Temporal-spatial evolution and source parameters quantitative inversion of microcracks in quartzite under post-peak cyclic loading

Deep underground engineering disasters stem from the cross-scale evolution and catastrophic failure of rock masses, with the occurrence and severity of such disasters being contingent upon the dynamics of crack propagation and energy release characteristics during the post-peak stage. To fundamentally elucidate the black-box processes involved in post-peak crack propagation and fracturing mechanisms in loaded rock, this study conducted mode-I fracture tests on quartzite beams under post-peak cyclic loading conditions. Through acoustic emission (AE) analysis, we investigated the spatiotemporal evolution of microcracks and developed a quantitative inversion method for microcracking source parameters based on forward modeling of the entire process, from source generation and wave propagation to signal reception. The evolution patterns of critical microcrack parameters, including type, scale, orientation, and damage, were systematically revealed, with mesoscopic inversion results validated against macroscopic measurements. Key findings include: During post-peak cycling, most microcracking events occur between the pre-peak damage stress and the post-peak stress drop period of the loading stage, with large-magnitude events concentrating in the upper-middle region of the crack. The AE Felicity ratio progressively decreases with the number of cycles, indicating weakening spatiotemporal memory effects. At the mesoscale, individual microcracks predominantly exhibit mixed-mode mechanisms. However, the global volume decomposition of microcracks shows that mode-I tensile components account for over 90% of the accumulated damage volume, where large-magnitude microcracks demonstrate horizontal tensile motion aligned with macroscopic fracture planes, confirming tensile-dominated mechanisms in three-point bending tests. Damage tensor analysis based on microcrack orientation and size reveals that preferentially oriented microcrack clusters dominate macroscopic fracture propagation. The increasing ratio between preferential and random damage components serves as a precursor for fracture progression. Additionally, the scale of microcracks and the energy dissipation range were quantitatively estimated. The average size of microcracks was found to be consistent with grain sizes, while the cumulative opening displacements of microcrack clusters exhibited a linear positive correlation with macroscopic crack opening displacement, thereby confirming the reliability of the inversion results. This research provides a quantitative methodology for representing rock fracture morphology, offering significant implications for understanding rock failure mechanisms and enhancing disaster prevention in rock engineering.