| During underground mining,room pillars,strip pillars,section protective pillars and fault protective pillars are bound to accumulate damage under repeated mining stress.Eventually,fatigue failure and instability will occur.Carrying out the acoustic emission basic test of fatigue damage evolution mechanism of coal under cyclic loading,and systematically studing the mechanical mechanism and acoustic emission precursor characteristics of fatigue damage evolution process of coal has important guiding significance in revealing the intrinsic relationship between coal fatigue failure mechanism and the acoustic emission process,designing the size of protective coal pillar scientifically,predicting the fatigue failure instability of engineering coal bodies by using acoustic emission monitoring method,ensuring the safety and efficient production of coal mine,and guaranteeing the long-term safety and stability of ground buildings(structures).In this paper,a reasonable acoustic emission test method which can place acoustic emission detector in the triaxial pressure chamber of MTS815.02 electro-hydraulic servo rock mechanics test system was adopted by theoretical analysis and laboratory test.Acoustic emission test in the process of fatigue damage failure of coal specimens under cyclic loading was carried out.The evolution mechanism of fatigue damage and the characteristics of acoustic emission precursors were systematically studied,and the following main results and conclusions were obtained.(1)Compared with hard and compact rocks such as marble,limestone and sandstone,the"threshold value" of fatigue failure of coal specimens under cyclic loading is relatively low,and increases with the increase of loading frequency and confining pressure.Axial deformation and circumferential deformation show different development laws in different stress levels.With the continuous increase of stress level,the axial strain and circumferential strain show a trend of gradual increase.Compared with the uniaxial cyclic loading test,the irreversible strain of coal samples under triaxial cyclic loading presents a more gentle development and change characteristics,which indicates that the existence of confining pressure not only restrains the circular deformation of coal samples,but also restrains the plastic deformation and irreversible damage deformation of coal samples,and delays the overall process of fatigue damage evolution of coal to a certain extent.In this process,the elastic modulus and damage evolution of coal samples show obvious staged development law.In addition to the ultimate stress level,the dissipated energy of coal samples under uniaxial(triaxial)cyclic loading presents a "L" type evolution law,while the "(?)" type evolution law appears at the ultimate stress level.(2)Acoustic emission tests under multi-stress horizontal cyclic loading with different loading frequencies(0.25 Hz and 0.5 Hz)under uniaxial(triaxial)loading were carried out.The important effects of stress levels and loading frequencies on acoustic emission characterization parameters(ringing counting rate,impact counting rate and energy counting rate)were emphatically discussed.A fatigue damage evolution model based on acoustic emission cumulative energy counts was established.(3)Distribution characteristics of scatter characteristics of acoustic emission RA value and average frequency AF value can be used to judge the crack propagation pattern of coal samples in the process of fatigue damage failure under cyclic loading.In the initial stress level stage,tensile crack is dominant,and then presents the form of tension-shear composite crack.Near fatigue failure,AE and RA value increasing sharply can be used to judge the crack propagation pattern of coal samples.Acoustic emission b value is an important parameter used to characterize the deformation and failure mechanism of coal.Compared with the uniaxial cyclic loading test,the small-scale micro-fracture of coal samples under triaxial cyclic loading is more common,and the acoustic emission b value generally presents a relatively stable development and change law.The higher the confining pressure is,the more stable the distribution of acoustic emission b value will be.Near fatigue failure,the acoustic emission b value shows a more obvious decrease,and the abrupt drop of b value can be used as a precursor of fatigue failure of coal under cyclic loading.(4)Compared with short-time Fourier transform and wavelet transform,Hilbert-Huang Transform has unique advantages in processing acoustic emission signals,avoiding the obstacles of preset window function and wavelet function.Based on Hilbert-Huang Transform theory,the acoustic emission signals of coal near fatigue failure under cyclic loading were analyzed by principal and secondary component analysis.Hilbert spectrum and Hilbert marginal spectrum were obtained by Hilbert transform.The results show that the energy fluctuation in the two-dimensional Hilbert time spectrum is mainly in the low frequency range,and the confining pressure density effect has a certain influence on the distribution of signal energy.The amplitude of acoustic emission wave in Hilbert marginal spectrum increases obviously in the low frequency range,which can be used as an important precursor information for fatigue failure of coal under cyclic loading.(5)According to the theory of non-linear dynamics of rock,combined with the characterization parameters of acoustic emission of cyclic loading test,the self similarity and catastrophe characteristics were systematically explored,and then the precursory characteristics(criteria)of fatigue failure of coal samples were analyzed.The self-similarity coefficient of AE state appears to be minimal,showing a "sudden drop-rotation" self-similarity model,which indicates the occurrence of fatigue failure and instability of coal samples.Based on the catastrophic eigenvalue ?(?=4p3+27q2)of cusp catastrophic model,the catastrophic phenomena of acoustic emission process of coal samples under cyclic loading were fully revealed. |
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