Analysis Of Structure And Mechanical Properties Of Sandstone Under Cyclic Impact

In the implementation of blasting engineering,blasting impact load is used to achieve the purpose of high-efficiency rock breaking,and it also induces cracking and cause irreversible damage to the rock mass near the excavation surface at the same time.This multiple blasting loads not only change the mechanical properties of rock mass,but also influences its bearing capacity,even endangers the safety of underground engineering.Based on the split Hopkinson pressure bar system with a confining pressure device,the cyclic impact tests on sandstone under different confining pressures and gas pressure were carried out.On this basis,the dynamic mechanical properties and damage energy consumption characteristics of sandstone under cyclic impact were studied as follows:The strain waveform characteristics of sandstone were studied,and its non-linearity of propagation was discussed based on FFT spectrum analysis.The results show that the amplitude of reflected waves increase while the transmitted waves amplitude decrease with the increase of impact number,and when the gas pressure and the confining pressure are the same,the time of peak voltage of reflected wave and transmitted wave are delayed accordingly.The sandstone behaves nonlinear response under impact loads and its waveform distorts obviously.The shockwave distance of the explored sandstone is calculated under different gas pressure,showing that there are no shockwave in sandstone during the test,and the shockwave distance decreases linearly with the increase of gas pressure.Based the theory of 1D elastic wave,the characteristics of stress-strain curve of sandstone under cyclic impact were studied,and the physical and mechanical properties of sandstone were analyzed under different confining pressures and different gas pressures.It is demonstrated that the sandstone behaves brittle failure when loaded without confining pressures,but it shows elastic-plastic characteristic under confining pressures.In a certain range,confining pressure can not only effectively improve the bearing capacity,but also improve the elastic deformation ability of sandstone,besides,larger gas pressure can reduce the elastic deformation ability of sandstone.Based on damage factor theory,energy consumption and damage characteristics of sandstone under cyclic impact were analyzed.The results show that the reflection energy and cumulative unit volume absorption energy rise gradually with the increase of impact number,while the transmission energy decreases when when the gas pressure and confining pressure are constant.A power function relationship exists between damage factor and average strain rate,and when the average strain rate is the same,the damage factor decreases with the increase of confining pressures,besides,thesandstone is broken when the damage factor exceeds 0.52.Sandstone acts crushing failure without confining pressure,and there are shear failure cracks on the surface of samples under confining pressure.As gas pressure increase,tensile failure forms appears along with shear failure.