Experimental Research On Dynamic Properties And Energy Evolution Of Persistent Jointed Granite Under Impact Loading

Rock masses are a typical heterogeneous material with complex geological structures formed in geological processes.A typical phenomenon is that natural joints,fractures and faults are widely distributed in rock masses.With the action of the engineering force,crack initiation,propagation and coalescence from these discontinuities may occur,leading to the catastrophic failure of rock mass engineering.Previous studies on rock mechanics have focused on the static mechanical properties of intact or defective(intermittent joints)rocks,rather than rocks with persistent joints.This paper conduct dynamic experimental research on the dynamic mechanical properties,failure modes and energy evolution characteristics of granite with different joint configurations by using the Split Hopkinson Pressure Bar(SHPB)and a highspeed camera system.Research and analyze the effect of loading rate on the dynamic characteristics of rock with persistent joints.The research work and the conclusive results can be summarized into the following main parts:(1)Dynamic uniaxial impact tests are carried out on granite specimen with different jointed configurations respectively to analyze the dynamic elastic modulus,peak stress and peak strain of persistent jointed rock.It is found that the elastic modulus has no significant difference with the increase of the number of joints,but the dynamic strength and peak strain gradually decreased.As the joint thickness increases,the dynamic peak stress and dynamic peak strain of rock samples both decrease significantly.However,the influence of the joint inclination angle on the dynamic strength and strain of the rock mass is more complicated.With the increase of the joint inclination angle,the peak strength and peak strain show a U-shaped change trend,decreasing first and then increasing.When the joint inclination angle is 60°,the dynamic elastic modulus,dynamic peak stress and strain of the persistent jointed granite specimen all reach the minimum value.And the dynamic compressive properties of rock mass have a great correlation with the strain rate.The dynamic uniaxial compressive strength of granite specimens increases continuously with the increase of strain rate.However,the existence of joints makes the rock mass show stronger sensitivity to the change of strain rate.There is a trend that with an increasing in the joint inclination angle,the dynamic response is first obtuse and then sensitive.When the inclination angle of the joint is 60°,the response coefficient of the strain rate reaches a minimum value.Hence,this specimen can resist a weaker dynamic impact before destruction with an increasing strain rate.(2)High-speed cameras are used to observe and analyze the progressive failure process of dynamic crack initiation of jointed rock mass.The paper reveal the crack initiation law,failure mode and deformation failure mechanism of rock mass with persistent jointed under impact load.When the joint inclination angle is 90°,the number and thickness of joints do not affect the initiation law and failure mode of rock mass.At this time,radial initial tensile cracks are mainly developed in jointed granite,and the ultimate failure mode is tensile splitting.As the inclination angle of the macroscopic prefabricated joint decreases,the initial cracks gradually change from tensile cracks to coplanar shear cracks along the joint direction.The failure mode gradually changes from splitting failure to tangential slip failure along the joint.When the inclination angles of joints are 30° and 45°,the failure modes are characterized by dislocation slip along the joint plane.With the increase of joint thickness,the specimen has a tendency to severe deformation failure behavior near the thick soft joint surface.(3)Digital image technology and strength theory are used to analyze the progressive fracture process and mechanism of rock mass.It is found that tensile strain concentration always occurred in axial direction for samples with large joint inclination angle.As the joint inclination angle decreases from 90° to 30°,the phenomenon of shear strain concentration becomes more obvious.For the specimens with joint inclination angles of 45° and 30°,the deformation and failure of the rock mass are dominated by both the shear strain along the joint and the tensile strain along the radial and joint directions.The analysis of strain concentration images after DIC treatment shows that the existence of persistent joint accelerates the strain concentration rate of rock mass.The paper analyzed the mechanism of crack initiation and deformation of the jointed rock mass,and found that with the decrease of the joint inclination,the shear stress along the joint direction gradually increased,and the normal stress gradually decreased.This phenomenon induces shear crack initiation along the joint direction.In addition,based on the Mohr-Coulomb failure criterion,it is found that the sample has a critical angle with weak shear failure resistance when the joint inclination angle is about 45°.And this sample is prone to shear failure.(4)This paper analyzes and summarizes the energy evolution law in the dynamic failure process of jointed rock samples with different joint configurations.It is found that the energy evolution can be roughly divided into three stages: compaction stage,steady energy growth stage and platform stage.In the dynamic impact process,the reflected,transmitted and absorbed energy grow linearly with the increase of incident energy.The rock sample with more joints and thicker joint interlayer needs to absorb and dissipate more energy under dynamic load.In addition,the energy absorption capacity of jointed rock is closely related to strain rate and dynamic compressive strength.With the increase of strain rate,the energy absorption capacity of rock mass is enhanced.Under similar strength conditions,with the increase of joint inclination angle,the energy absorption capacity of granite sample with persistent joint increases first and then decreases,and the energy absorption capacity of rock mass with joint inclination angle of 45° and 60° is the strongest.(5)On the basis of the existing viewpoint of composite damage mechanics and previous studies,this paper improves the generalized Bingham model and constructs the dynamic constitutive model of jointed rock mass with initial damage of persistent jointed granite.By comparing the measured results of the test with the model calculation results,it is found that the dynamic constitutive model of jointed rock mass can reflect the stress-strain relationship of granite with persistent joints in the pre peak stage under uniaxial impact,and can well describe the stress-strain relationship of jointed rock mass under dynamic impact load.However,with the increase of strain rate,the fitting accuracy of the dynamic constitutive model also decreases controllably.Based on the requirements of the research project,the dynamic test of granite samples with persistent joints at different strain rates are carried out by using the split Hopkinson pressure bar device and high-speed camera system.The conclusion of this paper reveal the influence of joint configuration on engineering dynamic failure of rock mass.This discovery can provide some valuable reference for the practical field work in the engineering protection and rock breaking application.