Study on localized failure simulation and electromagnetic radiation–acoustic emission signal characteristics of roof sandstone under low-intensity impact: Based on drop-weight impact tests

To address the limited understanding of the localized failure response characteristics of coal mine roof sandstone under low-intensity impact conditions, a drop-weight impact loading method was employed to simulate actual disturbance scenarios. The mechanical response, failure behavior, and the evolution of electromagnetic radiation (EMR) and acoustic emission (AE) signals during loading were systematically monitored. The results show that sandstone undergoes a nonlinear evolution process comprising compaction, yielding, critical fracturing, and eventual instability, with tensile failure as the predominant mode. The propagation velocity of Mode I (opening) cracks is negatively correlated with EMR frequency and positively correlated with AE energy intensity. With increasing impact velocity, the absorbed impact energy shows a slight rise, while the energy absorption rate decreases significantly. Meanwhile, the failure time shortens and the size of fragmented blocks increases. Due to differences in the sequence of energy release, EMR signals exhibit a distinct “precursor response” compared to AE signals. During crack propagation, intense dipole oscillations cause the EMR to emerge earlier and rapidly develop to its peak. In contrast, the coalescence of numerous cracks leads to the rapid release of elastic energy, resulting in the peak intensity of the AE signal. “The near-synchronous peaks” of EMR and AE signals mark the transition from localized instability to macroscopic failure. Accurate identification of these signal characteristics is of great significance for elucidating the localized failure behavior of sandstone roofs under low-intensity disturbances and for enhancing the early warning capabilities for dynamic hazards in coal mines.