Experimental study on the propagation behavior of multiple directional cracks under delay blasting

Refined delay blasting using digital electronic detonators has been widely used in rock breaking engineering, where notched borehole could control the direction of explosion-induced crack propagation. This study investigates how delay time affects the propagation behavior of multiple directional cracks through dynamic caustics experiment for delay blasting on specimens with multi-directional notched boreholes to unveil crack propagation path and mechanical properties of crack tip. Results show that multi-directional notched boreholes could effectively control the direction of crack propagation. In both groups of specimens, the main cracks generated by the explosion extended along the notched direction, with cracks from the first borehole longer than those from the second. At 30 μs delay time, cracks between the boreholes showed a "hook-like" distribution at their convergence point, with a larger deflection angle in the cracks from the rear borehole. Propagation velocities, dynamic stress intensity factors and dynamic energy release rates exhibited consistent trends, showing significant increase when the cracks approached each other. Cracks from the first borehole displayed pronounced fluctuations. Both cracks transformed into Ⅰ-Ⅱ mixed mode and then propagated alternately. At 180 μs delay time, the directional control provided by the second borehole's notch was weakened horizontally and formed a "Y-shaped" crack distribution, significantly affecting crack propagation velocity of the first borehole. This study offers references for optimizing delay times and improving rock breaking efficiency.