| It is of great significance to explore the fracture development in coal seam and the spatial distribution and evolution of coal reservoir fracture network with the stress change,so as to provide theoretical support for the safety production of coal and coalbed methane and dual energy production,and also the prediction of coal dynamic disasters.Important progress has been made in the study of characteristics of rock fracture,however,it needs further exploration to the more precise characterization of the three-dimensional structure of mesoscopic fractures in coal,and visualization and quantitative study on the real-time dynamic evolution of fractures in coal under triaxial compression.In this paper,high-precision X-ray industrial CT scanning system equipped with loading device was used to do the uniaxial compression,triaxial test and real-time CT scanning of two kinds of anthracite,which are from Zhaogu coal mine in Jiaozuo and no.3 coal seam of Sihe coal mine in Qinshui basin.The image processing technique,threedimensional reconstruction technique,fractal geometry and mathematical statistics were used to extract and analyze the parameters such as fracture geometry distribution,fracture volume,porosity,azimuth angle and numbers to show the dynamic evolution of fracture.At the same time,the numerical simulation method of discrete element particle flow software PFC was used to simulate the fracture development process under the loading of coal.The microstructure damage characteristics and mechanism of coal and rock under loading are studied by means of mesoscopic damage theory of rock mechanics.The main research results are as follows:(1)Combined with the image processing method and three-dimensional reconstruction technology,the spatial morphology and digital characteristic parameters of the coal fracture were extracted,and the coal fracture was characterized in detail.The extraction and analysis of the dynamic evolution of coal fracture damage in the process of triaxial loading are realized,and the correct operation flow and reasonable loading and scanning parameters are determined,which is helpful for the follow-up of the relevant CT scanning test of coal samples under the loading condition.(2)The gray value,the mean gray deviation,the volume of fracture,CT fracture porosity and fractal dimension can be used as quantitative characterization parameters to effectively represent the internal fractures of coal.The physical properties and damage evolution degree of the fracture are effectively and precisely characterized.MATLAB programs are used to realize the number of plane fracture length and angle and then statistics are done.The fractal dimension based on gray-scale image is proposed,which can cover the gray-scale information of coal with a resolution less than the scanning resolution,and avoid the error caused by the selection of the threshold value of binarization image.(3)Based on the coal samples from Zhaogu coal mine and Sihe coal mine,the failure modes,the physical and chemical properties and the law and characteristics of fracture evolution in single triaxial compression test are presented and described.The coal sample under triaxial compression failure process experienced four stages,and also the stress levels of each stage are given.The damage propagation characteristics of different layers of coal and rock are also different under the same stress state.It showed unevenness and localization of the damage evolution of coal under loading.In addition,the split fracture appears on the macroscopic surface,while in the center of the coal body,the shear zone is formed by mutual polymerization under the uniaxial loading.Therefore,it is concluded that the crack starts from the edge and develops,and then with the increase of pressure,the internal crack accumulates and integrates.Macroscopic surface cracks are not enough to describe the fracture morphology of coal.(4)The compressive strength and peak strain of coal under load are greatly reduced by containing initial fractures.The two coal samples show different characteristics of strength and ductility due to different fracture development,the final macro fracture damage and their internal primary fracture distribution is closely related to the interaction and extension gathered.The ultimate macroscopic fracture failure of coal mass under the action of load is closely related to the distribution of primary fractures and the extended aggregation.However,the fracture morphology of coal body was affected,but the failure of coal was mainly caused by the development of new fractures.The original fracture is an important factor of influencing the strength and failure mode of coal sample.The effect of primary fracture on the failure of coal under loading is revealed.(5)The greater the confining pressure is,the greater the compressive strength and peak strain are.However,the elastic modulus of dense coal is not affected,but affect the coal with many original fractures,whose elastic modulus increases with the increasing of confining pressure.At the same time,confining pressure also has an impact on the morphology and spatial distribution characteristics of coal fracture.Confining pressure is small,and the fracture shape is relatively simple and the fracture is wider,and vice versa.The effects of confining pressure on the compressive strength and fracture space development of coal are revealed.In this paper,the fracture rose diagram representing the expansion angle of the dominant fracture in the plane was drawn,the change of fracture trend between the original fracture and the fracture process in the plane is discussed,and the relationship between the distribution direction of the original fracture and the fracture direction under different confining pressures is studied.(6)Using the discrete element PFC2 D numerical simulation software to get the real coal sample model,and triaxial compression simulation experiment was carried out,the stress-strain of triaxial compression test can be better simulated.By monitoring the particle displacement,particle velocity vector,fracture evolution process and fracture number of the sample,the fracture propagation process of the sample under triaxial compression can be demonstrated visually.The detailed process of fracture development,the influence of primary fractures on the initiation and development of new fractures,the difference of fracture initiation location between primary and dense fractures,the process of stress redistribution in fracture evolution and the characteristics of localized regionalization are revealed. |
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