Fatigue Deformation Characteristics and Damage Model of Sandstones Subjected to Cyclic Loading: Implications for Fatigue Life Prediction

To study the fatigue deformation and damage characteristics of sandstone during multilevel cyclic loading and unloading (MCLU) with varying lower limits of stress (VLLS), this research conducted an MCLU test with VLLS by varying the main factors influencing fatigue, i.e., confining pressure, stress amplitude, and loading frequency. By fitting the evolution process of the axial strain, this study clarified the effects of confining pressure, amplitude, and frequency on the fatigue deformation of sandstone. The results demonstrate that the fatigue resistance of rock can be improved by increasing the confining pressure and frequency and decreasing the amplitude. During the MCLU with the VLLS, the development of fatigue deformation in the rock shows threshold effects and is mainly divided into an elastic stage and a rapid fracture growth stage. A variation in confining pressure mainly changes the rate of increase in axial strain in the rapid fracture growth stage, whereas an increase in amplitude can increase the rate of increase in axial strain in each stage and advance the rapid fracture growth stage. However, the influence of frequency lies in the acceleration of the increase in axial strain in the elastic stage due to a reduction in period duration, while such an effect weakens with the increase in cycle number. The analysis of the fatigue life and damage state of rock shows that the accumulation of local plastic deformation is the main factor causing fatigue failure, and the residual life and current fatigue level can be predicted with the axial strain. Moreover, the fatigue life of rock can be effectively increased when the amplitude decreases. According to the fracture characteristics, confining pressure mainly changes the size of the dimples on the fracture surfaces and then changes the rock plasticity. With increasing amplitude, the fracture characteristics gradually develop from stepped to curved to planar. An increase in frequency will increase the proportion of transgranular fractures. The research results provide an important scientific basis for the future design and construction of geotechnical engineering.