Scaling local instability to critical failure of coal under quasi-static loading through combined acoustic emission and electromagnetic radiation analysis

Catastrophic failure in geomaterials emerges from multi-scale processes that span diverse spatial and temporal dimensions. However, the precise mechanisms governing this progression from microscale instabilities to macroscopic rupture remain poorly understood. Here, we demonstrate that coal failure under quasi-static loading follows deterministic scaling patterns that connect microscale instabilities to macroscopic rupture through systematic precursor sequences. By simultaneously monitoring acoustic emission (AE) and electromagnetic radiation (EMR) during uniaxial compression, we reveal that these multi-physical signals capture complementary aspects of the damage evolution process, with EMR exhibiting heightened sensitivity to incipient microcracks during early deformation stages. Hudson plot analysis of over 1000 AE events demonstrates an unexpected oscillation in fracture mechanisms across loading phases, with tensile-dominated regimes (55% at failure) alternating with shear-dominated intervals, contradicting conventional failure models. High-resolution spatiotemporal mapping of AE hypocenters shows a transition from dispersed, vertically aligned microcracking to organized diagonal failure planes, reflecting the transformation from distributed damage to localized strain concentration. Crucially, we identify distinct stress drop signatures (Δσ = 0.3–2.0 kN) that precede ultimate failure under laboratory conditions, establishing a quantifiable link between microcrack coalescence dynamics and macroscopic stability thresholds. These findings provide a practical framework for interpreting precursory signals in geological materials and have potential implications for forecasting catastrophic failures in underground mining environments.