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A mechanism, based on some model of compressed atoms, for the production of electrons in rocks under compression up to fracture has been proposed. The electron emission and light emission during rock fracture and the explosive nature of the fracture can be satisfactorily explained in this way at a semiquantitative level. -English summary
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... During dynamic coal disasters, the energy stored in coal will be released in the form of acoustic energy and electromagnetic energy (Cai et al. 2014;Yamada et al. 1989). The generation and transfer of free charge occur during coal deformation and destruction, leading to an electric potential (EP) response (Enomoto et al. 1993;Freund et al. 2006;Guo et al. 1989;Niu et al. 2017). Under the action of stress, the internal damage of a coal mass continues to develop, resulting in the continuous generation, growth, and expansion of microcracks (Cartwright-Taylor et al. 2014). ...
... Consequently, free charges are produced on the crack surfaces (Cartwright-Taylor et al. 2014). Moreover, the outer electrons of particles will obtain a sufficient energy under the effect of the stress concentration at the crack tip and then escape (Cress et al. 1987;Guo et al. 1989). With the expansion and movement of cracks, the charges continuously migrate and accumulate. ...
... Many scholars have conducted detailed research on the response mechanism of EP. Guo et al. (1989) believed that due to the high concentration of stress at the microfracture tip, some outer electrons of the atoms have increased kinetic energy and escape. Cress et al. (1987) thought that new gravel particles formed where electrostatic charges were distributed on the crack surface. ...
Previous studies indicate that an electric potential (EP) signal is generated during the loading process of coal and that the EP response is related to the damage evolution. When coupled with gas, EP changes the pore structure and mechanical properties of a coal mass, promoting crack generation and growth and accelerating damage evolution. To study the EP response characteristics and investigate the damage of gas-bearing coal, a triaxial test was carried out with a gas-controlled confining pressure, and multiple types of data were measured and analyzed. The results show that with the change in stress, the EP response increases and fluctuates. This response reflects the stress and reveals the damage evolution, which could be verified with the variation in the acoustic emission response. For the mechanism analyses, the failure of the sample is caused by crack expansion and propagation under the coupling action of stress and gas. Consequently, microscopic charge separation and electron emission are the dominant mechanisms controlling the EP response. Furthermore, the constitutive damage equation of gas-bearing coal is established based on the EP response in view of continuous damage theory and the stress intensity distribution hypothesis. The calculation results of damage and stress based on the EP response are utilized for verification; the results indicate that the damage expressed by the EP response is reasonable and useful. This finding is helpful for understanding the damage evolution mechanism of gas-bearing coal.
... Qian et al., 1998; Stavrakas et al., 2007 ). Based on these experimental results, several models have been proposed to explain the universal features of the generation of EM signals from rocks, including piezoelectric effect (Nitsan, 1977; Yoshida et al., 1997), electrokinetic effect (Mizutani et al., 1976; Ren et al., 2012 ), micro-fracture electrification (Ogawa et al., 1985; Hayakawa, 1995, 1998 ), the emission of electrons and charged particles from fracture surfaces (Enomoto and Hashimoto, 1990), atomic oscillations upon crack surfaces (Frid et al., 2003 ), the bombardment of atmospheric gases due to the turbulent flow of net charged particles (Cress et al., 1987), and compressed atom and electron emission (Guo et al., 1989). However, these results and models have been based mainly on the experiments under axial compressive fracture, shear fracture, indentation fracture and stick–slip (friction) in lab and blasting in situ. ...
... It is clear that there are not any fluids inside the rock specimens used here, so it should be out of the mechanisms for our cases. Compressed atom and electron emission (Guo et al., 1989 ) often occurs during the explosive fracture of a rock under high pressure such as about 100 MPa or above (Guo et al., 1988). The used specimens in our experiment wereFig. ...
... Ogawa observed both positive and negative charges generated from the secondary crack surfaces . Guo et al. believes that the stress concentration around the crack tips leads to the contraction of atom and increase in electron energy . e electron then flees and therefore causes the charge separation. ...
The electrical charge characteristic of rock materials under compression is an important index for predicting the development of rock fractures and the failure of engineering structures. However, the charge behaviours of a preexisting rock sample have not been studied in depth. In this study, sandstone samples with a single fabricated precrack at different angles of inclination are prepared. The uniaxial compression tests are performed to study the charge behaviours associated with the initiation and propagation of secondary cracks, the mechanical properties, and the progressive failure of stressed rock samples. An improved analytical model based on the maximum tensile stress failure theory for brittle materials is also proposed for determining the crack growth paths of the single precrack rock samples under uniaxial compression. The friction factors of crack surfaces are computed. The results show that the step functions on the curves of charge accumulation over time correspond to the fluctuation of stress, indicating the initiation of microcracks. The sample with a crack inclination angle of π/4 shows the largest amount of both the first charge and the total accumulation. The analytical model shows a positive relationship between the crack face friction factors and the charge accumulation. The analytical solution of the crack development angles shows good agreement with the experimental results. This work may provide reference for the similar studies regarding the correlation of charging behaviours to the compressed rock materials.
