Load-dependent threshold behavior and fracture toughness interrelations in coal rock micromechanics

Coal rock, a porous organic–inorganic composite, exhibits micro-mechanical behaviors that are crucial for its macroscopic engineering performance, particularly in resource extraction processes such as coalbed methane recovery and coal–gas outburst mitigation. Based on traditional mechanical tests typically yielding bulk strength parameters while overlooking critical micro-scale phenomena—such as phase-specific mechanical responses and pore–fracture interactions—this study investigates the load-dependent micromechanical properties of Longtan Formation coal from the Sichuan Basin using gradient nanoindentation (10–100 mN). Polished coal samples were tested with a Berkovich indenter, and mechanical properties were extracted via the Oliver–Pharr method supplemented with energy-based analysis. The results show that both the elastic modulus and hardness stabilize above 30 mN, with greater scatter at lower loads attributable to surface roughness and instrument sensitivity. Fracture toughness correlates positively with elastic modulus and negatively with hardness, indicating that stiffer coals resist crack propagation more effectively, while harder coals display increased brittleness. A load threshold of ≥30 mN is recommended for reliable measurements. The indentation-size effect follows Meyer’s law at low loads, whereas plastic deformation and micro-cracking dominate at higher loads, governing energy dissipation. This study provides key insights into the scaling relationship between micromechanical response and indentation load, establishing a basis for modeling multi-scale coal behavior and improving stability predictions in coal-based engineering processes. Future research should consider the influence of interfacial transition zones, adsorption–mechanical coupling, and multi-field interactions to further elucidate the underlying mechanisms.