Experimental Investigation On The Impact Failure Features And Transient Electromagnetic Radiation Characteristics Of Gas-bearing Coal

During the deep coal mining operations,coal seams always contain methane gas which results in most gas disasters.This import property of underground gas-coal-rock system aggravates the complexity of system’s state and behavior for coal mining.Dynamic disasters such as gas outburst and rock bursts have become major threats to mine safety production for its various damage forms,high frequent occurrence and serious casualty and loss.To prevemt dynamic disasters,electromagnetic radiation（EMR）monitoring technology of coal-rock damage has become a crucial research branch due to the advantage of non-contact,continuous monitoring and less interference on coal mining operations.So far,scholars at home and abroad have conducted many field test and numerical analysis on electromagnetic radiation effect of the loaded coal.Although acheieving positive progresses,there still are several problems remaining,such as:（1）The role and influence of gas in the dynamic disaster is not clear and unified neither in laboratory test nor mechanism analysis.（2）Previous studies have focused on static or quasi-static slow loading at low strain rates,while little attention has been paid to the transient electromagnetic radiation effect of coal-rock damage under high strain rate impact load,especially the impact damage of gas-bearing coal.However,coal mine production operation,especially under rich gas state,can lead to strong dynamic load,such as blasting,tunneling with the pneumatic pick,tunnel roof collapse and fault slip.Therefore,it is necessary to focus on the impact failure features and transient electromagnetic radiation characteristics of gas-bearing coal,which will provide therotical and technological support for future application.In this paper,three main terms—gassy coal,impact damage and transient electromagnetic radiation—are studied by means of theoretical analysis,experimental investigation,numerical simulation and field test.On the theoretical aspects,the influence on coal of gas was probed by tentative quantity analysis.On the experimental side,impact damage of the gassy coal was tested by self-designed impact experimental system,acquisiting transient electromagnetic radiation signal simutanously.Then we analyzed the variation characteristics of the fluctuation amplitude,pulse number,cumulative energy and duration of the transient electromagnetic signal under different impact loads and types of coal and rock.In numerical simulation,we adapt ANSYS/LS-DYNA explicit kinetic analysis program to simulate the failure process of coal under different impact energy.On the site test,we analyzed the characteristic parameters of electromagnetic radiation in the process of shaking and blasting in the original coal seam,then,we studied the influence of gas parameters on the electromagnetic radiation signal by combining the variation of gas emission and gas concentration.The main contents and conclusions of this paper are illustrated as follows:（1）Theoretical analysis of EMR induced by impact damage of gas-bearing coalOn the basis of previous studies,the microscopic pore structure of coal and its internal relationship between gas distributions is analyzed.Then the stress distribution of gas and the theory of effective stress is summarized.The theoretical mechanism of coal strength weakening is discussed,and the internal mechanism of adsorbed gas or free gas in weakening coal body strength is quantified respectively according to the intensity of coal body.In order to distinguish gassy coal and common coal,the concept—static damage—is used.And this paper suggests using static damage variable to stand for the mechanical property of gas-containing coal.The macroscopic failure modes,crack propagation mechanism and dynamic constitutive characteristics of gas-bearing coal are analyzed,and the dynamic constitutive model of HJC with gas is established on the basis of the above analysis.The free charge separation and charge Movement of electromagnetic radiation phenomenon of a variety of mechanisms,and gives the characteristics of electromagnetic radiation waveform parameters of the definition and analysis methods（2）Experimental study on impact damage and transient EMR effect of gasOn the basis of the traditional drop hammer impact loading method,a set of experiment apparatus is designed and built to test the impact damage of gas-bearing coal and its transient electromagnetic effect.Using this test system and apparatus,we designed a series of impact tests,with different dropping height（1m,1.5m and 2m）,different drop hammer weight（6kg and 9kg）,different gas media（N₂,CH₄,CO₂）,different pressure gradient（0MPa¹.5MPa）,and different specimens type（raw coal sample and briquette coal sample）,to sample the rupture process of coal and its EMR characteristics.In addition,some factors such as the particle size used for molding briquette the molding pressure（50MPa⁴00MPa）and coal（<0.25 mm,0.25mm⁰.5mm,0.5mm¹mm & 1mm¹.25mm）were also included.After finishing all the tests,a combination of infinite response pulse filter,ensemble empirical mode decomposition and wavelet packet transform method（IIR-EEMD-WPT）is used to separate and extract the effective electromagnetic radiation signals.Then we determine some key wave characteristic parameter,such as fluctuation amplitude,cumulative energy,pulse number,duration to make a statistics analysis.The influence factors such as impact velocity,inflation pressure,inflatable medium,coal and rock type and electromagnetic signal characteristics were discussed according to the experimental scheme.The influence of gas on the mechanical properties of coal and the impact of electromagnetic damage on coal is discussed on the basis of data analysis.This research indicates that: 1).The transient electromagnetic signal generated by the coal sample under the impact load of the falling hammer has a large discretization.The