Study On The Failure Process Of Pre-crack Rocks Under Compression

The natural rock mass contains a large number of initial micro-cracks.These micro-cracks expand and penetrate continuously under stress and will have a significant impact on the mechanical properties of the rock mass.The research on the failure process of precast crack rock under compressive conditions has important practical value and engineering significance for the prediction of rock mass strength in geological engineering,the stability analysis of rock slope,and the stability evaluation of surrounding rock of underground roadway.In this paper,basalt and other brittle materials were simulated by formulating similar materials to produce single fracture and double fracture specimens with different fracture combinations.The YDS-3 rock mechanics multi-functional testing machine independently developed by the company is used for uniaxial and biaxial compression tests.The PIV particle imaging and velocity measurement system is used to record rock fracture surface deformation process in real time.Failure process and crack propagation law of fractured rock are studied.The main results were as follows:(1)The variation law of the maximum relative displacement value of the particles on the surface of the sample during the loading process can be used to characterize the fluctuation of the surface displacement of the sample.It has obvious stage characteristics and can effectively divide the fracture process of the fractured rock and determine the corresponding Characteristic stress value.The closing stress was judged by the variation law of the maximum relative axial displacement value,and the crack initiation and damage stress were judged by the lateral maximum relative displacement value.(2)In the process of failure,the characteristic strength is affected by the combination of lateral pressure and fracture,and the specific performance is as follows:The closing stress,crack initiation stress and peak stress of the specimen are basically in a positive linear relationship with the lateral pressure.The sensitivity of the closure stress to lateral pressure is relatively weak,and the sensitivity of the damage stress to side pressure is the strongest.Each characteristic stress value shows a “V” shape that decreases first and then increases with the increase of crack dip angle,and takes the lowest value when the crack inclination angle is 45°.When there is a crack joint in the loading direction(rock angles of 120° and 150°),the pre-existing fissure ends are prone to initiate tensile cracks extending in the direction of loading,and rapidly cause the sample to break through.(3)The stress and displacement field characteristics of the sample at each stage can better reflect the crack initiation and propagation evolution process.Lateral tensile stress is mainly concentrated in the rock bridge and end regions.of the precast crack.The axial compressive stress is mainly concentrated on the pre-fabricated fracture surface.The lateral displacement gradient zone can reflect the distribution of macroscopic cracks.The stress concentration belt has a good fit with the gradient of displacement and contour density,and both of them have good consistency with the distribution of the macro cracks of the specimen.(4)The effect of lateral pressure on the distribution of stress and strain on the surface of fractured rock is as follows: With the increase of lateral pressure,the axial compressive stress concentration area along the prefabricated fracture surface gradually increases;lateral tensile stress concentration area of the surface of the sample is concentrated.It also gradually increases,the corresponding lateral displacement discontinuity increases,microcracks increase,and the plasticity of the sample increases.The tensile stress concentration area distributed along prefabricated fracture surfaces and rock bridges under cyclic loading decreases.Axial stress manifests as an increase in the area of the overall compressive stress concentration on the specimen surface.The discontinuity of the lateral displacement field is relatively weak.(5)The mesoscopic displacement field reflects the entire development process of crack tip crack initiation,crack propagation,and finally penetration failure.It can be seen that not all interfaces at which the displacement direction line changes will produce cracks.Only when the difference in displacement vector reaches a certain value will macroscopic cracks occur.The change of displacement direction line is a necessary condition for crack generation.(6)Crack propagation and failure characteristics of cracked rock are affected by crack dip,lateral pressure,and rock bridge inclination.With the increase of fracture dip angle,crack initiation mainly evolved from inclined secondary cracks(0°,30°)to gradually dominated by wing cracks.Especially at 45° and 60°,the number of wing crack development was mostly.The typical splitting failure is formed at 90 degrees.With the increase of lateral pressure,the overall trend of the number of through cracks gradually decreases,wing cracks are gradually replaced by inclined secondary cracks as the confining pressure increases.The damage type is mainly transformed from tensile failure to shear failure.When the dip angles of the double-fractured rock are the same,with the increase of the dip angle of the rock bridge,the penetration of the rock bridge is mainly transformed from shear-through penetration to tensile continuity of the wing crack connection.When the pre-fabricated fracture overlaps along the loading direction,The wing cracks at the rock bridge pass through the two inner tips of the upper and lower prefabricated fissures to become the inner tip and the outer tip of the other one.When the fracture dip angle is 75°,the rock bridge dip angle will not be stretched at the rock bridge under the condition of the most favorable wing crack initiation and overlap(120°).Under other rock bridge inclinations(30°,90°,150°),no penetration phenomenon was observed at the rock bridge.