Influence of notch hole wall defects on crack propagation under notch blasting

The defective cracks were prefabricated on the wall of the notch holes by using polymethyl methacrylate (PMMA) material, which were parallel or vertical to the notch, and the distance from the defective cracks to the hole center was 2, 3, and 4 mm. The influence of notch hole wall defects on the crack propagation of notch blasting was investigated by using a digital dynamic caustic experimental system with numerical simulation. At the same time, triacetone triperoxide (TATP) explosives were employed as a charge, which served to mitigate the effect of gun smoke on the dynamic caustic experimental system and to improve the experimental design. The results demonstrate that the reflection of the stress wave at parallel defects results in a downward shift in the direction of crack initiation at the notch, but the refraction of the stress wave at vertical defects has no effect on the direction of crack initiation. The presence of wall defects in the hole impedes the impact of stress waves and blast gases on the cracks at the notch, resulting in a reduction in the length, expansion rate, and strength factor values of the cracks, and the degree of inhibition is contingent upon the distance of the defects from the centre of the borehole. As the distance between the parallel defects and the centre of the borehole increases, the inhibition effect of the parallel defects on both sides of the notch cracks gradually decreases. The inhibition effect of vertical defects on the far side of the notch cracks gradually decreases, while the inhibition effect on the proximal side of the notch cracks gradually enhances. The left and right notch cracks of vertical defects are more significantly affected by the boundary reflected stress wave than those of parallel defects. The notch cracks on the left side do not exhibit a clear pattern, owing to the pre-existing reflected stress wave at the defects. In contrast, the notch cracks on the right side are substantially diminished by the boundary-reflected stress wave as the vertical defects move away from the centers of the notch holes.