Damage Characteristics Of Coal Research Articles

Energy Dissipation and Fractal Properties of Coal under Cyclic Impact Loading

To investigate the periodic disturbance and damage characteristics of coal under different impact pressures, the stress‒strain failure and damage trends of weak, impact‐prone coal samples were analysed under different impact pressures and different numbers of impacts. Split‐Hopkinson pressure bar (SHPB) tests were conducted to study the stress‒strain curve, dynamic compressive strength, elastic modulus, failure strain, and absorbed energy responses with different numbers of impacts and different strain rates. Fractal theory and methods were applied to analyse the influence of different impact pressures and different numbers of impacts on coal sample failure to reveal the relationship between the number of impacts and the fractal size characteristics of coal fragmentation. The energy modes and failure modes of coal samples under different impact effects were analysed under dynamic loading. Regarding the energy absorption corresponding to different numbers of impacts, the energy absorption per unit volume (EAPUV) of three impacts was calculated to be 0.46 J/cm3. The number of impacts was directly correlated with the energy absorbed by the coal sample. Furthermore, the number of impacts was directly correlated with the severity of the deformation and damage of the coal sample and was inversely correlated with the fragmentation of the coal sample. The degree of coal sample fragmentation was directly proportional to the number of impacts, and the fractal dimension increased with the number of impacts. The study of the failure and fractal characteristics of coal samples under different impact pressures and different numbers of impacts provides important information for the prevention and control of coal mine rock bursts.

Study of the Mechanical Properties and Damage Characteristics of Coal after Interaction with Supercritical Carbon Dioxide

Study of the Mechanical Properties and Damage Characteristics of Coal after Interaction with Supercritical Carbon Dioxide

Study on Mechanical Properties and Crack Propagation of Raw Coal with Different Bedding Angles based on CT Scanning.

The deformation and damage characteristics of coal are the important foundation that affects the fracturing potential of coal reservoirs and the development plan of coalbed methane (CBM). To reveal the influence regulation of primary fractures and the bedding angle of coal on its failure and provide theoretical basis for CBM development, raw coal samples of no 16 coal seam in Wenjiaba Coal Mine, Zhijin County, Bijie City with different bedding angles were selected as the research object, and uniaxial compression tests were carried out on them, and CT scanning and crack reconstruction before and after sample failure were carried out. The results show that (1) the compressive strength, elastic modulus, and Poisson’s ratio of coal show a strong bedding angle effect, and the changing trend of each index is basically the same. The coal samples with bedding angles of 0 and 90° are the highest, while the coal samples with bedding angles of 30° are the lowest, and the overall distribution is an approximate “U” with the increase in bedding angle. With the increase in bedding angle of 0–90°, the failure modes of coal samples are tension-shear combined failure, shear-slip failure, and splitting tension failure in turn. (2) The observation of raw coal and CT scanning show that the primary cracks in coal samples are well developed, especially in the lower part of 0° samples, the cracks in 30° samples, 60° samples, and 90° samples are evenly distributed and develop at a certain angle with the weak bedding surface, and microcracks parallel to and nearly perpendicular to the weak bedding surface are developed in 45° samples. At the same time, banded minerals in coal and rock samples are also well developed. (3) The characteristics of crack propagation and evolution in coal samples with different bedding dip angles are significantly different. The bedding dip angles and primary cracks of coal seam have a great influence on crack propagation. With different bedding angles, the propagation modes are different. The crack propagation mainly includes two ways: forming a certain angle with bedding and extending along the bedding plane. (4) The fracture characteristic parameters of coal in the primary state and after failure have the same law with the bedding dip angle, showing a trend of high at both ends and low in the middle, which is an irregular “U”-shaped distribution and has a similar law with mechanical characteristic parameters.

Experimental investigation on mechanical properties of coal samples under different freeze–thaw cycles

Due to the extensive excavation of open-pit coal mines in northwest of China, the rock slopes formed by special environments are subjected to freeze-thaw (F-T) action, which has a certain impact on their stabilities. In order to evaluate the mechanical properties and micro damage characteristics of coal under different freeze-thaw cycles, uniaxial compression experiments combining acoustic emission tests were conducted. The results suggest that as the number of freeze-thaw cycles increased, elastic modulus of coal samples decreased, the samples showed ductile damage characteristics and initial compaction stage gradually increased. Compared with unfrozen-thawed coal sample, the compressive strength of the coal samples decreased by 23.27% after 10 F-T cycles, 31.06% after 15 F-T cycles, and 36.01% after 20 F-T cycles. The internal fissures in the coal samples transitioned from tensile fissures to shearing fissures, and the samples gradually showed tensile-shear combined failure. The final cumulative energy of the coal sample became lower, the cumulative energy duration increased and the time point of the energy surge was delayed with the increase of cyclic freeze-thaw times. A damage model based on the evolution law of the cumulative energy was established to bridge the gap between macro-micro damage mechanics.

