Experimental study on the influence of prefabricated fissure size on the directional propagation law of rock type-I crack

To investigate the influence of prefabricated fissure size on the directional propagation law of rock type-I cracks, a simple device for simulating rock crack directional propagation was developed. Subsequently, loading tests for white sandstone samples with different prefabricated fissure lengths and widths were performed. The evolution laws of the deformation field and acoustic emission (AE) were analyzed during the entire crack propagation process. Finally, the influence of fissure size on the crack directional propagation mechanism was elucidated. The obtained results indicate that: The initial crack angle was mainly influenced by the prefabricated fissure width. The initial crack angle increased with the increase in the prefabricated fissure width. The crack directional propagation process can be divided into four stages: local strain-band initiation, local strain-band development, crack initiation, and crack coalescence. The crack maximum relative strain increases with an increase in the prefabricated fissure length but decreases with an increase in the prefabricated fissure width. This phenomenon indicates that increasing the prefabricated fissure length is advantageous to crack propagation at the fissure tip. The AE evolution process includes quiet, slowly increasing, booming, and decreasing periods. When the pre-fabricated fissure length increased, the durations of the quiet and slowly increasing periods decreased, but the durations of booming and decreasing periods increased. However, the durations of quiet, slowly increasing, and booming periods decreased as the prefabricated fissure width increased. In other words, increasing the pre-fabricated fissure length can enhance the stability of crack directional propagation, but increasing the pre-fabricated fissure width leads to the instability of main crack propagation path. In underground engineering practice, increasing the prefabricated fissure length and decreasing the prefabricated fissure width may be excellent approaches to realizing the crack directional propagation of rock or coal fracturing.