Person:

Zhu, Cheng

Permanent Link

https://hdl.handle.net/1853/71896

Associated Organization(s)

Organizational Unit

Wallace H. Coulter Department of Biomedical Engineering

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Publication Search Results

Now showing 1 - 10 of 13

Analysis of microstructure, deformation and permeability of salt/sand mixtures during creep

(Georgia Institute of Technology, 2017-07) Shen, Xianda ; Zhu, Cheng ; Arson, Chloé

The 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.
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.
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.
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.
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.
Numerical study of the influence of fluid viscosity on wellbore spalling in drained fractured rock

(Georgia Institute of Technology, 2015-07) Jin, W. ; Zhu, Cheng ; Arson, Chloé ; Pouya, A.

The objective of this work is to model the influence of shear stresses induced by viscous fluid flow on wellbore spalling. We simulated a drop of stress and pore pressure at the wall of a meter-scale borehole with a plane strain Finite Element model. The rock mass was modeled as a jointed continuum. Block sliding was predicted from the tangential displacements in the joint after the shear failure criterion was reached. Simulations show that: (1) Higher far field stresses induce more normal stress in the joints, which prevents the occurrence of shear plastic strains in the joints and reduces block sliding at the wall; (2) Shear stresses and consequent shear plastic strains that are induced by viscous fluid flow in the joints are higher for higher fluid viscosities, and decrease over time as the blocks on each side of the joint slide on each other; (3) In joints that are in contact with the borehole, a change of one order of magnitude in the fluid viscosity results in a change in joint shear stress by a factor of 2. Results suggest that if drainage had been simulated over a longer period of time or for a smaller borehole diameter, the failure criterion would have been reached on a larger zone around the borehole, which could have a critical impact on the risk of borehole spalling. The numerical approach proposed in this work is expected to be useful to recommend wellbore operation modes so as to avoid excessive spalling and clogging.
Fabric-enriched Modeling of Anisotropic Healing induced by Diffusion in Granular Salt

(Georgia Institute of Technology, 2015-07) Zhu, Cheng ; Arson, Chloé

This study aims to model anisotropic damage (i.e. increase of porosity and loss of stiffness) and healing (i.e. recovery of stiffness) in salt rock subject to microcrack initiation, propagation, and rebonding. We introduce enriched fabric tensors in a Continuum Damage Mechanics model to link micro-crack evolution with macroscopic deformation rates. We carry out creep tests on granular salt assemblies to infer the form of fabric descriptors. We use moments of probability of fabric descriptors to find relationships between microstructural and phenomenological variables. Creep processes in salt include glide, cross-slip, diffusion, and dynamic recrystallization. We assume that healing is predominantly governed by diffusive mass transfer. We model the corresponding crack cusp propagation on grain faces by means of a two-dimensional diffusion equation. We calibrate this grainscale healing model against experimental measures of crack cusp propagation distance. We simulate the opening, closure and rebonding of three orthogonal families of micro-cracks during a compression-tension loading cycle. Multi-scale model predictions illustrate the evolution of stiffness, deformation, and crack geometry during the anisotropic damage and healing process, and highlight the increased healing efficiency with time. We expect that the proposed modeling approach will provide more precise and reliable performance assessments on geological storage facilities in salt rock.
Fabric-based modeling of thermo-mechanical damage and healing around salt caverns

(Georgia Institute of Technology, 2015-05) Zhu, Cheng ; Arson, Chloé

Geotechnical reservoirs and repositories in salt such as nuclear waste disposals, geothermal systems, and compressed air energy storage (CAES) are usually subject to complex thermo-mechanical conditions, leading to crack initiation, propagation, and rebonding. This work aims to model thermo-mechanical damage and healing around salt caverns, by enriching the framework of continuum damage mechanics with fabric descriptors. In order to infer the form of fabric tensors from microstructure observations, we carry out creep tests on granular salt under constant stress and humidity conditions. We simulate a stress path typical of CAES conditions at the material element level. The model presented in this paper is expected to improve the fundamental understanding of damage and healing in rocks at both macroscopic and microscopic levels, and the long-term evaluation of geological storage facilities.
Image processing of fabric evolution in granular salt subject to diffusive mass transfer

(Georgia Institute of Technology, 2015-05) Zhu, Cheng ; Arson, Chloé ; Jeong, J. H. ; Dutta, M.

Because of its favorable creep properties and low gas permeability, salt rock is viewed as an attractive host medium for nuclear waste disposals and natural resources storage. Under high stress and temperature conditions, diffusive mass transfer in salt rock can result in crack rebonding and strength recovery. In order to track the evolution of voids between salt crystals with lower load levels but higher healing rates than what is practically encountered in underground storage, we carried out creep loading tests on table salt. We used different loading conditions and inclusion materials to study the potential recurrence of topological patterns at grain boundaries. We developed a dedicated multi-stage image processing procedure to enhance microscopic image quality, and presented a slicing method to track the evolution of microstructure in different sections of the sample. This allowed us to analyze not only the evolution of average void size and orientation, but also the evolution of the fabric. We found that creep deformation is due to pore shrinkage along a diagonal direction across the sample, without significant grain rearrangement. It was noted however that basalt and sand inclusions rotated during the first 136 days of the creep tests. The proposed image processing techniques presented herein are expected to provide a methodology to track the evolution of microstructure descriptors that can be used to define alternative fabric tensors in thermodynamic models.
Theoretical study of damage accommodation in salt subject to viscous fatigue

(Georgia Institute of Technology, 2015-05) Zhu, Cheng ; Arson, Chloé ; Pouya, A.

Underground salt cavities used for compressed air energy storage undergo cyclic loads and are subject to a fatigue phenomenon that reduces rock strength and stiffness. Understanding such behaviors and developing relevant constitutive models require a micro-mechanical analysis. This study investigates damage and fatigue in salt rock, the extent of which is influenced by its polycrystalline nature, on the basis of self-consistent upscaling approaches for viscous heterogeneous materials. We develop a model that treats monocrystals as spherical inclusions embedded in an infinite homogeneous matrix with purely elastic inclusion-matrix interactions. To predict grain breakage and its subsequent impact, we also introduce a failure criterion. The model provides micro-mechanical interpretations of the common viscoplastic and fatigue behavior of salt such as damage and accelerated creep from grain breakage and the shakedown effect observed in elastoplastic media. Finite element (FE) simulations confirmed the macrostrain and microstress predictions obtained by homogenization. The FE program will be used in future studies to simulate inter-granular fracture propagation. This study provides new perspectives on research pertaining to the microscopic origin of fatigue in viscous polycrystalline materials.