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ItemChemo-Mechanical Damage and Healing of Granular Salt: Micro-macro modeling(Georgia Institute of Technology, 2016-06) Xianda, Shen ; Zhu, Cheng ; Arson, Chloé ; Georgia Institute of Technology. School of Civil and Environmental EngineeringA micro-macro chemo-mechanical model of damage and healing is proposed to predict the evolution of salt stiffness and deformation upon micro-crack propagation, opening, closure and rebonding, which is the result of pressure solution. We hypothesize that at a given grain contact, the surface area of the contact dictates which mechanism dominates the rate of healing. Based on thermodynamic equations of dissolution, diffusion and precipitation, we establish a formula for the critical contact area that marks the transition between diffusion-dominated kinetics and dissolution-precipitation-dominated kinetics. We relate the change of contact area to the change of solid volume in the Representative Elementary Volume, and we define net damage as the sum of the mechanical damage and the chemical porosity change. A continuum-based damage mechanics framework is used to deduce the change of salt stiffness with net damage. A stress path comprising a tensile loading, a compressive unloading, a creep– healing stage and a reloading is simulated. Stiffness degradation and residual strain development are observed with the evolution of damage under tensile loading. Unilateral effects of crack closure can be predicted by the model upon compression. Our micromacro model also allows predicting the evolution of the probability distribution of contact areas upon healing, as well as the consequent decrease of net damage and recovery of stiffness. The proposed modeling framework is expected to shed light on coupled healing processes that govern microstructure changes and subsequent variations of deformation rate, stiffness and permeability in salt rock, and to allow the assessment of long-term behavior of geological storage facilities in salt.
ItemMicrostructure-based modeling of damage and healing in salt rock with application to geological storage(Georgia Institute of Technology, 2016-06-24) Zhu, Cheng ; Arson, Chloé ; Frost, David ; Dai, Sheng ; Huber, Christian ; Pouya, Ahmad ; Civil and Environmental EngineeringMost mineral and energy resources such as ore, petroleum, natural gas, and geothermal energy are recovered from the earth. Nuclear waste repositories and CO2 storage systems are buried underground. Recovery of mineral resources, storage of energy, and disposal of waste involve changes in coupled mechanical and transport rock properties. The evolution of pores and cracks during thermo-hydro-chemo-mechanical coupled processes governs the variations of macroscopic properties. This research investigates the modeling of damage and healing in rocks with applications in geological storage. This presentation focuses on salt rock, which is used as a model material to study rock microstructure evolution under various stress paths, and to understand the microscopic processes that lead to macroscopic mechanical recovery. We developed two different techniques based on continuum damage mechanics (CDM) and micromechanics. The first method enriches the framework of CDM with fabric descriptors. We carried out creep tests on granular salt to infer the form of fabric tensors from microstructure observation. Net damage evolution is governed by a diffusion equation. Macroscopic and microscopic model predictions highlight the increased efficiency of healing with time and temperature. The other method is based on a self-consistent homogenization scheme, in which the viscoplastic and damage behavior of halite polycrystals is upscaled from mono-crystal slip mechanisms. The model provides micro-mechanical interpretations to important aspects of salt rock viscoplastic and fatigue behavior. We implemented the micromechanical model in a finite element program to characterize crack patterns in salt polycrystals and predict damage around a salt cavern used for high-pressure gas storage. This study is expected to improve the fundamental understanding of damage and healing in rocks, and the long-term assessment of geological storage facilities.
