Influence of Loading Rate on the Failure Characteristics of Composite Coal–Rock Specimens Under Quasi-static Loading Conditions

Interbedded rock layers are a typical geological structure in coal measures. In underground mining practices, the disturbance resulting from the extraction of long-wall panels changes the loading rate of roof–coal structures. To investigate the mechanical behavior and failure characteristics of roof–coal structures under different loading rates, composite coal–rock specimens with a height ratio of 1:1 was prepared and loaded under uniaxial compressive conditions with different loading rates. Acoustic emission signals and the deformation process of sandstone and coal were monitored. The fracture morphology was analyzed by scanning electron microscope (SEM), and the mean diameter of debris (da) was calculated to quantitatively evaluate the fragmentation. It is found that the sensitivity of the mechanical behavior to heterogeneity tends to decrease with the loading rate. The higher the loading rate, the less likely localized failure occurs in the pre-peak stage, the smoother the axial stress–axial strain curves before peak stress, and the greater the uniaxial compressive strength and Young’s modulus. The higher the loading rate, the more brittle the post-peak behavior and the severer the damage of the coal sample at the final loading stage. The coal sample tends to fail through intragranular cracking under a high load rate and intergranular cracking under a low loading rate. The sandstone sample exhibits deformation rebounding in the post-peak stage, providing direct evidence of energy transition from strain energy stored in the sandstone to cracking energy of the coal sample during coal failure. The higher the loading rate, the greater the rebounding speed of the sandstone, leading to severer damage to the coal sample.