amplitude range of the signal is about 10 mV 600mV;the effective duration of about 3ms¹500ms;between the signal energy according to the sample and test conditions change,the energy range of 0.1μJ¹000μJ;the number of pulse of one impact electromagnetic signal is change from 1 to 20.Moreover,the maximum amplititude,duration and singal energy of raw coal is larger than briquette,which respectively is 2.10,1.70 and 5.01 times of raw coal.2).As filling methane gas into the coal sample,the parameters of electromagnetic sigal such as maxmum amplititude,duration and number of pulse have decreased.These trends will be clearer if the gas adsorption rate is larger（CO₂>CH₄）;and the electromagnetic singal remain unchanged while filling N₂ in the coal.The participation of gas induces an overall reduction of the peak strength of electromagnetic signals,but the effect on the mean value of the electromagnetic signal is not obvious,the gas also reduces the electromagnetic signal fluctuations.3).The EMR signal produced of both briquette and raw coal special fall in the low frequency range of 0Hz⁴0Hz,the effect of gas on the frequency of EMR signal caused by coal damage is not obvious.4).Although the coal sample has good homogeneity and orientation,but still on the molding pressure,the size of coal,the amount of cementing agent and pressing time and many other factors are more sensitive to the higher signal dispersion.The lower intensity test signal is mostly single pulse,or less pulse count.Raw coal samples of the signal is also a single pulse,some samples will produce multiple pulses.5)The test results of the two electromagnetic antennas have high synchronization in most cases,but the number of sample failure test results show that the electromagnetic signals obtained by the two magnetic bar antennas are quite different in the magnetic field direction and the fluctuation characteristics.It is concluded that this is related to the relative orientation of the internal rupture of the coal body.（3）Numerical simulation study on dynamic failure process of gas-bearing coalBased on commercial dynamical analysis program ANSYS/LS-DYNA,a modified dynamic constitutive model and failure criterion method were introduced to simulate the dynamic shock damage process of gas-bearing coal.By proposing some static damage parameters,the strength weakening relation is derived,and then the mechanic parameters of gas-bearing coal are thus determined on the basis of the mechanical parameters of ordinary coal.The dynamic failure process of gas-bearing coal under different gas pressure and different impact velocity is simulated and analyzed.It is found that with the increase of impact velocity and the influence of gas on the dynamic failure characteristics of coal and the electromagnetic signal of coal rupture is discussed.The results show that the generation process of the transient electromagnetic signal and the influence of the gas on the transient electromagnetic effect of the coal body are explained by the cross comparison and verification of the experimental results,which can explain the influence of drop hammer on the effect and signal.（4）Verification of transient EMR characteristics during gas primitive coal miningIn the typical high-gas-out coal mine site,the electromagnetic radiation signal of coal-rock failure in the excavation space is monitored and collected.The parameters of the electromagnetic radiation characteristic of the vibration are measured,and the parameters such as gas emission and gas concentration.The influence of the parameters of the electromagnetic radiation on the electromagnetic radiation signal and the influence of the gas parameters on the electromagnetic radiation signal are studied under the action of the shock load.The results show that: 1).the electromagnetic radiation signals in two directions are very consistent and these produced by blasting shock show cluster pulse characteristics.Its energy is usually maintained between 0.1mJ 2mJ,which is about 10 times of the energy of the raw coal.2).There is a significant positive correlation between the energy of the pulsed electromagnetic signal and the explosive charge in the process of blasting,and the trend of the pulsed electromagnetic signal increases linearly with the increase of the number of pulses and the duration of the pulse.The frequency of the pulse part is about 200Hz;the electromagnetic signal in the vertical coal wall direction contains some small oscillation in addition to the pulse part,the amplitude and energy of the small oscillation are large,and the energy and frequency are smaller than the pulse part.3).the gas content of the coal seam before the gungrass is recorded after 30 minutes of the gun.It is found that with the increase of the methane gas contents,the strength of the coal body decreases,and the amplitude and energy of the electromagnetic signal showed a downward trend,but the vibration of the pulsed electromagnetic signal frequency seemed no significant changes.（5）The mechanism of the effect of gas on EMR of coal impact damageThe effect of gas on the electromagnetic signal is mainly two aspects: On one hand,the gas causes the macroscopic weakening of the strength of the coal.Due to the discharge of gas molecules in the high-strength electric field,the micro-pore structure of the coal is damaged and the energy storage capacity of the coal body can be reduced,so that the strength of the radiation weakened.One the other hand,the pore gas in the coal containing coal ions in the impact of ionization process,but also produce more free electrons,and the internal pressure of gas containing coal to lag behind the external pressure,the impact of instantaneous crushing to provide certain movement acceleration.These two aspects caused by electromagnetic radiation have a opposite role in promoting as the coal damaging.Therefore,when different mechanisms of influence dominate,the impact of gas on the impact of coal on the electromagnetic radiation signal has a different effect.