Experimental study on fatigue damage evolution mechanism of hard layered sandstone under cyclic loading

The deformation and cracking of tunnel basement surrounding rock under the disturbance of train dynamic load contain complex mechanical damage problems. The fatigue mechanics tests and electron microscope scanning tests are carried out to explore the evolution mechanism of fatigue damage of hard layered sandstone under cyclic loading. The results show that there is an aging correspondence between macroscopic cracks and dynamic stress-strain curves of layered sandstone. The more obvious the layer effects, the more frequent the stress drop and the more significant the hysteresis loop transition. The peak strength presents a trend of slow decay-fast decrease-sharp increase and the fatigue life is positively correlated with the peak strength. The elastic modulus presents a trend of sharp increase-slow increase-flattening or decrease, and the dividing points of the period ratio are 28.57% and 81.81%. The fracture mode is closely related to the layer effects, and it presents tensile fracture (Ⅰ and Ⅱ types), oblique shear fracture type and compound fracture type. The tensile fracture type Ⅱ has a compression rod effect and the roughness of fracture surface is slightly smaller. The roughness of the fracture surface of oblique shear fracture type decreases significantly, and compression-shear action leads to fracture zone and smooth fracture of the composite fracture section. The cyclic loading and unloading effects result in the evolution of critical damage of layer sandstone with an evolution trend of nonlinear rapid increase - approximate linear increase - nonlinear sharp increase. The logistic inverse function damage model can well describe the critical damage laws, and the order of fatigue sensitivity is oblique shear fracture type > compound fracture type > tensile fracture type (Ⅰ and Ⅱ).