes a random speckle pattern that is painted on the surface of the specimen. Images are captured by a CCD camera of a small portion of the painted region as the material creeps under load. A digital image correlation technique is used to analyze the movement of the speckle pattern over time. The resulting displacement field obtained from these images is then used to determine the strain on the surface of the specimen. This strain measurement technique has also been extended to the measurement of biaxial strains on the surface of thick sheets as well as thin films.

Erratum: Improved three-dimensional image correlation for surface displacement measurement [Optical Engineering 35(7), 1911 2004 (1996)]

Abstract: This PDF contains the Communications and Errata for Optical Engineering Volume 36, Issue 8.

Phase images in non destructive evaluation

Abstract: Optical image processing offers in many cases a reliable and very simple way to visualise phase images. The key concept of optical processing is that an expanded beam of coherent light reads the amplitude and phase of an image, and, when focused by an optical system, produces the bidimensional Fourier transform of the image, which is filtered by a suitable optical filter and reconstructed to give the processed image. Several methods are able to visualise phase images: those most known are the phase contrast, the strioscopy, the Schlieren and Shadowgraph techniques, and the technique of rotating derivative filters. An integrated interpretation of all the mentioned techniques basing on the general theory of optical processing of images constitutes an important theoretic result and is useful in the experimental practice, for instance in designing the optical system in relation to the characteristics and the desired aspect of the phase object to be visualised. From the visualised phase image is often possible to derive with a great precision the measure of characteristics of systems and materials and to analyse phenomena and processes. Optical processing is practically instantaneous, therefore it can follow very rapid processes; on the other hand it is essentially a planar technique, in the sense that only one plane is exactly focused, so that the examined images must be relatively thin.

New digital speckle intensity correlation search technique

Abstract: In this paper, a new digital speckle intensity correlation searching technique which is named as 'differential oriented search' is proposed. This method effectively makes use of the distribution characteristics of the correlation functions, which makes the calculating time be considerably reduced. Experimental results shows that, the method presented here is a kind of high speed, high accurate and practical correlation metrology.

Optical Image Correlation Using Wavelength Multiplexed Volume Holograms.

Abstract: : An optical image correlator using a spatial light modulator (SLM) for reference image recording and correlation readout has been demonstrated. The reference image correlation filters are recorded in a volume holographic optical element using wavelength multiplexing. Auto and cross correlations between similar and dissimilar images were compared to determine the image discrimination characteristics of the optical correlator.

Effect of composite structure on strain and failure of laminar and wafer composites

Abstract: The effect of composite structure on the distribution of strain and failure modes in wood laminar and wafer composites was investigated. Mechanical testing was performed using bending tests of solid wood, plywood, and oriented strandboard specimens (OSB) in accordance with ASTM D143-83, Standard Methods of Testing Small Clear Specimens of Timber [1], and ASTM D3043-83, Standard Method of Testing Structural Panels in Flexure [2], The strain distributions were measured using a new technique, the digital image correlation technique Strain distributions from bending tests on small, clear specimens of solid wood indicated close agreement with conventional beam theory and exact correlation with the failure patterns oflht specimens. The layered annual ring structure of the solid wood controlled the path of the tension cracks. Neither composite board followed conventional beam theory entirely. Of the two composites investigated in this study, the laminated veneer board exhibited behavior closer to conventional th...

Image correlation techniques for variable-aperture radio telescopes

Abstract: A new possibility of two-dimensional image creation for variable-aperture antennas is considered. An image correlation technique is proposed to recover two-dimensional images in a single passage of the source through the two-dimensional radiation pattern of the RATAN-600 radio telescope in the incoherent irradiation mode. High sensitivity of instantaneous mapping can be reached in spectrographic observations by both integration in the frequency band and long-term object tracking.