Study On The Weak Current Effect And Its Mechanism Of Stressed Coal During Damage

Coal will still be the most important energy resource in China for quite a long time.Coal and rock dynamic disasters such as rock bursts,coal and gas outbursts could occur during coal mining,which seriously threatens the safety and efficient production of coal mines.The stress increase,deformation aggravation and damage accumulation of coal and rock mass induced by mining and tunnelling activities are the main causes of these dynamic disasters.Therefore,the prediction of the dynamic disasters essentially lies in monitoring the stress,deformation and fracture of coal and rock mass.Previous research shows that weak currents can be stimulated from stressed rocks,but there is little research on the weak current effect of coals during damage.Therefore,this research is designed to explore the key scientific issues of weak current effect of stressed coals and its mechanism.Weak current stimulated during damage of coals under external load and the current response laws are analyzed.The damage evolution laws and energy conversion relationships based on current effect were investigated.The generation mechanisms of weak currents from coal during damage were investigated and both physical and mathematical models were established to describe the mechanisms.In addition,field tests were conducted in a coal mine with the risk of coal bumps to investigate the response characteristics of weak current during mining.The main conclusions are as follows:The systems for weak current measurement of coals under loads were established.Uniaxial loading,concentrated loading and impact loading tests on raw coal samples were carried out,and weak current were synchronously measured.The direction and response laws of currents from coals under loading were analyzed,and the quantitative relationship between weak current and mechanical behaviors in different deformation stages was investigated.The results show that weak currents can be produced from coals under different modes of loadings,and the current has directivity,flowing from the stressed area to the unstressed one.The change of this current is closely related to the mechanical behaviors of the coal.In plastic deformation phase,the weak current increases exponentially with the increase of strain rate,reaching its peak value when a fracture occurs and then decreases rapidly.Stress drops are always accompanied by a sharp increase of current,and the increment is proportional to the stress drop,which indicates that the abnormal current increases with the intensity of a fracture.What's more,transient currents can be stimulated from coals under impact loading,and the current increases with the stress drop.The current attenuation laws are studied based on the non-extensive statistical mechanics(Tsallis entropy),and the relationship between non-extensive ductility and stress level is investigated.The results show that under a constant stress or the stress is removed,weak current decays gradually and finally tends to be stable,and the stable current increases linearly with the constant stress.The existence of stress can reduce both the rate and degree of current attenuation.The current attenuation obeys nonextensive statistical mechanics,and the non-extensive parameter q is greater than 1.The non-extensive degree of current attenuation increases first and then decreases with the stress level,which can be used to evaluate the stress level of coals.The analytical expression of damage variable based on current effect is established.Damage evolution laws of coal in the progressive loading is studied,and the onedimensional damage constitutive model of coal under uniaxial compression based on cumulative charge is constructed.The variation law of dissipated energy density and electric current energy density of coals under disturbance stress and in creep process is studied.The results show that the damage variable defined based on the "normalized" cumulative charge can well reflect the damage evolution process of coals.Pulsating direct current is generated in the coal under disturbance loads,and its amplitude increases linearly with the amplitude of disturbance stress.The current energy density increases linearly with the dissipative energy density.Creeping occurs when coals are under a constant stress,and both the creep rate and degree increase with stress level.The current energy density increases with the dissipative energy density in a negative exponential power during creep process.Charge distribution on coal surfaces in the micro scale was investigated,and the charge distribution model was established.Combining the charge distribution model with the experimental results,the generation mechanism of weak current from coals during damage were revealed from a macro-micro perspective,and physical models were established to describe the processes of charge transfer and current generation.The mathematical models were also established to induce the expression of charge density in different deformation phases.The results show that the carriers of the weak currents from stressed coals are mainly free electrons,and the distribution of the charge obeys the tip effect,that is,the charge tends to enrich towards the tip of a crack,and the closer to the tip,the greater the charge density.The generation mechanism of weak currents during coal damage is the accumulation and release of free charge because of the tip effect and diffusion effect.The currents are stimulated because of charge diffusion during coal deformation whiles because of the release of accumulated charges when a rupture occurs.The current value is dependent on strain,strain rate,charge density and the change rate of charge density,and the charge density is a function of strain rate.The dominant factors of weak current are different in different deformation phases.Weak current attenuation is essentially a carrier relaxation process,which is related to the electric field edge of phase interface.The current relaxation time decreases firstly and then increases with the stress level.Field tests on weak current measurement in the coal mining face were carried out using the self-developed weak current monitor.The spatial distribution laws and timedomain evolution laws of weak currents in the mining process were studied.The results show that the distribution of weak currents in surrounding coal mass is consistent with the stress distribution,and the weak current shows a trend of “increase-decrease-stable”.Weak current responses well to the advancing process of the working face,showing a stepped increasing trend.Weak current increases gradually in the mining period,while fluctuates in mining-stop period.In addition,weak current response to mining tremors in advance,and the acceleration of weak current can be used as the precursor of mining tremors or coal failures.The weak current method has a wide application prospect due to its advantages of strong anti-interference ability,sensitive response and advanced disaster warning.Research results can provide a new idea and a new method for monitoring and early warning of coal and rock dynamic disasters.It is of great theoretical significance and practical value for coal stress monitoring,stability evaluation and for coal and rock dynamic disaster prediction.There are 108 figures,23 tables and 220 references in this dissertation.