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Application of Stereovision to the Mechanical Characterisation of Ceramic Refractories Reinforced with Metallic Fibres
Abstract
Optical methods that yield displacement or strain fields are now emerging significantly in the mechanical sciences. At the Research Center on Tools, Materials and Forming Processes (CROMeP) at École des Mines d'Albi, a binocular stereovision system has been developed that can be used to measure: (a) the three-dimensional (3D) shape of a static object, or (b) the surface strains of an object undergoing some 3D mechanical or thermal stress. In this paper, the application of the stereovision technique to investigate the behaviour of ceramic refractories reinforced with metallic fibres is presented: (i) after the rupture of the sample for assessing the 3D orientation of the fibres in order to correlate a micro-mechanical model of fibre pullout with the macro-mechanical results of tensile tests, and (ii) during a tensile test for measuring the 3D displacement/strain field around a notch.
... Numerous studies have utilized digital image correlation to understand the mechanical behavior of fiber composites [7][8][9][10][11][12][13]. Two recent studies [14,15] have incorporated DIC to examine the mechanical behavior of a 2-D C-fiber polymer matrix composite. ...
... Strain error also decreases with increasing step size of displacements and strains over a subset. In contrast, when h f > h sub , the errors follow the expected scaling relationship[12]: RMS ∝ v RMS / h f . ...
Digital image correlation (DIC) is assessed as a tool for measuring strains with high spatial resolution in woven-fiber ceramic matrix composites. Using results of mechanical tests on aluminum alloy specimens in various geometric configurations, guidelines are provided for selecting DIC test parameters to maximize the extent of correlation and to minimize errors in displacements and strains. The latter error is shown to be exacerbated by the presence of strain gradients. In a case study, the resulting guidelines are applied to the measurement of strain fields in a SiC/SiC composite comprising 2-D woven fiber. Sub-fiber tow resolution of strain and low strain error are achieved. The fiber weave architecture is seen to exert a significant influence over strain heterogeneity within the composite. Moreover, strain concentrations at tow crossovers lead to the formation of macroscopic cracks in adjacent longitudinal tows. Such cracks initially grow stably, subject to increasing app lied stress, but ultimately lead to composite rupture. Once cracking is evident, the composite response is couched in terms of displacements, since the computed strains lack physical meaning in the vicinity of cracks. DIC is used to identify the locations of these cracks (via displacement discontinuities) and to measure the crack opening displacement profiles as a function of applied stress.
... In addition to its non-contact, non-destructive and full-field measurement capability, the 3D DIC technique is well known for its simple experimental set-up and preparation, low environmental vulnerability , easy and automatic processing, and widespread applications. The powerful technique has been successfully applied to many applications2345 in the area of experimental solid mechanics, and is currently attracting more and more attention from researchers and engineers in broader fields. Due to their excellent mechanical properties such as high strength-to-weight ratio, high stiffness-toweight ratio and high impact resistance, carbon fibre composite materials have been chosen to fabricate satellite antennae which usually work under severe ambient conditions for long periods of time. ...
... From this procedure, it can be concluded that there are two crucial steps in the implementation of the 3D DIC technique: (i) camera calibration (i.e. stereovision sensor calibration [4, 5]) and (ii) the stereo matching of points. ...
Application of the three-dimensional digital image correlation technique (3D DIC) to the accurate measurement of full-field surface profile of a 730 mm-diameter carbon fibre composite satellite antenna is investigated in this article. The basic principles of the 3D DIC technique are described. The measured profile was compared with the one measured with a three-dimensional coordinate measuring machine. The results clearly indicate that the 3D DIC technique is suitable for full-field surface profile measurement of small satellite antenna, and further application of the 3D DIC technique to the measurement of thermal deformation of the antenna is expected.
... Before the analysis of the data is performed the histogram of all the frames is normalized 6.5. Solid deformation measurement using stereo-vision In the paper [102] the strain of the object is obtained between two frames, the initial undeformed state and the final deformation. To obtain the value of the deformation a triangle mesh is built and the distortion of the triangles is measured as the strain value of the deformation. ...
In this work is presented an effcient parallel implementation of a Lagrangian mesh-free method based on the Smoothing Particle Hydrodynamics technique to analyse the behaviour of solids and fluids. The proposed solution makes use of different techniques to improve the computational complexity of the simulation. To improve the performance, a parallel solution of the SPH numerical solution is implemented on the GPU using the NVIDIA CUDA architecture. Both the serial and the parallel implementation of SPH used benchmark experiments to compare effectiveness under different conditions. Results are provided with a set of discriminants that allow the program to select which of the implementation (serial or parallel) is best suited to solve the model studied. The SPH modelling implementation is used to solve fluid flows simulation in a soil core's pore structure. The study of this problem is important to understand how heterogeneous soil structure affects water and solute transfer. This helps to obtain accurate predictions of contaminants residence time (average amount of time that a contaminant particle spends inside the pore network) and the quantity leached in the zone represented by the structures. In order to have a method to quantify the accuracy of the results provided by the SPH implementation the commercial software COMSOL was used to obtain the numerical results on the same models solved by the SPH implementation. Finally both results are compared against the expected theoretical results. The second application of the developed computational implementation is the study of the deformation of solid objects under stress deformation. The SPH model in conjunction with a stereo technique provides a method capable of defining the solid material variables. The obtained variables can be used in solid deformation modelling using SPH.
... Robert [19] conducted four-point bending and tensile tests on a fiber reinforced refractory castable and the DIC results were better than using the measurements of classical beam detection that demonstrated the ability to determine Young's modulus from heterogeneous strain fields. Orteu [22] investigated the tensile mechanical behavior of ceramic refractories reinforced with metallic fibers and correlated a micro-mechanical model with macro-mechanical results. Xie [23] evaluated the aluminum edge stretching limit using DIC. ...
This paper introduces an aviation industrial application of digital image correlation (DIC) technique on the measurement of residual tensile strength (RTS). In order to investigate multi-site damage (MSD) that is common in the fuselage of aging aircraft, RTS of 2024-T4 aluminum alloy sheet with MSD was evaluated using DIC technique. Firstly, the four-factor and three-level orthogonal experiment was designed to optimize the DIC method to control the strain calculation error by considering subset size, interpolation tap, calibration score and step size. Secondly, RTS and strain fields were generated to analyze the path of crack propagation. The results show the optimal factor combination is 0.018 of calibration score, 23 pixels of subset size, step size is 1/4 of subset size and the filter size of interpolation calibration is 8 pixels. With the increase of spacing between adjacent holes, the RTS increases and the collinear cracked specimen becomes more perilous than that of non-collinear cracked while the hole spacing is 25 mm from the statistical analysis. Based on the Net section yield criterion, the RTS was calculated, which can give a conservative prediction of RTS.
... Thus, with an increasing fibre content, the ceramic specimens presented a more ductile behaviour. It is worth mentioning that Orteu et al. [32] and Ozyurt et al. [33] reported increased ceramic ductility with fibre substitution because the fibres prevented fissure propagation. The results obtained from the thermal shock tests, in triplicate for each mixture, are reported in Table 4. ...
