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1929 results
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1 - 10 of 1929
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ItemHomogenization of dentin elastic properties based on microstructure characterization, statistical back-analysis and FEM simulation(Georgia Institute of Technology, 2017-07) Jeanneret, Romain ; Arson, Chloé ; Vennat, ElsaThe dentinal tissue is made of tubules surrounded by peri-tubular dentin (PTD), embedded in a matrix of inter-tubular dentin (ITD). Hashin and Rosen found exact stiffness bounds for hexagonal patterns of hollow fibers. But State-of-the-Art micro-macro models rely on simplified microstructure representations and lack experimental validation. The Poisson’s ratios of dentin microstructure components cannot be determined by direct experimental methods. By contrast, we apply Hashin’s homogenization scheme to a non-uniform PTD distribution, determined from image analysis. According to Finite Element simulations, a cube containing 60 tubules is a Representative Elementary Volume. Microscopy, nanoindentation and Resonant Ultrasound Spectroscopy data were collected from each dentin sample studied for model calibration. Despite the high variability of microstructure descriptors and mechanical properties, statistical analyses show that Hashins bounds converge and that the proposed model can be used for backcalculating the microscopic mechanical properties of dentin constituents.
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ItemAnalysis of microstructure, deformation and permeability of salt/sand mixtures during creep(Georgia Institute of Technology, 2017-07) Shen, Xianda ; Zhu, Cheng ; Arson, ChloéThe impact of impurities on salt healing properties is studied through creep tests performed on brine saturated granular salt with various quartz contents. Quartz grains act as shields that reduce dissolution at salt grain contacts and decrease the creep rate. Non-smooth creep curves are obtained for specimens with 50% quartz contents, due to sequential pore collapse. Micro-CT images acquired before creep, after creep, and after unloading, show that pore-to-pore distances decrease with quartz contents and that the creep rate decreases as the mean area of the salt grain contacts increases. Based on grain scale thermodynamic models, we show that creep deformation is controlled by diffusion - not dissolution-precipitation. Permeability evolution is less sensitive to porosity than to void radius and spacing, which control pore connectivity. The proposed modeling framework can be used in any crystalline material to relate microscopic reaction rates to macroscopic deformation rates.
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ItemModeling of Tensile and Compressive Damage in Layered Sedimentary Rock: A Direction Dependent Non-Local Model(Georgia Institute of Technology, 2017-06) Wencheng, Jin ; Arson, ChloéThis paper presents the theoretical formulation and numerical implementation of an anisotropic damage model for materials with intrinsic transverse isotropy, e.g. sedimentary rocks with a bedding plane. The direction dependent mechanical response is captured by utilizing four types of equivalent strains, for tension and compression, parallel and perpendicular to the bedding plane. The model is calibrated against triaxial compression test data, for different confinement and loading orientations. The variations of uniaxial tensile and compressive strengths with the orientation of the loading relative to the bedding follow the trends and magnitudes noted in experiments. Anisotropic non-local equivalent strains were used in the formulation to avoid localization and mesh dependence encountered with strain softening. Two different internal length parameters are used to distinguish the non-local effects along and perpendicular to the bedding. An arc length control algorithm is used to avoid convergence issues. Results of three-point bending tests confirm that the nonlocal approach indeed eliminates mesh dependency. Results show that the orientation and size of the damage process zone are direction dependent, and that materials with intrinsic transverse isotropy exhibit mixed fracture propagation modes except when the bedding aligns with the loading direction. Further research towards a multiscale hydro-mechanical fracture propagation scheme is undergoing.
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ItemNon-Local Micromechanical Anisotropic Damage Modeling Of Quasi-Brittle Materials: Formulation And Implementation(Georgia Institute of Technology, 2017-06) Wencheng, Jin ; Arson, ChloéA nonlocal anisotropic damage model is proposed for quasi-brittle materials, such as concrete and rock. The local anisotropic damage model is formulated by combining a free energy derived from micromechanics with phenomenological yield criteria and damage potentials. The trace of the total strain is used to distinguish tensile and compressive loading paths, and to account for the influence of the confining pressure on the propagation of compression cracks. Yield criteria in tension and compression are expressed in terms of equivalent strains, which depend on the difference between principal strain components. A non-local measure of strain is used to avoid localization. Constitutive parameters are calibrated against published experimental data for concrete and shale. Simulations of three-point bending tests show that non-local enhancement is necessary and efficient to avoid mesh dependency upon strain softening. Simulations of borehole excavation damage zone show that the damage model is not mesh dependent upon stress hardening. Numerical predictions are in agreement with experimental observations and the model can capture unilateral effects, tensile softening, compressive hardening and confinement dependent compressive behavior.
