Study on the dynamic expansion mechanism and behavior of coal rock fractures under impact load

Abstract

To investigate the dynamic expansion mechanism and behavior of coal rock cracks under impact loading, impact tests were conducted on side-open single-crack semi-circular plates using a large-diameter split Hopkinson pressure bar. The average crack propagation speed and the failure modes of the semi-circular coal samples were analyzed. Using ABAQUS software, the variation of dynamic fracture toughness of coal rock cracks was examined under different impact velocities and distances between pre-existing cracks and the central axis of coal rock specimens. The results indicate: (1) The greater the impact velocity, the more severe the final damage to the coal rock. The larger the distance between the pre-existing crack and the specimen’s central axis, the greater the crack deflection angle. When the pre-existing crack coincides with the central axis, the crack expands along the axis. When the crack deviates from the central axis, it initially deflects to the sides, then expands along the axis. I-II mixed-mode cracks are more susceptible to crack arrest compared to pure I-mode cracks. (2) The speed of crack propagation is relatively high during the initial stage of cracking, and then gradually decreases and fluctuates within a certain range. The maximum propagation speed of specimen H-0–5.2-A at an impact velocity of 5.2 m/s reached 842.11 m/s. The average propagation speed of the cracks increased with the rise in impact velocity. (3) When the pre-existing crack coincides with the central axis, both the dynamic initiation toughness and dynamic propagation toughness of the coal rock specimens increase with impact velocity. The dynamic propagation and arrest toughness of all specimens were lower than their dynamic initiation toughness. (4) At a constant impact velocity, as the distance between the pre-existing crack and the specimen’s central axis increases, the dynamic initiation, propagation, and arrest toughness of I-mode cracks remain unchanged, while those of II-mode cracks significantly increase, especially the propagation toughness. When the distance is x = 7 mm, the propagation toughness of II-mode cracks surpasses their initiation toughness.