Showing papers on "Digital image correlation published in 2004"

Polarization-sensitive complex Fourier domain optical coherence tomography for Jones matrix imaging of biological samples

Abstract: A polarization-sensitive Fourier domain optical coherence tomography (PS-FD-OCT) system is demonstrated. This OCT system is based on a spectral interferometer, does not require mechanical axial scanning, and enables phase information to be used in an OCT image. Owing to this phase information, the system requires only two measurements for determining the Jones matrix images and Muller images of biological samples. This system reveals the birefringence properties of the inner surface of a porcine esophagus.

Digital Image Correlation to Evaluate Shear Banding in Dilative Sands

Abstract: Experimental results are presented of plane strain compression experiments on dense sands, in which the technique of Digital Image Correlation (DIC) was used to quantify localized displacements. The technique is described in detail. Results of accuracy tests of the DIC technique indicated that localized displacements were measured to an accuracy of ±0.008 mm. Typical DIC analyses produced approximately 250 displacement points within a shear band, which served as input for regression based strain computation, yielding highly accurate volumetric strain data for determining evolutions to critical state. Displacement data also were used to investigate volume changes outside the shear band. The DIC technique also enabled measurement of shear band inclination and thickness throughout the deformation process.

Adaptive Cross Correlation for Imaging Displacements in Soils

Abstract: Digital image correlation (DIC) is used in this paper to study two-dimensional spatial soil deformations nonintrusively. Adaptive cross correlation (ACC), which is an advanced cross-correlation alg...

Mesure tridimensionnelle de champs cinématiques par imagerie volumique pour l'analyse des matériaux et des structures

Abstract: Surface digital image correlation techniques are extended to the analysis of volume data obtained from X-ray computed tomography with micrometric resolution or serial reconstruction obtained under plane laser illumination for transparent materials. The local image contrast is provided by markers much smaller than the characteristic dimensions of the gradients to analyze and which do not disturb notably the latter, as checked by numerical simulations. In case of computed tomography, small dense particles are randomly added, while optical scatterers are used in transparent materials. The resolution of the displacement measurement, evaluated on rigid body motions, can be close to 0.01 voxel but can be strongly degraded on noisy images. New matching criteria, less sensitive to noise, are proposed and discussed. Presented applications concern plastic homogeneous flat or notched samples under tension, as well as particulate metal matrix composites.

Procedure for the Determination of True Stress-Strain Curves From Tensile Tests With Rectangular Cross-Section Specimens

Abstract: The problem of determining true stress-strain curves from flat tensile specimens beyond the onset of necking has been investigated based on finite element analyses under consideration of experimental accessible data using digital image correlation (DIC). The displacement field on the specimen surface is determined by in-situ deformation field measurement. A three-dimensional finite element study with different stress-strain-curves has been carried out to develop a formula, with which it is possible to calculate the true stress subject to the strain in the necking region. The method has been used to evaluate the true stress-strain curve with a so-called micro flat tensile specimen, which is normally used to determine the material properties in the material gradient around thin weldments.

Investigation of Bond between Fiber Reinforced Polymer and Concrete Undergoing Global Mixed Mode I/II Loading

Abstract: A novel experimental method using modified double cantilever beam specimens and a customized test frame are introduced to evaluate bond characteristics and toughness of fiber reinforced polymer (FRP) composite overlays and a concrete substrate under mixed mode loading. A computer vision system is used to measure the crack location, near-tip deformations and crack opening displacement during the crack growth process. Digital image correlation is used to determine the crack opening displacement (COD) for flaws growing in the vicinity of the FRP-concrete interface. Results from this study indicate that during crack growth, the Mode I component of COD is dominant for all angles of specimen loading; the magnitude of the local Mode I component of COD is maximized when good bond quality is present and crack extension occurs within the mortar/concrete near the FRP-concrete interface; and good agreement exists between independent energy release rate estimates based upon both an approximate elastic double cantilever beam formulation and also use of the measured components of COD in a classical linear elastic expression.

A method to determine the macroscopic toughness scatter of brittle materials

Abstract: An experimental procedure is developed to determine the scatter of the macroscopic toughness of brittle materials. First, samples are precracked to obtain a sharp precrack. The toughness is then determined by using a standard three-point flexural test. Digital image correlation is used to analyze displacement fields of cracked samples. Based upon the resolution and the spatial resolution of the measurement technique, a detection criterion is proposed and validated. It allows for an accurate estimate of the crack tip location so that the presence of a crack and its size at arrest can be monitored. As an example, the toughness distribution of 18 samples made of silicon carbide is evaluated. By using a simple macro-micro transition, an analysis of the scatter in toughness is related to that in strength for the material with no macrocracks.

