The initiation and propagation of discontinuities in brittle materials is of great interest to engineers, at several scales. Discontinuities can be detrimental for structures (borehole for nuclear waste disposal, cavity for storage, tunnel for transportation) but discontinuities are necessary to extract energy resources (hydraulic fracturing for oil, gas, geothermal reservoir). A number of numerical tools are available to model fracture propagation at macro-scale in brittle solids. However, the fundamental inception mechanisms at micro-scale are not fully understood. Therefore, this doctoral research explores computational models to understand the processes that govern the initiation and propagation of micro-cracks in mixed mode in crystalline and porous media, and to predict the transition from a material that contain diffused micro-cracks with high density to a portion of discrete macro-fracture. We firstly developed two constitutive laws coupling micromechanics with thermodynamic principles. The phenomenological behaviors at meso-scale, such as nonlinear behavior, induced anisotropy, unilateral effect, are captured fundamentally with microstructure evolution, such as mixed open crack propagation, secondary wing crack development for closed crack. Dilute homogenization is used to connect the two scales. We, then, proposed a computational framework to model the cross-scale fracture propagation from diffused micro-cracks development to localized macro-fracture propagation, by coupling a nonlocal anisotropic damage model with a cohesive zone model. The transition point from continuum damage to discrete fracture is rigorous calibrated, and the correct amount of energy is ensured to dissipate at micro/macro scales. We further extended the framework of damage-fracture transition to simulate multiscale fracture propagation driven by fluid injection in transversely isotropic porous media. After implemented the model using extended finite element method, we investigated the influence of material and stress anisotropy on hydraulic fracturing paths. This study is expected to provide advanced numerical tools to understand and model the mechanical behavior of brittle material at several scales, and to apply them for practical engineering.
Dr. Chloe Arson
Dr. David Frost, Dr. Richard Neu (ME), Dr. Glaucio Paulino (CEE), Dr. Joshua White (Lawrence Livermore National Lab), and Dr. Ronaldo Borja (Stanford University)