Experimental Study On Edge Crack Propagation And Fracture Mechsnism In Brittle Rock-like Materials

Rock fracture mechanics is an advanced research area in modern rock mechanics and mining engineering.Rock fracture propagation is a major hazard for mining and tunnel excavation in fractured rock masses and coal seams.A longwall mining panel with a typical dimension of 100m?300m(width)×1000m?3000m(length)×1.5m?3m(height)can be considered as an open edge crack.Understanding the edge crack propagation chaaracteristics and fracture mechanisms i5 a key step for the prevention and control of floor water inrush in longwall mining panels.Theoretical analysis,laboratory tests,AE monitoring tests,injection of colored fluid and numerical simulation are used in this study.The edge crack propagation characteristics and fracture mechanisms in rock-like materials are studied from three aspects including edge crack propagation,energy and stress evolution mechanism and fracture spatial distribution characteristics.The main study achievements and findings are given as follows.(1)Uniaxial compression tests of rock-like specimens were carried out to analyse the fracture mechanism and fracture propagation pattern of 2D edge crack under uniaxial loading.The uniaxial compression strength and fracture initiation strength of the specimen decreased with the increase of the length of pre-existing 2D edge crack.During the tests,wing(tensile)cracks were first observed at the tip of the pre-existing edge crack.This was followed by anti-wing(tensile)cracks,secondary(tensile)cracks and shear cracks as the loading increasing.The final failure of the specimens however was dominated by tensile cracks throughout the specimens.Numerical situlations were conducted to explain propagation process of the 2D edge crack with different length and the distribution of stress and displacement.(2)Specimens containing pre-existing 3D edge crack were tested to study the fracture propagation mechanisms under uniaxial loading.The average uniaxial compression strength and fracture initiation strength of the specimens decreased with the increase of the length of pre-existing 3D edge crack.Tensile cracks were the dominant fractures,followed by wing(tensile)cracks,anti wing(tensile)cracks and shear cracks.Specimens failed due to combined transverse tensile fracturing and vertical splitting.AE occurred earlier with the increase of the length of pre-existing 3D edge crack.A new method was developed to map the spatial shapes of specimens by injecting colored fluid into the fractures.The failure patterns of specimens were shown as "?" and "Y".(3)The propagation mechanisms of pre-existing 3D edge cracks under true triaxial loading were investigated.The failures of specimens were mainly due to shear cracks and tensile crack propagation was limited.Meanwhile,the pre-existing 3D edge cracks were closed during loading.With the increase of the length of the pre-existing 3D edge crack and intermediate principal stress,the specimen axial strain and AE energy rate increased.Moreover the total number of AE events increased in a nonlinear form.With the increase of the length of the pre-existing 3D edge crack and decreased of the intermediate principal stress,the failure modes of specimens were changed from a single dominant shear zone into a "X"-type double shear zones.(4)The floor fracture propagation and the formation process of water inrush channel were studied by physical modelling using a test system for water inrush in coal mining.The forms of the water inrush channel through floor fractures and edge cracks propagation were analyzed and compared.The formation mechanisms of water inrush channel were discussed by using rock fracture mechanics theory.