Study On Rock Failure Mechanism At Different Strain Rates Based On Acoustic Emission Characteristics

Rock failure is accompanied by active acoustic emission signals.Acoustic emission（AE）is the release and rapid propagation of elastic waves accompanying the generation and propagation of microcracks in rock.The main AE parameters of rock fracture include count,energy,absolute energy,amplitude,average frequency（AF value）,RA value,b value,etc.In uniaxial compression test,different stages of rock are accompanied by different acoustic emission characteristics.The stress loading stage of rock can be judged by monitoring the AE signal of rock,which reflects the change of the average strength of rock and the development and expansion of internal microcracks.Based on acoustic emission monitoring technology,further analysis of rock interior can clarify the failure mechanism of rock,and analyze the failure mode of rock instability failure,which is a key to the study of the types of cracks in rock interior and the failure mechanism of rock.The evolution process of rock damage is the key to study rock damage.Further exploration of acoustic emission characteristic parameters can analyze the evolution process of rock damage.In this paper,MTS815 rock mechanics test system is used for uniaxial compression test of rock,and acoustic emission test system is used for acoustic emission monitoring of rock.Four kinds of hard rocks were respectively loaded with five-order strain rates(10^-5/s,5×10^-5/s,10^-4/s,5×10^-4/s,10^-3/s)to study the acoustic emission laws of four kinds of hard rocks at different strain rates.Comparing the changes of the relevant acoustic emission signals in the loading process can effectively analyze the rock failure mechanism,which is of great significance for the study of the changes inside the rock at different stages.The main research results are as follows:（1）By comparing the brittleness of the four kinds of rocks,it is found that the overall acoustic emission signal intensity of the rocks with greater brittleness is higher during loading,and the distinction between the calm period and the active period is more obvious.Among them,granite has the strongest AE signal,and less brittle sandstone has relatively small AE parameter magnitude during loading process.The AE signal of rock is active in a small range at the early stage of loading,which is caused by the initial closure of micro-cracks in rock at the stage of closing stress.In the stage of initiation stress,the AE signal is less and stable.At the stage of damage stress,the activity of AE signal increases until the AE signal of rock failure disappears after reaching the peak stress.Therefore,the stress loading stage of rock can be judged by comparing the variation trend of AE energy and count of rock during the whole loading process.When exploring the fracture mechanism of rock under different strain rates,the values of AE parameter index RA（rise time/amplitude）and AF（average frequency）can analyze the fracture mechanism of rock.The AE events corresponding to tensile fracture have small RA value and large AF value.The AE event corresponding to shear fracture has a larger RA value and a smaller AF value.By analyzing the AF and RA values of rock under uniaxial compression,it is found that the proportion of shear failure of rock increases with the increase of strain rate in the whole failure process,but the fracture form inside the rock is always dominated by tension fracture,so the strain rate does not affect the failure mechanism of rock.The variation law of the overall AE b value and real-time AE b value of rocks at different strain rates is analyzed.The overall b value and real-time b value of four kinds of rocks at grade 5 strain rate are obtained by using AE b value processing method,as well as AE b₁value and AE b₂value of rocks at crack initiation-damage stage and damage-peak stage.According to the change of real-time b value during the whole loading process,the change trend of real-time b value before rock failure is analyzed and summarized.（2）Damage evolution analysis of rock.The stress damage curve and strain damage curve of rock and the Hurst index of acoustic emission of rock are obtained.The evolution of strain damage and the change rule of Hurst index are analyzed in the loading process of rock.The stress damage point of rock is estimated according to the Hurst index peak value,and a new method for determining the stress damage point of rock is summarized.The results show that the AE characteristics of rock can reflect the crack evolution in rock,the AE b value can reflect the crack size in rock,and the strain rate will affect the proportion of failure forms in rock,but not the failure mechanism.By comparing the stress-strain damage curves of rocks,the method to analyze the damage evolution law of different rocks under different strain rates is determined.The Hurst index of acoustic emission of rock is used to obtain the phenomenon of the sudden increase of rock on the eve of damage and failure,which is one of the characteristics of rock entering the damage stage.Based on the stress damage points of rock determined by CVS method,the stress damage points determined by Hurst index peak value of rock energy（count）rate and Hurst index peak value of accumulated energy（count）are compared.It is found that the stress damage points determined by Hurst index peak value of cumulative count of rock have the smallest error.A method to determine the stress damage point of rock is determined by the Hurst index peak value of rock cumulative count.In this paper,acoustic emission signals of rock under uniaxial compression are obtained through acoustic emission monitoring technology,and the acoustic emission characteristics of rock during the whole process of loading are analyzed according to the stress-strain curve.Based on this,the development process of cracks in rock is analyzed,and the fracture mechanism of rock is summarized.According to damage index and Hurst index,the damage evolution law of rock is studied.The effects of strain rate on stress-strain,acoustic emission characteristics,fracture mechanism and damage evolution process of rock are analyzed.