A micromechanics based model for sandstone under monotonic and cyclic compression

This work aims to investigate some typical fatigue behaviors of a sandstone under cyclic loading. At the microscopic scale, the heterogeneous sandstone is viewed as a composite comprising a solid matrix and randomly distributed microcracks. Within the framework of thermodynamics, three main dissipation mechanisms, plastic deformation of matrix, microcrack propagation and frictional sliding, are considered in monotonic compression. However, during the cyclic compression period, the material’s progressive degradation is mainly attributed to microcrack growth and frictional sliding, which are separately described by a fatigue damage law and a non-associated flow rule. The fatigue lifetime (the number of cycles to failure) is regarded as an equivalent time with regard to the loading frequency and physical time. After calibrating the parameters, the proposed model is validated on describing mechanical behaviors of a sandstone in both conventional and cyclic triaxial compression tests. Compared with test data, the model can well predict fatigue lifetime and three-stage deformation under various loading conditions, including confining pressure, upper limit stress, and loading frequency.