Despite their high hardness and thermal stability, refractory ceramics have a tendency to undergo failure due to fatigue and thermal shock. In this study, the effects of rice husk silica and steel fibres on the mechanical and thermal properties of refractory ceramic materials were investigated. Samples were fabricated by replacing kaolin clay with 20% rice husk silica and 3%, 6% or 9% steel fibres in volume percentages. These specimens were analysed for their mineralogical composition, density, apparent porosity, tensile strength in a three-point bending test, thermal shock and microstructural analysis by scanning electron microscopy. For materials with silica and clay, a reduction in porosity and increase in mechanical resistance was observed. Ceramics with 9% steel fibre had the greatest ductility, while materials with 3% and 6% steel fibre presented the highest resistance against thermal shock.
... metal, composite, polymer, wood, biological materials) subjected to different types of loadings (thermal, mechanical or other) ( Bastawros et al., 2000;Wattrisse et al., 2001;Abanto-Bueno et al., 2002;Périé et al., 2002;Chevalier et al., 2001;Xiang et al., 2007;Chiang, 2008;Feissel et al., 2009Feissel et al., , 2012Feissel et al., , 2013. Recently, numerous works were carried out to better understand the behaviour of refractory materials thanks to this technique such as the characterisation of mechanical behaviour of fiber reinforced refractory castables ( Orteu et al., 2007;Robert et al., 2007;Dusserre et al., 2013), the thermal strain behaviour of Al 2 O 3 -MgO castables (Kamio, 2011;Kamio et al., 2011) or the asymmetric behaviour of aluminium titanate Al 2 TiO 5 ceramics and identification of their stress-strain law ( Leplay et al., 2010Leplay et al., , 2012). ...
Refractory materials are present in a large panel of industrial applications due to their specific properties. They are applied in several parts of many installations designed to face severe working environments such as high temperature, corrosion and thermal shock solicitations. Among the required properties, thermal shock resistance is a key parameter to enhance their service life. This thermal shock resistance is usually directly related to a specific mechanical behaviour induced by a voluntary micro-cracked microstructure. Indeed, depending on micro-cracking level, the mechanical behaviour will change from pure elastic to a “non-linear” one.
In order to accurately determine constitutive laws for such materials to feed finite element method (FEM), it was necessary, during the past 15 years, to develop uniaxial tests taking into account that these materials are characterized by a low level of strain-to-rupture. Nowadays, accurate instrumented tensile test using extensometers are available. However, such tensile devices involve some complexity for tests management (sample machining and well aligned loading grips). Fortunately, the occurrence of optical techniques such as digital image correlation (DIC) which allows to obtain the overall strain fields on a given loaded sample, coupled to FEM modelling, give today new opportunities for experimental investigations of refractory materials exhibiting a non-linear mechanical behaviour.
In this context, the main objective of the present PhD was to enrich the mechanical characterization of refractory materials by evaluating the efficiency of DIC applied on quite common mechanical tests in this scientific community: such as four-points bending test, Brazilian test and Wedge splitting test. For this purpose, it was necessary first for SPCTS laboratory to master this new DIC measurement tools and to apply them on refractory materials which exhibit low level of strain-to-rupture.
This research activity has been developed in the framework of the Federation for International Refractory Research and Education (F.I.R.E) which aims to promote a worldwide collaboration between academic institutes and industrial companies to pool the expertise at master and PhD levels in the field of refractories. In this purpose, multi-partners programs are regularly launched to support research activities. The present PhD (funded by the region of Limoges) has been part of F.I.R.E project D which took place between 2011 and 2014 and which was devoted to “dense refractories with enhanced flexibility for thermal shock”. This research program was organized through a collaboration between two laboratories (SPCTS-Limoges-France, RWTH-Aachen-Germany) and four industrial partners (Alteo-France, RHI-Austria, Tatasteel-Netherland and Tenaris-Argentina).
Even if the objective of the present PhD was to apply DIC techniques to refractory characterization, one should note here that absolutely no DIC expertise was present at SPCTS laboratory before the present work. Thus, in addition to the previously indicated partnerships, and in order to take advantage of the great expertise in photo-mechanics which has been developed for many years in Pprime Institute-Poitiers, a close collaboration has been also established with these colleagues. Nevertheless, due to the very low level of strain-to-rupture of refractory materials, it has been necessary to improve DIC techniques for our own purpose.
The first chapter is dedicated to establish the state of the art concerning thermal shock of refractories and non-linear behaviour by mechanical and energetic approaches. Then, optical methods used to complete the mechanical investigation of refractories such as DIC and mark tracking method have been introduced. Besides, in order to overcome the problem of mechanical characterization of the non-linear behaviour, kinematic fields obtained by DIC are conjugated to FEM in the framework of the identification technique by finite element method updating (FEMU-U).
The chapter II aims to present different experimental characterization techniques. The mechanical ones are associated to optical methods which are here described in details. Then, the studied materials chosen in order to develop and to valid efficiency of DIC are introduced. Among these materials, a model one based on aluminium titanate (AT) developed for academic purposes, and some industrial ones based on magnesia spinel systems delivered by industrial partners.
The main objective of the first investigation, detailed in chapter III, is to demonstrate the efficiency of the DIC technique as an effective tool to complete the mechanical analysis of refractory materials. This first study highlights the specific non-linear mechanical behaviour of AT during four-points bending test at room temperature.
In chapter IV, the acquired DIC expertise has been used to complete the characterization of industrial magnesia spinel refractories. Even if spinel inclusions are used to promote micro-cracking by thermal expansion mismatch with the magnesia matrix, the low level of flexibility of these industrial materials which is less accentuated than AT, pushed us to improve the accuracy of DIC measurements.
After having studied experimentally the non-linear mechanical behaviour of several refractories, chapter V is dedicated to the numerical development of FEMU-U for linear elastic behaviour materials under four-points bending test, then, the developed approach has been applied for flexible AT materials.
... Metallic fibers are randomly oriented inside the cementitious matrix because of vibration during concrete pouring into molds. As shown in previous works [16,[26][27][28], the introduction of short metallic fibers in a refractory concrete allows to avoid its brittle behavior. On the one hand, diffuse damage develops in the reinforced refractory because of stress fields induced during the firing cycle, by a different dilatometric behavior between fibers and concrete constituents. ...
Hybrid propulsion represents a good alternative to the more widely used liquid and solid systems. This technology combines some important specifications of the latters, as the possibility of reignition, thrust modulation, a higher specific impulse than solid systems, a greater simplicity and a lower cost than liquid systems. Nevertheless the highly oxidizing environment represents a major problem as regards the thermo-oxidation and ablative behaviour of nozzle materials. The main goal of this research is to characterize a silicon carbide based micro-concrete with a maximum aggregates size of 800 μm, in a hybrid propulsion environment. The nozzle throat has to resist to a highly oxidizing polyethylene/nitrous oxide hybrid environment, under temperatures up to 2900 K. Three tests were performed on concrete-based nozzles in HERA Hybrid Rocket Motor (HRM) test bench at ONERA. Pressure chamber evolution and observations before and after tests are used to investigate the ablated surface at nozzle throat. Ablation behaviour and crack generation are discussed and some improvements are proposed.
