Acoustic emission statistical characteristics and fracture evolution of sandstones with different grain sizes

This study investigates the effects of grain size on the acoustic emission (AE) statistical characteristics and fracture evolution of fine-, medium-, and coarse-grained sandstones. Uniaxial compression tests with real-time AE monitoring were conducted, integrated with avalanche dynamics, fractal theory, RA-AF crack classification, and scanning electron microscopy fracture surface observations. Results show that all three sandstones exhibit four typical loading stages (compaction, elasticity, crack propagation, and post-peak instability), with uniaxial compressive strength decreasing as grain size increases. The probability density of AE absolute energy, Omori aftershock decay, and waiting-time distributions all follow power-law behavior, and their exponents increase synchronously with grain size, indicating a transition in fracture energy release from high-energy concentrated events to multi-source, small- to medium-scale events, accompanied by enhanced short-term clustering and local abruptness. Macroscopic fractures are predominantly tensile, with the tensile crack proportion rising with grain size. The microscopic failure mechanism shifts from relatively continuous intergranular propagation in fine-grained sandstone to a composite mode involving intergranular cracking, transgranular cracking, interface debonding, and grain pull-out in coarse-grained sandstone. The correlation dimension exhibits a fluctuating increase during loading, a pre-peak decline, and low values at instability, reflecting a progressive scaling process from dispersed microcrack development to localized coalescence of major macrocracks. These findings provide a statistical physics basis for understanding fracture evolution in heterogeneous rocks and offer reference value for rock engineering stability assessment.