Mechanisms And Numerical Simulation Methods For Strain Localized Progressive Failure Of Sandstones

Sandstones preserved in the deep underground environment are accompanied by many undesirable properties,such as high porosity,strong spatial heterogeneities,and sensitivity to the mechanical environment.Under external loading or alteration of preservation environment,this sort of material is conductive to strain localization instability,which is adverse to the development and utilization of deep underground engineering.The study of strain localized progressive failure characteristics in such materials is of great practical importance to deep underground engineering problems,such as the stability of borehole,the recovery of underground fossil energy,and the storage of greenhouse gases,etc.Under conditions of high-stress states in deep underground,strain localization in sandstones is mainly featured by particle migration and breakage inside a narrow region with concentrated reduction of porosity,and is prone to show spontaneous unstable accelerating propagation under stationary external perturbations.In the meantime,the inter-particle cementation degradation can further promote the inceptions of localized deformation.In this context,this thesis has utilized theories of phenomenological constitutive and controllability for strain localization to identify the inceptions of quasi-instantaneous and delayed localized instabilities in sandstones.This thesis has further detected particle breakage processes inside strain localized zones in granular materials,as well as to reveal the key factors involved in spatial breakage mechanisms.In addition,through high-order continuum theories,this thesis has proposed numerical methodologies for analyzing the mechanisms for spatial propagation of strain localization instability in sandstones preserved in deep underground.The main contents of this thesis involve the following aspects:(1)Systems of nonlinear ordinary differential equations(OEDs)are set up using a viscoplastic assumption and the theory of controllability,as such,to describe the kinematics of deformation inside strain localization bands.On this basis,a viscoplastic localization criterion is derived to identify the inceptions of quasi-instantaneous strain localization,as well as to detect accelerating responses of the spontaneous propagated delayed localized deformation under stationary external perturbations.This work can remedy the deficiencies of the classical bifurcation theory in the field of viscoplasticity.Using constitutive laws for sandstones,numerical analyses of material point and boundary value problems have demonstrated the correspondence between the positive/negative sign of the proposed criterion,the pulses of overstress,and the decaying/accelerating responses of strain localization.(2)By taking into account the influences provided by the Lode angle,i.e.,the third stress invariant,the proposed localization criterion is extended to three-dimensional(3D)stress circumstances.Using numerical simulations of progressive instability processes of a deep excavated borehole,the applicability of the proposed theory on identifying compaction shear localization,shear enhanced compaction localization,and compaction localization emerging in three-dimensional(3D)stress states is detected.The interplay between in-situ stress configurations and the versatile instability modes of the excavated borehole are tested.As such,conditions for excavation-induced strain localization instability and diffusive plastic deformation in natural formations are obtained.This work further investigates and interprets the triggering and propagation mechanisms of versatile deformation modes through the Lode angle enhanced localization criteria.(3)The evolution patterns of local particle breakage commonly observed in the strain localization bands inside sandstones are investigated.This work is based on a homogenization method of microscale material properties for replicating measurements from X-ray computed tomography to finite element model and the use of continuum breakage mechanics.The specific effects of the initial material spatial heterogeneities on the spatial pattern of particle breakage are analyzed,and the perturbations provided by boundary friction are further considered to illustrate the competition interplay between the boundary friction and material heterogeneity on dominating the nucleation and propagation of spatial particle breakage.(4)Through the generalized micromorphic approach,a micro-strain is introduced to serve as an additional kinematic variable.A coupled Helmholtz partial differential governing equation is converted from a micro-force balance containing higher-order generalized stresses using the virtual power method and the second law of thermodynamics.Based on the modified Drucker-Prager model,a rate-type constitutive integration algorithm and nonlinear coupled finite element procedure are implemented.As such,a numerical framework of implicit gradient theories for simulating strain localization processes in geomaterials is proposed.Numerical examples have shown that the proposed methodology is capable of producing mesh-independent numerical solutions.The plastic dissipation analysis further verifies the ability of the proposed model to regularize ill-posed boundary value problems.(5)Through the equivalent nonlocal theory,the numerical framework of implicit gradient theory is used to enhance the advanced constitutive laws for sandstones.The aim is to simulate the compaction localization with progressive failure processes in sandstones under high confining pressure.Through controllability analyses and finite element numerical simulations,the necessary conditions for obtaining a full regularization under the mixed influences provided by non-associated flow characteristics and material softening are revealed.Based on the proposed gradient model,two sorts of triggering mechanisms for compaction localization are illustrated.The discrete distributed compaction banding mode induced by material hardening and the thickening compaction banding mode caused by material softening are reproduced from a numerical point of view.