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Microscopic damage evolution of Anisotropic rocks under indirect tensile conditions: Insights from acoustic emission and digital image correlation techniques

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International Journal of Minerals,Metallurgy and Materials 2023年第9期30卷 1680-1691页

作者： Chaoqun Chu Shunchuan Wu Chaojun Zhang Yongle Zhang School of Civil and Resource Engineering University of Science and Technology BeijingBeijing 100083China Department of Civil&Mineral Engineering University of TorontoToronto ON M5S 1A4Canada Faculty of Land Resources Engineering Kunming University of Science and TechnologyKunmiing 650093China China Railway Sixth Group Limited Company Beijing 100036China

The anisotropy induced by rock bedding structures is usually manifested in the mechanical behaviors and failure modes of rocks.Brazilian tests are conducted for seven groups of shale specimens featuring different bedding angles. Acoustic emission (AE) and digital image correlation (DIC) technologies are used to monitor the in-situ failure of the specimens. Furthermore, the crack morphology of damaged samples is observed through scanning electron microscopy (SEM). Results reveal the structural dependence on the tensile mechanical behavior of shales. The shale disk exhibits compression in the early stage of the experiment with varying locations and durations. The location of the compression area moves downward and gradually disappears when the bedding angle increases. The macroscopic failure is well characterized by AE event location results, and the dominant frequency distribution is related to the bedding angle. The b-value is found to be stress-dependent.The crack turning angle between layers and the number of cracks crossing the bedding both increase with the bedding angle, indicating competition between crack propagations. SEM results revealed that the failure modes of the samples can be classified into three types:tensile failure along beddings with shear failure of the matrix, ladder shear failure along beddings with tensile failure of the matrix, and shear failure along multiple beddings with tensile failure of the matrix.

关键词： Anisotropic rock failure mechanism acoustic emission digital image correlation Brazilian test

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Multiscale modeling of gas-induced fracturing in anisotropic clayey rocks

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Journal of rock Mechanics and Geotechnical Engineering 2024年第6期16卷 2091-2110页

作者： Jianxiong Yang Jianfeng Liu Zhengyuan Qin Xuhai Tang Houquan Zhang State Key Laboratory of Hydraulics and Mountain River Engineering College of Water Resource and HydropowerSichuan UniversityChengdu610042China School of Resources and Safety Engineering Wuhan Institute of TechnologyWuhan430073China School of Civil Engineering Wuhan UniversityWuhan430072China State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering China University of Mining and TechnologyXuzhou221116China

In the context of repositories for nuclear waste,understanding the behavior of gas migration through clayey rocks with inherent anisotropy is crucial for assessing the safety of geological disposal facilities.The primary mechanism for gas breakthrough is the opening of micro-fractures due to high gas pressure.This occurs at gas pressures lower than the combined strength of the rock and its minimum principal stress under external loading conditions.To investigate the mechanism of microscale mode-I ruptures,it is essential to incorporate a multiscale approach that includes subcritical microcracks in the modeling framework.In this contribution,we derive the model from microstructures that contain periodically distributed microcracks within a porous material.The damage evolution law is coupled with the macroscopic poroelastic system by employing the asymptotic homogenization method and considering the inherent hydro-mechanical(HM)anisotropy at the microscale.The resulting permeability change induced by fracture opening is implicitly integrated into the gas flow equation.Verification examples are presented to validate the developed model step by step.An analysis of local macroscopic response is undertaken to underscore the influence of factors such as strain rate,initial damage,and applied stress,on the gas migration process.Numerical examples of direct tension tests are used to demonstrate the model’s efficacy in describing localized failure characteristics.Finally,the simulation results for preferential gas flow reveal the robustness of the two-scale model in explicitly depicting gas-induced fracturing in anisotropic clayey rocks.The model successfully captures the common behaviors observed in laboratory experiments,such as a sudden drop in gas injection pressure,rapid build-up of downstream gas pressure,and steady-state gas flow following gas breakthrough.

关键词： Deep geological repositories Mode-I microcracks Time-dependent damage Fracturing process Anisotropic rock

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Behavior of tunnels excavated with dip and against dip

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Underground Space 2021年第6期6卷 709-717页

作者： Osvaldo P.M.Vitali Tarcisio B.Celestino Antonio Bobet Lyles School of Civil Engineering Purdue UniversityWest LafayetteIN 47907USA Sao Carlos School of Engineering University of Sao PauloSao CarlosSP 13566Brazil

rock masses with remarked rock structure are likely to be highly anisotropic;thus,the relative orientation of the tunnel with respect to the rock structure will affect the tunnel behavior.Displacements measured at the walls of tunnels in highly Anisotropic rock masses often reveal an asymmetric deformation pattern around the tunnel perimeter,which is often interpreted as the result of rock heterogeneity or changing geology.However,numerical and analytical investigation of tunnels in Anisotropic rock masses show that the tunnel axis misalignment with the principal directions of stress and material anisotropy induce anti-symmetric axial displacements(i.e.,axial distortion of the tunnel cross section)far behind the tunnel face.Near the face,the axial distortion is partially constrained,causing asymmetric radial displacements.In this study,tunnels excavated with and against dip of the major rock mass discontinuity are explored using 3D FEM modeling.A detailed analysis on the mechanisms associated with the direction of excavation is provided.It is shown that the asymmetric displacements near the face may strongly affect the displacements measured at the tunnel walls,which may explain the asymmetric displacements commonly observed on tunnels in Anisotropic rock.

关键词： Tunnel Tunnel misalignment Anisotropic rock Face effects

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A method to model the effect of pre-existing cracks on P-wave velocity in rocks

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Journal of rock Mechanics and Geotechnical Engineering 2020年第3期12卷 493-506页

作者： Haimeng Shen Xiaying Li Qi Li Haibin Wang State Key Laboratory of Geomechanics and Geotechnical Engineering Institute of Rock and Soil MechanicsChinese Academy of SciencesWuhan430071China University of Chinese Academy of Sciences Beijing100049China

Crack closure is one of the reasons inducing changes of P-wave velocity of rocks under compression.In this context,a method is proposed to investigate the relationships among P-wave velocity,pre-existing cracks,and confining pressure based on the discrete element method(DEM).Pre-existing open cracks inside the rocks are generated by the initial gap of the flat-joint model.The validity of the method is evaluated by comparing the P-wave velocity tested on a sandstone specimen with numerical result.As the crack size is determined by the diameter of particles,the effects of three factors,i.***.number,aspect ratio,and orientation of cracks on the P-wave velocity are discussed.The results show that P-wave velocity is controlled by the(i.***.number) of open micro-cracks,while the closure pressure is determined by the aspect ratio of crack.The reason accounting for the anisotropy of P-wave velocity is the difference in crack number in measurement paths.Both of the number and aspect ratio of cracks can affect the responses of P-wave velocity to the applied confining pressure.Under confining pressure,the number of open cracks inside rocks will dominate the lowest P-wave velocity,and the P-wave velocity of the rock containing narrower cracks is more sensitive to the confining pressure.In this sense,crack density is difficult to be back-calculated merely by P-wave velocity.The proposed method offers a means to analyze the effect of pre-existing cracks on P-wave velocity.

关键词： Pre-existing cracks P-wave velocity Discrete element method(DEM) Anisotropic rock

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