Arson, Chloé

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Now showing 1 - 10 of 110
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    Simulation of compound anchor intrusion in dry sand by a hybrid FEM+SPH method
    ( 2022-09) He, Haozhou ; Karsai, Andras ; Liu, Bangyuan ; Hammond III, Frank L. ; Goldman, Daniel I. ; Arson, Chloé
    The intrusion of deformable compound anchors in dry sand is simulated by coupling the Finite Element Method (FEM) with Smoothed Particle Hydrodynamics (SPH). This novel approach can calculate granular flows at lower computational cost than SPH alone. The SPH and FEM domains interact through reaction forces calculated from balance equations and are assigned the same soil constitutive model (Drucker-Prager) and the same constitutive parameters (measured or calibrated). Experimental force-displacement curves are reproduced for penetration depths of 8 mm or more (respectively, 20 mm or more) for spike-shaped (respectively, fan-shaped) anchors with 1 to 6 blades. As the number of blades increases, simulations reveal that the granular flow under the anchor deviates from the vertical and that the horizontal granular flow transitions from orthoradial to radial. We interpret the strain field distribution as the result of soil arching, i.e., the transfer of stress from a yielding mass of soil onto adjoining stationary soil masses. Arching is fully active when the radial distance between blade end points is less than a critical length. In that case, the normal stress that acts on the compound anchor at a given depth reaches the normal stress that acts on a disk-shaped anchor of same radius. A single-blade anchor produces soil deformation and failure similar to Prandtl’s foundation sliding model. Multiblade anchors produce a complex failure mechanism that combines sliding and arching.
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    Effect of the intermediate principal stress on pre-peak damage propagation in hard rock under true triaxial compression
    ( 2022) Wu, Zhuorui ; Xu, Tingting ; Arson, Chloé
    It is of foremost importance to understand the mechanisms of damage propagation in rock under true triaxial stress. True triaxial compression tests reported in the literature do reflect the effect of the intermediate principal stress (σ2), but predictive models are still lacking. In this paper, an enhanced version of the Discrete Equivalent Wing Crack Damage (DEWCD) model initially proposed in (Jin and Arson, 2017) is calibrated and tested to bridge this gap. The original DEWCD model can predict most mechanical nonlinearities induced by damage but it cannot capture dilatancy effects accurately. To overcome this limitation, a dependence of the energy release rate on the first and third stress invariants is introduced in the damage potential. The enhanced DEWCD model depends on eight constitutive parameters. An automated calibration procedure is adopted to match pre-peak stress-strain curves obtained experimentally in (Feng et al., 2019) during true triaxial compression. The model successfully captures the differences in deformation and damage in the three principal directions of loading and accurately predicts that an increase of compression σ2 yields a decrease of the intermediate (tensile) deformation, a triggering of damage at a lower value of σ1 −σ2, as well as a decrease of cumulated damage in the direction of σ2 and an increase of cumulated damage in the direction of σ3 at the stress peak (pre-softening). During the true triaxial compression stage, a higher intermediate principal stress hinders dilatancy such that the volumetric strain at the peak of σ1 changes from dilation to shrinkage. The enhanced DEWCD model shows good performance in axis-symmetric compression and true triaxial compression, both for monotonic and cyclic loading. A comparison of three true triaxial stress paths at constant/variable mean stress/Lode angle suggests that: (i) the mean stress controls damage hardening and the sign of the volumetric strain rate at damage initiation, (ii) the second stress invariant is the primary control factor of the direction of the irreversible deviatoric strain rate during triaxial loading and of the sign of the total volumetric strain rate at failure; (iii) the Lode angle controls the direction of the total deviatoric strain rate.
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    Anisotropy and microcrack propagation induced by weathering, regional stresses and topographic stresses
    (American Geophysical Union., 2022) Xu, Tingting ; Shen, Xianda ; Reed, Miles ; West, Nicole ; Ferrier, Ken L. ; Arson, Chloé
    This paper presents a new model for anisotropic damage in bedrock under the combined influences of biotite weathering, regional stresses, and topographic stresses. We used the homogenization theory to calculate the mechanical properties of a rock representative elementary volume made of a homogeneous matrix, biotite inclusions that expand as they weather, and ellipsoidal cracks of various orientations. With this model, we conducted a series of finite element simulations in bedrock under gently rolling topography with two contrasting spatial patterns in biotite weathering rate and a range of biotite orientations. In all simulations, damage is far more sensitive to biotite weathering than to topographic or regional stresses. The spatial gradient of damage follows that of the imposed biotite weathering rate at all times. The direction of micro-cracks tends to align with that of the biotite minerals. Relative to the topographic and regional stresses imparted by the boundary conditions of the model, the stress field after 1,000 years of biotite weathering exhibits higher magnitudes, wider shear stress zones at the feet of hills, more tensile vertical stress below the hilltops, and more compressive horizontal stress concentrated in the valleys. These behaviors are similar in simulations of slowing eroding topography and static topography. Over longer periods of time (500 kyr), the combined effects or weathering and erosion result in horizontal tensile stress under the hills and vertical tensile stress under and in the hills. These simulations illustrate how this model can help elucidate the influence of mineral weathering on Critical Zone evolution.