... (2) The induced charge generated during the deformation process can be measured using a non-contact charge sensor, and the characteristics of charge signals in granite, sandstone and coal are discussed in the literature . Based on the electromagnetic emission observed during the failure process, a compressed atomic model is proposed . (3) From theoretical research, the electromagnetic field theory and double layer theory are used to calculate the amount of charge generated when coal and rock samples fail . ...
Using an analysis of the uniaxial compression process of Datong coal samples, the change of transient charge signals on coal surfaces is observed, and the influence of sampling directions (perpendicular to bedding planes and parallel to bedding planes) on the transient charge signals is studied. The intensity in perpendicular to bedding planes is 4.6~10.2 MPa, parallel to bedding planes is 2.1~5.3 MPa. The results show that the change of the charge signals on sample surfaces is instantaneous and pulsing, and such a change is always in accord with stress change and the alternation of positive and negative charge occurring over a short time period. Under uniaxial compression, the surface charge signal characteristics of coal sample in perpendicular to and parallel to the bedding are different. With a higher value of limiting stress, the transient charge signals on coal sample surfaces perpendicular to the bedding exhibit higher strength than those of coal samples oriented parallel to the bedding. However, the number of signal pulses during the failure process, for the samples perpendicular to the bedding, is less than that for the samples oriented parallel to the bedding. According to the variation law for transient charge signals on coal surfaces, we conclude that changes in the transient charge can serve as a tool to characterize crack propagation within coal specimens and provide an important reference for the prediction of coal and rock dynamic disasters.
... Since the discovery in 1988 that satellite thermal infrared (TIR) anomalies have occurred days or weeks before some moderate to strong earthquakes , a considerable amount of TIR radiation detection work has been conducted during the past decade. This work, carried out in the context of understanding the mechanism of earthquake lights , has involved studies of electromagnetic radiation emanating from loaded rock samples. The work has revealed that there exist complex and interesting electromagnetic radiation phenomena during rock fracturing. ...
... In addition, scholars from China and other countries carried out extensive researches on EME effects and mechanisms due to coal rock damage. Based on fracture mechanics and electrodynamics, they established some micromathematical and physics models of coal rock EME and applied them for preliminary theoretical analyses [7,8,. However, most of these models were microscopic and difficult to meet the requirements of practical applications and especially challenged by the determination of warning critical value and hazard level, etc. ...
Dynamic collapses of deeply mined coal rocks are severe threats to miners. In order to predict the collapses more accurately using electromagnetic emission (EME), we established a loaded coal rock EME electromechanical coupling model based on statistical damage mechanics. By using it, we numerically simulated both the accumulative pulse and strain ratios. We further improved the model with the Weibull pattern parameter, which has important effects on simulated results and can be applied to judge coal's homogeneity, and determined the pattern parameter and its value domain. Based on the revised model and the characteristics of coal rock deformation and fracture, we setup EME graded warning criteria against coal rock dynamic collapses by determining static critical coefficient and dynamic trend coefficient. We have applied this model to predict and deal with coal and gas outburst and rock burst occurring at Xie I and Taoshan Mines, respectively. All these verifications show that the model has many advantages and provides more sensitive and accurate warning for dynamic collapses.
... The interaction between coal rock particles in nature is realized by the actions of the electric field and charges. When the interface between the adjacent particles produces non-uniform deformation, the charge balance at the interface is destroyed, resulting in the accumulation of free charges (mainly electrons) at the stretched interface and opposite charges inside the pressed particles, in other words, resulting in the formation of an electric dipole . The heterogeneity of coal rock mass subject to stress could cause constant changes of the local stress and changes in the distances between particles, which eventually lead to changes in these electric dipoles. ...
... Although there is no clear mechanism of the generation of EMR from concrete, coal or rock, several hypotheses have been proposed to explain the phenomenon. These explanations include the acceleration and deceleration of dislocations , the movement of positive and negative charges along the sides of fractures , electrokinetic effects , piezoelectric effects , compressed atom and electron emissions , and motion from the variable velocity of charged particles . Electrokinetic effects always involve fluids , but there are obviously no fluids in the concrete specimens so this mechanism can be excluded. ...
