Study On Mechanical Response Of Granite Under Impact Load

As a natural material,rocks contain internal defects such as joints,laminations,fissures and pores,which give them varying degrees of natural damage,resulting in different degrees of dynamic disturbance on its structural stability in rock engineering excavation.As the underground tunnel project continues to extend deeper and deeper,the high ground stress environment endowed by the surrounding rock becomes more and more complex,resulting in the frequent occurrence of rock engineering disasters;and the excavation of deep rock engineering is actually a mechanical process in which ground stress and dynamic disturbance act together.Therefore,it is of practical engineering significance to study the influence of rock damage degree on its dynamic mechanical properties and the damage characteristics of the tunnel surrounding rock by frequent disturbances under high ground stress conditions.Based on this,this paper mainly develops the following two aspects of research.(1)Different levels of impact damage to granite by using spilt Hopkinson pressure bar(SHPB),establishing the determination of the degree of rock damage expressed by deformation modulus;critical air pressure impact test studies were conducted on rock samples with different degrees of damage.The test results show that: the reflected wave gradually increases with the increase in the degree of damage,while the transmitted wave has a gradually decreasing trend;the stress-strain curves of specimens with different degrees of damage mainly go through the elastic phase,crack extension phase and rebound phase;the stress-strain curves at the time of damage mainly go through the crack extension phase and damage phase,and there is basically no rebound phenomenon in the unloading damage phase after the peak.With the increase of damage degree,the peak stress gradually decreases,while the dynamic peak strain,damage strain and strain rate have an increasing trend;the damage pattern of specimens with different damage degree has obvious differences,and the damage block of rock samples tends to be more broken with the increase of damage degree.(2)Cyclic impact tests on granite with vertical holes under different axial prestress conditions were performed by a modified split Hopkinson pressure bar test,and the damage of the specimens was recorded with a high-speed camera process.The test results show that under the same air pressure cyclic impact,the rock specimens mainly undergo the compaction-fatigue-failure transition.As the axial prestress increases,the compaction-fatigue phase gradually weakens,and the combined strength of the test piece shows an increasing trend.When the axial prestress is 42% of the UCS and 62% of the UCS,the rock specimen shows a certain "strengthening" effect during the initial cyclic impact stage.During the failure of the rock specimens,the axial prestressing effect limited the initiation of some transverse cracks,and a mixed tensile-shear failure mode appeared.The rock specimens with an axial prestress of 62% of the UCS showed energy release during cyclic impact.To some extent,the probability of "rock bursts" has been induced.Based on the one-dimensional stress wave theory,the damage variables of wave impedance during the cyclic impact loading of the rock with vertical holes is improved.It is found that when the rock specimen is in the stage of compaction and fatigue damage,the damage is small,and the damage is even reduced.