Study On Damage Evolution Law In Loading And Unloading Process Of Coal-Rock Combination

With the gradual depletion of shallow coal resources,coal mining has gradually developed to the deep.The deep stress environment is complicated,and the small mining disturbance will induce the instability of surrounding rock and cause serious disasters.Aiming at the rock burst disaster caused by excavation and unloading disturbance in deep mines,the deformation and fracture mechanism of roadway surrounding rock under three-way stress state was studied.Using coal-rock combination as the research object,uniaxial loading test,and triaxial loading-unloading test were carried out by using true triaxial testing machine and acoustic emission monitoring system of rock mechanics.The influence of the height ratio of rock to coal on the mechanical properties of composite coal rock was analyzed,and the influence degree of initial lateral stress and unloading rate on the fracture of coal-rock combination was studied,the propagation laws of different types of cracks in the composite coal rock were inverted combined with acoustic emission signal characteristic parameter RA-AF,the energy dissipation of the coal-rock combination under lateral stress path of unloading and increased axial pressure and lateral stress path of unloading and constant axial pressure was compared,the damage evolution equation of the coal-rock combination was derived based on the damage energy dissipation rate,and the damage constitutive model of the composite coal rock coal-rock combination under different unloading conditions was established and verified.(1)The basic parameters of coal-rock combination were obtained from uniaxial tests,a characterization model of the relationship between the height ratio of coal rock and the elastic modulus of coal-rock combination was established.Reasonable triaxial loading and unloading test parameters were set up,and the mechanical properties,deformation,and fracture characteristics of coal-rock combination under different loading and unloading stress paths were studied,the damage characteristics of coal-rock combination were studied based on the evolution of failure forms of coal-rock combination from a macroscopic perspective.The results show that the increase of the height ratio and lateral stress will aggravate the axial deformation of coal-rock combination during triaxial loading-unloading,and improve the bearing capacity of coal-rock combination.Shear failure was the main form of failure under the triaxial loading path,and tensile shear composite failure was the main form of failure under the triaxial unloading path.With the increase of the height ratio of rock to coal and the initial lateral stress of unloading,the shear slip phenomenon was obvious during the triaxial unloading process.(2)In order to further clarify the degree of damage development of coal-rock combination,the determination basis of shear crack and tensile crack was proposed by using the RA-AF of acoustic emission physical signal characteristic parameters.The crack development degree of coalrock combination was studied microscopically in the triaxial loading and unloading process.The research results show that: the increase of height ratio and confining pressure was the main reason for the increase of crack number and shear crack proportion;the development of internal cracks coal-rock combination was mainly concentrated in the lateral stress unloading stage under triaxial unloading conditions;the higher the initial lateral stress and the faster the unloading rate,the more cracks under the lateral stress path of axial unloading.(3)The initiation,propagation,and penetration of cracks are the direct causes of damage,and the energy dissipation during crack development is the fundamental cause of damage.According to the law of thermodynamics,the energy-damage evolution process of coal-rock combination was studied in the loading-unloading process,which further revealed the damage evolution law of the coal-rock combination From the perspective of energy dissipation,the damage evolution equation of coal-rock combination was derived combined with the damage mechanics theory under the triaxial unloading and loading equal stress path,and the damage constitutive model of coal-rock combination was established under the multi-stress path.The deformation,damage,and failure processes of coal-rock combination in the triaxial loading and unloading process are simulated and verified.The research shows that coal-rock combination damage constitutive model based on energy damage and energy consumption rate had good rationality,accuracy,and applicability.This paper has 45 figures,51 tables and 77 references.