Fatigue damage characteristics and degradation mechanism of mechanical properties in shale under different water saturation durations

The investigation of the effects of water saturation and fatigue stress on rock's mechanical properties is crucial for designing and maintaining water-rich roadways. Fatigue loading and uniaxial compression tests were performed on shale samples subjected to varying water saturation durations. Mechanisms governing the deterioration of the samples' mechanical properties due to water saturation and fatigue loading were analyzed. Both factors reduced the peak strength and peak strain of the samples. Longer saturation durations increased global damage. As saturation duration increased, the stress-strain curve under fatigue loading showed greater irregularities, and fatigue damage between hysteresis loops progressively diminished. The effects of water saturation and fatigue damage on mechanical properties were interdependent. Their combined impact was less than additive, following a “1 + 1 < 2” pattern. Fatigue loading's influence on deformation, strength, and damage decreased with increasing saturation duration. Prolonged saturation also reduced the cumulative Acoustic Emission (AE) parameters and plastic strain under fatigue loading. In terms of failure characteristics, under the same proportion of fatigue loading, fatigue failure degree was negatively correlated with damage extent from saturation. By integrating water saturation duration, AE parameters, and plastic strain, a water saturation and fatigue damage model based on the complex plane was developed. This model, with cumulative AE parameter analysis, identified a threshold for the interaction between water saturation and fatigue-induced damage. Beyond this threshold, the sample's sensitivity to fatigue damage decreased.