Mechanical Properties And Crack Evolution Mechanism Of Discontinuous Three-fissure Yellow Sandstone

There are many joints,fissures,holes and other defects in natural rock mass.Rock mass instability and failure is essentially the expansion and development of original defects or the initiation,development and penetration of new defects.This paper takes the dip Angle and vertical offset of the center of the conjugate discontinuous crack as the research variables to analyze the influence of these variables on the mechanical properties and crack propagation of the yellow sandstone with three discontinuous cracks.Using hydraulic servo rock press on precast intermittent fissure yellow sandstone specimen under uniaxial compression test,analysis of the prefabricated crack of intermittent three yellow sandstone the influence law of mechanical properties,experiment using high-definition cameras monitoring process,record the crack extension and penetration process,combining with digital image correlation technique,the analysis of strain fields in crack propagation process evolution.Finally,the discrete element numerical simulation software was used to further analyze the failure mechanism.Research shows that:(1)With the increase of central fracture dip Angle?(15°,30°,45°),the uniaxial compressive strength of yellow sandstone in the three discontinuous fractures changes as follows:when h=2mm,6mm,and 8mm,the compressive strength of yellow sandstone in the three discontinuous fractures decreases gradually with the increase of Angle;When h=4mm,the compressive strength of the yellow sandstone with three discontinuous fractures gradually increases with the increase of Angle.When h=10mm,the compressive strength of yellow sandstone with three discontinuous fractures decreases first and then increases with the increase of Angle in a V-shaped trend.(2)With the increase of vertical offset H(2mm,4mm,6mm,8mm,10mm),the uniaxial compressive strength of yellow sandstone with three discontinuous fractures changes:when?=15°,the compressive strength of yellow sandstone with three discontinuous fractures increases with the increase of vertical offset.When?=30°,the compressive strength of yellow sandstone with discontinuous three fractures increases first and then decreases with the increase of vertical offset.When?=45°,the compressive strength of yellow sandstone with three discontinuous fractures increases first,then decreases and then increases with the increase of vertical offset.(3)With the increase of the dip Angle?of the central crack,the initiation,propagation and failure rules of the discontinuous three-crack yellow sandstone crack are as follows:(1)The initial cracks are all initiated in the area of the rock bridge between the conjugate crack and the central crack;(2)The main crack occurs at the ends of the central crack or conjugate crack tips(A₁,C₃),and extends along the upper and lower end faces of the specimen and the edges of both sides.(3)The secondary cracks appear near the tip of the central crack and propagate to both sides of the specimen.The failure modes are mainly shear failure and tensile failure.(4)With the increase of vertical offset h,the rules of crack initiation,propagation and failure of discontinuous three-crack yellow sandstone are as follows:(1)The initial crack initiates in the rock bridge area between the conjugate crack and the central crack of the sample;(2)The main crack is a transfixion crack starting from the crack tip and extending to the upper and lower end faces of the sample,and gradually becomes a crack starting from the crack tip and extending to the lower end faces and edges of the sample.The failure modes are mainly shear failure and axial splitting failure.(5)Through the analysis of DIC digital image processing technology,the following results can be obtained:(1)With the increase of?,the influence range of strain field near the central crack increases,and the sample is mainly affected by shear strain field;(2)With the increase of H,the influence of axial strain field gradually increases,and the influence of shear strain gradually decreases,and the strain concentration area of the sample gradually approaches to the center crack.(6)The discrete element was used to simulate the distribution of stress field in sandstone and the laboratory test results showed that:(1)With the increase of?,the compressive stress distribution region of the sample was transferred from near the crack tip to the upper and lower end faces,and the distribution of tensile stress field was more dispersed.(2)With the increase of H,the compressive stress concentration area moves from the crack area to the upper and lower end faces of the sample,and the tensile stress is almost all over the sample.