Evolution and Deterioration Mechanism of the Mechanical Behavior of Gas-Bearing Coal under Low-Amplitude Cyclic...

Engineering projects related to deep underground resource development and deep space utilization are inevitably accompanied by dynamic loading, which exerts a significant impact on the stability of rocks. This paper focuses on low-amplitude cyclic impact loading, conducts static-dynamic combined loading tests on gas-containing coal in the laboratory, and employs low-field nuclear magnetic resonance (LF-NMR) to analyze the influence of impact loading on the multi-scale structure of coal. The research findings indicate that under a high number of impact cycles, the effect of dynamic loading amplitude F on sample strength manifests as initial strengthening followed by subsequent degradation. The peak strength of the samples decreases exponentially with the increase in the number of impact cycles. As the axial static load and pore pressure increase, the peak strength changes following a quadratic function, first increasing and then decreasing. Low-amplitude impact loading has a strengthening effect on the elastic modulus of coal. Under a low number of impact cycles, the growth rate of the elastic modulus ω increases exponentially with the increase in F; as the number of impact cycles increases, ω decreases exponentially. With the increase in axial static load, ω shows a quadratic function trend of first decreasing and then increasing, while the opposite trend is observed for pore pressure. The axial irreversible deformation ε_vd of the samples shows a linear increase with the number of impact cycles, and pore pressure promotes the occurrence of irreversible deformation in coal under dynamic loading. As the axial static load increases, ε_vd rises exponentially. After the action of impact loading, the porosities of micropores, macropores, and total pores in the samples all increase significantly, while the mesopore porosity decreases.