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ItemA Method to Estimate the Energy Distribution During the Confined Comminution of Granular Materials(Georgia Institute of Technology, 2017-06) Arson, Chloé ; Wang, PeiDuring the confined comminution of granular materials, the work input is transformed into elastic energy stored in the grains, breakage energy used to generate new surfaces, energy dissipated by friction between grains in contact, and redistribution energy dissipated by the relative movement of crushed fragments. We assume that the expression of Particle Size Distribution (PSD) in a crushed sample is a function of a fractal distribution and a uniform distribution. This allows calculating the breakage parameter, the increase of surface energy, and finally the energy dissipated by breakage. By summing the contact energy at all the contacts within a sample, we calculate the elastic energy stored. The calculation of friction-dissipated energy requires calculating relative movements of contacts, which are highly unpredictable especially when crushing is involved. Thus we include the dissipation that results from the relative displacement of grains in contact (including both crushed fragments and surrounding intact grains) in the friction-dissipated energy. We obtain the friction-dissipated energy by subtracting the elastic energy stored in the grains and the breakage energy from the input energy. The results show that the energy distribution is stress sensitive and changes a lot with the increase of compressive stress. Energy dissipation by friction plays a major role during confined comminution.
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ItemNumerical Study of Thermo-Mechanical Effects on the Viscous Damage Behavior of Rock Salt Caverns(Georgia Institute of Technology, 2017-06) Cheng, Zhu ; Shen, Xianda ; Arson, Chloé ; Pouya, AhmadUnderground cavities in rock salt have received increased attention for the storage of oil, gas, and compressed air energy. In this study, the transition between secondary and tertiary creep in salt is determined by a micro-macro model: The initiation of grain breakage is correlated with the acceleration of viscoplastic deformation rate and with the initiation of damage at the macroscopic scale. Salt stiffness decreases when macroscopic damage increases, which allows predicting the evolution of the damage zone around salt caverns used for geological storage. After implementing the phenomenological model into the Finite Element Method (FEM) program POROFIS, two thermo-mechanical coupled stress paths are simulated to analyze stress concentrations and viscous damage around a 650-m-deep cavern in axisymmetric conditions. Numerical results indicate that, despite the pressurization or depressurization-induced temperature variation, internal gas temperature always tends to approach the primary surrounding rock mass value. The viscous deformation induced by thermo-mechanical couplings significantly affects the original stress field at the cavern wall and induces high damage at the most concave sections of the cavern. Results reveal the significant influences of idle time, gas pressure range, and injection and withdrawal cycles on stress, strain and temperature distributions in the vicinity of the cavern. More analyses are needed to confirm the influence of thermo-mechanical cycles of pressurization and depressurization, and to design long-term cavern operations.
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ItemBio-Inspired Fluid Extraction Model for Reservoir Rocks(Georgia Institute of Technology, 2017-06) Patino-Ramirez, Fernando ; Arson, ChloéBiological and engineering flow systems maximize their efficiency by following the path of minimum energy over the domain they are embedded in. This fact motivates the present research, since industrial fluid extraction and injection processes are designed to minimize the implementation cost (energy, materials) and maximize the volume of fluid injected (or withdrawn). This work presents a bio-inspired fluid flow model to optimize the path that connects resource-rich pores in a rock. We explain the commonalities between the equations governing flow in a porous medium and growth of slime mold, an organism that dynamically deploys tube-like structures and adapts them as a function of their contribution to the overall network. We perform several simulations to analyze the influence of the pore size distribution and of pore spatial distribution on the topology of the extraction network predicted by the slime mold growth algorithm. We discuss the suitability of the biomimicry model to design fracture patterns for optimal fluid extraction from a porous rock.