AE and Damage Characteristics of Coal with Different Water Soaked Depths Under Uniaxial Compression

AE and Damage Characteristics of Coal with Different Water Soaked Depths Under Uniaxial Compression

Damage Characteristics Caused by Deep-Hole Blasting near Normal Fault and Its Effects on Coal and Gas Outbursts

To understand the stability of coal and rock in areas of normal faults disturbed by the dynamic loads of blasting, the damage characteristics of coal and rock within a normal fault are investigated using similarity simulation tests. The mechanism for coal and gas outbursts within a normal fault is analyzed theoretically, and the results indicate that the maximum tensile stress in the vertical direction of the blasting hole in the normal-fault model is 1.17 times that in the no-fault model. The propagation of cracks near the blasting hole produces crushing circles, and more cracks are produced in the normal-fault model, causing severe damage to the coal seam and floor rock adjacent to the upper wall of the normal fault. Meanwhile, coal on the surface of the coal seam falls off. The cracks extend to the roof rock through the footwall of the normal fault. Cracks in the adjacent strata and coal seam interpenetrate those around the blasting hole, which is a potentially dangerous area for coal and gas outbursts. The cumulative damage caused by blasting vibrations increases the extent and scope of the damage to coal and rock, and broken coal and rock provide weak surfaces and gas flow channels that can lead to dynamic gas disasters. The research results will provide a theoretical basis for gas dynamic disasters induced by blasting disturbance in normal fault structures. Based on cases of coal and gas outbursts in the Didaoshenghe Coal Mine in Heilongjiang Province, China, an important reason for such incidents is considered to be the blasting areas of normal faults being disturbed by air-powered coal drilling.

Experimental study on mechanical and damage characteristics of coal under true triaxial cyclic disturbance

A true triaxial test system was used to study the mechanical and damage characteristics of coal under tiered cyclic maximum principal stress (σ1), intermediate principal stress (σ2), and minimum principal stress (σ3). The influence of each principal stress cycle on the deformation, energy evolution and damage characteristics of coal was systematically analyzed, and the mechanical evolution characteristics of coal under the disturbance action of true triaxial excavation were revealed. In the process of true triaxial tiered cyclic loading, it is found that: the stress-strain curves of different principal stress cycles processes are quite different, but they will all cause plastic deformation or plastic hysteresis loop of principal strain in each principal stress directions of coal, and the principal strain corresponding to the cyclic principal stress direction is the dominant deformation, and its deformation is relatively large. According to the true triaxial strength curve and experimental results, the cyclic loading or unloading σ2 may cause instability and failure of coal. The variation trend of residual deformation caused by σ1 and σ3 is the same: with the increase of the amplitude of true triaxial tiered cyclic loading, the absolute value of the cumulative maximum and minimum residual principal strains increase exponentially and quadratic parabola, respectively. And the absolute value of the relative maximum and minimum residual principal strains decrease first and then increase, showing U-shaped and arch shaped changes, respectively. While the changes of cumulative and relative residual intermediate principal strains are all small. The total dissipation energy of coal increases exponentially with the increase of the amplitude of true triaxial tiered cyclic loading. Combined with the energy dissipation characteristics, the damage variable equation of coal under true triaxial cyclic loading is newly defined. With the increase of the amplitude of true triaxial tiered cyclic loading, the damage variable presents an obvious S-type change trend of deceleration acceleration re-deceleration.

Damage characteristics of coal under different loading modes based on CT three-dimensional reconstruction

Due to the heterogeneous characteristics of coal materials, it is difficult to carry out repetitive tests and quantitative analysis of internal damage of coal. Therefore, the damage characteristics of coal under uniaxial and true triaxial loading was studied by numerical simulation based on 3D reconstruction modeling, and the corresponding mechanics experiment was used for auxiliary parameter validation. Firstly, the CT scanning system was used to scan the coal specimen, and the Otsu method was used to segment the original CT images to extract the three-dimensional fracture network and the distribution characteristics of mineral components. Secondly, the internal fracture and mineral distribution network model of the coal was reconstructed and imported into FLAC3D for modeling, and the numerical model was validated by fractal geometry method. The average error of fractal dimension between the original CT image and the corresponding numerical model is 1.422%, showing that the numerical model is basically consistent with the CT image of the coal, and the numerical model could reflect the distribution characteristics of the internal fractures and mineral components of the coal. Then, the basic mechanical parameters and acoustic emission (AE) response characteristics of CT reconstructed coal samples were obtained. And the mechanical parameters of the numerical model were calibrated. It can be concluded that the stress–strain curve of the coal is basically consistent with that of the numerical model, and the AE response characteristics of the coal are basically consistent with the evolution of the plastic zone of the numerical simulation, indicating that the numerical model and parameters are reasonable in this work. Moreover, the damage variable evolution of coal specimen in uniaxial compression and true triaxial loading was studied. Finally, the damage equation of coal under uniaxial compression and the relationship between damage variable and body strain energy under true triaxial loading were established.

Simulation and Experimental Study on the Discontinuous Dynamic Impact on Unidirectional Confined Coal‐Rock Damage

The deformation and the damage characteristics of coal and rock under different restraint static load and different impact velocity were studied with the FEM software LS‐DYNA. Based on this, the influence of impact load on the damage of coal rock under one‐dimensional constraint condition was studied by using the self‐developed constrained pendulum impact dynamic loading test device and ultrasonic testing device. The results show that the deformation of coal and rock increases with the increase of impact velocity and decreases firstly and then remains constant with the increase of constraint static load; when the constraint static load exceeds the compression strength, the displacement increases rapidly and the coal collapses at a strike; the damage quantity of coal and rock has a cumulative effect, and the damage quantity increases with the increase of impact number; when the constraint static load is identical, the increase of impulse contributes to microcrack propagation, and the damage quantity of coal and rock accelerates with the increase of single impact impulse; when the impulse is identical, the constraint static load restrains the microcrack propagation, and the damage quantity of coal and rock decelerates with the increase of constraint static load; the complete damage quantity range of coal and rock is 0.65∼0.75. In order to fully destroy the coal and rock, if the single horizontal impulse is greater, the number of shocks needed is less; if the constrained static load is greater, the more impact times are needed.