... Therefore, EP signals can potentially be utilized as precursor information to monitor the failure of coal rock mass (Darnet et al. 2006;Nguyen et al. 2015). Guo et al. (1989) recorded the electron emission in uniaxial compression experiment of granite and constructed the quantum chemical model. Yoshida et al. (1998) found EP signals were produced by expansion and shear evolution of when cracks grew inside rock, meanwhile electric current caused concurrently. ...
During the mining activities, coal–rock dynamic disasters have caused grievous casualties and massive property losses. It is the severe problem for regional monitoring of abnormal stress. Electrical potential (EP) can be produced on coal rock under loading, and its response is related closely with the loading stress and damage evolution. Meanwhile, electric field inversion has the advantage to realize regionalization monitoring in the space. To identify the abnormal stress localization characteristics in mining coal seam, after theoretical derivation, the strategy of EP inversion imaging on is studied under bilateral model. Further, simulation experiment of coal rock under loading is conducted. The abnormal probability zones of sample can be identified with EP inversion imaging, while it corresponds with severe damage zones with significant crack propagation. It can be utilized to reveal the localized characteristics of damage and failure of coal rock spatially. Finally, the results of EP inversion in the mining coal seam indicate that the abnormal probability zones can be considered as stress concentration and dynamic hazard areas. Its effectiveness is verified by microseismic monitoring and rock-burst hazard assessment. The study provides a new idea to monitor abnormal stress zone regionally and forecast dynamic disasters in the field.
The characters, the contents and their regularity of electromagnetic emission (EME) memory effect during the deformation and fracture of raw coal and molding coal are studied by the experimental methods of uniaxial compression. Its mechanism is discussed and analyzed. The results show that EME of coal or rock has an ability of memorizing previous maximal stress, longitudinal strain, lateral strain and cubical strain. The direct memory contents of coal or rock EME are the previously received maximal stress, longitudinal and lateral strain. Its memory ability rapidly weakens or is entirely lost when the previous stress level exceeds 70% ∼ 80%. The irreversibility of EME process is decided by the irreversibility of coal or rock damage fracture process. The irreversibility of coal or rock EME process is the direct reason why EME during the deformation and fracture of coal or rock has memory effect. The theory of coal or rock EME memory effect has broad application prospects in the measurement of crustal stress, the determination of different stress areas in surrounding rock of tunnel and the prediction of coal and rock dynamic disasters, and so on.
Physical information monitoring is significant for the prediction of dynamic disasters such as coal and gas outburst under unloading. The gas permeability and charge induction experiments were carried out to the gaseous coal rock under the confining pressure unloading using the charge collection device developed. The gas permeability and charge induction were found to link closely to the deformation and damage of the gaseous coal rock in the process of confining pressure unloading. With the increase of the unloading rate, a large number of micro cracks were developed, which increases the probability of partial bound charge bacoming the free charge in the internal cracj s of coal rock. Consequently, the inductive charge had a significant increase in the main rupture pricess of coal rock. The confining pressure unloading weakened the circular restrictions of coal rock. New gas channels were produced inside the coal rock and led the permeability of coal rock to increase. The streaming current was formed by the friction action between the coal particles carried by gas and the hole wall of coal rock, which contributed an additional amount of the charge signal.
We have analyzed the relations between electromagnetic emission (EME) pulse numbers and damage parameter and discussed the EME producing mechanism and electro-mechanical coupling laws of EME memory effect during deformation and fracture of coal or rock, established the EME memory effect electro-mechanical coupling model of coal or rock containing gas deformation and fracture under conditions of constant confining pressure and triaxial compression, tested and analyzed the EME memory effect electro-mechanical coupling law of coal or rock containing gas under condition of uniaxial compression. The results show that the EME memory effect electro-mechanical coupling model of coal or rock containing gas primely reflects the characteristic laws and microcosmic damage mechanism of EME memory effect during deformation and fracture of coal or rock.
In order to study the free charge migration law of different types of coal and rock samples during the process of tensile failure, the charge monitoring system for coal and rock tensile instability and failure is developed under the frame of the Brazil disk splitting test. The mechanical characteristics and free charge migration law are obtained through the real-time monitoring process of sandstone, mudstone and coal samples during tensile instability and failure. The results show that there exist free charges during the tensile process of coal and rock mass. Charge signal abnormal area is corresponding to the stress mutation process in the samples splitting test; and the free charges produced by coal is richer than that produced by sandstone and mudstone during tensile failure. Compared with sandstone and mudstone, the residual strength of coal after fracture still makes the charge signal have larger fluctuation. Charge induction law is different between tensile and compression failure of coal and rock. One of the important reasons for the abnormal charge signal is the damage localization caused by crack propagation under the tensile stress during the tensile instability and failure of coal and rock mass.