... 6 The resulting strain error scales with m RMS /h f , where h f is a characteristic dimension used for strain averaging (i.e., the gauge length). 6,7 For gauge lengths employed in computing the average tow-segment strains, the strain error is estimated to be in the range of (1-3) 9 10 À4 . ...
We present a methodology for calibrating the thermoelastic properties of constituent tows in a woven C/SiC composite using a combination of experimental measurements and finite element simulations of strain distributions that arise upon heating. Because of the nonuniform distribution of the matrix phase and the presence of matrix microcracks, tow properties cannot be reliably predicted a priori using micromechanical models alone; instead, some can only be inferred from coupled experimental/numerical studies of the kind presented here. A small number of iterations of finite element simulations is required to achieve satisfactory agreement in all thermal strain components. The number of iterations is minimized by first performing simulations using model-based estimates of the effective tow properties, followed by judicious adjustments to select property values. Comparisons between measurements and simulations are performed on the basis of strains averaged over a characteristic area dictated by tow dimensions. The study reveals that, although each segment of a particular tow genus is nominally equivalent to all others, the strains within each population exhibit considerable variation. This is attributable to variations in both tow geometry and connectivity between adjacent tows. The virtual tests predict strain variations of comparable magnitude.
... It is also of sound interest to have an experimental setup that gives sufficient surface deformation data to describe more precisely 3D computational models [4]. To solve this problem we introduce an effective stereo vision system that can obtain synchronised image pairs at a rate of up to 83 frames per second (depending on lighting and exposure time). ...
Elastic surface deformation of an object is described using depth information obtained at video frame-rate with camera calibration and stereo matching techniques. Two synchronised USB cameras acquire successive image pairs describing the surface deformation. Images are corrected to exclude lens distortion and generate rectified stereo pairs. The depth information is computed with a simplified version of the belief propagation stereo matching algorithm. Experimental results are related to computer simulation of elastic deformation based on the mesh-free smoothed particle hydrodynamics technique. The obtained strain, stress, and internal energy of the object help calibrate the physical properties of the computational model and lead to more realistic simulation of elastic deformation.
... They succeeded in measuring very small strains (< 10 −4 ). Orteu et al. [17] have applied the 3-D DIC technique to characterize the behavior of fiber reinforced refractory castables (FRRC) by performing 3-D reconstruction of metallic fibres and 3-D displacement measurement during a tensile test using virtual extensometers. ...
Refractory castables exhibit very low fracture strain levels when subjected to tension or bending. The main objective of this
work is to show that 3-D digital image correlation (3-D DIC) allows such low strain levels to be measured. Compared to mechanical
extensometer measurements, 3-D DIC makes it possible to reach similar strain resolution levels and to avoid the problem of
position dependance related to the heterogeneous nature of the strain and to strain localization phenomena. First, the 3-D
DIC method and the experimental set-up are presented. Secondly, an analysis of the 3-D DIC method is performed in order to
evaluate the resolution, the standard uncertainty and the spatial resolution for both displacement and strain measurements.
An optimized compromise between strain spatial resolution and standard uncertainty is reached for the configuration of the
experimental bending test. Finally, the macroscopic mechanical behavior of a fiber reinforced refractory castable (FRRC) is
studied using mechanical extensometry and 3-D DIC in the case of tensile and four-point bending tests. It is shown that similar
results are obtained with both methods. Furthermore, in the case of bending tests on damaged castable, 3-D DIC results demonstrate
the ability to determine Young’s modulus from heterogeneous strain fields better than by using classical beam deflection measurements.
... The phenomenon occurring in that zone corresponds to local buckling that is not accounted for by the simple kinematics assumed herein. To analyze accurately the kinematics in that zone, one would need to resort to a full 3D evaluation of the displacement field by using, for instance, stereovision [41,42]. ...
It is proposed to develop a digital image correlation procedure that is suitable for beams whose kinematics is described by an Euler–Bernoulli hypothesis. As a direct output, the degrees of freedom corresponding to flexural and axial loads are directly measured. The performance of the correlation algorithm is evaluated by using a picture of a cantilever beam experiment. One load level is analyzed with the present algorithm. The latter is validated by comparing the displacement field with that given by a finite element based correlation algorithm. It is also shown that a locally buckled zone is detectable with the present procedure.
... This is clearly dependent on the interfacial bond strength, which thus affects both fibre pull-out and fibre plasticity. There have been various observations, over an extended period [16,18,20,21,[24][25][26][27][28][29][30][31][32], concerning the extent and nature of fibre pull-out and plastic deformation in MFCs. In general, it is recognised [20][21][22]24] that strong interfacial bonding is likely to impose greater constraint on fibre plasticity, and hence limit the plastic work done. ...
A model is presented for prediction of the fracture energy of ceramic–matrix composites containing dispersed metallic fibres. It is assumed that the work of fracture comes entirely from pull-out and/or plastic deformation of fibres bridging the crack plane. Comparisons are presented between these predictions and experimental measurements made on a commercially-available composite material of this type, containing stainless steel (304) fibres in a matrix predominantly comprising alumina and alumino-silicate phases. Good agreement is observed, and it’s noted that there is scope for the fracture energy levels to be high (∼20 kJ m−2). Higher toughness levels are both predicted and observed for coarser fibres, up to a practical limit for the fibre diameter of the order of 0.5 mm. Other deductions are also made concerning strategies for optimisation of the toughness of this type of material.
Digital Image Correlation (DIC) technique is used to collect the whole‐field strain of concrete subjected to bending loads under different temperature change cycles, cooling rates, and water‐binder ratio. The bending damage behavior of concrete under large‐difference temperature circulation is analyzed with the whole‐field strain cloud map and data. In addition, with the whole‐field strain obtained by 3D‐DIC technology, the strain‐based damage variable is defined. Furthermore, the damage degree of concrete specimen in the bending process is quantified, and the bending damage evolution process of concrete under the action of temperature circulation is discussed. The results show that the damage of concrete in areas with large temperature differences is related to the number of temperature change cycles as well as the cooling rate. Moreover, the increase in the number of temperature change cycles and the cooling rate will aggravate the bending damage of the concrete. Additionally, the higher the water‐binder ratio, the more serious the adverse effect of temperature circulation on the concrete. This paper provide certain reference for the evaluation of concrete bending deformation damage in areas with a large temperature difference.
