Research On Quantitative Characterization And Failure Mode Of Meso Scale Key Cracks In Coal

The occurrence conditions and human disturbance cause the complex physical changes of coal structure,promote the development of micro-structures such as pores and cracks in coal,and directly affect the physical and mechanical properties and failure types.At present,the internal microstructure morphology,distribution characteristics and expansion evolution law of pore-fractured coal body and its understanding of the macroscopic damage of coal body are not deep enough,which leads to the unclear macroscopic damage mechanism of this kind of coal body affected by the change of mesostructure.In this paper,the quantitative characterization,visual reconstruction and statistical analysis of coal pore and fissure structure are carried out from a mesoscopic point of view by using a combination of macroscopic and mesoscopic physical and mechanical experiments,digital image processing technology,and mathematical statistics.The large-scale cracks that lead to the instability and failure of the coal body are defined as the key cracks of the coal body.The spatial location,geometric size,failure mode and propagation path of key cracks are determined,which provides theoretical basis and technical support for the macro-mechanical failure mechanism caused by the change of the mesostructure of coal body.The main results and conclusions of this paper are as follows:(1)Through the scanning electron microscope experiment,the microstructure and microscopic composition of coal were analyzed.The results show that the coal macerals are vitrinite and inertinite,mainly including clastic vitrinite and filamentous body of plant cell pores.The interior of the coal is rich in metamorphic pores with various shapes,the pores are basically disconnected,and the permeability is weak.The mesomorphology of the coal body is rough and has a large number of defects.(2)Through uniaxial compression experiments,the mechanical strength characteristics and size effects of coal were analyzed.The results show that the uniaxial compressive strength of coal body is in the range of 10.79MPa to 32.71MPa,with an average of 18.62MPa,which belongs to medium hard coal body.The elastic modulus range is 0.12GPa to 2.43GPa,the average is 1.37GPa,the normalized proportion of the standard coal body is negatively correlated with the uniaxial compressive strength,and positively correlated with the elastic modulus,that is,the smaller the size of the standard piece,the greater the uniaxial compressive strength and the smaller the elastic modulus.It shows that coal has size effect.(3)The correlation between the mesostructure characteristics and mechanical strength of coal before and after uniaxial compression was compared.The results show that the original pore and fracture structure of coal before compression is negatively correlated with the mechanical strength,that is,the greater the complexity of micro-structure is,the lower the mechanical strength.The secondary pore and fracture structure of coal after compression is positively correlated with mechanical strength,that is,the greater the mechanical strength of coal,the greater the disturbance time under uniaxial loading,resulting in the development,expansion and complexity of micro-structure.(4)The definition,research multiple,definition method and characteristic parameters of two-dimensional key cracks in coal were determined and verified,and the mathematical model of coal porosity and uniaxial compression strain considering the pore compaction stage was constructed.The results show that the average porosity change of the 500 times magnified electron microscope image of the coal body before and after compression is 80%.The perimeter of the two-dimensional key crack is 165.84-806.46μm,and the area is 224.14-535.63μm~2.The average area of the two-dimensional key crack in the coal body and the uniaxial compressive strength are negative exponential correlations with a goodness of fit of 0.65.That is,the larger the crack area,the lower the uniaxial compressive strength.(5)Through CT scanning experiments,a three-dimensional reconstructed body model was constructed when the coal was loaded to 50%of the peak force,the spatial distribution characteristics of pores and fractures in the coal body were analyzed,and the spatial location and characteristic parameters of key cracks in coal body are determined.The results show that the porosity corresponding to 50%of the peak force of the coal body is 4.43%.At this time,the three-dimensional surface area of the key crack of the coal body is 500.51μm~2 and the volume is 5705.79μm~3.(6)Through the acoustic emission monitoring experiment,the key crack mode generated when the coal is loaded to 50%of the peak force,and the growth path evolution law of the key crack developed through uniaxial compression through other mesoscopic cracks or macroscopic damage were determined.The results show that the macroscopic damage of coal starts from the development and expansion of key cracks,and forms high-energy acoustic emission events.The failure direction of the coal body is the direction of the connection line between the critical crack and the macroscopic failure position,and the distance between the macroscopic crack and the critical crack is the closest.The paper has 46 figures,11 tables,and 85 references.