ItemSelf-consistent micromechanical approach for damage accommodation in rock-like polycrystalline materials(Georgia Institute of Technology, 2017) Pouya, Ahmad ; Zhu, Cheng ; Arson, Chloé ; Laboratoire Navier ; Georgia Institute of Technology. School of Civil and Environmental Engineering ; Rowan University. Department of Civil and Environmental EngineeringIn quasi-brittle polycrystalline materials, damage by cracking or cleavage dominates plastic and viscous deformation. This paper proposes a micromechanical model for rock-like materials, incorporating the elastic-damage accommodation of the material matrix, and presents an original method to solve the system of implicit equations involved in the formulation. A self-consistent micromechanical approach is used to predict the anisotropic behavior of a polycrystal in which grain inclusions undergo intragranular damage. Crack propagation along planes of weakness with various orientation distributions at the mineral scale is modeled by a softening damage law and results in mechanical anisotropy at the macroscopic scale. One original aspect of the formulated inclusionmatrix model is the use of an explicit expression of the Hill’s tensor to account for matrix ellipsoidal anisotropy. To illustrate the model capabilities, a uniaxial compression test was simulated for a variety of polycrystals made of two types of mineral inclusions with each containing only one plane of weakness. Damage always occurred in only one mineral type: the damaging mineral was that with a smaller shear modulus (respectively higher bulk modulus) when bulk modulus (respectively shear modulus) was the same. For two minerals with the same shear moduli but different bulk moduli, the maximum damage in the polycrystal under a given load was obtained at equal mineral fractions. However, for two minerals with different shear moduli, the macroscopic damage was not always maximum when the volume fraction of two minerals was the same. When the weakness planes’ orientations in the damaging mineral laid within a narrow interval close to the loading direction, the macroscopic damage behavior was more brittle than when the orientations were distributed over a wider interval. Parametric studies show that upon proper calibration, the proposed model can be extended to understand and predict the micro-macro behavior of different types of quasi-brittle materials.
ItemMechanical Behavior and Microstructure Development in Consolidation of Nominally Dry Granular Salt(Georgia Institute of Technology, 2016-06) Ding, Jihui ; Chester, Frederick M. ; Chester, Judith S. ; Zhu, Cheng ; Arson, Chloé ; Georgia Institute of Technology. School of Civil and Environmental Engineering ; Texas A & M University. Department of Civil EngineeringUniaxial consolidation of granular salt is carried out to study the mechanical behavior and fabric development in a material that deforms by microscopic brittle and intracrystalline-plastic processes. Dry granular salt is sieved to produce well-sorted size fractions. The granular salt is consolidated in a heated cell at axial stresses up to 90 MPa and temperatures of 100 - 200 ˚C to document stress-consolidation relationships and microstructural development. Polished and chemically-etched petrographic sections of salt samples prior to and after deformation at 150˚C are studied using transmitted- and reflected-light optical microscopy. We show that temperature has profound effect on porosity reduction during consolidation. At tested conditions, the dominant deformation mechanism is crystal plasticity; brittle deformation is largely suppressed. Samples consolidated at higher maximum axial stress develop higher overall dislocation densities. The distribution of dislocations, however, is strongly heterogeneous from grain to grain because of the complex grain-scale loading geometries and the distribution of intragranular flaws such as fluid inclusions. Static recrystallization occurs in some highly strained areas, but overall is minor at 150˚C. The experiments help to improve our understanding of consolidation, and serve to guide the fabrication of synthetic rock salt as experimental material, as well as to inform and test constitutive models of deformation of granular salt for engineering needs.