Effects of heat treatment on the compressive deformation behavior of open cell aluminum foams

Abstract: This paper presents a study of effects of heat treatment on compressive deformation behavior of Duocel® open cell aluminum foams in as-fabricated (F), annealed (O) and T6-strengthened (T6) conditions. Following an investigation of the effects of heat treatment on foam strength and stress–strain behavior, heterogeneous nature of the compressive deformation was then elucidated using a combination of experimental investigation and digital image correlation (DIC) technique. DIC analyses were carried out on image sets that were recorded using an in situ digital camera during compressive testing. The significant effects of heat treatment processes on foam compressive strengths, stress–strain behavior, and deformation mechanisms are then attributed to the changes of the microstructure and micro-scale mechanical property of individual struts due to annealing and T6-strengthening processes.

Mechanics of thin films and microdevices

Abstract: This paper discusses the latest developments in nanomechanics of thin films with applications in microelectromechanical systems (MEMS) and microelectronics. A precise methodology that combines in situ atomic force microscopy (AFM) surface measurements of uniaxially tension-loaded MEMS specimens and strain analysis via digital image correlation (DIC) achieving 0.1 pixel spatial displacement resolution is presented. By this method, the mechanical deformation of thin films was obtained in areas as small as 4 /spl times/ 4 /spl mu/m and with 1-2 nm spatial displacement resolution supporting the derivation of interrelations between the material microstructure and the local mechanical properties. This methodology provided for the first time the values of Young's modulus and Poisson's ratio from specimens with cross-sections as small as 2 /spl times/ 6 /spl mu/m. The value of properties derived via AFM/DIC demonstrated very limited scatter compared to indirect mechanical property measurement methods. The application of this technique on nonuniform geometries resolved nanoscale displacement and strain fields in the vicinity of ultrasharp elliptical perforations achieving very good agreement with finite element models. Furthermore, the stochastic and deterministic material failure properties described via Weibull statistics and fracture toughness, respectively, are illustrated for brittle thin films. Failure initiated at notches was found to be influenced by the local radius of curvature and the stress concentration factor. Precise fracture toughness values for MEMS materials were obtained from MEMS specimens with atomically sharp cracks. These studies were supported by measurements of displacements/strains conducted for the first time in the vicinity of mathematically sharp cracks via the AFM/DIC method. The method can be applied to a variety of thermomechanical reliability problems in multilayered thin films and inhomogeneous/anisotropic materials.

Ultrasound-modulated optical computed tomography of biological tissues

Abstract: An optical imaging technique called ultrasound-modulated optical computed tomography is demonstrated for tomographic imaging of biological tissues. Ultrasound-modulated optical signals are extracted from scattered light to provide projection data for the image reconstruction. A filtered back-projection algorithm is implemented to reconstruct an image reflecting optical tissue properties from angular and linear scans of an ultrasonic beam across a sample. This reconstruction-based imaging technique provides a way to obtain images of cross sections containing the scanned ultrasonic axis in biological tissues, which enables three-dimensional ultrasound-modulated optical imaging. The technique combines the contrast advantage of optical waves and the resolution advantage of ultrasonic waves.

Integrated method for 3-D rigid-body displacement measurement using fringe projection

Abstract: A novel method using a charge-coupled device (CCD) camera is developed to measure 3-D rigid-body displacement of an object. This method combines fringe projection and digital image correlation (DIC) methods into one optical system. In this method, sinusoidal fringes are projected on an object using a liquid crystal display (LCD) fringe projector. Images of the object's surface are captured by a CCD camera and stored for further processing. With the aid of the Fourier transform, the fringes in the images are removed while the background intensity variation is preserved. DIC is subsequently used to obtain in-plane displacement using the fringe-free images. The original images are also processed by fast Fourier transform (FFT) to obtain information on the shape of the object. Based on the in-plane displacement obtained by DIC, the reference and measured profiles are compared to obtain out-of-plane displacement. Experiments are conducted on a small coin, and the results demonstrate that both in-plane and out-of-plane displacements can be accurately measured using the proposed method.

Biological image correlation device and correlation method thereof

Abstract: A biological image correlation device includes: an image quality priority decision section (40) for evaluating image quality for each of a plurality of first image data consisting of biological images relating to the same object, according to a particular index which is in relation of a monotone function with verification accuracy of biological verification, assigning priority to each of the first image data according to the evaluation result, and outputting them; first image storage sections (6, 81) for storing the first image data to which the priority is assigned and output from the image priority decision section (40); second image storage sections (8, 61) for storing second image data used for comparison and correlation with the first image data; an image correlation section (7) for comparing and correlating the reference image data stored in the second image storage section with the first image data stored in the first image storage sections (6, 81) and outputting the comparison/correlation result; and a control section (9) for controlling the aforementioned components. The control section (9) makes the image correlation section (7) compare/correlate the second image data stored in the second image storage sections (8, 61) with the first image data stored in the first image storage sections (6, 81) according to the aforementioned priority and output the correlation result from the image correlation section (7).

