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Zhu, Cheng

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End date: 2016 × Start date: 2016 × search.filters.applied.f.isOrgUnitOfPublicationTitle: School of Civil and Environmental Engineering × Type: Text ×

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Now showing 1 - 6 of 6
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    Prediction of viscous cracking and cyclic fatigue of salt polycrystals using a joint-enriched Finite Element Model
    (Georgia Institute of Technology, 2016-09) Zhu, Cheng ; Pouya, Amade ; Arson, Chloé
    We present a new Joint-enriched Finite Element Method (JFEM) to predict viscous damage and fatigue in halite polycrystals in 2D. Different visco-plastic finite elements are used to represent grains of different orientations, and joint elements are used for modeling crack propagation. Simulations of uniaxial creep tests show that, as it could be predicted theoretically, viscous shear deformation in grains causes geometric incompatibilities. Numerical results also show that the transition between secondary and tertiary creep corresponds to inter-granular crack coalescence. The JFEM model captures the mechanical behavior of halite under cyclic loading, mainly: (a) Higher stress amplitude, lower confining stress, and lower loading frequency increase deformation and damage; (b) The polycrystal’s Young’s modulus decreases exponentially with the number of cycles; (c) The behavior is similar for different loading directions. Simulations with intra- and inter- granular joint elements show that most stress concentrations occur in intra-granular joints where several angular grains are in contact. Results of creep tests obtained with the JFEM are compared to those obtained with an inclusion-matrix model that accounts for damage accommodation due to grain breakage. Both the JFEM and inclusion-matrix models are calibrated against experimental creep tests to: (a) Produce a Young’s modulus of 23 GPa for the polycrystal; (b) Match secondary creep strain rates; (c) Match the time of tertiary creep initiation. In the inclusion-matrix model, the absence of grain geometric rearrangement results in a brutal failure just after the first grain breakage that triggers tertiary creep. Moreover, the JFEM model highlights the development of crack patterns upon viscous deformation. The JFEM is of great promise to understand complex phenomena of viscous accommodation coupled with grain interface debonding.
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    Microstructure-based modeling of damage and healing in salt rock with application to geological storage
    (Georgia Institute of Technology, 2016-06-24) Zhu, Cheng
    Most 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.
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    Damage 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, Chloé
    In 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.
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    Mechanical 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
    Uniaxial 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.
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    Chemo-Mechanical Damage and Healing of Granular Salt: Micro-macro modeling
    (Georgia Institute of Technology, 2016-06) Xianda, Shen ; Zhu, Cheng ; Arson, Chloé
    A 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.
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    Micro-mechanical analysis of salt creep tests with a joint-enriched Finite Element model
    (Georgia Institute of Technology, 2016-06) Zhu, Cheng ; Pouya, Ahmad ; Arson, Chloé ; Ding, Jihui ; Chester, Frederick M. ; Chester, Judith S.
    In this study, micro-mechanisms that govern the viscous and damage behavior of salt polycrystal during creep processes are investigated. A Finite Element model is designed with POROFIS, in which surface elements represent salt grains and joint elements represent inter-granular contacts. Microscopic observations of salt thin sections serve as a basis to design the mesh, which includes voids. We compare three strategies to predict microscopic damage in the salt polycrystal: (1) inter-granular damage represented by damage propagation in joint elements; (2) intra-granular damage represented by stiffness degradation in grain surface elements; (3) damage in both surface and joint elements. We simulate creep tests in conditions typical of Compressed Air Energy Storage. The three models capture polycrystal stiffness degradation and the initiation, propagation and coalescence of cracks that originate from geometric incompatibilities and local stress concentrations. The model with damageable joints presents a more ductile behavior and captures a smooth transition between steady state and tertiary state creep. This research is expected to improve the fundamental understanding of viscous damage mechanisms in salt rock for geostorage applications, and bring new insights on numerical modeling of multi-scale damage processes in crystalline materials.