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In this study, cyclic loading–unloading experiments on coal samples with different moisture contents were conducted. The damage mechanism was investigated through acoustic emission (AE) monitoring. The validity of the average frequency and rise angle (RA–AF) correlation for the fracture evolution of water-bearing coal samples under cyclic loading was verified. The damage evolution equation of coal samples based on energy dissipation theory was proposed. The results show that at the same growth rates of damage variables for the three water-bearing coal samples, there is a characteristic stress σ' (σ' = 5.36 MPa, N = 3) in the evolution of damage variables of the samples. Under cyclic loading, the moisture content was negatively correlated with the damage variable during the initial damage stage, while the two were positively correlated during the accelerated damage stage. The total strain energy of the water-bearing coal samples increases with the increase in the loading cycles, and the higher the moisture content, the greater is the total strain energy. The results of cyclic loading–unloading experiments demonstrate that there is a correlation between the damage process caused by moisture content and stress in coal samples, and the evolution of dissipation energy and the development of hysteresis loops are both related to the presence of the characteristic stress σ'. The RA–AF scatter plot and probability density plot feedback of each period correspond to the characteristics of damage evolution. This study verifies the feasibility of RA–AF qualitative evaluation of fracture type evolution mode of water-bearing coal samples under cyclic loading. Highlights: The coal pillar dams of the underground reservoir are simplified to the model of coal samples with different moisture contents under cyclic loading. There is a characteristic stress σ' in the evolution of damage variables of the water-bearing coal samples under cyclic loading–unloading. Based on energy dissipation theory, the equations describing the damage evolution of water-bearing coal samples are established. Qualitative characterization of the evolution of fracture types within each cycle using RA–AF correlation was studied. [ABSTRACT FROM AUTHOR]

Transforming intact coal seams into broken coals that are convenient to transport is one of the technical problems of fluidized coal mining. In this paper, cyclic freeze-thaw by liquid nitrogen (CFT-LN2) was adopted to improve the fragmentation degree of coal seams. The coal pore structure evolutions and coal breakage properties were explored using the acoustic emission (AE) parameter method (the kernel density estimation method), the fractal theory, and the nuclear magnetic resonance spectra. The main factors discussed include the number of freeze-thaw cycles, the LN2 freezing time and the moisture content of the coal sample. Variations of these factors alter the mechanical and physical characteristics and pore structures of coal samples treated with CFT-LN2. The results reveal that the kernel density estimation method can distinguish the density region determined based on the rise angle (RA) and average frequency (AF) values and visualize the rupture modes of coal samples formed after CFT-LN2. As the number of freeze-thaw cycles grows, the tensile rupture mode gradually transits to shear rupture mode; the fractal dimension gradually increases; the equivalent average size gradually decreases; and the mass of different fragment sizes tend to be more uniform. Only freezing in LN2 without CFT-LN2 cannot notably change the rupture mode or fragmentation degree of a coal sample. During CFT-LN2, the coal matrix is subjected to repeated action of contraction in LN2 and expansion at room temperature. As a result, new micro-pores are formed easily in the matrix, and micro-pores incline to connect into larger pores. Meanwhile, increasing the moisture content is conducive to coal fracturing through CFT-LN2. [ABSTRACT FROM AUTHOR]

To study the weakening mechanism of the shear characteristics of water-bearing rock joints, four types of artificial saw-tooth joints (including tooth inclination angles of 0°, 15°, 30° and 45°) were prepared in sandstone with four moisture content (0%, 1.55%, 3.1% and 4.65%). This work adopted the non-destructive water intrusion, X-ray diffraction (XRD), acoustic emission (AE) and direct shear tests to mainly investigate the water and inclination angle-induced changes in shear mechanical properties, failure modes and AE characteristics of sandstone rock joints. The results show that the sandstone specimens contain quartz (91.04%), kaolinite, illite and montmorillonite, among which the content of quartz decrease while that of kaolinite and illite increase under the action of water, except that montmorillonite is a little controversial. The shear failure modes of the tested rock joints are divided into climbing, climbing-gnawing combined and gnawing. The inclination angle, followed by normal stress, has the greatest effect on the shear failure modes, and the moisture content has the least effect by comparison. The cohesion and internal friction angle have a positive linear relationship with the inclination angle and a negative linear relationship with the moisture content. According to the test results and Mohr–Coulomb model, a shear strength calculation model of rock joints considering the effect of inclination angle and moisture content is established. The intensity of the AE signal released during specimen failure increases with increasing inclination angle and decreases with moisture content. [ABSTRACT FROM AUTHOR]

At the laboratory scale, locating acoustic emission (AE) events is a comparatively mature method for evaluating cracks in rock materials, and the method plays an important role in numerical simulations. This study is aimed at developing a quantitative method for the measurement of acoustic emission (AE) events in numerical simulations. Furthermore, this method was applied to estimate the crack initiation, propagation, and coalescence in rock materials. The discrete element method-acoustic emission model (DEM-AE model) was developed using an independent subprogram. This model was designed to calculate the scalar seismic tensor of particles in the process of movement and further to determine the magnitude of AE events. An algorithm for identifying the same spatiotemporal AE event is being presented. To validate the model, a systematic physical experiment and numerical simulation for argillaceous sandstones were performed to present a quantitative comparison of the results with confining pressure. The results showed good agreement in terms of magnitude and spatiotemporal evolution between the simulation and the physical experiment. Finally, the magnitude of AE events was analyzed, and the relationship between AE events and microcracks was discussed. This model can provide the research basis for preventing seismic hazards caused by underground coal mining. [ABSTRACT FROM AUTHOR]

• The enhancement of CFRP jackets on the stress threshold was quantitatively studied. • The coal sample microcrack development was analyzed based on RA-AF correlation. • The evolution of AE event locations in CFRP-jacketed coal samples was studied. • A damage constitutive model was established based on coal sample energy evolution. Fiber-reinforced polymer (FRP) wrapping is a potentially attractive coal-pillar reinforcement technique. To study the effect of carbon fiber-reinforced polymer (CFRP) jacketing on mechanical properties, crack evolution, and energy dissipation mechanism of coal specimens, a series of uniaxial compression tests with acoustic emission (AE) monitoring were conducted on coal specimens with 0–2 CFRP-jacket layers. The results showed that the CFRP jacket had visible effects on the core failure process of coal specimens. (1) With an increase in the number of CFRP-jacketed layers, the specimens showed stronger strain-softening characteristics in the unstable crack growth stage, and the damage stress threshold increased significantly. (2) The correlation analysis between average frequency (AF) and rise angle (RA) demonstrated that with an increase in the number of CFRP-jacketed layers, the tendency of crack classification to change from tensile mode to shear mode increased. (3) The spatiotemporal evolution of AE event locations showed that CFRP jackets changed the AE event location concentration mode from a longitudinal distribution along the coal specimen to a distribution near the fracture. Moreover, the strain energy density evolution of coal specimens with 0–2 layers of CFRP jacketing was analyzed based on the mechanical equilibrium and energy conservation of CFRP-jacketed coal specimens. The dissipation strain energy ratio was introduced to define the damage variable, and the damage constitutive model with regards to CFRP jackets was constructed; the model describes the CFRP-jacketed coal specimen stress–strain relationship for strain-hardening, strain-softening, and sudden failure. [ABSTRACT FROM AUTHOR]