Fracture behavior and fracture surface smoothness of rock-like model subjected to axial decoupling spherical charging

The objective of presplit blasting is to achieve directional fracturing of rock and obtain a smooth fracture surface. Axial decoupling spherical charge presplit blasting (PSB-ADSC) is a novel design method. To investigate the smoothness of the fracture surface and the formation and development process of pre-cracks under PSB-ADSC conditions, a spherical charge blast load was applied to a cement mortar model with seven boreholes. The characteristics of the fracture surface and the formation process of the pre-cracks were analyzed. Firstly, fractal theory was utilized to study the spatial variation in fractal dimension on the fracture surfaces. Secondly, high-speed photography was employed to examine the formation and development of pre-cracks. Finally, based on digital image correlation method, the strain development process on the free surface was revealed. The results indicate that the PSB-ADSC method can produce pre-cracks, fracture surfaces, and half-cast holes similar to those of traditional presplit blasting. Moreover, the distribution of the charges has a significant impact on the effectiveness of PSB-ADSC. Therefore, in the axial direction of the borehole, the fractal dimension increases with the distance from the spherical charges. In the borehole diameter direction, the smoothness of the fracture surface gradually decreases with the distance from the borehole. Macroscopic cracks and strain concentration areas first appear at the central position of the specimen, where the explosive stress wave superposition is strongest. The quasi-static action of the detonation gas plays a major role in the formation of the fracture surface and pre-cracks. Once the constraint effect weakens, the pre-cracks will rapidly propagate.