Cette thèse s'inscrit dans le cadre du projet CPER Bramms, dont un des objectifs était de développer une méthode d'acquisition de la surface du buste féminin. Les travaux menés ont donc eu pour but la conception, le développement et la réalisation d'une machine de mesure tridimensionnelle adaptée aux objets vivants. Parmi le nombre important de méthodes de mesures tridimensionnelles existantes, l'attention a été portée sur la mise en correspondance par stéréovision ainsi que sur l'utilisation de lumière structurée. La mise en correspondance par stéréovision consiste à retrouver les pixels homologues dans deux images d'une même scène, prise de deux points de vue différents. Une des manières de réaliser la mise en correspondance est de faire appel à des mesures de corrélation. Les algorithmes utilisés se heurtent alors à certaines difficultés : les changements de luminosité, les bruits, les déformations, les occultations, les zones peu texturées et les larges zones homogènes. L'utilisation de lumière structurée a permis essentiellement d'ajouter de l'information dans les zones homogènes lors des travaux menés. En développant cette approche, une méthode de reconstruction originale basée sur l'exploitation d'un motif particulier projeté sur la surface a ainsi été conçue. Un appariement basé sur la comparaison de signatures de points particuliers du motif a été mis en place. Ce procédé permet une reconstruction éparse en une unique acquisition et simplifie l'étape de gestion du nuage de points pour en faire un maillage surfacique
This study proposes a methodology to predict the capacity of aluminum welded connections. In order to evaluate the material characteristics within the heat-affected zone, an inverse analysis methodology, using full-field measurements of the strain field using digital image correlation, was developed during uniaxial tensile tests on specimens extracted from gas metal arc welded 6061-T6 aluminum alloy plates. The identification of the constitutive law problem was formulated within the Virtual Fields Method. The inverse analysis methodology was compared with an identification process of the material in the vicinity of the weld using a fully coupled multiphysics simulation considering thermal, metallurgical, and mechanical mechanisms during heating and cooling. The simulation accounts for the nonhomogeneous hardening properties within the heat-affected zone to extract the constitutive material laws of a welded join. The proposed simulation methodology was used to analyze the structural response of a plate–square hollow structural section (SHSS) joint subjected to tensile loading. The predicted capacity of the specimens was compared with the experimental findings as well as analyses using Canadian code recommendations. It is shown that it is possible to improve the prediction of the capacity of welded aluminum connection using the Canadian recommendations if the width of the heat-affected zone is reduced to 15 mm instead of the original 25 mm.
White light has often been used for surface illumination to acquire images for digital image correlation (DIC) analysis. In recent years, fluorescent imaging technique has been introduced for illumination, surface deformation and topography measurements with applications on various materials including biomaterials (biofilms, etc.) at the microscale. Traditional imaging, with the use of white light, encounters technical issues such as specular reflection owing to moisture or smooth shiny surfaces (e.g., metallic or glass surfaces). As an alternative to white light, fluorescent imaging serves as a solution to resolve the issues of specular reflection. Fluorescent DIC techniques, especially the fluorescent stereo DIC, allow 3D surface profilometry and deformation measurements at the microscale and submicron scale. Fluorescent stereo imaging under a microscope utilizes emission wavelengths that are different from illumination wavelengths to ensure clear images on any surface that might give reflections at certain angles when white light is used, allowing accurate metrology and deformation measurement. In addition microscopic fluorescent imaging provides nanoscale resolutions surpassing Abbe’s diffraction limit. This paper provides a review of the recent advances in fluorescent DIC.
This chapter primarily concerns composite material comprising (discontinuous) metallic fibers within a ceramic matrix, at levels of the order of 5–20 vol%. Such material offers considerable potential for a combination of microstructural stability at high temperature and good mechanical properties – in particular a high toughness – over a wide temperature range. There are several possible production routes for such material, but attention is focused here on a manufacturing procedure that is in fairly extensive commercial use (allowing cost-effective “casting” of large and/or complex components), which involves infiltration of a ceramic slurry into an assembly of fibers in a mold. While various materials could be used for the two constituents, the commercial products concerned are mainly comprised of stainless steel fibers in a matrix that is predominantly alumina. Setting occurs via chemical reactions, and no sintering treatments are needed. Information is presented about the microstructure of this type of material, including the spatial distribution of the fibers, followed by an approach for estimating the fracture energy of such composites, which is dominated by the contributions from fiber pull-out and, particularly, from plastic deformation and rupture of the fibers. There is then a study of the stability of this type of material at high temperature, and hence its tolerance to service conditions. Finally, there is a very brief survey of current industrial usage. It is noted that there may be scope for this usage to expand, particularly if viable measures can be identified for control of the level and nature of the porosity in these materials, which is commonly present at about 10–20%.
Fire ranks high among the potential risks faced by most buildings and structures. A full understanding of temperature effects on fiber reinforced concrete is still lacking. This investigation focuses on the study of the residual compressive strength, stress strain behavior and surface cracking of structural polypropylene fiber-reinforced concrete subjected to temperatures upto 300 oC. A total of 48 cubes was cast with different fiber dosages and tested under compression after exposing to different temperatures. Concrete cubes with varying macro (structural) fiber dosages were exposed to different temperatures and tested to observe the stress-strain behavior. Digital Image Correlation (DIC), an advanced non-contacting method was used for measuring the strain. Trends in the relative residual strengths with respect to different fiber dosages indicate an improvement upto 15% in the ultimate compressive strengths at all exposure temperatures. The stress-strain curves show an improvement in post peak behavior with increasing fiber dosage at all exposure temperatures considered in this study.
Background Surface and Volume Digital Image Correlation Errors and Uncertainties Stereo-Correlation or 3D-DIC Conclusions Bibliography
Studying the performance of concrete and in particular its fracture properties is necessary for assessing the integrity of existing structures. Concrete fracture mechanics parameters are also needed in order to model concrete structures subjected to extreme loading such as earthquakes. The main objective of this paper is to develop a new methodology for the identification of fracture parameters using Digital Image Correlation (DIC) for measuring crack extension. Experimental results from the wedge splitting tests (WST) are used as reference where the crack mouth opening displacement (CMOD) is measured by clip-on-gauge. These results are subsequently compared to the data obtained from the Digital Image Correlation using the ARAMIS system. The identification of the fracture properties is carried out by solving an inverse problem. The associated forward problem is based on the cracked hinge model capable of taking into account the softening phenomenon. This approach approximates the behaviour of WST and allows an analytical solution. The results of inverse identification are confirmed using the finite element analysis (FEA) along with the obtained fracture energy.
One of the most critical demands in the industrialized world lies in the assessment of the integrity of existing infrastructures and the monitoring of their safety. Accurate assessment methods allow engineers to improve maintenance and repair strategies of structures based on reliable and objective data. Techniques based on non-contact, non-disturbing methods proved to be valuable for generating accurate structural measurements. In particular, the monitoring of the concrete fracture is of the most importance. In this research paper, the Digital Image Correlation (DIC) is employed to characterise the development of the fracture process using the wedge splitting test. The experimental observations are thoroughly discussed; with special attention being placed on the monitoring of the crack path, the size of the fracture process zone, the traction free zone (crack extension), the variation of dissipated energy and tensile damage along the crack path.
