Study On The Effect Of Loading Rate On Fracture And Acoustic Emission Characteristics Of Rocks

Rock is a kind of polycrystalline and anisotropic material,and its failure is essentially a process of the generation and propagation of internal micro-cracks,resulting in the ultimate macro rupture.This process will emit energy as forms of elastic waves that are referred to as acoustic emission(AE),which is different significantly under various failure modes.The crack propagation characteristics of rocks are of great importance in rock mechanics field,and AE parameters are one of the most useful means to investigate the rock failure mechanisms.To study the AE signal characteristics and its relationship with crack propagation by AE parameters and Cluster analysis method,impact loading and uniaxial compression tests at different loading rates and Brazilian split test on four different materials of rocks were conducted by the split Hopkinson pressure bar(SHPB)system and MTS322 servo-controlled rock mechanical test system.The main contents and results of this paper are as follows:(1)Under uniaxial compression,with the increase of strain rate,the macroscopic fracture degree of rock sample increases.Under splitting load,with the increase of loading rate,the macroscopic crack tends to be a straight line extending along the axial direction.The cumulative count under uniaxial compression is much larger than that under splitting load and its decrease trend is more significant with the increase of loading rate.(2)Under uniaxial compression,the low frequency signal increases with the increase of strain rate,and the amplitude of increase is greater than that of the low frequency signal under splitting load with the increase of loading rate,and the curves are convex.Under uniaxial compression and splitting load,with the increase of loading rate,the proportion of high-frequency signals decreases and the amplitude is similar,and the curves are "concave".(3)Under uniaxial compression,with the increase of strain rate,values of AF in approaching the peak stress drop is more obvious,the RA value downward trend is more obvious in consolidation stage,under the splitting load,with the increase of loading rate,AF value increasingly focused on the consolidation phase,in approaching the peak stress,the RA value decline more obvious,more and more reflect the characteristics of tensile damage.Under uniaxial compression and splitting load,the acoustic emission b-value of rock decreases with the increase of loading rate,and the decrease of b-value under uniaxial compression is greater than that under splitting load,and the fracture toughness of rock increases with the decrease of b-value.(4)With the increase of loading rate,the microscopic fracture mode of rock gradually changes from extending along grain boundary and soft plane to transgranular failure,and the main crack expands along straight line under splitting load.