| During the exploitation of coal resources,the probability of coal and rock dynamic disasters due to disturbances has increased with the increase of mining depth and the complexity of geological conditions,which often leads to serious casualties and economic losses.Therefore,it has been a problem that many researchers have tried to solve and perfect the problem by carrying out effective early warning measures on both small-scale experimental study and suitable field scale.There are two types of prediction methods widely used in mining.The first one is the conventional index prediction method,which uses the traditional indicators such as the amount of drill cuttings and the initial velocity of gas emission.There are shortcomings such as low accuracy and high manpower consumption.The second one is geophysical methods with the use of precursory signals accompanied by energy release during dynamic disasters,including acoustic emission methods based on sound,infrared methods based on light,and electromagnetic radiation based on electricity,etc.This kind of method can carry out continuous and real-time monitoring of the mining area without affecting the production of coal mines.It has good prospects for further development,but all of them have certain shortcomings and respective application scopes.The use of anomalous changes in the geoelectric field to predict geological hazards is a common geophysical method that can be applied to the prediction of large-scale rock dynamic disasters such as earthquakes and volcanoes.At the same time,in view of the dynamic disasters in a relatively small scale such as mines and dams,researchers have begun to study the phenomenon of charged anomalies in coal and rock materials and have made some progress.Studies show that all the changes in surface potentials,currents,induced charges or the electromagnetic radiation that occur during the deformation and destruction of coal and rock materials are related to the generation of internal free charge,that is,the generation and change of free charge is the basis for the above-mentioned electric effect.This has been widely verified both theoretically and experimentally.At the same time,it is discovered that there are dramatic changes in positive and negative polarities and signal amplitudes in a short time when coal and rock are deformed or destroyed,which cause changes in other physical parameters.However,current researches on the electrical effects of material damage focus on surface potentials and currents,as well as induced charges and electromagnetic radiation.These indicators can only reflect the degree of accumulation of free charge on the material surface over a certain period of time during the destruction of the sample.The induction effect does not accurately reflect the instantaneous change in surface charge,which can lead to a more direct and accurate correspondence with the failure of the specimen,sudden change in stress,and the dynamic tendency of other signals in a short time.In order to solve the above problems,this paper makes following attempts based on previous studies.Firstly,the process of deformation of coal and rock under external loads,as well as the mechanism of internal crack propagation,bifurcation,and penetration are summarized and analyzed.With the charge generation mechanism,the"stress-charge" coupling model under uniaxial compression of coal and rock is established.Then,using the self-made experimental system,different coal and rock samples,including raw coal,briquette,rock,cement,etc.,are subjected to uniaxial compression and failure.The surface transient charge signals generated during the destruction process are collected and the relationship between stress and energy has been analyzed,as well as the various factors affecting their characteristics.At the same time,the synchronism between the monitored microseismic signal and surface transient charge on the time scale is compared,and the mechanism of the crack-induced transient charge generation is verified.Finally,using ABAQUS software and the extended finite element method to establish a similar model,and the experimental results and related theories are confirmed.The research results in this paper make up for the gaps in the characteristics of surface transient charge changes caused by deformation and failure of coal and rock materials.It is of great significance to explain the electrification effects in coal-rock dynamic disasters and improve disaster monitoring and early warning technologies.The main research contents and conclusions of the paper are as follows:(1)The surface transient charge test system is designed and constructed,and destruction results in different coal rock materials are compared.According to the characteristics of surface transient charge generated during deformation and failure process of coal and rock materials,an experimental system for surface transient charge is established in laboratory conditions.By designing relevant experimental schemes,surface transient charge generated during the destruction process is monitored and collected.The crushing characteristics and corresponding surface transient charge of different types of samples are compared and analyzed.The main factors for experimental results are the physical and mechanical properties of the material and the differences in internal structure.