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ItemA mixed damage model for unsaturated porous media(Georgia Institute of Technology, 2009-04) Arson, Chloé ; Gatmiri, BehrouzThe aim of this study is to present a framework for the modeling of damage in continuous unsaturated porous geomaterials. The damage variable is a second-order tensor. The model is formulated in net stress and suction independent state variables. Correspondingly, the strain tensor is split into two independent thermodynamic strain components. The proposed framework mixes micro-mechanical and phenomenological approaches. On the one hand, the effective stress concept of Continuum Damage Mechanics is used in order to compute the damaged rigidities. On the other hand, the concept of equivalent mechanical state is introduced in order to get a simple phenomenological formulation of the behavior laws. Cracking effects are also taken into account in the fluid transfer laws.
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ItemOn damage modelling in unsaturated clay rocks(Georgia Institute of Technology, 2008-10) Arson, Chloé ; Gatmiri, BehrouzThe aim of this paper is to present the main problems encountered in the modelling of damage in an unsaturated quasi-brittle rock mass. Micromechanical damage models are based on a physical definition of damage, related to fracturing. Phenomenological formulations are less straightforward, but offer huge modelling possibilities by means of economical computation processes. Due to the dissipative aspect of damage, the Inequality of Clausius–Duhem (ICD) has to be satisfied. Strain softening and crack localization are regularized by means of a non-local formulation, founded on microstructure concepts, homogenisation and space averaging or gradient-enhancement. In an unsaturated damaged porous medium, suction effects combine with mechanical loading and fracturing, which induces complex couplings. On the one hand, Continuum Damage Mechanics well represents stiffness degradation for dry materials. On the other hand, fracture network models give a good estimation of complex flows. It is difficult to reconcile both theories. A new mixed model, HHMD, is proposed. It is a fully coupled formulation, involving independent state variables.
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Itemθ-STOCK, a powerful tool of thermohydromechanical behaviour and damage modelling of unsaturated porous media(Georgia Institute of Technology, 2008-09) Gatmiri, Behrouz ; Arson, ChloéA brief review of the basic points of a suction-based heat, moisture transfer and skeleton deformation equations for an unsaturated medium is presented. The main issues such as: two temperature-dependent state surfaces of void ratio and degree of saturation which are used to present the coupling effects of temperature, moisture content and deformation of skeleton; the new thermoelastoplastic constitutive law, etc. are briefly mentioned. The Bubnov–Galerkin integral form of field equations has been developed as the basis of spatial and temporal discretized matrix form. The single-step integration in time is described. The numerical solution algorithm of the finite element package, θ-STOCK, is presented. Some application cases are presented and discussed to show the strong ability of presented model and the prepared numerical package.
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ItemSite-specific Spectral Response of Seismic Movement due to Geometrical and Geotechnical Characteristics of Sites(Georgia Institute of Technology, 2008-04) Gatmiri, Behrouz ; Maghoul, P. ; Arson, ChloéIt is well-known that the response of a site to a seismic solicitation depends on local topographical and geotechnical characteristics. Many aspects of seismic site effect still need to be studied in more detail and they can be incorporated in the seismic norms after quantification. The purpose of this paper is to contribute to establishment of a simple method to include complex site effects in a building code. Horizontal ground movements in various points of two-dimensional (2D) irregular configurations subjected to synthetic SV waves of vertical incidence are calculated. The parametric studies are achieved by means of HYBRID program combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The results are shown in the form of pseudo-acceleration response spectra. For the empty valleys, we can classify the spectral response according to a unique geometric criterion: the “surface/angle” ratio, where surface is the area of the valley opening, and angle denotes the angle between the slope and horizontal line in the above corner. To assess the influence of the 2D effect on the spectral response of filled valleys, the response of alluvial basins are compared with the response of one-dimensional columns of soil. Finally, an offset criterion is proposed to choose a relevant computation method for the spectral acceleration at the surface of alluvial basins.
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ItemSeismic site effects by an optimized 2D BE/FE method. II. Quantification of site effects in two-dimensional sedimentary valleys(Georgia Institute of Technology, 2007-10) Gatmiri, Behrouz ; Arson, ChloéThis paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.
