Investigation on fracture evolution and acoustic emission of rocks during drilling under different stress reconstruction levels

Understanding the damage evolution of stress-reconstructed rocks in rotary drilling is critical for assessing rock fragmentation in deep engineering. This research adopts brown-red sandstone, marble, purple sandstone and granite to study how lithology, confining pressure and stress reconstruction affect drilling-triggered crack growth. Tests with and without stress reconstruction were performed at stress levels of 10 %–50 % of each rock's uniaxial compressive strength. Combined digital drilling, triaxial compression and real-time acoustic emission monitoring were used to analyze drilling characteristics, acoustic emission ringing counts and b-value variation. Stress reconstruction reshapes rock initial damage, energy storage and fracture propagation. Reconstructed rocks have stronger nonlinear ringing fluctuations, higher confining pressure sensitivity and uneven energy release. Average ringing counts reflect steady drilling damage accumulation, whereas peak values correspond to sudden energy release from major crack propagation. Marble is the most sensitive sample, with average and peak ringing counts increased by 45.8 % and 21.0 %. Without stress reconstruction, stable b-values indicate uniform small crack initiation and slow expansion. Post-reconstruction b-values drop and fluctuate sharply, concentrating crack growth and coalescence into macro-fractures, notably in hard brittle rocks. Lower drilling thrust and torque align with acoustic emission changes, proving drilling resistance degradation comes from internal stress-induced damage. This work links microcrack behaviors to macroscopic drilling properties to evaluate stress history, drillability and damage for deep rock engineering.