Experimental Study On Characteristics Of Acoustic Emission And Fractal In Rock

Rock is a kind of natural geological body,which has been formed by tectonic movement and weathering denudation of hundreds of millions of years,and which hasbeen discontinuous,anisotropic and affected by environmental conditions.Rock acoustic emission,as the external acoustic phenomenon of the location of rock damage,contains a lot of damage and fracture information.Fractal is a new branch of rock mechanics,and it is a powerful tool to study the rules of the seemingly complex disorder.The studies have shown that the damage process of rock is fractal in space and time.Analyzing and discussing the characteristics of basic parameters and fractal parameters of acoustic emission in the damage process of rock are helpful to further understand the mechanism of rock damage and fracture.In order to understand the rock damage process and further predict fracture precursor of rock,the acoustic emission tests of uniaxial compression complete process for the four types of rocks,sandstone,gabbro,granite,limestone,have been carried out to analyze the statistical relation between the evolution process of acoustic emission parameters and the stage of stress and deformation,and identifythe precursory characteristics of rock damage;to calculate and analyze the fractal evolution process of rock acoustic emission time series and identifythe precursory characteristics of rock damage.The main contents and achievements are as follows:(1)The complete process of uniaxial compression of rock specimens was tested by 0.1mm/Min,and the acoustic emission signals were monitored in real time.The damage evolution process of three parameters,such as events,ringing counts and energy,are analyzed and discussed.The precursory characteristics of rock damage are identified.(2)The acoustic emission events,ringing counts and energy of the rock specimens in the compaction stage,inthe elastic deformation stage and the accelerated failure stage are increased,and events increaseis the most slow,and the energy surge is the most obvious.In the compaction stage,the elastic deformation stage and the accelerated failure stage,the absolute amount of rock acoustic emissionevents from small to large are sandstone,gabbro,limestone and granite;The relative amount of acoustic emissionevents,increased with the increase of rockelastic modulus,are increased in the compaction stage and the elastic deformation stageanddecreased in the accelerated failure stage.The regularity of ringing counts and energy isnot obvious in compaction stage and in accelerating failure stage.However,in the stage of elastic deformation,average value,maximum value and the accumulative value of ringing counts and average value,maximum value and the accumulative value of energy,are all increased with rock elastic modulus;the accumulativeringingcounts ratio and the accumulative energy ratio also showthat the largest is limestone and the least is granite.(3)The rock specimen fracture precursory stress ratio of feature point(s)of acoustic emissionparametersshows generally that stress ratio of featurepoint(s)of accumulativeringing countsless than that of accumulativeenergy,andmore than that of events.The feature pointof accumulativeringing countsis always very close to the yield point.Analyzing the basic parameters evolution curve of acoustic emission by contrast,fracture precursor feature point of acoustic emission events curve may not be obvious,and that of energy accumulative curve may obscure small effective information so that it can’t apply well,but the accumulative ringing counts curve can reflect the real state of rock damage process intuitively and effectively.(4)Compiling routine(GP-m.m)is used to calculateand analyze the value and influence of phase space dimension(m).When the value of m is too small,the phase space overlaps;when the m value is too large,the amount of calculation is greatly increased,and the fractal range is narrowed,so the optimal phase space embedding dimension m=10.Compiling routine(GP-D.m)is used to calculate the phase point distance mean and the correlation dimension of the complete process of rock damage under different strain ratios,and to analyze that of it and identify the precursory characteristics of rock damage.(5)In the stage of elastic deformation,the fractal parameter(phase point distance mean)of the acoustic emission of rock damage in the complete process of uniaxial compression,presents toincreasewith the large of rock initial damage degree,but steady decliningwith the small of rock initial damage degree;In accelerating failure stagewhich increasesvastly and reaches the maximumat peak stress;In the post peak stage,it decreases with the stress drop,increases with the stress step and remains stable in the residual stress.In the complete process of uniaxial compression,the fractal parameter(correlation dimension)presents theevolution law "wave-rising-falling-surge and wave".(6)The surge point of phase point distance meanand the "V" inflection point of the correlation dimension can be used as the feature point of rock fracture precursor,and also can be used as the basis for the indirect determination of rock yield point.In view of the sensitivity,intuition and accuracy of the basic parameters of rock acoustic emission and the stability of fractal parameter,the author suggest that the feature point(s)of acoustic emission ringing counts is used as the rock fractureprecursor.The feature point ofmean value of the phase point distance and the correlation dimension can be used as the accuracy identification ofthe feature points of acoustic emissionringing countsfor the rock fractureprecursor.(7)The damage constitutive model based on acoustic emission ringing counts canreflect the complete process of rock damage and fracture;The constitutive model based on Weibull distribution and logarithmic normal distribution can well predict the damage process of rock before peak stress.