Experimental Study on the Influence of the Reflected Explosive Stress Waves on the Dynamic Crack Propagation characteristics

Using dynamic photoelastic experimental method, the effect of reflected explosive stress waves on dynamic cracks and the penetration of dynamic crack propagation in specimens containing cracks are studied. The full-field photoelastic fringe series of horizontal explosion cracks generated by cutting explosives in specimens containing cracks were recorded by a high-speed camera. From the initiation of the explosion to the penetration of the crack, there are three stages in total: Firstly, the explosion from the cutting explosive generates the crack and the incident blast stress wave propagated and acted on pre-existing vertical crack; Secondly, the reflected blast stress wave interacted with the crack; Thirdly, the propagating crack penetrated to the pre-existing crack and released unloading stress waves. By comprehensively considering the singular and non-singular stresses at the near- crack-tip region, three constant stresses controlled by the far field were adopted. Newton-Raphson iteration method was applied to analyze and calculate the stress intensity of mixed modes in dynamic crack initiation under the reflected stress wave loading. The results show that the leading wave of the reflected pressure wave (PrP) is a tensile wave, while the trailing wave is a compressive wave. The tensile wave of PrP applies tensile stress to the crack tip, increasing the dynamic intensity factor and promoting crack propagation. Conversely, the compressive wave of PrP applies compressive stress to the crack tip, reducing the dynamic stress intensity factor and inhibiting crack propagation. The reflected shear wave (PrS) causes unstable propagation of cracks, including changes in direction and velocity, resulting in a wavy outward expansion of the cracks. After the dynamic crack penetrated to the pre-existing crack, the stored elastic energy near the crack is rapidly released in the form of unloading waves. Stress concentration occurs at the tip of the stationary crack due to the unloading wave, inducing the generation of secondary cracks in the pre-existing crack.