Study On Evolution Of Time-frequency Characteristics Of Acoustic Emission From Rockburst Under Different Gradient Stress

With the rapid development of social and economic needs and infrastructure construction,the scope of human activities continues to extend to the deep underground,and rockburst has gradually become a geological disaster that cannot be ignored in the process of deep underground mining and construction.Studies have shown that a series of acoustic emission information will be generated in the surrounding rock of the chamber before the rockburst disaster occurs.How to reasonably use the acoustic emission information of rockburst and find its general evolution law is of great significance to formulate the prevention and control strategy of rockburst geological disasters.At present,the rockburst model tests are mostly based on small-size specimens,and the vertical loading stress is evenly distributed.The influence of the tangential stress gradient distribution of surrounding rock on the rockburst failure is not considered,and most of them are relatively isolated to study the evolution of acoustic emission time-domain characteristics and frequency-domain characteristics.It is difficult to comprehensively analyze the evolution law of acoustic emission time-frequency characteristics in the process of rockburst inoculation.Based on this background,the strength and rockburst proneness of gypsum specimens were tested by uniaxial compression test,and the appropriate water-gypsum ratio was selected.Under the same conditions,four large-scale rock-like material specimens with the same specifications were made.On the independently developed YB-A rockburst model test system,the rockburst model tests of four specimens with three-direction,six-side and different gradient stress loading at the top were carried out,respectively.With the help of the high-definition camera,stress-strain and acoustic emission real-time monitoring,the macroscopic failure phenomenon of the rockburst inoculation process,and the change of time-frequency information of stress and acoustic emission with the loading process were recorded and analyzed.The time domain characteristics,frequency domain characteristics and time-frequency distribution characteristics of acoustic emission under different gradient stresses are studied.The main research contents and results of this paper are as follows:（1）Through the gas-liquid composite loading device,the rockburst model test of large-scale rock-like material specimens under three-way six-side loading and one-side unloading under different gradient stresses was carried out.Based on the macroscopic failure phenomenon of specimens under different gradient stresses,taking the maximum gradient loading stress and the maximum loading gradient stress ratioσ_1-1/σ_1-1max as the characterization scale,the failure process of rockburst inoculation under different gradient stresses is divided into three stages:initial damage stage（I）,damage development stage（II）and damage failure stage（III）.It is found that with the increase of the loading stress gradient coefficient n,the peak load of the specimen is smaller when the failure occurs,and the rockburst inoculation process of the specimen is more rapid from the damage development stage（II）to the damage failure stage（III）,and the dynamic failure of the specimen is more obvious.（2）The time domain characteristics of acoustic emission signals under different gradient stresses are extracted,and the evolution process of acoustic emission impact number and acoustic emission ring count with the loading stress ratioσ_1-1/σ_1-1max under different gradients stresses are analyzed.The evolution law of acoustic emission b value in each stage of rock specimen explosion damage is also analyzed.It is found that the loading stress gradient has an important influence on the time-domain characteristic evolution of the acoustic emission signal of the specimen,the active degree of the acoustic emission signal inside the specimen,the degree of damage and rupture,and the evolution of the proportion of large-scale rupture are positively correlated with the stress gradient coefficient.（3）The frequency domain characteristics of acoustic emission signals under different gradient stresses are extracted,and the evolution process of main frequency and main frequency amplitude of acoustic emission signals with the loading stress ratioσ_1-1/σ_1-1max under different gradient stresses is analyzed.The distribution and evolution of low frequency and high amplitude acoustic emission signals in each stage of damage and failure of specimens are analyzed.It is found that the loading stress gradient has an important influence on the evolution of the frequency domain characteristics of the acoustic emission signal of the specimen.The increase of the stress gradient coefficient makes the large-scale shear cracks gradually concentrate to the damage and failure stage（III）.（4）The evolution of time-frequency characteristics of acoustic emission signals is comprehensively analyzed.The ringing count and main frequency amplitude of acoustic emission are selected as the characteristic parameters to characterize the time domain and frequency domain characteristics of acoustic emission,and the unified time-frequency coordinate system is constructed by normalization.The time-frequency distribution scatters plot is used to characterize the time-frequency distribution of acoustic emission signals under different gradient stresses,and the evolution law of time-frequency comprehensive characteristics is studied.The characteristic parameters of acoustic emission in the time domain and frequency domain are analyzed by the nearest neighbor method,and the evolution of time-frequency distribution characteristics of acoustic emission waveform signals under different stress gradients is quantitatively analyzed by NNI.The results show that the loading stress gradient has an important influence on the evolution of the time-frequency distribution characteristics of acoustic emission signals.The time-frequency distribution aggregation degree of acoustic emission waveform signals increases with the increase of the stress gradient coefficient n at each stage of damage and failure of the specimen.The high-stress gradient causes the time-frequency distribution aggregation degree of acoustic emission waveform signals near the rockburst to decrease rapidly.