Crack Propagation and High Efficiency Rock Fragmentation Mechanisms in Rotary Percussive Drilling

In deep hard-rock rotary–percussive drilling, energy efficiency is often constrained because only a fraction of the input energy is converted into effective volumetric fragmentation. The combined action of high-frequency impacts and bottom-hole rotational loading promotes localized crushing, excessive fines generation, and non-directional crack growth, so the crack network fails to evolve into a dominant fracture system capable of effectively detaching rock fragments; instead, substantial energy is dissipated through secondary crushing and high-frequency vibration, yielding only marginal gains in fragmentation while increasing the risk of borehole-wall damage. To address this limitation, the present study relates drilling efficiency to bottom-hole fracture modes by clarifying how crack connectivity and propagation mechanisms govern effective breakage work. A coupled tooth–rock stress-field and fracture-evolution framework is developed to systematically evaluate how tooth geometry, cutter layout, and operational parameters steer crack-network development, and an energy-based metric is formulated to interpret trends in specific energy. The framework is validated against laboratory experiments and dynamic numerical simulations using cuttings size distribution, energy consumption, and borehole-wall damage as verification targets; based on the validated model, practical design and operating windows are identified to increase the fraction of effective breakage work and provide actionable guidance for high-efficiency drilling.