| Under the influence of bedding structure,shale exhibit obvious anisotropic characteristics in mechanics.The shale can be simplified as a transversely isotropic medium when analyzing its mechanical response under load.It is of important theoretical significance and engineering practical value for guiding shale gas production that analyzing the influence of the transverse isotropy of shale on its damage and fracture properties,and establishing the constitutive model and fracture criterion that can reflect the transverse isotropy of shale.In this paper,the physical and mechanical anisotropy characteristics of Longmaxi stratified shale are systematically analyzed by laboratory experiment and numerical simulation.At the same time,based on the elastic theory of transversely isotropic medium,the establishment of constitutive model,the solution of stress intensity factor and the analysis of fracture mechanism of shale are deeply studied.The main content and results of the paper are as follows:(1)Uniaxial compression tests,conventional triaxial compression tests,and Brazilian splitting tests were conducted on shale specimens with different bedding inclinations.The mechanical anisotropy characteristics of shale were systematically analyzed from the failure modes,strength characteristics,energy evolution,threshold values of characteristic stress and strain,and distribution rule of failure envelopes inπ-plane.At the same time,the internal mechanism of mechanical anisotropy of shale was revealed by combining the analysis results of micro-structure and P-wave velocity distribution.(2)According to the results of uniaxial compression test and Brazilian splitting test,the transverse isotropic elastic parameters,apparent elastic modulus and apparent Poisson’s ratio of shale were calculated,and the rationality of using the transverse isotropic constitutive model to describe the elastic deformation characteristics of shale was verified.Furthermore,a transversely isotropic statistical damage constitutive model of shale was established by introducing a modified three-dimensional transversely isotropic H-B criterion and Weibull probability density function,and the model was verified by uniaxial and triaxial tests,as well as the physical significance of model parameters was discussed.(3)Based on the discrete element theory of the particle flow,the numerical model of shale specimen was established by the Particle Flow Code in three-dimensions(PFC3D),and the simulation research on the anisotropic mechanical characteristics of shale under triaxial compression was carried out.Then,the meso-mechanical characteristics of shale were analyzed from the perspectives of microcrack evolution,force chain distribution of contact and particle displacement evolution.Meanwhile,the damage evolution mechanism of shale in the process of deformation and failure was revealed on the microscopic level,and the influence of meso-parameters on the macro-mechanical characteristics of shale was discussed.(4)Utilizing the complex variable function method and combined with the Fourier expansion series of boundary conditions,the analytical solution of the stress of a transversely isotropic disk under boundary load with different distribution forms was derived,as well as the solution formula of the stress concentration factors.Moreover,the convergence performance of the solution and the influence of elastic parameters on the stress concentration factors were analyzed.Then,a weight function method for solving the crack stress intensity factor based on the cracked straight-through Brazilian disc(CSTBD)structure was proposed,and the feasibility of the method was verified by using the existing solution data.(5)Fracture mechanics tests on shale under different loading modes were carried out with CSTBD specimens.The failure strength,crack propagation morphology,failure mode and temporal distribution characteristics of acoustic emission of the specimens were analyzed.Furthermore,based on the test results,the fracture toughness of shale was calculated by using the finite element method and weight function method to solve the crack stress intensity factor in transversely isotropic medium.The anisotropic characteristics of fracture toughness of shale were analyzed,and the polynomial function between the fracture toughness and crack inclination angle was obtained,as well as the polynomial function between the fracture toughness and bedding inclination angle.(6)Based on the fracture mechanics theory of transversely isotropic medium,and considering the influence of T-stress and plastic zone at the crack tip,the modified maximum tangential stress fracture criterion of transversely isotropic medium.At the same time,the applicability of the fracture criterion was verified by the results of fracture mechanics tests of shale. |
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