Dynamic Mechanical Property And Energy Evolution Of High-stressed Sandstone Under Triaxial Unloading Impact Compression

In the process of deep projects excavation,rock often experiences a full stress process as “three-dimensional high stress +excavation unloading + disturbance stress”.To study the dynamic characteristics of high-stressed rock subjected to unloading and impact loads,triaxial unloading impact compression tests were conducted on red sandstone using a modified split Hopkinson pressure bar(SHPB).For compression,uniaxial pre-stressed impact compression tests were also conducted(without considering the effect of confining pressure unloading).The influence of dynamic mechanical response,failure models and energy dissipation evolution were studied.The main research contents and results are as follows:(1)In triaxial unloading impact compression tests,the dynamic strength of red sandstone shows an obvious strain rate effect.With an approximately equal strain rate or incident energy,the dynamic strength of red sandstone in triaxial unloading impact compression test is significantly less than that in uniaxial pre-stressed impact compression test,meaning that confining pressure unloading causes a distinct dynamic strengthweakening effect.In addition,under same incident energies,confining pressure unloading causes a higher strain rate in pre-stressed rock.(2)The dynamic strength-weakening factor(DSWF)is defined to quantitatively describe the dynamic strength-weakening effect caused by confining pressure unloading.The result shows that the DSWF decreases with the increase of strain rate.Under the same initial confining pressure,the DSWF of the red sandstone sample decreases most obviously when the axial pre-stress is 70 MPa.Under the same axial pre-stress,initial confining pressure has little influence on the DSWF.(3)Under the coupled static and dynamic compression tests,the dynamic stress-strain curve of the rock changes from type-II curve to typeI curve with the increase of incident energy.Compared with the uniaxial coupled static and dynamic compression tests,the confining pressure unloading will reduce the critical incident energy.The reduction of critical incident energy increases with the increase of the axial pre-stress,but initial confining pressure has little influence on the reduction of critical incident energy.Under the coupled static and dynamic compression tests,the failure modes of the red sandstone specimens are tension-shear failure modes.(4)The energy dissipation evolution of red sandstone in dynamic failure process was obtained.Under the same stress path,the absorbed energy density of red sandstone increases linearly with the increase of incident energy.When the axial pre-stress is low,the absorbed energy density of the specimen subjected confining pressure unloading is greater than that in uniaxial pre-stressed impact compression test.When the axial pre-stress is high and near the yield strength of specimen,the absorbed energy density of the specimen under confining pressure unloading is lower than that in uniaxial pre-stressed impact compression test.