Mechanical Properties And Fracture Characteristics Of Damaged Sandstone Under Dynamic Loading

Compared with undisturbed rocks,rocks that occur in nature are usually in different degrees of damage due to earthquakes,floods,and human excavation activities.Joints and fissures with different sizes,scales and occurrences are randomly distributed in the damaged rock,which degrade the internal structure of the rock and weaken the strength.In view of this,the structural design,stability analysis and danger relief measures of rocks in geotechnical engineering are also different from those of undisturbed rocks.Although the degree of mechanization of rock breaking work continues to increase with technological innovation,some areas where mechanical operations cannot adapt to still use blasting.The blasting vibrations and shock waves induced by blasting operations affect the stability of adjacent rocks,not only burying hidden dangers to the safety of the project,but also threatening the lives of construction personnel.Therefore,studying the mechanical properties and fracture characteristics of damaged rocks under dynamic load is of great significance to safe production and protection.In this paper,damaged sandstone is taken as the research object.The damage degree,fracture mode and velocity structure evolution of the sandstone are analyzed by acoustic emission and time-lapse double-difference tomography.The cyclic static load with different upper limits is applied to the sandstone,and the damage degree of the sandstone is characterized by CT.Then,high-speed camera and Split Hopkinson Bar are used to carry out impact dynamic tests on sandstones with different degrees of damage.The strength change,energy evolution,crack propagation and fracture characteristics of damaged sandstone under impact load are explored.Finally,the crack propagation and fracture characteristics of sandstone damaged by explosion load are analyzed by using industrial CT scanning device.The results are as follows:The acoustic emission hit ratio HR is defined to characterize the damage degree of the loaded sandstone.A mechanical model that can quantitatively describe the damage degree of the loaded sandstone under different stress levels is constructed,which is able to describe the damage in two areas including the primary fracture compaction closure stage and the post-peak stage.According to the characteristics of the monitored acoustic emission signals,the sandstone loading process is divided into three stages,which are the initial hit phase,the hit stability phase,and the hit instability phase.The fracture modes of these three phases are shear failure,tension failure and shear failure,respectively.Considering the heterogeneity of the rock,the time-lapse double-difference tomography is used to invert the velocity structure of any section of the loaded sandstone under different stress levels.The velocity structure increases with the increase of the load,and the damage of the rock also increases gradually,and the growth rate and the degree of damage reach the maximum during the failure and instability stage.In addition,after the loaded sandstone enters the plastic stage,a small part of the rock block is in the "isolated state".With the combined effect of the isolation of the fissure and the expansion of the rock,the velocity structure of the rock block still increases in the failure and instability stage.The damage of sandstone after cyclic loading is consistent with the damage of maximum stress in stress history.The damage of sandstone weakens its dynamic mechanical strength.Under the compression and splitting action of the impact load,with the increase of the damage degree,the number,length and width of the initial cracks increase,and the crushing degree of damaged sandstone and the kinetic energy of broken rock also increase.When the damage degree of sandstone is constant,the number,length and width of the initial cracks,the degree of fragmentation,the kinetic energy of the broken rock block,and the mechanical behavior of the compression-shear zone all increase with the increase of the impact load.Under the splitting tension of impact load,the fracture range and extent of the compression-shear region of the bar-rock contact surface increase with the increase of the damage degree and impact air pressure.Under the action of impact load,the proportion of dissipated energy,as the damage degree of sandstone and impact air pressure increase,both show an exponential function increasing law.The expansion range,width,and number of the blast cracks on the bottom surface of the damaged sandstone under the blast load increase with the increase of the upper limit of the cyclic static load stress.Inside the sandstone,the size of the blasting crater and the craters also increase with the increase of the damage degree.It can be seen from the test results of CT imaging that the damage degree,damage region,crack size,fracture degree and penetrate the crack along the direction parallel to the side length in the sandstone under explosive load gradually increase with the increase of the initial damage degree.Affected by the end hoop effect and the size effect of the sandstone during the cyclic static load,all the cracks in the sandstone that penetrate the center of the blast hole to the boundary of the sandstone under the explosion load are generated along the direction of the static load.Figure[92] table[17] reference[278]...