Optical interferometry combines two or more light waves in such a way that interference occurs between them. This principle is widely utilized in engineering applications to carry out high-sensitivity full-field measurements. The information on the quantity to be determined - spatial coordinates, displacements, strains, etc. - are encoded in a pattern of interference fringes. These fringes form because a reference wavefront interferes with a modulated wavefront experiencing some change in optical path with respect to the reference beam. From the interference pattern, one can obtain a full-field map of phase related to the difference in optical path between object and reference wavefronts. For each point of the specimen, the value of the quantity to be measured depends linearly on the corresponding value of phase. Sensitivity is the main parameter characterizing interferometric techniques and identifies the measurement scale range: it represents the change in the measured quantity between points located on two adjacent interference fringes. Optical interferometry techniques allow us to perform measurements at very different scales, ranging from nanometers to millimeters. It is worthy noting that the level of accuracy achieved in the measurements does not depend on the size of the investigated specimen. Coherent-light interferometry and white-light interferometry differ by the nature of the light used for illuminating the investigated object. Coherent illumination (for example, a laser source) allows sensitivity to be reduced down to a fraction of the wavelength of the light. White-light interferometry allows to carry out measurements on rather large objects without spending the large amount of energy which would instead be required in the case of coherent illumination. Moiré, speckle and holography are the interferometric techniques most widely used in engineering measurements. The chapter reviews the theory behind those methods and presents five cases taken from disparate fields: (i) calibration of gauge blocks; (ii) determination of residual stresses in thin films; (iii) monitoring of failure modes in electronic components subject to thermal cycling; (iv) mechanical characterization of composite laminates for aeronautical use; (v) measurement of deflections and determination of buckling loads for large-scale aircraft panels. The examples illustrated in the chapter cover a wide range of scales both in terms of magnitude of the different quantities to be measured and specimen sizes. Results demonstrate clearly the reliability, robustness and versatility of optical interferometry in engineering applications.
The objective of the thesis is to formulate a strategy that gives a robust identification of constitutive law from full-field measurements taking into account 3D effects. Model validation from global response of samples or structures has shortcomings that can be overcome by the use of full-field measurement techniques. Full-field measurement techniques offer the opportunity to acquire large amount of experimental data that might be useful in the context of identification of constitutive law parameters. Among the full field measurement techniques the most popular is digital and stereo image correlation. The existing strategies to make use of full-field data like the Virtual Field Method, the Equilibrium Gap Method, the Constitutive Equation Gap Method and the Reciprocity Method were limited to 2D applications. However, for a specimen with finite thickness 3D effects must be included. Most importantly, for the case of plasticity, stress triaxiality plays an important role. Its effect must therefore be accounted for in the modelling of the constitutive behaviour of the material. Thus in this thesis we propose a method to identify the parameters of an elasto-plastic constitutive law in which the mechanical model can have 3D kinematics. The strategy has been shown to be noise robust, almost independent of initial parameter guess and mesh refinement and allows differentiating between constitutive models with same global response on the basis of mean correlation error. The identification is shown to be good for both single and multiple cameras. The strategy validation is done for stainless steel. The global identified load displacement response of the strategy for mono and stereo mechanical image correlation is very close to the experiments. Lastly, the material parameters have been identified with very different initial guess but all converge to the same final values which show the robustness of the proposed strategy.
The goal of this work is to develop the methodology for obtaining accurate, three-dimensional measurements of object deformation using a scanning electron microscope. Calibration methods are being developed for the SEM that include novel distortion correction approaches. The 3D reconstruction of the specimen surface is computed using multiple views acquired through motion of the SEM specimen's stage. Since the displacement information obtained from typical SEM stages is not sufficiently accurate for 3D deformation measurements, we are employing a photogrammetric approach to estimate the movements of the sample using specimen images. Using these results, the 3D deformation measurements are computed using a bundle adjustment method that is also common to a range of photogrammetric methods.
Multiple cracking of beams made of cement-based composite was extensively studied by means of a suitable non-contact measurement technique. Images of a grid-pattern bonded onto the surface of a tested specimen were captured by a digital camera. Image processing by means of an appropriate software provided the displacement field within the zone under investigation. Cracks were revealed by discontinuities in the displacement field. Results are presented for three specimens tested in three- or four-point bending. Location, actual length and width are easily and precisely determined for each of the cracks within the zone under investigation. High sensitivity, early crack detection and no need to anticipate crack location are among the main features of this full-field measurement method. Crack propagation is derived from crack diagrams obtained at successive loading levels. Initial length, spacing, opening, propagation and profile of cracks are discussed in terms of mode of loading and bond properties of the material.
Digital image correlation (DIC) is a classic pattern recognition technique that is widely used for experimental measurements,
mainly for: (1) surface displacement analysis in solid mechanics, and (2) particle image velocimetry (PIV) in fluid mechanics.
DIC is based on using a correlation function to locate the best matching position of two images thus predicting movements.
An advanced form of DIC called adaptive cross correlation (ACC) has been adapted for this research. In this chapter, the basis
of the technique and its application are explained. The anticipated errors of using DIC and ACC are also evaluated.
Image Correlation for Shape, Motion and Deformation Measurements provides a comprehensive overview of data extraction through image analysis. Readers will find and in-depth look into various single- and multi-camera models (2D-DIC and 3D-DIC), two- and three-dimensional computer vision, and volumetric digital image correlation (VDIC). Fundamentals of accurate image matching are described, along with presentations of both new methods for quantitative error estimates in correlation-based motion measurements, and the effect of out-of-plane motion on 2D measurements. Thorough appendices offer descriptions of continuum mechanics formulations, methods for local surface strain estimation and non-linear optimization, as well as terminology in statistics and probability. With equal treatment of computer vision fundamentals and techniques for practical applications, this volume is both a reference for academic and industry-based researchers and engineers, as well as a valuable companion text for appropriate vision-based educational offerings. © Springer Science+Business Media, LLC 2009. All rights reserved.
An innovative technique for measuring both the shape, the displacement, the strain and the temperature fields at the surface
of an object using a single stereovision sensor is proposed. The sensor is based on two off-the-shelf low-cost high-resolution
uncooled CCD cameras. To allow both dimensional and thermal measurements, the sensor operates in the visible and near infrared
(NIR) spectral band (0.7–1.1 μm), and a radiometric and geometric calibration of the sensor is required. This technique leads
to a low-cost camera-based simplified instrumentation that gives simultaneously dimensional/kinematical and thermal field
measurements.
A multiple-camera system (more than two cameras) has been developed to measure the shape variations and the 3D displacement
field of a sheet metal part during a Single Point Incremental Forming (SPIF) operation. The modeling of the multiple-camera
system and the calibration procedure to determine its parameters are described. The sequence of images taken during the forming
operation is processed using a multiple-view Digital Image Correlation (DIC) method and the 3D reconstruction of the part
shape is obtained using a Sparse Bundle Adjustment (SBA) method. Two experiments that demonstrate the potentiality of the
method are described.