Iterative Fourier reconstruction for laser surgery and other optical applications

Abstract: Methods, systems and software for determine an optical surface model for an optical tissue system using Fourier transformation algorithms. A method of reconstructing optical tissues of an eye comprises transmitting an image through the optical tissues of the eye. The surface gradients from the transmitted image are measured across the optical tissues of the eye. A Fourier transform algorithm is applied to the surface gradients to reconstruct an optical surface model that corresponds to the optical tissues of the eye.

Stress Intensity Factor of Wood from Crack-Tip Displacement Fields Obtained from Digital Image Processing

Abstract: Stress intensity factor of radiata pine (Pinus radiata) in Tangential-Longitudinal opening mode was determined from crack-tip displacement fields obtained from digital image correlation in conjunction with orthotropic fracture theory. For lower loads, experiments agreed with the linear elastic fracture theory but for higher loads, stress intensity factor and load relationship was nonlinear. For 41% of the specimens tested, tip-displacement based stress intensity factor agreed with that based on the ASTM standard formula for lower loads but deviated for higher loads closer to failure. The tip displacement plots showed that the nonlinear behaviour is due to large displacements which we attributed to large plastic deformations and/or micro-cracking in this region. The other 59% specimens showed a similar trend except that the crack-tip based stress intensity factor was consistently higher than the value obtained from the standard formula. The fracture toughness from tip displacements was larger than the standard values for all specimens and the two were related by a logarithmic function with an R2 of 0.61. The study also established that fracture toughness increases with the angle of inclination of the original crack plane to the Radial Longitudinal plane.

System and method for improving accuracy in a speckle-based image correlation displacement sensor

Abstract: A system and method for improving the accuracy of a speckle-based image correlation displacement sensor provides ultra-high accuracy by ensuring that, in the absence of motion, the speckle image does not vary over time. In one embodiment, the speckle image is stabilized by reducing or compensating for laser diode wavelength changes. Various methods for stabilizing the wavelength include thermoelectric temperature control, measuring and correcting the wavelength by any suitable means, or providing a specific wavelength of light feedback from an external grating. Image stabilization may also be accomplished by monitoring the warm-up process of the system. Once the system is determined to have completed the warm-up process, an indicator is provided to the user to indicate that the system is ready for use. Sensor geometric configurations that reduce or eliminate wavelength-related errors are also disclosed.

Size effects in metal foam cores for sandwich structures

Abstract: The shear response of aluminum foam, including size effects, is measured and quantified for a closed-cell aluminum foam. The shear stiffness is shown to depend linearly on density, whereas the strength exhibits a power law dependence. The linear response is shown to be independent of strain rate up to rates of 0.17/s, whereas the strength and energy absorption increase with increasing strain rate. The density dependence of the stiffness is reproduced analytically based on the composite cylinders model. Optical techniques are used to measure the strain field of the experimental specimens throughout the loading program. By evaluation of concentric subregions of the sample, a sample size of 18 mean cell diameters is determined to be the dimension below which the uncertainty in the predicted shear modulus of an aluminum foam sample increases significantly. This length scale threshold is replicated in a periodic finite element structure with randomly distributed imperfections. I. Introduction M ETAL foams represent an attractive alternative for sandwich structure cores for multiple reasons. First, with metal foam cores, the adhesive substrate of a sandwich structure may be eliminated with in-production integral bonding to metallic face sheets, stiffening the sandwich and broadening its range of operating environments. Second, metal foams exhibit a compressive stress‐strain response that is ideal for energy absorption and impact alleviation with a long, constant stress, plastic strain plateau. 1 Third, an opencell metal foam offers an opportunity to eliminate the catastrophic nature of water or cryogenic gas permeation that has crippled the long-term use of sandwich constructions with honeycomb cores. 2 Fourth, an open-cell construction also allows for active cooling of the sandwich structure, elevating its range of acceptable operating temperatures. For integration into sandwich structures, the shear behavior of metal foam must be understood. Some disparate results regarding shear behavior currently exist in the literature. One study found a linear relationship between shear strength and density, 3 whereas a cubic lattice model subjected to shear loading predicted a nonlinear power law dependence. 4 Another investigation offers only a few data points for shear stiffness and strength of melt-foamed aluminum. 5 Furthermore, these experiments involved thin specimens, with no account for size effects. The present paper offers the full shear response curves for a broad range of density. The density dependence of stiffness and strength are found experimentally with the former being reproduced analytically. The strain rate dependence of the shear response is also considered. The effect of specimen size, relative to the mean cell size, is analyzed experimentally with a unique approach involving digital image correlation. The observed behavior is reproduced with a finite element model. These analyses identify a threshold in the ratio of specimen size to cell size, below which the shear response of a given sample is associated with a significant amount of uncertainty.

