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Zhu,
Cheng
Zhu,
Cheng
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ItemSelf-consistent micromechanical approach for damage accommodation in rock-like polycrystalline materials(Georgia Institute of Technology, 2017) Pouya, Ahmad ; Zhu, Cheng ; Arson, ChloéIn 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.
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ItemMicro-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.
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ItemMicro-Macro Approach of Salt Viscous Fatigue under Cyclic Loading(Georgia Institute of Technology, 2015) Pouya, Ahmad ; Zhu, Cheng ; Arson, ChloéThe 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.