To study the mechanical properties and energy evolution characteristics of the shallow fractured coal in the Western China mining area, a series of triaxial compression tests was carried out on fractured coal specimens. The stress–strain curve, mechanical properties and failure characteristics were analyzed. Then, the fractal characteristics and energy evolution law were investigated. Results show that with the increase of prefabricated crack number, the post-peak stage of the stress–strain curve changed from a linear drop to a stepped drop, indicating that the damage degree tends to moderate. Both the elastic modulus and peak strength decreased as the prefabricated crack number increased, whereas the peak strain did not exhibit a clear trend. The failure mode changed from overall tensile failure to tensile-shear mixed failure as the prefabricated crack number increased. With the increase of prefabricated crack number, the failure shape dimension increased linearly. The total energy and elastic energy decreased gradually with the increase of prefabricated crack number, whereas the dissipation energy increased gradually. The results can provide a better understanding of the failure mechanism of fractured coal and its application for control design.
To study the influence of fissure angle on the rock damage process and energy evolution characteristics, uniaxial cyclic loading and unloading tests were conducted on fractured rock specimens with different prefabricated fissure angles. The stress–strain curves, mechanical properties, and failure characteristics were analyzed. Subsequently, the energy evolution characteristics and failure mechanisms were investigated. The results showed that the stress–strain curves of fractured specimens fluctuated in the pre-peak phase and rapidly declined in post-peak phase. The peak stresses and strains of fractured specimens initially decreased and then increased with an increase in the fissure angle, whereas the elastic modulus first increased and then decreased. With an increase in the fissure angle, specimen failure changed from shear damage to tensile damage. The input, elastic, and dissipation energies of fractured specimens non-linearly increased with an increase in cyclic loading and unloading. As the number of cycles increased, the energy density decreased in segments with an increase in the fissure angle, and there was a rapid increase in the dissipation energy density before failure occurred. The results can provide a reference for the study of fractured rock failures and their prevention and control design in the field.
Uniaxial compression tests were performed on coal specimens with five fissure angles to study the mechanical behaviors and acoustic emission (AE) characteristics of fractured coals. AE and video monitoring techniques were used to examine crack propagation in the fractured specimens. The stress–strain curves, mechanical properties, and cracking processes at different fissure angles were analysed. The AE counts and dominant frequency characteristics, during the failure processes of the specimens, were investigated. In addition, five types of AE signals were classified according to the AE spectral frequency analysis, and low-frequency–high-energy signals were used to accurately predict the brittle fracture processes of the fractured specimens. Finally, a comparison with sandstone specimens revealed the influence of primary cracks on the strength of brittle coal specimens containing preexisting fissures under uniaxial compression. The test results are helpful to elucidate the mechanical behavior and failure mechanism in underground engineering, such as hydraulic slotting in mines.
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