| A large number of mine accidents are caused by the failure of the main bearing structure after the external load acting on the large structure coal and rock mass exceeds the bearing capacity of the coal and rock mass.There are certain geophysical signals before the occurrence of coal and rock dynamic disasters.If the precursor information of the dynamic disaster can be fully utilized,the rock burst,large mine earthquake and other disasters can be effectively warned.In order to study the mechanical characteristics of large-scale coal bodies such as coal mine roofs and the spatial-temporal precursory information of failure,this thesis conducts Brazilian splitting experiments on coal with different thickness-diameter ratios based on the experimental platform of“Acoustic Emission（AE）-Electromagnetic Radiation（EMR）-Deformation Multi-Parameter Comprehensive Monitoring System”.The synchronous variation law of AE-EMR-deformation time-frequency spatial information of coal tensile fracture is obtained,and the mechanical characteristics of coal tensile fracture,the precursory characteristics of time-frequency spatial response and the evolution process of energy damage are analyzed.Combined with the experimental results,the spatial-temporal response precursory characteristics of large-energy tensile fracture events in coal mine roofs are analyzed.Draw the following main conclusions:（1）The mechanical parameters,energy and AE parameters of coal tensile fracture have obvious size effect:the bearing capacity,peak axial displacement,elastic modulus,fracture toughness,peak input energy,peak energy rate and AE peak cumulative count of coal increase linearly with the increase of diameter,and the AE peak cumulative energy increases quadratically with the increase of diameter.The tensile strength and peak strain of coal decrease exponentially with the increase of diameter,and finally stabilize at a fixed value.The tensile strength tends to 1.1 MPa and the peak strain tends to 4×10-3.（2）According to the AE time-frequency response characteristics,the tensile fracture process of coal can be divided into crack compaction stage,crack initiation stage,crack stable propagation stage,crack unstable propagation stage and failure stage.In the unstable propagation stage of crack,the energy,count and amplitude of acoustic-electric events increase greatly,the AE events with low main frequency and high amplitude increase rapidly,and the frequency band of AE signal widens obviously.This phenomenon can be used as the precursor characteristics of AE signals of coal tensile failure.The precursory response of EMR time-frequency signal is earlier than that of AE time-frequency signal.Under the action of load,the two ends of the prefabricated crack of the specimen first concentrate on the fracture event,resulting in the accumulation of damage in the region and the generation of cracks,and then the cracks expand and develop along the vertical direction of the prefabricated crack.When the AE positioning point is dense into a straight line through the sample,it indicates that the sample has been seriously damaged and the damage is coming.The closer the specimen is to the prefabricated crack,the larger the surface deformation of the specimen is.When the displacement field and strain field banded cloud map will penetrate the surface of the specimen,it indicates that the macroscopic fracture surface is about to form.（3）Based on the damage early warning model of AE and EMR parameters,the characteristics of tensile fracture damage evolution of coal can be characterized.In the unstable propagation stage of crack,the AE and EMR damage variables of the sample increase sharply,which indicates that the damage of the sample accumulates rapidly.The peak value of the simulated stress curve based on the inversion of AE and EMR parameters is obviously ahead of the peak value of the experimental stress curve,and there is a plastic yield stage with a larger span,which can be used as a precursor warning for the tensile failure of coal.The acoustic-electric b value shows a“V-shaped”variation law of sudden drop and sudden increase in the unstable propagation stage of the crack,indicating that the large fracture event occurs rapidly inside the sample,and this law can be used as a precursor feature of coal tensile failure.（4）The daily energy,daily maximum energy and daily frequency of microseismic events before the occurrence of large-energy roof fracture events show a“V-shaped”variation characteristic of sudden drop and sudden increase,while the b value of microseismic events shows a significant downward trend.After the fracture event,the b value of microseismic events increases rapidly.This phenomenon can be used as a precursor to the time series of large-energy roof fracture events.Before the occurrence of the large energy fracture event of the roof,a large number of large and medium energy microseismic events in space are concentrated around it,and the stress concentration occurs in the area,and a large number of fracture events occur,and the large energy fracture event of the roof occurs in the mining face area,special attention should be paid to the disaster prevention and control in the area.There are 64 figures,4 tables and 82 references in this thesis. |
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