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ItemSeismic site effects by an optimized 2D BE/FE method I. Theory, numerical optimization and application to topographical irregularities(Georgia Institute of Technology, 2007-10) Gatmiri, Behrouz ; Arson, Chloé ; Ngyuen, K.V.This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.
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ItemFuzzy Image Processing in Sun Sensor(Georgia Institute of Technology, 2003-06) Mobasser, Sohrab ; Liebe, Carl Christian ; Howard, Ayanna M.Sun sensors are widely used in spacecraft attitude determination subsystems to provide a measurement of the sun vector in spacecraft coordinates. At the Jet Propulsion Laboratory, California Institute of Technology, there is an ongoing research activity to utilize Micro Electro Mechanical Systems (MEMS) processes to develop a smaller and lighter sun sensor for space applications. A prototype sun sensor has been designed and constructed. It consists of a piece of silicon coated with a thin layer of chrome, and a layer of gold with hundreds of small pinholes, placed on top of an image detector at a distance of less than a "eter. Images of the sun are formed on the detector when the sun illuminates the assembly. Software algorithms must be able to identify the individual pinholes on the image detector and calculate the angle to the sun. Fuzzy image processing is utilized in this process. This paper will describe how the fuzzy image processing is implemented in the instrument. Comparison of the Fuzzy image processing and a more conventional image pmcessing algorithm is provided and shows that the Fuzzy image processing yields better accuracy then conventional image processing.
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ItemA Real-Time Autonomous Rover Navigation System(Georgia Institute of Technology, 2000-06) Howard, Ayanna M. ; Seraji, HomayounTo enable real-time autonomous navigation, a mobile robot is equipped with on-board processing power, image-processing algorithms, and a fuzzy computation engine that allow the rover to safely navigate to a designated goal while avoiding obstacles and impassible terrains. The underlying architecture discussed in this paper utilizes real-time measurement of terrain characteristics and a fuzzy logic framework for onboard analysis of terrain traversability. The overall navigation strategy, consisting of terrain-traverse and goal-seeking behaviors, requires no a priori information about the environment, and uses the on-board traversability analysis to enable the rover to select easy-to-traverse paths to the goal autonomously. The rover navigation system is tested and validated with a set of physical rover experiments. These experiments demonstrate the real-time capability of the navigation system.
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ItemA Learning Methodology for Robotic Manipulation of Deformable Objects(Georgia Institute of Technology, 2000-06) Howard, Ayanna M. ; Bekey, George A.The majority of manipulation systems are designed with the assumption that the objects being handled are rigid and do not deform when grasped. This paper address the problem of robotic grasping and manipulation of 3- D deformable objects, such as rubber balls or bags filled with sand. Specifically, we have developed a generalized learning algorithm for handling of 3-D deformable objects in which prior knowledge of object attributes is not required and thus it can be applied to a large class of object types. A description of our learning methodology will be given in this paper. We outline our methodology for modeling the object deformation and learning the required minimum forces for grasping. Evaluation of the overall algorithm demonstrates that we can achieve an error level of 14% with respect to the minimum physical lifting force.
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ItemReal Time Intelligent Target Detection and Analysis with Machine Vision(Georgia Institute of Technology, 2000-06) Howard, Ayanna M. ; Padgett, Curtis ; Brown, Kenneth R.This paper presents an algorithm for detecting a specified set of target objects embedded in visual imagery for an Automatic Target Recognition (ATR) application. ATR involves processing images for detecting, classifying, and tracking targets embedded in a background scene. We address the problem of discriminating between targets and non-target objects located within a cluttered environment by evaluating 40x40 image blocks belonging to a segmented image scene. Using directed principal component analysis, the data dimensionality of an image block is first reduced and then clustered into one of n classes based on a minimum distance to a set of n cluster prototypes. Following clustering, each image pattern is fed into an associated trained neural network for classification. A detailed description of our algorithm will be given in this paper. Evaluation of the overall algorithm demonstrates that our detection rates approach 96% with a false positive rate of less than 0.03%.