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    A perspective on Darcy's law across the scales: from physical foundations to particulate mechanics
    ( 2022) O’Sullivan, Catherine ; Arson, Chloé ; Coasne, Benoît
    This paper puts forward a perspective or opinion that we can demonstrate Darcy’s law is valid at any scale where fluid can be modeled/analyzed as a continuum. Darcy’s law describes the flow of a fluid through a porous medium by a linear relationship between the flow rate and the pore pressure gradient through the permeability tensor. We show that such a linear relationship can be established at any scale, so long as the permeability tensor is expressed as a function of adequate parameters that describe the pore space geometry, fluid properties and physical phenomena. Analytical models at pore scale provide essential information on the key variables that permeablity depends on under different flow regimes. Upscaling techniques based on the Lippman-Schwinger equation, pore network models or Eshelby’s homogenization theory make it possible to predict fluid flow beyond the pore scale. One of the key challenges to validate these techniques is to characterize microstructure and measure transport properties at multiple scales. Recent developments in imaging, multi-scale modeling and advanced computing offer new possibilities to address some of these challenges.
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    Self-consistent approach for modeling coupled elastic and visco-plastic processes induced by dislocation and pressure solution
    ( 2021) Xu, Tingting ; Arson, Chloé
    This paper pesents a self-consistent approach in which coupled non-linear time-dependent deformation mechanisms are estimated by an affine approximation and elastic and visco-plastic macroscopic properties are calculated for the most general form of anisotropy. Phase changes from crystal to pore are accounted for, via a grain breakage mechanism. As an example, we study dislocation and pressure solution, two non-linear deformation mechanisms common to a wide range of polycrystalline materials. To this date, the couplings between the two are not fully understood. We analyze the sensitivity of the behavior of halite polycrystals to stress, brine content, grain size and grain breakage. Results indicate that: Pressure solution yields mechanical healing only if grain breakage is ignored; Otherwise, pressure solution accelerates dislocation creep, which results in an abrupt increase of elastic and viscoplastic compliance components; Higher stress and/or higher brine content enhance the coupled effects of pressure solution and dislocation; Pressure solution is delayed by the occurrence of larger grains and by the entrapment of fluid in isolated pores. An interesting feature of the model is the representation of the pore space, which allows distinguishing the deformation mechanisms of these isolated pores from those of the pores where precipitation occurs.
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    Micro-mechanical Modeling for Rate-Dependent Behavior of Salt Rock under Cyclic Loading
    ( 2021) Shen, Xianda ; Ding, Jihui ; Arson, Chloé ; Chester, Judith S. ; Chester, Frederick M.
    The dependence of rock behavior to the deformation rate is still not well understood. In salt rock, the fundamental mechanisms that drive the accumulation of irreversible deformation, the reduction of stiffness and the development of hysteresis during cyclic loading are usually attributed to intracrystalline plasticity and diffusion. We hypothesize that at low pressure and low temperature, the rate-dependent behavior of salt rock is governed by water-assisted diffusion along grain boundaries. Accordingly, a chemo-mechanical homogenization framework is proposed, in which the Representative Elementary Volume (REV) is viewed as a homogeneous polycrystalline matrix that contains sliding grain-boundary cracks. The slip is related to the mass of salt ions that diffuse along the crack surface. The rate of diffusion is calculated by a pressure solution model. The relationship between fluid inclusion-scale and REV-scale stresses and strains is established by using the Mori-Tanaka homogenization scheme. The proposed rate-dependent homogenization model is calibrated against cyclic compression tests. It is noted from the model that a lower strain rate and a larger number of sliding cracks enhances stiffness reduction and hysteresis. Thinner sliding cracks (i.e. thinner brine films) promote stiffness reduction and accelerate stress redistributions in the crack inclusions. Higher roughness angles lead to an increased difference of normal stress along the different segments of the crack plane and to a reduced diffusion path, which both amplify the reduction of stiffness and the development of hysteresis. The larger the volume fraction of the crack inclusions, the larger the REV deformation and the larger the hysteresis. Results presented in this study shed light on the mechanical behavior of salt-rock that is pertinent to the design of geological storage facilities that undergo cyclic unloading, which could help optimize the energy production cycle with low carbon emissions.
