Study on fracture propagation and evolution characteristics of coal samples under uniaxial compression

To investigate the propagation and evolution characteristics of coal fractures, uniaxial compression tests were conducted on coal samples using a coal sample loading system and an acoustic emission (AE) testing system. AE parameters during loading were monitored to characterize coal damage and failure behaviors. PFC2D numerical software was employed to reconstruct fractured coal samples, enabling analysis of the micro-fracture propagation and evolution process under loading. The results indicate that coal samples undergo four sequential stages under uniaxial compression: initial stage, elastic stage, yield stage, and failure stage. Correspondingly, the internal fracture propagation and evolution of coal samples exhibits distinct stage-dependent features. In the initial stage, primary fractures within coal samples gradually tend to close under external loads. The elastic stage is characterized by minimal secondary fracture generation. When the external load reaches 80% of the coal sample's peak strength, the coal sample transitions from the elastic to the yield stage, accompanied by a sharp increase in AE ring counts and the initiation of internal fracture propagation and development. During the yield stage, internal fractures propagate and develop significantly, leading to a marked increase in coal volumetric strain. In the failure stage, fracture propagation and development culminate in the formation of a macroscopic fracture surface. Numerical simulation results demonstrate that secondary fractures inside coal samples experience a process from slow growth to rapid proliferation. Fractures initiate, propagate, and coalesce near or at the tips of primary fractures, ultimately resulting in coal failure.