Plastic Surface Strain Mapping of Bent Sheets by Image Correlation

Abstract: A technique using a single CCD camera, a precision rotation/translation stage, a telecentric zoom lens, and digital image correlation software is described for measuring surface profiles and surface plastic strain distributions of a bent thin sheet. The measurement principles, based on both parallel and pinhole perspective projections, are outlined and the relevant mathematical equations for computing the profiles and displacement fields on a curved surface are presented. The typical optical setup as well as the experimental measurement and digital image correlation analysis procedure are described. The maximum errors in the in-plane and out-of-plane coordinates or displacements are about ±5 and ±25 μm, respectively, and the maximum errors in surface strain mapping are about 0.1% or less based on a series of evaluation tests on flat and curved sample surfaces over a physical field of view of 15.2 × 11.4 mm2. As an application example, the shape and surface plastic strain distribution example, the shape and surface plastic strain distributions around a bent apex of a flat 2 mm thick automotive aluminum AA5182-O sheet, which underwent a 90° bend with three bend ratios of 2t, 1t, and 0.6t, are determined using the proposed technique.

A super-resolution particle image velocimetry interrogation approach by means of velocity second derivatives correlation

Abstract: The present study proposes a super-resolution approach for the analysis of particle image velocimetry (PIV) recordings. The method is based on image correlation with respect to the second spatial derivatives of the particle image displacement distribution. The direct measurement of the displacement second derivatives (spatial curvature) is obtained by maximizing the product of deformed particle image patterns. The paper describes the performance of the cross-correlation approach in terms of spatial resolution in analogy with linear filters (moving average) as directly applied to the displacement distribution over the interrogation window. The proposed method aims at reducing the evaluation error and introduces a correlation scheme, which directly measures the local second derivatives of the displacement distribution over the interrogation window. The window product is maximized separately for each spatial derivative term in order to reduce the large computational cost associated with image deformation resampling and image product. The method's performance is assessed first by evaluating the modulation transfer function using synthetic PIV images with a one-dimensional sinusoidal displacement, where results show a factor of three spatial resolution enhancement. The extension to the two-dimensional case is obtained by simulation of homogeneous random fluctuations. The measurement uncertainty is kept at the same level as that of the window deformation iterative and multi-grid method (WIDIM). The assessment of the method's performances in actual experimental conditions is made by analysing a wall jet flow, focusing the attention on the steep velocity profile across the free shear layer. The assessment compares the velocity and vorticity profiles obtained by varying the size of the interrogation window. The factor of three improvement of the spatial resolution is also confirmed for the experimental case. However, a slight increase of the measurement uncertainty is observed.

Determination of forming limit curves using 3d digital image correlation and in-situ observation

Abstract: The use of finite element analysis in process design of sheet metal forming has become a standard practice. Forming limit curves are used in finite element simulations of sheet metal forming to check for possible failure of the sheet. Forming limit curves are typically determined using the limiting dome height test and the strain circle technique. With the development of the three-dimensional digital image correlation technique, it has become possible to determine the forming limit curves with a higher accuracy and less effort typically by comparing images of the test specimens at the end and at the beginning of the test. A limiting dome height tool has been designed such that it permits in-situ observation of the part while it is being formed. Combining in-situ observation with three-dimensional digital image correlation, it is possible to monitor the evolution of strain throughout the sheet from the beginning of the forming to the failure of the sheet. This eliminates the need for an abort criterion since limiting strains can be obtained by considering the deformation stage just before the onset of localized necking. Using this technique, forming limit curves for different aluminum alloys have been determined. The effects of anisotropy and lubrication on limiting strains have also been investigated.

Mixed Mode (KI+KII) Stress Intensity Factor Measurement by Electronic Speckle Pattern Interferometry and Image Correlation

Abstract: The surface displacement fields of a fatigue precracked compact tension sample under tensile load were registered by electronic speckle pattern interferometry and image correlation. The in-plane elastic strain fields calculated from the displacement data were used to obtain the first stress invariant, which for the case of plane stress is proportional to the real part of the first complex potential in Muskhelishvili's approach. Solutions for the stress fields around the crack tip, KI and KII were sought in the form of the Fourier series using Muskhelishvili's complex stress functions. The Fourier series coefficients were calculated from the displacement data using multiple point overdeterministic method (MPODM). The nominal and inferred KI values differ by around 10%; this is probably due in part to mixed mode (KII) loading introduced by some degree of misalignment during the experiment.