Experimental Study On Relationship Between Rock Characteristic Stress And Acoustic Signal Under Uniaxial Compressioncompression Compression

For hard brittle rock,the stress damage and failure usually pass through five stages:compression,linear elastic deformation,crack stable growth,crack instability growth,and post-peak failure.The boundary points of the five deformation failure stages are called crack closure stress(?_cc),crack closure stress {?_ci),damage stress {(?_cd)and peak stress(?_c).The above-mentioned rock damage process is usually accompanied by the incubation,propagation,coalescence,and nucleation evolution of internal microcracks,which will cause the change of rock wave velocity.At the same time,the evolution of microcracks caused by rock damage is accompanied by the change of acoustic emission signal.In this paper,taking red sandstone and granite as examples,rock acoustic emission tests under uniaxial compression are carried out to study the characteristics of wave velocity and acoustic emission signal in different characteristic stress intervals.By comparing the parameters of wave velocity and acoustic emission,the evolution law of microcracks in rock is revealed,and the response law of wave velocity and acoustic emission signal in different characteristic stress ranges is explored.The main research contents and achievements are as follows:(1)The experimental results show that there is a certain correlation between the wave velocity of rock and the characteristic stress,and the wave velocity of rock decreases when the stress drops.There are significant differences in the amplitude of wave velocity variation among different lithology specimens.The maximum amplitude of wave velocity increase of granite is 8%?11%,while that of red sandstone is 29.5%?35%.(2)The wave velocity variation rate and the maximum wave velocity are determined as the basis for dividing the wave velocity anomaly points.Four-wave velocity outliers are determined,respectively V₁,V₂?Vmax?V₃.According to the wave velocity anomaly,it is divided into four different stages:rapid growth,steady growth,fluctuation,and rapid decrease.The correspondence between the wave velocity anomaly and the characteristic stress point is studied,in which V₂?V₃ and ?_ci??_c is very close,V₁.V_max is earlier than ?cc and ?cd?(3)Two kinds of acoustic emission sensors with different response frequency bands are used to study the various characteristics of high and low-frequency acoustic emission signals of red sandstone and granite in different characteristic stress ranges.The results show that the low-frequency acoustic emission signals of granite samples are mainly concentrated in the compaction stage,and the high-frequency acoustic emission signals mainly appear at the end of the elastic stage;the low-frequency acoustic emission signals of red sandstone samples are mainly concentrated in the compaction stage and the middle stage of the elastic stage and the high-frequency acoustic emission signals mainly appear at the end of the elastic stage.(4)The main frequencies of AE of granite and red sandstone are mainly concentrated in three frequency bands,and the main distribution range is slightly different.The main frequency distribution ranges of granite samples are 100?150kHz,200?270khz,and 300?350khz,while that of red sandstone samples are 100?150kHz,200?250kHz,and 270?320khz.In the whole stressful process,the proportion of lower and higher dominant frequency bands is more than 80%,the proportion begins to increase greatly near the damage stress point(?_cd),and reaches the maximum at the peak stress point(?_c),and the proportion of dominant frequency in the high-frequency band of red sandstone is higher than that in the high-frequency band of granite.The increase and broadening of the main frequency band of acoustic emission reflect the rapid propagation and penetration of microcracks in rock specimens,which can be used as the precursor characteristics of rock specimen failure.(5)The difference of two different rock damage characterization models of wave velocity and acoustic emission parameters in different characteristic stress intervals is compared.The damage accumulation of the damage model based on wave velocity begins after the crack initiation stress point(?_cd),and the damage suddenly increases at the peak stress point(?_c).In the damage model established by acoustic emission characteristic parameters,the granite specimen shows obvious damage accumulation after damage stress,while the red sandstone specimen shows obvious damage accumulation earlier in the elastic stage.Both of them show the phenomenon of sharp damage increase after the damage point.At the same time,the rock damage characterized by different AE parameters has different manifestations and response degrees.The rock damage represented by acoustic emission event number changes smoothly,while the rock damage represented by acoustic emission ringing count and energy has the obvious mutation,and the rock damage starting point represented by acoustic emission event number is earlier than that represented by acoustic emission ringing count and energy,and the rock damage defined by energy can more accurately reflect the real damage of rock specimen.