Fracture Mechanics in Anisotropic Quasi-Brittle Material: Hydraulic Fractures in Shale, Damaging Composites

摘要

Anisotropic quasibrittle materials, such as shale and carbon fiber composites, are heterogeneous materials with brittle constituents, and widely appear in various engineering structures. The failure of these materials is typically characterized by a large fracture process zone that is not negligible to the structure dimensions. Further, quasibrittle damage often occurs in combination with tri-axial softening damage whose localization is governed by a finite material characteristic length. Objective mathematical description of these phenomena is vitally important in oil and gas industry, and automotive industry as well.;In the first part of the thesis, a simple but realistic gas transport model is proposed. This model can be used to estimate the hydraulic crack spacing under the shale reservoir. From this model and the stability analysis, we conclude that there should be a system of million intersecting cracks during the hydraulic fracture process. Based on this conclusion, we developed a hydraulic fracture model combining finite elements for deformation and fracture with volume elements for water flow in order to model the lateral crack branching. The key idea of this model is to include both Darcy-type flow and Poiseuille-type flow in the fluid equation, then using the new three-phase medium concept to couple the fluid and crack interaction. Since shale is anisotropic quasi-brittle material, a constitutive model using the new spherocylindrical microplane model is developed to capture the inelastic fracturing behavior of transversely isotropic property of shale.;In the next part of this dissertation, an innovative experimental protocol for characterizing the fracturing behavior of transversely isotropic woven composites is proposed with application to many different industries such as wind energy production, blast protection, civil application, naval, and especially automotive field. The key success of this direct testing method of gradual postpeak softening of fiber composites is due to the newly enhanced grip stiffness and mass. This method allows users to be able to calculate the the full fracture energy available in composites, which had been believed to be a highly brittle material for decades. The quasi-brittle character can be shown by the stable post-peak softening load-displacement curve of the composite specimen in compact tension test. The fracture energy value given by this new testing method can match very well with the value obtained from the size effect testing, which is an indirect way to get the fracture energy.