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The model of compressed atoms and electron emission of rock fracture

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Abstract

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. ...
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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 [9]. Guo et al. believes that the stress concentration around the crack tips leads to the contraction of atom and increase in electron energy [10]. e electron then flees and therefore causes the charge separation. ...
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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 [19][20][21][22]. Based on the electromagnetic emission observed during the failure process, a compressed atomic model is proposed [23][24]. (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 [25][26][27]. ...
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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 [1], 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 [2][3][4][5][6], 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,[21][22][23][24]. 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. ...
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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 [23][24][25]. 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 [44], the movement of positive and negative charges along the sides of fractures [45], electrokinetic effects [46], piezoelectric effects [25], compressed atom and electron emissions [47], and motion from the variable velocity of charged particles [24]. Electrokinetic effects always involve fluids [46], 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. ...
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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.
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