ItemCatch bonds govern adhesion through L-selectin at threshold shear(Georgia Institute of Technology, 2004-09) Yago, Tadayuki ; Wu, Jianhua ; Wey, C. Diana ; Klopocki, Arkadiusz G. ; Zhu, Cheng ; McEver, Rodger P. ; Oklahoma Medical Research Foundation. Cardiovascular Biology Research Program ; University of Oklahoma Health Sciences Center. Dept. of Biochemistry and Molecular Biology ; University of Oklahoma Health Sciences Center. Oklahoma Center for Medical Glycobiology ; Zhongshan University. School of Life Sciences ; Georgia Institute of Technology. Dept. of Biomedical Engineering ; Emory University. Dept. of Biomedical Engineering ; Georgia Institute of Technology. School of Mechanical EngineeringFlow-enhanced cell adhesion is an unexplained phenomenon that might result from a transport-dependent increase in on-rates or a force-dependent decrease in off-rates of adhesive bonds. L-selectin requires a threshold shear to support leukocyte rolling on P-selectin glycoprotein ligand-1 (PSGL-1) and other vascular ligands. Low forces decrease L-selectin–PSGL-1 off-rates (catch bonds), whereas higher forces increase off-rates (slip bonds). We determined that a force-dependent decrease in off-rates dictated flowenhanced rolling of L-selectin–bearing microspheres or neutrophils on PSGL-1. Catch bonds enabled increasing force to convert short-lived tethers into longer-lived tethers, which decreased rolling velocities and increased the regularity of rolling steps as shear rose from the threshold to an optimal value. As shear increased above the optimum, transitions to slip bonds shortened tether lifetimes, which increased rolling velocities and decreased rolling regularity. Thus, force-dependent alterations of bond lifetimes govern L-selectin–dependent cell adhesion below and above the shear optimum. These findings establish the first biological function for catch bonds as a mechanism for flow-enhanced cell adhesion.
ItemP-Selectin Glycoprotein Ligand-1 Forms Dimeric Interactions with E-Selectin but Monomeric Interactions with L-Selectin on Cell Surfaces(Georgia Institute of Technology, 2013-02-25) Zhang, Yan ; Jiang, Ning ; Zarnitsyna, Veronika I. ; Klopocki, Arkadiusz G. ; McEver, Rodger P. ; Zhu, Cheng ; Georgia Institute of Technology. School of Mechanical Engineering ; Georgia Institute of Technology. Dept. of Biomedical Engineering ; Emory University. Dept. of Biomedical Engineering ; University of Oklahoma. Health Science Center ; University of Oklahoma. Dept. of Biochemistry and Molecular Biology ; University of Oklahoma. Cardiovascular Biology Research ProgramInteractions of selectins with cell surface glycoconjugates mediate the first step of the adhesion and signaling cascade that recruits circulating leukocytes to sites of infection or injury. P-selectin dimerizes on the surface of endothelial cells and forms dimeric bonds with P-selectin glycoprotein ligand-1 (PSGL-1), a homodimeric sialomucin on leukocytes. It is not known whether leukocyte L-selectin or endothelial cell E-selectin are monomeric or oligomeric. Here we used the micropipette technique to analyze two-dimensional binding of monomeric or dimeric L- and E-selectin with monomeric or dimeric PSGL- 1. Adhesion frequency analysis demonstrated that E-selectin on human aortic endothelial cells supported dimeric interactions with dimeric PSGL-1 and monomeric interactions with monomeric PSGL-1. In contrast, L-selectin on human neutrophils supported monomeric interactions with dimeric or monomeric PSGL-1. Our work provides a new method to analyze oligomeric cross-junctional molecular binding at the interface of two interacting cells.
ItemModeling the Influence of Thermo-Mechanical Crack Opening and Closure on Rock Stiffness(Georgia Institute of Technology, 2013-07) Zhu, Cheng ; Arson, Chloé ; Georgia Institute of Technology. School of Civil and Environmental EngineeringA thermodynamic framework is proposed to model the effect of mechanical stress and temperature on crack opening and closure in rocks. The model is based on Continuum Damage Mechanics with damage defined as the second-order crack density tensor. The free energy of damaged rock is expressed as a function of deformation, temperature and damage. The damage criterion controls mode I crack propagation, captures temperature-induced decrease of rock toughness, and accounts for the increase of energy release rate necessary to propagate cracks induced by damage. Crack closure is modeled through unilateral effects produced on rock stiffness. Simulations show that: (1) under anisotropic mechanical boundary conditions, crack closure occurs during cooling, (2) the thermo-mechanical strain energy necessary to close cracks during cooling is larger than the strain energy needed to close the cracks by mechanical compression. Parametric study highlights the thermo-mechanical stress redistributions occurring during closure. The proposed framework is expected to bring new insights in the design and reliability assessment of geotechnical reservoirs and repositories.