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ItemMicrocrack Network Development in Salt-Rock During Cyclic Loading at Low Confining Pressure(Georgia Institute of Technology, 2017-06) Ding, Jihui ; Chester, Frederick M. ; Chester, Judith S. ; Xianda, Shen ; Arson, ChloéTriaxial compression tests of synthetic salt-rock are conducted to investigate microfracture development in a semibrittle polycrystalline aggregate. The salt-rock is produced from uniaxial consolidation of granular halite at 150 °C. Following consolidation, the sample is deformed by cyclic loading at room temperature and low confining pressure (Pc = 1 MPa). Load cycles are performed within the elastic regime, up to yielding, and after successive increments of steady ductile flow. At the tested conditions, the samples exhibit ductile behavior with slight work hardening. The microstructure at different stages of deformation indicates that grain-boundary cracking is the dominant brittle deformation mechanism. Microcracking is influenced by the loading configuration and the geometric relationships between neighboring grains. These microcracks display a preferred orientation parallel to the load axis. With cyclic loading, microcracks increase in density and form linked arrays parallel to the direction of loading. As the linked arrays lengthen, grain contacts are progressively opened, which eventually leads to loss of cohesion along surfaces parallel to the loading direction. The observations of crack-network development in salt-rock can improve our understanding of progressive damage and spalling at salt cavern walls.
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ItemExperimental Characterization of Microstructure Development for Calculating Fabric and Stiffness Tensors in Salt Rock(Georgia Institute of Technology, 2017-06) Xianda, Shen ; Arson, Chloé ; Ding, Jihui ; Chester, Frederick M. ; Chester, Judith S.Uniaxial consolidation tests were conducted on reagent-grade granular salt in dry conditions at 150 C. 2Dmicroscopic images, parallel to the axis of consolidation, were obtained at several stages of progressive consolidation from 15% to 3% porosity. Microstructure image analyses were performed to obtain probability density functions (PDFs) of the area, solidity, coordination number, orientation, elongation and roundness of the grains, as well as the PDFs of the branch lengths, branch orientations and solid volume fraction, defined locally over polygons with edges matching grain centroids. It is found that sample deformation is mostly due to grain rearrangement and that upon consolidation, grains become less convex, and elongate in the direction perpendicular to the loading axis. Four fabric tensors were calculated to assess microstructure anisotropy induced by grain orientation, branch length orientation, grain solidity and local solid volume fraction. Fabric tensors were diagonal and orthogonal. Therefore, their product was used to define a global fabric tensor, which was introduced in the expression of the stiffness tensors. The constitutive parameters were calibrated against the consolidation tests. The approach paves the way to enrich continuum damage and healing mechanics model with fabric descriptors that can play the role of internal variables.
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ItemDamage and healing model of stiffness and permeability for salt rock: microstructure imaging, fabric processes and continuum mechanics(Georgia Institute of Technology, 2016-06) Zhu, Cheng ; Arson, ChloéIn this study, we proposed a fabric-enriched Continuum Damage Mechanics model to investigate the coupled influence of damage and healing on the mechanical and transport properties of salt rock. In order to infer the form of fabric tensors, we carried out creep tests on granular salt assemblies under constant temperature and humidity conditions and used micro-computed tomography for microstructure characterization. Using microscope imaging and micro-CT scanning, we analyzed the probability distributions of crack radius, void areas and crack spacing and used them as a basis to derive macroscopic evolution laws. A stress path comprising a tensile loading, a compressive unloading, a creep-healing stage, and a reloading was simulated. As expected, stiffness decreases (respectively increases) and permeability increases (respectively decreases) upon damage (respectively healing). Results also highlight the increased efficiency of healing with temperature. The micro-macro relationships established by statistical image analysis also provide the evolution of microstructure descriptors during the test. Simulations show that permeability changes are controlled by changes in crack connectivity, which dominate changes of porosity. The proposed framework is expected to improve the fundamental understanding of coupled processes that govern microstructure changes and subsequent variations of stiffness and permeability in salt rock, which will allow the assessment of the long-term performance of geological storage facilities.