KeywordsMultiple cameras–Multiple-camera calibration–Digital Image Correlation (DIC)–Sparse Bundle Adjustment (SBA)–Shape Measurement–3D Displacement Measurement–Single Point Incremental Forming (SPIF)
An experimental investigation to determine the strain distribution and collapse behaviour for AA6061-T6 square cross-sectional
extrusions with and without circular discontinuities under quasi-static axial compressive loading was completed. Three-dimensional
digital image correlation (DIC) was utilized for strain assessment. In order to validate the results of the optical strain
measurement system, tensile tests were first conducted employing both the DIC technique and a traditional extensometer. Strain
observations from both methods were found to be very consistent prior to strain localization in the test specimen. Quasi-static
axial crushing tests were then conducted. Extrusions considered for the present research had a nominal side width, wall thickness
and length of 38.1mm, 3.15mm, and 200mm, respectively. A centrally-located circular hole with diameter of either 14.29mm,
10.72mm or 7.14mm was incorporated into the extrusion. Square tubes without any discontinuities were also considered in
the experimental testing program. Testing results showed that the collapse mode of the extrusion altered from global bending
to a cutting and splitting deformation mode with the presence of the circular discontinuity. Strain localization occurred
near the vicinity of the holes for all specimens. For discontinuities sized 14.29mm and 10.72mm the location of strain localization
and the initiation of material fracture was at the edge of the discontinuity while the location for extrusions with a 7.14mm
hole was found to occur at the intersection of the extrusion side walls. Maximum values of the effective strain were found
to vary from approximately 60% to 100%. The region of strain localization was consistent with the location where material
fracture initiated.
KeywordsAA6061-T6–Aluminum alloy extrusion–Circular geometrical discontinuities–Digital image correlation–Strain distribution
Optical methods that give displacement or strain fields are now widely used in experimental mechanics. Some of the methods can only measure in-plane displacements/strains on planar specimens and some of them can give both in-plane and out-of-plane displacement/strain fields on any kind of specimen (planar or not). In the present paper, the stereovision technique that uses two cameras to measure 3-D displacement/strain fields on any 3-D object is presented. Additionally, a quite inclusive list of references on applications of stereovision (and 3-D DIC) to experimental mechanics is given at the end of the paper.
Optical methods that give displacement or strain fields are now emerging significantly in the mechanical sciences. Much work has been done on two-dimensional (2D) displacement/strain measurement from a single camera but the proposed methods give only in-plane strains. A binocular correlation-based stereovision technique has been developed:(a)to measure the three-dimensional (3D) shape of a static object or(b)to measure the strains of an object undergoing some 3D mechanical or thermal stress.In this paper, the application of the stereo-correlation technique to measure accurately the 3D shape of a stamped sheet metal part or the surface strain field undergone by the part during the stamping process is presented.
Today heat resistant cast steels are the nominal solution for Ti-SPF forming die manufacturing. Nevertheless, this materials present some drawbacks related to delivery time and cost. A fibre reinforced refractory castable (FRRC) is proposed as a new solution for prototype SPF die manufacturing. Due to the general brittleness of refractory castables, a short fibre reinforcement has been investigated in order to avoid catastrophic failure during the forming process. General macroscopic behavior of such materials is very complex and presents large evolutions with the testing temperature. The paper addresses the important benefits of the reinforcement for refractory castable in the case of loading on a complex structure. The capability of the material to support several cracks is shown in the case of a technological sample with a complex shape.
Optical methods that give displacement or strain fields are now emerging significantly in the mechanical sciences. In the Material Research laboratory at École des Mines d'Albi, we have developed a binocular stereovision system that can be used a) to measure the 3D shape of a static object or b) to measure the deformations of an object undergoing some 3D mechanical or thermal stress. In this paper, we will focus on two current applications of the stereo-correlation method: 1. the measurement of local deformations on stamped sheet metal parts, 2. the measurement of 3D displacement and strain fields on an inflated elastomer membrane undergoing large elongations (>100%).
De nombreux domaines concernant le comportement mécanique des matériaux posent le problème de la mesure des déplacements ou des déformations. Pour ce type de mesure, les méthodes optiques se sont largement imposées en raison de leur caractère non intrusif, de leur grande résolution spatiale, de leur sensibilité élevée, de l'importance du champ examiné à tout instant et des progrès de l'informatique qui permet le traitement automatique d'un grand volume d'information.
Dans ce contexte, nous avons développé un système de mesure de formes 3D ou de champs de déplacements 3D par stéréovision (en particulier par stéréo-corrélation).
Cette technique permet : (1) la mesure de la forme 3D d'un objet à partir d'une simple paire d'images stéréoscopiques de l'objet, (2) la mesure de champs de déplacements 3D à partir d'au moins 2 paires d'images correspondant à différents instants de déformation de l'objet (en général analyse d'une séquence de paires d'images acquises en cours de déformation).
Les points développés dans la thèse sont : le calibrage fort d'une caméra ou d'un capteur de vision stéréoscopique, la reconstruction 3D par stéréovision (en particulier par stéréo-corrélation), la mesure de champs de déplacements 3D à partir du couplage de la stéréo-corrélation et du suivi de pixels dans une séquence d'images par corrélation. Compte tenu de la finalité métrologique de ces travaux, nous accordons une attention toute particulière à la précision des méthodes mises en oeuvre (qualité du calibrage, qualité de la mise en correspondance des images, corrélation subpixel,...).
Ces travaux ont été appliqués à l'emboutissage de tôles minces (mesure de formes 3D d'emboutis et mesure de champs de déformations à la surface d'emboutis 3D), à la mise en forme de polymères (mesure de champs de déplacements 3D sur des membranes en élastomère soufflées), et à l'étude du comportement mécanique de bétons réfractaires renforcés de fibres métalliques (mesure de champs de déplacements 3D lors d'essais de traction).
This monograph by one of the world's leading vision researchers provides a thorough, mathematically rigorous exposition of a broad and vital area in computer vision: the problems and techniques related to three-dimensional (stereo) vision and motion. The emphasis is on using geometry to solve problems in stereo and motion, with examples from navigation and object recognition.
Image rectification is the process of applying a pair of 2D projective transforms, or homographies, to a pair of images whose epipolar geometry is known so that epipolar lines in the original images map to horizontally aligned lines in the transformed images. We propose a novel technique for image rectification based on geometrically well defined criteria such that image distortion due to rectification is minimized. This is achieved by decomposing each homography into a specialized projective transform, a similarity transform, followed by a shearing transform. The effect of image distortion at each stage is carefully considered
Presents a vision system for autonomous navigation based on stereo perception without 3D reconstruction. This approach uses weakly calibrated stereo images, i.e. images for which only the epipolar geometry is known. The vision system first rectifies the images, matches selected points between the two images, and then computes the relative elevation of the points relative to a reference plane as well as the images of their projections on this plane. We have integrated this vision module into a complete navigation system. In this system, the relative elevation is used as a shape indicator in order to compute appropriate steering directions everytime a new stereo pair is processed. We have conducted initial experiments in unstructured, outdoor environments with an wheeled rover
Image rectification is the process of applying a pair of 2 dimensional projective transforms, or homographies, to a pair of images whose epipolar geometry is known so that epipolar lines in the original images map to horizontally aligned lines in the transformed images. We propose a novel technique for image rectification based on geometrically well defined criteria such that image distortion due to rectification is minimized. This is achieved by decomposing each homography into a specialized projective transform, a similarity transform, followed by a shearing transform. The effect of image distortion at each stage is carefully considered. 1 Introduction Image rectification is an important component of stereo computer vision algorithms. We assume that a pair of 2D images of a 3D object or environment are taken from two distinct viewpoints and their epipolar geometry has been determined. Corresponding points between the two images must satisfy the so-called epipolar constraint. For a g...
