Study On Fatigue Property Of Brittle Rock Under Repetitive Impact Loading

In engineering practice,rock and rock masses are often subjected to cyclic/repetitive impact loadings.However,studies on the mechanical properties of fatigue damage and failure modes of rock under repetitive impacts are insufficient.Therefore,research on determining fatigue damage evolution and deformation characteristics of rocks under repetitive impacts will have high scientific value and engineering significance.In this study,the fatigue characteristic of granite under repetitive impacts was studied,with a self-designed split Hopkinson pressure bar(SHPB)system of 38.1mm in diameter.The experiment consists of two parts,i.e.repetitive compressive impacts and repetitive tensile impacts.For repetitive compressive impact,impact tests with seven different repetitive impact stresses were sequencially conducted to obtain the relationship between the repetitive impact stress and fatigue life of rocks.The influences of repetitive impact on the dynamic Young's modulus,axial strain and energy absorption of rock specimens were also investigated.It isfound that there exists a critical fatigue impact stress?_c(between 0.55?_d to 0.58?_d which is defined as the threshold value of incident stress for breaking intact granite sample into two large blocks),below which,almost no material deterioration would be incurred with increasing impact number.The fatigue life of the granite samples exponentially decreased with increasing of the impetitive impact stress.With an increase of the impact numbers under a certain impact stress,the dynamic Young's modulus of specimen generally decreased,while the axial strain corresponding to the peak stress increased gradually.Wave slowness,which is adopted to reflect damage degree,increases with increasing repetitive impact numbers,but the responses of P-wave velocities measured along radical and axial directions are different during dynamic repetitive impacts.P-wave slowness in radial direction is severe than that in axial direction.This may be attributed to the fact that the cracks were mainly generated and developed along the axial direction as detected by the X-ray micro-CT scanning,which have weaker influence on velocity along the axial direction.The process of fatigue failure of specimens under repetitive tensile impacts was studied via the repetitive Brazilian splitting tests.The energy absorption and P-wave slowness,which is utilized to reflect damage evolution inside the specimen,along axial and radial directions were studied.Experimental results showed that P-wave velocities along axial and radial directions both were slowed with increasing of the repetitive tensile impacts,but the wave slowness extent was relative small.This indicates that the tensile fatigue resistance of rock is weaker compared to the compressive fatigue resistance,that means tensile failure of the granite sample immediately occured with a relatively lower accumulation of fatigue damage under repetitive tensile impacts.Overall,the absorption energy and P-wave velocities were measured to study the evolution of fatigue damage inside rock specimens under the repetitive compressive and tensile impacts.Through analyzing energy absorption behaviours,it is found that the laws of fatigue damage evolution under repetitive compressive impacts was similar to that under repetitive tensile impacts,i.e.,fatigue damage accumulated in a linear form.Through analyzing the fatigue damage evolution using P-wave velocities,it shows that it is more appropriate to utilize the radical-wave velocity along radical directions as damage variable to study damage development inside rocks,compared to the one along axial direction.