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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Now showing 1 - 10 of 11

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.
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.
Micro-Macro Approach of Salt Viscous Fatigue under Cyclic Loading

(Georgia Institute of Technology, 2015) Pouya, Ahmad ; Zhu, Cheng ; Arson, Chloé

The objective of this work is to explain the origin of fatigue observed in salt rock subject to cyclic loading. We used a self-consistent homogenization scheme to upscale the viscoplastic and damage behavior of halite polycrystals from mono-crystal viscous glide and breakage mechanisms. We modeled mono-crystals as spherical inclusions embedded in an infinite homogeneous matrix, and we assumed purely elastic inclusion/matrix interactions. We introduced a failure criterion at the mono-crystal scale in order to predict grain breakage and the subsequent damage effects on salt rock elastic moduli. We wrote an algorithm that allows computing macroscopic and microscopic stresses and strains during creep and cyclic axial loading. Although some simplifying assumptions were made in our micro- macro approach, the model provided micro-mechanical interpretations to important aspects of salt rock viscoplastic and fatigue behavior, which had not been explained so far, such as strain hardening, creep recovery, as well as damage and accelerated creep due to grain breakage. Moreover, incremental viscoplastic strains decreased over the cycles, which is in agreement with the phenomenon of "shakedown" observed in elasto-plastic media. Salt rock can be viewed as a model material. More generally, this research is expected to bring new perspectives to study the microscopic origin of fatigue in viscous polycrystalline materials.
A model of damage and healing coupling halite thermo-mechanical behavior to microstructure evolution

(Georgia Institute of Technology, 2014-07) Zhu, Cheng ; Arson, Chloé

Creep processes in halite (salt rock) include glide, cross-slip, diffusion and dynamic recrystallization. Diffusive Mass Transfer (DMT) can result in crack rebonding, and mechanical stiffness recovery. Crack rebonding driven by DMT occurs within a few days at room temperature and low pressure. DMT is enhanced at higher temperatures, which could be beneficial for the sustainabilty of geological storage facilities in salt mines. On the one hand, visco-plastic laws relating creep microscopic processes to microstructure changes are empirical. On the other hand, theoretical models of damage and healing disconnect thermodynamic variables from their physical meaning. The proposed model enriches the framework of Continuum Damage Mechanics (CDM) with fabric descriptors. In order to infer the form of fabric tensors from microstructure observation, creep tests were carried out on granular salt under constant stress and humidity conditions. The evolution of net damage is governed by a diffusion equation, in which the characteristic time scales with the typical size of halite crystals, and the diffusion coefficient is a function of temperature. A stress path comprising a tensile loading, a compressive unloading, a creep healing stage and a reloading was simulated. Macroscopic and microscopic model predictions highlight the increased efficiency of healing with time and temperature. 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 assessment of geological storage facilities.
Theoretical Bases of A Thermo-Mechanical Damage and DMT-Healing Model for Rock

(Georgia Institute of Technology, 2014-02) Zhu, Cheng ; Arson, Chloé

A theoretical framework is proposed to model thermo-mechanical (TM) crack opening, closure, and healing in rock. The model is based on Continuum Damage Mechanics and thermodynamics. The postulated free energy is a polynomial of deformation, temperature, damage and healing. The damage-driving force captures damage evolution due to mechanical or TM tensile stresses, as well as the decrease of material toughness at elevated temperature. Crack closure is modeled by adopting the concept of unilateral effect on rock stiffness. A mixed variable is introduced to account for anisotropic TM damage and rate -dependent healing. Crack rebonding is assumed to result from Diffusive Mass Transfer (DMT) processes, and accordingly, the healing evolution law is governed by the diffusion equation. Contrary to other models for rock, the healing deformation is not a creep volumetric deformation, but the difference between the deformation before and after DMT. A parametric study illustrates the model capabilities: the simulation of TM stress paths with higher degree of mechanical recovery for longer healing time or higher healing temperature. The proposed model is expected to better predict the long-term behavior of self-healing rock materials – containing clay of halite minerals for instance.
A Thermo-Mechanical Damage Model for Rock Stiffness during Anisotropic Crack Opening and Closure

(Georgia Institute of Technology, 2013-11) Zhu, Cheng ; Arson, Chloé

A 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.
Modeling the Influence of Thermo-Mechanical Crack Opening and Closure on Rock Stiffness

(Georgia Institute of Technology, 2013-07) Zhu, Cheng ; Arson, Chloé

A 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.