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The foundations of two- and three-dimensional image correlation, as well as recent developments, are described in detail. The versatility and robustness of these methods are illustrated through application examples from diverse areas including fracture mechanics, biomechanics, constitutive property measurement in complex materials, model verification for large, flawed structures and nondestructive evaluation, A detailed description of experimental and data-reduction procedures is presented for the application of the too-dimensional image-correlation method to thin-sheet mixed-mode I/II fracture studies, local crack-closure measurements using optical microscopy and the measurement of constitutive properties. Application examples using three-dimensional image correlation include profiling of components for reverse engineering and manufacturing and the measurement of full-field surface deformation during wide cracked panel tensile tests for verification of buckling and crack-growth models. Results from nearly sixteen years of use have demonstrated that both two-dimensional and three-dimensional image-correlation methods are robust and accurate tools for deformation measurements in a variety of applications. The range of uses for the two- and three-dimensional image-correlation methods is growing rapidly as scientists and engineers begin to understand their true capabilities.
Mineral polymer cements are new synthetic refractory materials similar to natural tecto-alumino-silicates. They can be used as binder phase in refractory concretes. Such concretes present some attractive properties such as low shrinkage and low thermal expansion coefficient which make them good candidates for high temperature applications. The aim of this study is to characterize a new refractory material made of a mineral polymer cement reinforced with cordierite aggregates for applications as ceramic moulds for titanium superplastic forming at 925 °C. In a first part, the structural transformations of the concrete during the first heating have been characterized. A set of optimal conditions has been determined taking into account the stabilization heat treatment in order to decrease the binder shrinkage influence and to minimize crack formation. In a second part, the mechanical properties have been measured by three point bending tests and compressive tests from room temperature up to 925 °C.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
We present in this paper a stereovision system that has been developed in Ecole des Mines d'Albi Material Research Center laboratory for the automatic measurement of 3D deformed surfaces, in collaboration with the LAAS-CNRS laboratory. This method uses off-the-shelf lenses, CCD cameras and frame grabber, and requires that a predefined pattern be applied to the sheet surface before stamping. The system works in three steps: (i) the stereovision system is first calibrated, (ii) two images of the part to be measured are taken and the 3D coordinates are computed, (iii) the strains are calculated from these 3D coordinates.
The pullout behaviour of steel fibres from a refractory castable was characterized through high temperature pullout tests and scanning electron microscopy observations. The effects of the firing temperature, the testing temperature and the inclination angle on the pullout performances were quantified. Complementary optical microscopy observations allowed to identify the pullout micromechanisms induced by the fibre inclination angle such as fibre bending, matrix spalling and the development of a concentrated friction load at the fibre exit point from the matrix. From these results, an analytical pullout model of inclined fibre was developed. The model is based on the strength of materials theory and takes into account the pullout micromechanims previously described. Two domains are considered. The first one corresponds to the free fibre length outside the matrix and models the fibre bending. The second one corresponds to the embedded fibre segment and manages the concentrated friction load, the fibre extraction and the matrix spalling. A good agreement was obtained between the predicted and the experimental pullout curves for all the considered conditions of firing and testing temperatures. A parametric study was then performed. The effects of the friction coefficient, the fibre Young’s modulus, the concrete rupture strength and the testing temperature on the pullout performances were quantified and discussed.
The extraction behaviour of stainless steel fibres from a ceramic refractory matrix has been studied by way of pullout tests and scanning electron microscopy observations. The effect of first heating of such composites has been characterised both by room temperature pullout tests and by dilatometric experiments. High temperature pullout tests have been performed too. Mechanical and microstructural evolutions are discussed with the help of analytical models.
Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.
The use of fibers in a cement-based matrix can fundamentally improve its mechanical properties. Such improvement may lead to a new class of cement-based materials. Further developments depend on an understanding of the interaction between different fibers and cement-based matrices. The current knowledge on the mechanical behavior of fiber-reinforced cement-based composites is summarized. Toughening mechanisms, interface properties, and tensile response of fiber-reinforced cement-based composites are presented. Various theoretical approaches used to describe the mechanical behavior of fiber-reinforced composites are reviewed.
The application of digital image correlation and stereoscopic principles is used to determine three-dimensional displacements. Two pairs of stereo images of a speckled surface before and after deformation are digitized and correlated to determine the three-dimensional displacements. The images are interpolated so as to account for subpixel displacements. A sequential decision technique and a coarsefine search are employed to increase computer efficiency and decrease run time. Very accurate results are obtained, expecially when the magnification is increased. The effect of camera tilt is shown to be negligible. Theory and experimental verification are presented.
The foundations of two- and three-dimensional image correlation, as well as recent developments, are described in detail.
The versatility and robustness of these methods are illustrated through application examples from diverse areas including
fracture mechanics, biomechanics, constitutive property measurement in complex materials, model verification for large, flawed
structures and nondestructive evaluation. A detailed description of experimental and data-reduction procedures is presented
for the application of the two-dimensional image-correlation method to thin-sheet mixed-mode I/II fracture studies, local
crack-closure measurements using optical microscopy and the measurement of constitutive properties. Application examples using
three-dimensional image correlation include profiling of components for reverse engineering and manufacturing and the measurement
of full-field surface deformation during wide cracked panel tensile tests for verification of buckling and crack-growth models.
Results from nearly sixteen years of use have demonstrated that both two-dimensional and three-dimensional image-correlation
methods are robust and accurate tools for deformation measurements in a variety of applications. The range of uses for the
two-and three-dimensional image-correlation methods is growing rapidly as scientists and engineers begin to understand their
true capabilities.
Recently, digital-image-correlation techniques have been used to accurately determine two-dimensional in-plane displacements
and strains. An extension of the two-dimensional method to the acquisition of accurate, three-dimensional surfacedisplacement
data from a stereo pair of CCD cameras is presented in this paper.
A pin-hole camera model is used to express the transformation relating three-dimensional world coordinates to two-dimensional
computer-image coordinates by the use of camera extrinsic and intrinsic parameters. Accurate camera model parameters are obtained
for each camera independently by (a) using several points which have three-dimensional world coordinates that are accurate
within 0.001 mm and (b) using two-dimensional image-correlation methods that are accurate to within 0.05 pixels to obtain
the computer-image coordinates of various object positions. A nonlinear, least-squares method is used to select the optimal
camera parameters such that the deviations between the measured and estimated image positions are minimized.
