Study on Compression Fracture Evolution and Damage Mechanism of Coal–Rock Composite Specimen Under Different Coal Interlayer Thickness

Abstract

This study investigates how coal interlayer thickness influences the compression fracture evolution and damage mechanism of coal–rock combinations. Pure coal, pure siltstone, and siltstone–coal–siltstone specimens with 2‐ and 3‐cm coal interlayers were examined through uniaxial compression tests combined with acoustic emission (AE), digital image correlation (DIC), FLAC 3D simulation, and fractal analysis. Results show that the presence of a weak coal interlayer reduces the strength of the combination, and increasing the coal interlayer thickness from 2 to 3 cm decreases the peak strength by 19.1%. Meanwhile, the shear‐event proportion increases to 52.9%, the maximum principal strain rises from 37.36% to 50.29%, and the crack fractal dimension increases from 1.24 to about 1.33. Overall, coal interlayer thickening weakens cooperative load bearing and promotes a transition from rock‐dominated tensile cracking to coal–interface–controlled shear instability. Damage also localizes progressively from siltstone to coal interfaces.