The strain characteristics and corresponding model of rock materials under uniaxial cyclic load/unload compression and their deformation and fatigue damage analysis

The fatigue characteristics of rock materials are usually studied by cyclic load-unload tests, and the deformation and damage development reflect their weakening characteristics. In this paper, according to the mechanical characteristics of rock materials during load/unload cycles, the total strain can be separated into three types, that is, elastic strain, viscoelastic strain, and viscoplastic strain. The elastic strain is linear with stress, and viscoelastic strain exhibits a special behavior after unloading, the viscoplastic strain also displays its own unique features and reflects the damage in rocks. Based on their unique characteristics, we establish elastic, viscoelastic, and viscoplastic submodels, then an elastic-visco-plastic model can be obtained by connecting three submodels in series, which can reflect the development of the law of different strains. In order to verify the reliability of the model, red sandstone samples are selected for cyclic load/unload tests. The results show that the collected strain-time data are well fitted by the model. In addition, the characteristics of strain-time curves imply the deformation and damage development of rocks during load/unload cycles.