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Zhu, Cheng

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Author: Arson, Chloé × Author: Chester, Frederick M. ×

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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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    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.