With the self developed charge induced equipment,the experimental researched effects of different confining pressure and pore pressure on charge signal during deformation and fracture of coal containing gas under triaxial compression.The results show that as the pore pressure increases,the charge induction signal weakened when the confining pressure is a certain value.The pore pressure plays a weak effect role in the charge induction signal produced during the coal sample deformation and fracture process.As confining pressure increases,the charge induction signal enhanced when the pore pressure is a certain value.The confining pressure plays an enhancement effect role in the charge induction signal produced during the coal samples deformation and fracture process.
Experiments and regular analyses on the electromagnetic emission (EME) during the deformation and fracture of loaded coal have been conducted, and the EME mechanism was discussed and analyzed. The results show that the EME can be produced during the deformation and fracture of coal, and the EME and the acoustic emission (AE) are not strictly synchronous. EME signals are stronger than AE signals, and are closely related to the coal or rock deformation and fracture process. The EME method can be used to disclose the deformation and fracture mechanism of coal or rock, to forecast earthquakes, and to investigate catastrophic dynamical phenomena of coal or rock in mines and the stability of rock and concrete buildings.
The experimental system for collecting surface potential was founded. The research was made on the characteristic of surface potential signal during the deformation and fracture of coal and concrete. The results show that surface potential signal generates during the process and exists during the whole deformation and fracture of coal and concrete. As a whole, the change tendency of the surface potential with the load is the same. When there is a break in load, the surface potential signal will change as well and the break extent of the surface potential and the load are positive correlation.
In this contribution, an effective method has been proposed to modulate the band gap of ZnO single-wall nanotube (SWNT). By adjusting the doping Cu concentration and the concentration gradient, the band gap can vary from 4.5 eV of the perfect ZnO SWNT to 1.95 eV. When the Cu concentration is fixed, the band gap decreases as concentration gradient decreases. Similarly, with the fixed concentration gradient, the band gap decreases as the concentration decreases. Thus, the band gap can be modulated through constructing appropriate impurity concentration and concentration gradient.
► The electromagnetic emission due to load-induced stress in coal-rock is presented. ► The microcosmic-scale generation mechanism of EME during fracturing is explained. ► A laboratory experiment is built to measure EME and stress induced by uniaxial loading. ► The EME amplitude and the pulse amount are in agreement with theoretical functions.
Strata pressures are common potential troubles during coal mining, serious ones can cause great catastrophe. This paper presents a new comprehensive electromagnetic radiation (EMR) monitoring technique to monitor and predict these disasters. In this paper, we studied the relationship of EMR emitted by various, uniaxially loaded, coal rock samples (coal, rock, roof–coal–floor composite) in their whole deformation and failure processes to the applied loads, and found that EMR signal is linearly related to the applied loads, while the number of EMR pulses is a third power function of the applied loads. Therefore, EMR signal is capable of reflecting the stress (load) state, deformation and fracture strength, and internal stress state of coal rock mass. Based on the above, we proposed three methods for measuring rock pressure distribution, periodic pressure, and internal stress distribution of coal rock mass on working faces, and conducted field measurements and verifications. The results showed that (i) EMR has a certain correspondence to support resistance of working faces, and can reflect more accurately the face pressure distribution; (ii) in the mining impacted area located within 100m in the front of working face, the stress distribution has unimodal and bimodal forms; (iii) EMR signal changes periodically with periodic pressure, and can be used to qualitatively observe and evaluate periodic pressure; (iv) tested with antenna in the borehole into coal rock mass, EMR signal can effectively reflect the internal stress distribution and shifting, which was verified by using traditional drilling cuttings method. In addition, EMR can also be used to check the effect of destress blasting as one of the emergency measures. In conclusion, the results are of practical significance for using EMR to monitor rock pressure and guiding safe underground mining.
An electromagnetic radiation evaluation method for the relative stress state of coal bed under stress was proposed in this paper. The stress distribution of mine roadway or working face, as well as high stress zone or stress gradient zone, was analyzed by the method. The main advantages of the technique are its characteristics of non-contact, orientability, and regional monitoring. Correlation analysis of electromagnetic radiation with relative stress was carried out in coal mines and tunnels. The results indicate that the electromagnetic radiation technology has a wide application prospect in the evaluation of mine pressure.
Based on the coupling mechanism of stress and electricity of EME of coal or rock and the theory of statistic damage mechanics,
the coupling model of stress and electricity of EME of coal or rock is established in this paper, which is modified by simulation
study on Weibull morphological parameter m. Then this model becomes more accurate and useful. The value of m and EME of different
coal or rock samples are simulated by using the modified coupling model of stress and electricity of EME of coal or rock.
The simulation results of EME of coal or rock are consistent with experiment results, which have very important significance
in numerical simulation of EME and homogeneity analysis of coal or rock.
Key wordsEME couple of stress and electricity-morphological parameter m-numerical simulation
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