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Computational and Experimental Characterization of the Behaviors of Anisotropic Quasi-Brittle Materials: Shale and Textile Composites

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Anisotropic quasi-brittle materials represent a large group of geological and structural materials commonly seen in various engineering applications. A thorough study of their behaviors, especially anisotropic elastic response, damage, and fracturing behaviors, are essential for better understanding, optimizing, and utilizing their engineering performance. Two representative anisotropic quasi-brittle materials, shale rocks and textile composites, were investigated in this work through both numerical and experimental methods. They are considered as transversely isotropic and orthotropic media respectively, which represent the two most common types of anisotropy among engineering materials. For shale rocks, numerical methods were adopted to investigate their anisotropic elastic behaviors, inelastic responses, and directional-dependent failure modes under various loading conditions. The extension of microplane formulation to transverse isotropy was explored, and its applicability to modeling shale anisotropic elastic behaviors was investigated. Furthermore, a multiscale framework based on a lattice discrete particle method and a mathematical homogenization scheme was adopted to simulate the heterogeneous deformation of the random granular internal structure of shale and the macroscopic mechanical responses. Material anisotropy was introduced at the level of constitutive laws according to an approximated geometric description of the material's internal structure. In addition, the numerical method was augmented by a dual lattice system at the grain level to simulate fluid flow along pores and cracks. The newly developed multiphysics discrete model was used for computational analysis of the shale fracture permeability behavior. Lastly, experimental characterization of the fracture properties of Marcellus shale was performed through size effect tests, and measures of anisotropic fracture energy/toughness, effective fracture process zone length as well as brittleness of the investigated specimens were obtained. For textile composites, a constitutive model based on the spectral stiffness microplane formulation was developed to simulate the orthotropic stiffness, pre-peak nonlinearity, and post-peak softening and fracturing behavior of composites with three dimensional (3D) fiber reinforcement. Extensive experimental characterization of a 3D carbon-epoxy woven composite was also performed to provide a comprehensive database of the orthotropic mechanical properties of the investigated material. The properties were measured in three principal directions of the material, and various damage mechanisms and failure modes were identified. Lastly, the results of fracture size effect tests revealed remarkable size effect of the investigated 3D textile composite and demonstrated improved ductility and damage tolerance featured by the 3D fiber reinforcement.

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  • 04/09/2019
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