Propagation mechanism of stationary and dynamic cracks under reflected explosion stress waves

The influence of reflected explosion stress waves on dynamic crack propagation behavior , as well as the connection between dynamic cracks and pre-existing cracks, was studied using dynamic photoelastic experiments. A high-speed camera was used to capture the full field photoelastic isochromatic fringe pattern of horizontally expanding explosive cracks. The explosive crack is a directional crack generated by detonating explosives in a blast hole containing a horizontal V-shaped groove. The propagation process of explosive cracks can be divided into three different stages. In the first stage, explosive detonation produces dynamic cracks. Simultaneously incident explosion stress waves propagate and interact with prefabricated vertical cracks. In the second stage, the reflected explosion stress waves interact with dynamic cracks. In the third stage, dynamic cracks connect with pre-existing cracks and release unloading stress waves. Considering both singular and non-singular stresses in the near-crack-tip region, three far-field-controlled constant stresses were adopted. The mixed mode stress intensity factor of dynamic cracks under the action of reflected stress waves was analyzed and calculated using the Newton-Raphson iteration method. The results indicate that the leading edge of the reflected pressure wave acts as a stretching wave and the trailing edge behaves as a compression wave. The tensile component of the reflected pressure wave applies tensile stress to the crack tip, increasing the dynamic stress intensity factor K_Ⅰ and promoting crack propagation. On the contrary, the compressive component of the reflected pressure wave applies compressive stress to the crack tip, resulting in a decrease in the dynamic stress intensity factor K_Ⅰ and suppressing crack propagation. Reflected shear waves can cause unstable crack propagation. It causes changes in the direction and velocity of crack propagation, resulting in a wavy crack trajectory. After the penetration of dynamic cracks and prefabricated cracks, the elastic energy stored near the crack tip is rapidly released to generate unloading waves. Due to the action of the unloading wave, stress is concentrated at the tip of the pre-existing crack, causing the formation of a secondary crack at the tip of the pre-existing crack.