ItemFlow-enhanced adhesion regulated by a selectin interdomain hinge(Georgia Institute of Technology, 2006-09) Lou, Jizhong ; Yago, Tadayuki ; Klopocki, Arkadiusz G. ; Mehta, Padmaja ; Chen, Wei ; Zarnitsyna, Veronika I. ; Bovin, Nicolai V. ; Zhu, Cheng ; McEver, Rodger P. ; Oklahoma Medical Research Foundation. Cardiovascular Biology Research Program ; University of Oklahoma Health Sciences Center. Dept. of Biochemistry and Molecular Biology ; Georgia Institute of Technology. Institute for Bioengineering and Bioscience ; Georgia Institute of Technology. Dept. of Biomedical Engineering ; Emory University. Dept. of Biomedical Engineering ; Georgia Institute of Technology. School of Mechanical Engineering ; Institut bioorganicheskoĭ khimii im. M.M. Shemi︠a︡kina i I︠U︡. A. OvchinnikovaL-selectin requires a threshold shear to enable leukocytes to tether to and roll on vascular surfaces. Transport mechanisms govern flow-enhanced tethering, whereas force governs fl ow-enhanced rolling by prolonging the lifetimes of L-selectin–ligand complexes (catch bonds). Using selectin crystal structures, molecular dynamics simulations, site-directed mutagenesis, single-molecule force and kinetics experiments, Monte Carlo modeling, and flow chamber adhesion studies, we show that eliminating a hydrogen bond to increase the fl exibility of an interdomain hinge in L-selectin reduced the shear threshold for adhesion via two mechanisms. One affects the on-rate by increasing tethering through greater rotational diffusion. The other affects the off-rate by strengthening rolling through augmented catch bonds with longer lifetimes at smaller forces. By forcing open the hinge angle, ligand may slide across its interface with L-selectin to promote rebinding, thereby providing a mechanism for catch bonds. Thus, allosteric changes remote from the ligand-binding interface regulate both bond formation and dissociation.
ItemModeling of processes in cell adhesion(Georgia Institute of Technology, 1994-02) Zhu, Cheng ; Georgia Institute of Technology. Office of Sponsored Programs ; Georgia Institute of Technology. School of Mechanical Engineering
ItemA Thermo-Mechanical Damage Model for Rock Stiffness during Anisotropic Crack Opening and Closure(Georgia Institute of Technology, 2013-11) Zhu, Cheng ; Arson, Chloé ; Georgia Institute of Technology. School of Civil and Environmental EngineeringA thermodynamic framework is proposed to couple the effect of mechanical stress and temperature on crack opening and closure in rocks. The model is based on continuum damage mechanics, with damage defined as the second-order crack density tensor. The free energy of the damaged rock is expressed as a function of deformation, temperature, and damage. The damage criterion captures mode I crack propagation, the reduction in toughness due to heating, and the increase in energy release rate with cumulated damage. Crack closure is modeled through unilateral effects produced on rock stiffness. The model was calibrated and verified against published experimental data. Thermo-mechanical crack opening (resp. closure) was studied by simulating a triaxial compression test (resp. uniaxial extension test), including a thermal loading phase. The degradation of stiffness due to tensile stress and recovery of stiffness induced by both mechanical and thermo-mechanical unilateral effects are well captured. The thermo-mechanical energy release rate increases with thermal dilation and also decreases with ambient temperature. It was observed that there is a temperature threshold, below which the rock behaves elastically. A parametric study also showed that the model can capture hardening and softening during thermo-mechanical closure (for specific sets of parameters). These numerical observations may guide the choice of rock material used in geotechnical design, especially for nuclear waste disposals or compressed-air storage facilities.