Characteristics of damage evolution of deep coal based on CT three-dimensional reconstruction

In order to study the damage evolution characteristics and failure mechanism of deep coal under different stress conditions,the coal at the No. 17220 working face of the Ji-15 coal seam in No. 12 Pingdingshan Coal Mine was sampled for this study. The CT real-time testing of uniaxial compression was carried out. Combining with the meso-sta- tistical damage mechanics,a method of defining the damage variable based on the gray value of CT images was pro- posed to quantitatively analyze the damage evolution characteristics during the uniaxial compression of coal samples. Based on CT scanning experiment,mercury intrusion experiment and indoor basic mechanics experiment,a three-di- mensional numerical geometric model of deep heterogeneous coal sample was established to reflect the distribution of solid matrix. And,a reasonable meshing was carried out. In addition,the constitutive models and physical and mechan- ical parameters of different material components were determined. Initially,the numerical simulation of uniaxial com- pression was carried out to qualitatively study the damage evolution and failure mechanism of coal samples using dis- placement controlling loading. Furthermore,on the basis of numerical simulation of uniaxial compression,the triaxial numerical simulations of coal samples under five different confining pressure conditions were carried out by applying different circumferential stresses to the coal sample. The damage evolution of deep coal under triaxial compression was qualitatively analyzed from the aspects of stress-strain curve,fracture morphology and fracture angle of coal sample. The results show that the stress-strain curve and damage evolution characteristics obtained by the numerical simulation of uniaxial compression are in good agreement with the results obtained by CT real-time scanning experiment. With the increase of axial stress,the meso-damage evolution of coal sample occurs in four stages:the zero damage stage,the lo- cal damage generation stage,the damage linear and nonlinear steady growth stage and the accelerated damage growth stage. When the specimen is destroyed,its maximum shear strain rate region and plastic zone are approximately parallel or perpendicular to the interface between the coal matrix and the coal inclusion,and the two main damage surfaces are perpendicular to each other. The coal sample mainly undergoes tensile failure during uniaxial compression,and the shear failure occurs after the yield stress due to the uneven deformation. The damage evolution characteristics under triaxial compression obtained by numerical simulation based on CT reconstruction can be well matched with the six classical stages of rock damage evolution. The coal sample mainly undergoes shear failure. With the increase of confi- ning pressure,the peak strength,the stress at the expansion point and the residual strength of coal all increase. The rupture angle decreases with the increasing confining pressure,and displays an approximately negatively linearly curve with the confining pressure. In this paper,the numerical simulation was optimized for meshing,establishment of geo- metric model,determination of material parameters and constitutive models and calculating method of stress and strain, and had achieved some good results of damage evolution. The numerical simulation method also can eliminate the in- fluence of the difference of experimental samples,and can qualitatively describe the damage evolution law under uniax- ial and triaxial compression intuitively and accurately.