| As a non-invasive geophysical method,electromagnetic radiation(EMR)technology has achieved relatively successful engineering applications in coal-rock dynamic disaster monitoring and early warning.However,the generation mechanism of EMR induced by coal and rock fracture is still in the hypothesis stage,which seriously restricts the further improvement of its application hence becoming an urgent unresolved issue in safety science and engineering.With this motive,the mechanical and electrical characteristics of coal and rock’s micro-surface and the in-situ evolution of these characteristics under different loading strengths were experimentally studied.The contribution of micro-surface potential to the EMR’s generation and the influence mechanism of micro-surface mechanical characteristics on crack’s generation and propagation under loading was analyzed.Meanwhile,the time-frequency evolutionary characteristics and correlation of EMR and micro-vibration(MV)during coal-rock failure process under load were revealed by employing a pioneer experimental study.A new mechanism of coal and rock fracture-induced EMR defined as the electricity-vibration coupling mechanism was proposed.The electricity-vibration coupling model for EMR generation was constructed,and the experimental verification and error analysis of this model was carried out.The main research results are as follows:(1)Atomic force microscopy(AFM)was used to study the mechanical and electrical characteristics of coal and rock’s micro-surface.It is found that the microsurface topography of coal and rock is uneven,with an overall fluctuation of tens to one hundred nanometers,reflecting the micro defects of coal and rock.The stiffness and elastic modulus of coal and rock’s micro-surface show a heterogeneous distribution,the micro-surface stiffness and elastic modulus of rock are higher than the coal on the whole.It is inferred from the obtained results that the region with lower micro-surface stiffness and elastic modulus more prone to deformation and fracture,producing micro-pores,cracks and other defects.The micro-surface potential of coal and rock can reach tens to hundreds of millivolts,and its distribution is heterogeneous,some local regions might be dominated by positive/negative electric properties.Based on electrostatic theory,the equivalent surface charge density corresponding to the micro-surface potential of coal and rock under static conditions is estimated,ranging from 3.65×10-7 C/m2 to 4.23×10-6 C/m2.It is concluded that the micro-pores(defects)on the surface of coal and rock provide a habitat for the existence and accumulation of charge,which is one of the reasons for the formation of the potential.Besides,the different polar groups in coal macromolecular structure and the derived charged groups generated on the crystal surface of different rock components will also have an impact on the potential.Also,the non-uniform distribution of micro-pores and polar groups/derived charged groups makes the micro-surface potential of coal and rock also show a heterogeneous distribution.(2)In-situ evolution of the mechanical and electrical characteristics of coal and rock’s micro-surface under different loading strengths were investigated by AFM and micro-loading device.The micro-surface topography of coal and rock increases and decreases locally with increasing load,which reflects the non-uniform transfer and distribution of the stress on the micro-surface.The micro-surface elastic modulus shows complex changes of local increase and decrease with the load,and the amplitude difference of various points is diverse.Based on the fracture mechanics theory,it is supposed that the non-uniform distribution of the microsurface elastic modulus and its local complex changes with the load is the internal reasons for the rough crack surface,uncertain crack path and crack branches,and it can indirectly lead to the vibration phenomenon during crack propagation.With the increase of load,the local high and low potential regions on coal and rock’s microsurface show a transferred distribution,and the potential change at the same point can reach tens of millivolts,indicating that there is a transfer and redistribution of surface charge before fracture.The existence of the micro-surface potential under static conditions and the redistribution of the potential under dynamic conditions prove that the coal and rock’s micro-surface can store charge,and the newly generated charge during the fracture can still exist on the surface for a certain time,which is supposed to be the electrical basis of the EMR generated during fracture.(3)The time-frequency evolution characteristics and correlation of EMR and MV during coal-rock failure process under uniaxial compression were studied by the three-axis electromagnetic antenna and miniature MV acceleration sensors.The main crack of coal and rock under uniaxial compression usually expands along the axial direction(at an acute angle to the axial direction)with an unstable propagation direction,the resulting crack surface is irregular,erratic and rough.The EMR and MV signals are usually generated synchronously with the fracture,and their overall duration is similar.Besides,a positive correlation among the peak value of EMR,MV and load drop has been found,indicating that the EMR and MV might be coupled in time series domain.Both EMR and MV are typical low-frequency signals,almost every fracture generates similar(with a difference of 1-2 kHz)or the same main frequency EMR and MV signals,indicating a noticeable correlation in the frequency domain.During the same fracture,the Z-axis of the three-axis electromagnetic antenna has the most times monitoring the maximum value of the peak intensity of the EMR signal,followed by the X-axis and the Y-axis.The directional characteristics of EMR are highly consistent with the analysis of the direction distribution characteristics of EMR excited by moving charge(vibrating charged crack),which indicates that the hypothesis that EMR is generated by vibrating charged cracks is reasonable.(4)Based on the above findings,a new mechanism of coal and rock fractureinduced EMR defined as the electricity-vibration coupling mechanism is proposed,stating that,the vibrating charged crack generates the EMR.In short,the original charge on the crack surface and the charge generated by the new crack surface are the electrical basis of fracture-induced EMR,and the vibration of the crack surface acts as the power source of EMR.Subsequently,based on the vibrating charged cracks,a generation model of EMR is constructed that helps in explicating such a mechanism.The model is verified by the EMR-MV synchronous monitoring experiments of different types of coal and rock Brazilian tests,uniaxial compression and different loading rates uniaxial compression tests.The EMR waveform calculated by the model and measured by the antenna shows good correspondence,indicating that the model is not affected by the type of sample,loading rate and loading mode,both the effectiveness of the model and the correctness of the electricity-vibration coupling mechanism are comprehensively verified.The research results are of the great theoretical value for further understanding the phenomenon and mechanism of coal and rock fracture-induced EMR,as well as laying important theoretical foundation for the development and application of EMR monitoring and early warning technology.This doctoral thesis contains 99 figures,16 tables,and 258 references. |
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