Experimental Study on Fatigue Mechanical Properties and Acoustic Emission Behavior of Hard Layer Sandstone under Cyclic Disturbance

Abstract

The fatigue deformation of sedimentary rock at tunnel bottom under the train disturbance load contains the mechanical damage behavior. The fatigue mechanics test and acoustic emission (AE) monitoring are carried out to investigate the fatigue deformation and AE response characteristics of layer sandstone under fatigue load. This study showed that there is a time-sensitive correspondence between hysteresis loop and layer degradation effect. The hysteresis loop undergoes intensive distribution, strain transition, sparse distribution and fracture instability, and the continuous stress drops induces a significant strain transition. The layer structure has a significant deterioration effect on bearing strength and fatigue life. The cycle peak strength is degraded by 15.05%-28.19% compared with static peak strength. The fatigue life is positively correlated with peak strength and fatigue life decreases first and then increases with the layer angle increasing. The fracture modes of layer sandstone has anisotropic properties, including tensile fracture I, tensile fracture Ⅱ, shear fracture and composite fracture, and the fractal dimension is negatively correlated with layer degradation effect. AE amplitudes at low-medium-high stress levels are distributed in banded shape, tower shape and semi-tower shape, showing a time-corresponding relationship with the stress paths. There is a competitive relationship between compaction strengthening and fracturing damage. The evolution stages of AE damage variable include damage calm phase, damage acceleration phase and damage burst phase, and 0.20 is the starting point of damage acceleration and mutation damage (DAE−C) near the peak stress indicates a complete fracture. The stronger the layer deterioration effect, the smaller the DAE−C. The result may be represents a useful complement to mechanical properties of sedimentary rocks and is of important for ensuring the rock engineering safety.