Investigations on Fatigue Properties of Red Sandstone under Positive and Negative Pure Bending Loads

Brittle materials such as rock and concrete may be subjected to positive and negative cyclic loading effects such as earthquakes and construction disturbances, and bending damage is one of the most likely forms of damage. A four-point bending fixture was designed to conduct fatigue tests with positive and negative stress ratios under pure bending to investigate the influence of cyclic loading on the form and mechanism of damage. The fracture energy calculation equation based on crack mouth opening displacement (CMOD) was derived, and the fracture criterion based on fracture toughness and fracture energy was established, which was in good agreement with the experimental results. Under the pure bending fatigue load, the hysteresis loop of the P-CMOD curve shows a loose-dense-loose law, and the damage curve can be divided into three stages of initial, stable, and accelerated law. Stress level and stress ratio are two main factors affecting fatigue life. The fatigue life of negative stress ratios is greater than that of positive stress ratios because there is a crack closure process in negative stress ratios, which affects the fatigue life. The final damage occurs for positive stress ratio specimens when the fatigue dissipation energy is greater than the static fracture energy. Static CMOD starting and final values were introduced to define the fatigue damage variables, and a damage evolution model was established better to simulate the evolution with positive and negative stress ratios.