Mechanical behavior and damage evolution of sandstone under different fatigue loading modes

Abstract To investigate the complex disturbance effects of sandstone in goaf roofs during deep coal mining, mechanical tests and acoustic emission (AE) monitoring were carried out for sandstone specimens subjected to multiple fatigue loading conditions. The mechanical responses and AE characteristics were examined to clarify the mechanisms underlying sandstone damage. The results indicate that as the stress level increases, the growth rate of the elastic modulus decreases, indicating a gradual weakening of hardening and an intensification of damage. At higher loading frequencies, peak strength decreases, irreversible strain increases, the initial elastic modulus decreases, the average total strain energy decreases, and AE activity becomes more pronounced. Increasing stress levels also cause a reversal in the trends of average irreversible strain and the AE b -value, reflecting a transition between the dominance of hardening and damage effects. Analyses based on AE peak frequency, RA–AF values, and multifractal spectrum features show that variable-frequency loading produces more large-scale fracture events than constant-frequency loading. The number of AE event locations increases with stress level and accelerates rapidly near failure. The AE intelligent clustering results align with the RA–AF classification, highlighting the non-negligible contribution of medium-scale mixed failure. During fatigue loading, hardening dominates the compaction stage, damage dominates the plastic stage, and both interact through a synergistic–competitive coupling relationship during the elastic stage. The findings of this study provide important guidance for ensuring the safety and stability of goaf roofs in deep coal mining.