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    Finite Element model of concrete repaired by High Molecular Weight Methacrylate (HMWM)
    ( 2021) Ji, Koochul ; Gao, Nan ; Stewart, Lauren ; Arson, Chloé
    Epoxy is widely used to fill concrete cracks that are less than a millimeter in width to protect the steel rebars from corrosion. However, the mechanical behavior of epoxy-repaired concrete remains vastly unknown. In order to understand whether or not epoxy can be used to recover the mechanical properties of damaged concrete, we provide a quantitative assessment of concrete repaired by injection of High Molecular Weight Metacrylate (HMWM). Uniaxial compression tests and three-point bending tests were conducted on cut-and-repaired specimens. The experiments were simulated with the Finite Element Method (FEM), in which concrete was assigned a constitutive model that combines continuum damage mechanics and plasticity and in which the concrete/HMWM interface was modeled with bilinear softening cohesive zone elements (CZEs). The numerical model was calibrated and validated against the experimental results. Steel-reinforced concrete (RC) beams were subjected to three-point bending to produce cracks. The beams were then repaired and reloaded. We used Digital Image Correlation (DIC) to identify the zones of high maximum principal strain after the first loading cycle. These zones were modeled with repaired concrete elements and HMWM CZEs to simulate the second load cycle. The load-displacement curves, damage distributions and strain fields obtained numerically are in agreement with those obtained experimentally, which validates the proposed model. Our simulation results suggest that HMWM can penetrate cracks of width 0.01 mm and above by gravity. We also found that HMWM reparation increases concrete stiffness and strength if crack in concrete members are over 0.1 mm in width, in which case, the load capacity of repaired RC beams is 30 to 40% higher than that of as-built RC beams.
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    Deformation and failure mechanisms of granular soil around pressurised shallow cavities
    ( 2021) Patino-Ramirez, Fernando ; Ando, Edward ; Viggiani, Gioacchino ; Caicedo, Bernardo ; Arson, Chloé
    The deformation patterns and failure mechanisms of pressurised cavities at shallow depth are of relevance to many geotechnical applications, including tunneling and horizontal directional drilling. In this paper, we present an experimental study of a reduced-scale pressurised cavity under geostatic stress, in order to measure the effect of cavity length, vertical stress and soil density on soil deformation and failure. x-ray computed tomography is used to acquire images of the system at key stages of the cavity inflation process. A closed shaped failure region developed around the cavities, beyond which, shear planes of elliptic paraboloid shape formed, extending from the bottom of the cavities all the way to the free surface. The plane strain assumption did not hold beyond the central portion of the longest cavity tested (L = 6D). The volumetric strain and porosity changes inside the shear bands showed significant dilation in dense specimens, but contraction in loose specimens. The average orientation and the thickness of the shear bands were in agreement with those found in the literature for passive arching mechanisms (anchoring). The orientation of the principal strains around the cavity follows catenary shapes, similar to those displayed in active trapdoor mechanisms.
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    Tensile strength of calcite/HMWM and silica/HMWM interfaces: A Molecular Dynamics analysis
    (Georgia Institute of Technology, 2020-03) Arson, Chloé ; Ji, Koochul (K.)
    The mechanical behavior of interfaces between high molecular weight methacrylate (HMWM) and concrete minerals (calcite and silica) is investigated from a Molecular Dynamics (MD) perspective. MD simulations of pullout tests shows that interfaces debond at the surface of contact between HMWM and the mineral substrate, and that the interfacial strength decreases in the presence of moisture, under low strain rate, or at high temperature. Silica/HMWM interfaces are stronger than the calcite/HMWM interfaces. Additionally, the work of separation is mostly done by van der Waals forces, in agreement with previous studies. We use published experimental data at low strain rate along with our MD results at high strain rate to calibrate Richeton’s model and Johnson-Cook model. We show that, if more experimental results were available for validation, MD results could be extrapolated to predict the tensile modulus of HMWM at low strain rate and the HMWM/mineral interfacial strength for a broad range of temperatures and strain rates. The sensitivity analysis of the model confirms that HMWM should be applied on dry surfaces and in concrete exposed to lower temperatures. Additionally, MD results suggest that HMWM is more likely to last in concrete with high silica contents than in concrete with high calcite contents.
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    Fabric evolution and crack propagation in salt during consolidation and cyclic compression tests
    ( 2020) Shen, Xianda ; Ding, Jihui ; Lordkipanidze, Ilia ; Arson, Chloé ; Chester, Judith ; Chester, Frederick
    It is of great interest to describe and quantify the evolution of microstructure for a better understanding of rock deformation processes. In this study, 2D microstructure images of salt rock are analyzed at several stages of consolidation tests and cyclic compression tests to quantify the evolution of the magnitude and orientation of solidity, coordination, local solid volume fraction and crack volume. In both the consolidation and the cyclic compression tests, the deformation of aggregates achieved by grain rearrangement is greater than that achieved by the deformation of an individual grain. In the consolidation tests, the aggregates are rearranged into horizontal layers of coordinated grains, the orientation distribution of grain indentations is quasi-uniform, and the size of the pores reduces and becomes more uniformly distributed. As a result, salt rock microstructure becomes more homogeneous. The increase of local solid volume fraction in the lateral direction is correlated with an increase of the oedometer modulus. In the cyclic compression tests, grain-to-grain contact areas decrease due to the redistribution of grains and the propagation of intergranular cracks. Aggregates are reorganized into columns of coordinated grains. Intergranular opening-mode cracks tend to develop in the axial direction, while intergranular shear-mode cracks propagate preferentially in the lateral direction. The lateral components of the fabric tensors of coordination and local solid volume fraction decrease, which results in an increase of the Poisson's ratio. The fabric descriptors used in this work allow a better quanti cation and understanding of halite deformation processes and can be used in other types of rocks encountering similar deformation mechanisms.