Using multiple orientations of the cameras, the accuracy of the methodology is tested by performing translation tests. Using
theoretical error estimates, error analyses are presented. To verify the methodology for actual tests both the displacement
field for a cantilever beam and also the surface, three-dimensional displacement and strain fields for a 304L stainless-steel
compact-tension specimen were experimentally obtained using stereo vision. Results indicate that the three-dimensional measurement
methodology, when combined with two-dimensional digital correlation for subpixel accuracy, is a viable tool for the accurate
measurement of surface displacements and strains.
A new approach to surface strain measurement is described in which two complete views of an object are digitized in two dimensions
using either a photographic or an automated vision system, corresponding points are identified, and the three-dimensional
coordinates are determined from the relationship between the views. Strains are computed in each triangular region into which
the surface is divided based on the assumption of pure homogeneous deformation in each triangle, and the strain contours are
displayed graphically on the deformed geometry. Results are presented and discussed in terms of known sources of error.
A new technique, which allows to characterize the local strain field over a domain representative of the microstructure of a heterogeneous material, is described. It is based on scanning electron microscopy, microelectrolithography, image analysis and in situ tensile tests. The in-plane components of the local strain field are characterized by their averages per phase and their distribution functions. The results are accurate for global strains between 5 and 15%. It is also possible to get contour plots of these components of the local strain field over the considered domain. The obtained strain maps give a powerful qualitative information on the strain localization modes during the deformation. This technique has basically been developed for two-phase elastoplastic materials, namely iron/silver and iron/copper blends, submitted to uniaxial tensile tests; it could also be used for polycrystals or other composite materials and for other mechanical tests.RésuméOn présente une technique expérimentale permettant de caractériser le champ de déformation dans un matériau hétéŕogène, sur un domaine représentatif de sa microstructure. Elle utilise la microscopie électronique à balayage, la microélectrolithographie, l'analyse d'image et des moyens d'essais mécaniques in situ. Les composantes planes du champ de déformation sont caractérisées par leurs valeurs moyennes par phase et leurs fonctions de distribution. La précision des résultats est satisfaisante pour des déformations macroscopiques comprises entre 5 et 15%. On peut également tracer des cartes de déformations sur le domaine étudié, fournissant de précieux renseignements qualitatifs sur les modes de localisation au cours de la déformation. Bien que développée pour des matériaux élastoplastiques biphasés, en l'occurrence des mélanges fer/argent et fer/cuivre, sous traction simple, la technique peut être utilisée pour des polycristaux ou d'autres matériaux composites, soumis à d'autres types de sollicitations.
In this paper, we consider the problem of finding the position of a point in space given its position in two images taken with cameras with known calibration and pose. This process requires the intersection of two known rays in space and is commonly known as triangulation. In the absence of noise, this problem is trivial. When noise is present, the two rays will not generally meet, in which case it is necessary to find the best point of intersection. This problem is especially critical in affine and projective reconstruction in which there is no meaningful metric information about the object space. It is desirable to find a triangulation method that is invariant to projective transformations of space. This paper solves that problem by assuming a Gaussian noise model for perturbation of the image coordinates. The triangulation problem may then be formulated as a least-squares minimization problem. In this paper a noniterative solution is given that finds the global minimum. It is shown that in certain configurations, local minima occur, which are avoided by the new method. Extensive comparisons of the new method with several other methods show that it consistently gives superior results.
Camera calibrating is a crucial problem for further metric scene measurement. Many techniques and some studies concerning calibration have been presented in the last few years. However, it is still difficult to go into details of a determined calibrating technique and compare its accuracy with respect to other methods. Principally, this problem emerges from the lack of a standardized notation and the existence of various methods of accuracy evaluation to choose from. This article presents a detailed review of some of the most used calibrating techniques in which the principal idea has been to present them all with the same notation. Furthermore, the techniques surveyed have been tested and their accuracy evaluated. Comparative results are shown and discussed in the article. Moreover, code and results are available in internet.
Optical methods that give displacement or strain fields are now emerging significantly in the mechanical sciences. Much work has been done on two-dimensional (2D) displacement/strain measurement from a single camera but the proposed methods give only in-plane strains. A binocular correlation-based stereovision technique has been developed:(a)to measure the three-dimensional (3D) shape of a static object or(b)to measure the strains of an object undergoing some 3D mechanical or thermal stress.In this paper, the application of the stereo-correlation technique to measure accurately the 3D shape of a stamped sheet metal part or the surface strain field undergone by the part during the stamping process is presented.
This paper is a survey of the theory and methods of photogrammetric bundle adjustment, aimed at potential implementors in the computer vision community. Bundle adjustment is the problem of refining a visual reconstruction to produce jointly optimal structure and viewing parameter estimates. Topics covered include: the choice of cost function and robustness; numerical optimization including sparse Newton methods, linearly convergent approximations, updating and recursive methods; gauge (datum) invariance; and quality control. The theory is developed for general robust cost functions rather than restricting attention to traditional nonlinear least squares.
An improved digital correlation method is presented for obtaining the full-field in-plane deformations of an object. The deformations are determined by numerically correlating a selected subset from the digitized intensity pattern of the undeformed object. The improved numerical correlation scheme is discussed in detail. The displacements of a simple object, as computed by the correlation routine, are shown to agree with theoretical calculations.
With the current development of nano-technology, there exists an increasing demand for three-dimensional shape and deformation measurements at this reduced-length scale in the field of materials research. Images acquired by Scanning Electron Microscope (SEM) systems coupled with analysis by Digital Image Correlation (DIC) is an interesting combination for development of a high magnification measurement system. However, a SEM is designed for visualization, not for metrological studies, and the application of DIC to the micro- or nano-scale with such a system faces the challenges of calibrating the imaging system and correcting the spatially-varying and time-varying distortions in order to obtain accurate measurements. Moreover, the SEM provides only a single sensor and recovering 3D information is not possible with the classical stereo-vision approach. But the specimen being mounted on the mobile SEM stage, images can be acquired from multiple viewpoints and 3D reconstruction is possible using the principle of videogrammetry for recovering the unknown rigid-body motions undergone by the specimen.
The dissertation emphasizes the new calibration methodology that has been developed because it is a major contribution for the accuracy of 3D shape and deformation measurements at reduced-length scale. It proves that, unlike previous works, image drift and distortion must be taken into account if accurate measurements are to be made with such a system. Necessary background and required theoretical knowledge for the 3D shape measurement using videogrammetry and for in-plane and out-of-plane deformation measurement are presented in details as well. In order to validate our work and demonstrate in particular the obtained measurement accuracy, experimental results resulting from different applications are presented throughout the different chapters. At last, a software gathering different computer vision applications has been developed.
A technique is presented for calibrating and rectifying in a very
efficient and simple manner pairs or triplets of images taken for
binocular or trinocular stereovision systems. After the rectification of
images, epipolar lines are parallel to the axes of the image coordinate
frames. Therefore, potential matches between two or three images satisfy
simpler relations, allowing for simpler and more efficient matching
algorithms. Experimental results obtained with a binocular and a
trinocular stereovision system are presented, and a complexity analysis
is provided
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