Dynamic Cracking Behaviors and Energy Evolution of Multi-flawed Rocks Under Static Pre-compression

Understanding the dynamic cracking behaviors and energy evolution of flawed rocks is highly relevant to underground rock engineering. In this study, multi-flawed rock specimens are tested under coupled static–dynamic compression using a modified SHPB system combined with high-speed photography and DIC monitoring. We systematically investigated the influences of pre-stress ratio, flaw inclination angle and strain rate on the dynamic progressive cracking mechanism and energy evolution of multi-flawed rocks. Experimental results show that the dynamic/total strength generally increases with increasing strain rate, featuring evident rate-dependence. With increasing flaw inclination angle from 15° to 60°, the dynamic/total strength initially decreases and subsequently increases with the minimum achieved around 45°. With the pre-stress ratio increasing from 0.2 to 0.8, the dynamic strength persistently decreases while the total strength initially increases and subsequently decreases with the maximum achieved at 0.6. Furthermore, based on the displacement trend lines method, a novel crack classification method is developed to analyze the progressive cracking mechanism of multi-flawed rocks using high-speed photography and DIC technique. Generally, mixed cracking dominates the failure of multi-flawed rocks under coupled static–dynamic compression. With increasing flaw inclination angle form 15°–60°, the predominant cracking mechanism changes from mixed tensile-shear cracking to mixed compression-shear cracking. The increasing pre-stress ratio promotes shear cracking under lower flaw inclination angles while facilitates tensile cracking under higher flaw inclination angles. In addition, the energy evolution for coupled static–dynamic SHPB tests is re-evaluated and a new energy calculation formula is proposed. The results show that the increasing strain rate reduces the energy utilization while promotes the energy dissipation density. Both the energy utilization and energy dissipation density increase with increasing pre-stress ratios.