(2)Mechanisms of surface transient charge generation is analyzed and a"stress-charge" coupling model in the deformation process of coal and rock under loading is establishedThe mechanism of the surface transient charge in each loading stage is discussed.In the starting stage of loading,the friction between the sample particles is the main factor.In the middle and later stages,the stress concentration and crack propagation are the main factors.The deformation process and internal crack propagation in coal and rock under uniaxial compression are analyzed,and the corresponding relationship between them is discussed.According to the theory of statistical damage mechanics,a"stress-charge" coupling model suitable for surface transient charge and stress changes is established based on the relationship between electromagnetic radiation and stress.By comparing the parameters of surface transient charge and deformation rupture signals,including amplitude and energy,it is shown that there is a good synchronization between the two on the time scale,and it proves that the generation of cracks during deformation of coal and rock samples is the main cause of surface transient charge.(3)The change law of surface transient charge produced on raw coal,briquette,rock and cement during uniaxial compression process is studied,and external environment and internal influence factors of samples are analyzed.Five characteristic parameters such as charge signal strength,pulse number,duration,charge accumulation amount,charge energy,and cumulative value are proposed to facilitate the subsequent analysis of surface transient charge generation and change characteristics.According to the experimental results,it is obvious that:1.Using a comprehensive denoising method including base signal remove,ensemble empirical mode decomposition,and wavelet threshold,the original signal is filtered and effective signal is obtained.2.In the deformation process of raw coal samples,there are two types of failure modes due to the different direction of bedding distribution.When the bedding direction is perpendicular to the loading direction(vertical bedding specimens),the peak stress can reach 4.6?10.2MPa,and the corresponding value of surface transient charge signal is 560?5117pc.When the bedding direction is parallel to the loading direction(parallel bedding samples),the peak stress is 2.1-5.3MPa,and the charge signal range is 188-800pcs.It shows that the distribution direction of internal bedding is the main factor affecting the transient charge change on the surface of raw coal.3.In the briquette samples,the amplitude of compressive strength and surface transient charge are all lower than that of raw coal and rock samples,which range from several tens of pcs to nearly 100 pcs.Comparing four groups of specimens with different particle size(<0.25mm,0.25mm?0.5 mm,0.5mm?1mm,1mm?2mm),the amplitudes of surface transient charge decrease or even undetectable as the particle size increase,while their abundance increase.4.As the reference sample,cement samples have the highest homogeneity.The peak stress range is 0.8?2.2MPa,and there are obvious surface transient charge signals during the whole loading process with the range of 55?230pc.5.In addition to the physical properties,bedding distribution,particle size,external factors such as the loading method and rate,environmental noise,and electrodes during the experiment will also have an impact on the surface transient charge changes.(4)Stage characteristics and energy release law of surface transient charge during deformation and failure of coal and rock materials are analyzed.According to the relationship between surface transient charge and stress of coal and rock over time,it is found that the generation process of surface transient charge can be divided into three stages.The first stage is the generation of trace transient charge,which corresponds to the stage of compaction and elastic deformation of the sample.The second stage is a large number of transient charge generation stages,corresponding to the non-elastic deformation stage of the sample.And the third stage is the extreme transient charge generation stage,corresponding to the main rupture process of the sample.Considering that the transient charge will accumulate on an area of the loaded surface in a short time,the integration operation is performed to obtain the variation law of surface charge accumulation with time.During the release of elastic energy in the main rupture of samples,there is an energy release critical point that has a high synchronization with the sudden change of the surface transient charge signal energy,indicating that the surface transient charge also has an increase during energy release.(5)Internal crack propagation process and surface transient charge characteristics during failure of different coal samples are numerical simulated.The results of ABAQUS numerical simulation show that:1.When the bedding direction is different(0°,30°,60°,90°),the crack propagation process is quite different.The larger the angle between the bedding direction and the vertical direction is,the greater the compressive strength of the sample is.And when the stress is increased to the limiting value,the speed of complete deformation is higher.2.In the briquette samples,the expansion process of the cracks has a relatively uniform trend.However,with the decrease of particle size,the crack propagation also delays at the early stage and accelerates at the later stage.3.The energy release process and the surface transient charge generation process in the uniaxial compression process show a good corresponding relationship,and the change law of the surface transient charge along with the bedding direction and particle size is the same with the crack propagation.4.The relationship among internal crack propagation,elastic energy release and surface transient charge change with bedding and particle size further confirmed the relationship between surface transient charge and crack propagation in experimental measurements,indicating that crack propagation is the main factor that causes surface transient charge. |
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