Deterioration mechanisms of coal mechanical properties under uniaxial multi-level cyclic loading considering initial damage effects

With the increase in global energy demand, coal remains a vital energy resource. However, during coal mining, coal often experiences both initial damage and cyclic loading, which leads to the deterioration of its mechanical properties. In this study, uniaxial multi-level cyclic loading experiments were performed on initial damage coal specimens (IDCSs) to examine their deformation evolution, energy characteristics, acoustic emission (AE) characteristics, and internal fracture distribution. The results indicate that a higher degree of initial damage leads to a shorter fatigue life of IDCSs, a more significant reduction in strength, and a noticeable decline in the deformation modulus. This indicated that the internal microfractures in the high IDCSs were more fully developed, leading to a reduction of the effective bearing area, an increase in the linear energy dissipation capacity, and the percentage of dissipated energy. Meanwhile, the AE characteristics revealed intense large-scale fracture extension in the coal specimens with higher initial damage, lower AE b -values, and an increased percentage of shear cracks. The spatial and temporal distribution of AE events demonstrated a positive correlation with initial damage. The AE signal parameters of IDCSs exhibited multifractal characteristics, further indicating a more complex failure mode in highly damaged specimens. Additionally, the three-dimensional fracture volume percentage and fractal dimension, which are quantitative measures of internal fractures, also increased with initial damage. Meanwhile, localized areas of high IDCSs were more likely to form domino structures. Ultimately, the macroscopic and microscopic deterioration mechanisms of coal's mechanical properties under the combined influence of initial damage and cyclic loading were revealed. The interaction of initial damage and cyclic loading accelerated the expansion of internal pores and microfractures, reduced inter-particle forces, and significantly deteriorated the coal's mechanical properties. This study enhances the understanding of the deterioration mechanisms in IDCSs and provides a scientific basis for coal mine disaster prevention.