Study On Dynamic Mechanical Properties Of Granite With Different Shapes Of Holes

With the steady increase of the exploitation of mineral resources,the volume of mined-out areas formed in the mining process is also increasing year by year.The existence of mined-out areas not only greatly reduces the bearing capacity of mine rock mass,but also increases the probability of accidents caused by mine instability in the mining process,and also affects the surrounding buildings and people’s lives in mined-out areas.In order to further explore the dynamic characteristics,crack propagation process and energy dissipation law of rocks with different shapes of holes under impact load,Hopkinson impact tests with different strain rates were carried out on rocks with three shapes of holes(circular,rectangular and elliptical).The purpose is to provide a certain theoretical basis for mining engineering.The research contents and results are as follows:Firstly,the Hopkinson impact tests of granite with different hole shapes,different distances H and different strain rates are carried out,and the stress-strain curves and dynamic strength obtained from the tests are analyzed to explore the rock position effect,hole effect and strain rate effect,and the influence of hole shapes,distance H and strain rate on the dynamic strength of rock is judged by the range analysis method.The results show that the influence of hole shape,strain rate and distance H on dynamic strength decreases in turn.Firstly,the existence of holes will deteriorate the dynamic strength of rock,and the dynamic strength of different hole shapes will deteriorate differently,with the ellipse short axis being the biggest,rectangle and circle being the second,and ellipse long axis being the smallest.H has little influence on the stress-strain curve,but mainly on the dynamic strength,in which the circular and rectangular holes first increase and then decrease with the distance H,and σmax is at 30 or 40mm;The elliptical long-axis hole shows the fluctuation change of first increasing,then decreasing and then increasing,and σmax is at h of 30mm;The short-axis hole decreases first and then increases,and σmax appears at the h of 20 mm.The dynamic strength of rock holes is positively correlated with strain rate;At the same time,DIC method is used to analyze the strain field in the failure process.All specimens have tensile cracks,while rectangular specimens also have shear cracks.Therefore,the rectangular hole specimens are subjected to tension-shear mixed failure and compression-shear mixed failure,and the rest are subjected to tension failure and tension-shear mixed failure.In addition,it is found that the strain rate and distance h mainly affect the damage degree of the specimen and the particle size of the fragment,and the former has a greater influence.Different shapes of holes lead to different shapes of fragments formed after impact.Elliptic long axis,rectangular and circular hole fragments are mainly long strips,which gradually become shorter and wider with the increase of strain rate.Elliptic short axis is mainly square-like,which gradually becomes shorter and thinner with the increase of strain rate.Secondly,the energy dissipation characteristics of granite samples with different shapes of holes are studied from the angle of energy,and the energy dissipation law of granite samples with different shapes of holes under impact load is revealed from many angles.It is found that all four kinds of energy show obvious strain rate effect,among which the incident energy is much larger than the other three kinds of energy,and the ratio of reflected energy to transmitted energy is about 2: 1.Although the absorption energy shows an overall increasing trend,it still accounts for the smallest ratio of total energy.Energy utilization rate and energy absorption density are introduced as analysis indexes,and it is found that the indexes show obvious strain rate effect,and the change with distance h is consistent with the trend of dynamic strength,which indicates that the energy absorbed by the specimen is used for the destruction of the specimen,and the higher the energy utilization rate,the greater the dynamic strength of the specimen.Finally,discrete element(PFC2D)software is used to simulate the Hopkinson impact test of rock with holes.The results show that the deformation and failure process of granite samples in numerical simulation under different conditions has experienced four stages: primary crack closing stage,crack initiation and stable propagation stage,crack unstable propagation stage and post-peak rebound stage.In order to explore the signs and difficulty of rock crack stable and unstable propagation under different conditions,the cracking stress,damage stress and cracking stress ratio of rock are analyzed.It is concluded that the ratio of cracking stress to damage stress of rocks with different holes is about 1/2,and the change trend with distance h is consistent with the change of dynamic strength.The crack initiation stress ratio and damage stress ratio of elliptical hole are the largest,which indicates that the crack initiation time of this kind of hole rock is the latest,and the long duration of crack stable propagation stage makes the dynamic strength the highest,which is consistent with the experiment.In addition,the tensile cracks in the specimen are mainly parallel to the impact direction,and the compressive stress concentration occurs on the upper and lower sides of the hole,and the tensile stress concentration is mainly around the compressive stress and the crack.At the same time,the dimensional analysis method is used to analyze the influencing factors of granite with holes,and the corresponding dynamic mechanical characteristics influencing formula is deduced.