Study on the Crack Propagation Mechanism of Single Cracks in Red Sandstone Based on Stress Field Evolution Characteristics

To elucidate the influence of fracture dip angle on the mechanical response of red sandstone and the mechanism of crack evolution, this study employs the discrete element software PFC2D to construct a single-fracture sandstone model. Numerical uniaxial compression tests are conducted under varying fracture dip angles α. By integrating the ‘stress field–microcrack’ coupling criterion with fracture initiation displacement field characteristics, the study quantitatively derives the segmented enhancement patterns of peak strength and fracture initiation stress, systematically elucidating the mechanisms of microcrack initiation and propagation. Results indicate that both peak strength and initiation stress increase overall with rising α, exhibiting a gradual increase when α ≤ 45° and a significant rise when α > 45°. Crack propagation follows a pattern in which wing-type cracks initiate first, followed by dominant secondary coplanar cracks that achieve full penetration. The initiation location shifts from the mid-section of the fracture (α < 30°) to the fracture tip (α ≥ 30°) as α increases. Microcrack numbers exhibited exponential growth, though tensile cracks predominated, leading to tensile failure of the rock specimens. Stress evolution and crack propagation initially showed non-coincidence at α ≤ 30°, where stress concentration at the tip was counteracted by elevated failure thresholds due to high normal clamping forces, favouring initiation in the low-constraint zone near the fissure surface. As loading progressed, crack evolution progressively aligned with high-stress pathways. At α ≥ 45°, crack propagation closely followed the evolution path of high-stress zones. During initial cracking, the instantaneous displacement of particles around the fissure decreased with increasing α, indicating diminished crack-induced initiation. As the fissure dip angle increased, the strain required for the first acoustic emission signal grew, the strain range for emission events narrowed, and the peak ringing count increased. This study elucidates the influence of fracture dip angle on rock microcrack mechanisms, providing theoretical support for assessing the stability of fractured rock masses and ensuring the safety of underground engineering construction.