Effects of cyclic dynamic loading on the mechanical properties of intact rock samples under confining pressure conditions

A series of laboratory tests were performed to assess the effects of confining pressure on the mechanical properties and fatigue damage evolution of sandstone samples subjected to cyclic loading. Six levels of confining pressure (2.0, 10.0, 20.0, 30.0, 40.0 and 50.0 MPa) were applied during axial cyclic loading at a 1.0 Hz frequency using a MTS-815 Rock and Concrete Test System. Results from the cyclic dynamic loading tests indicated that the level of confining pressure had a significant influence on the cyclic dynamic deformation and fatigue damage evolution of the sandstone samples tested. With increasing confining pressure, the axial strain at failure increased, as did the residual volumetric strain at the initiation of dilatancy. The residual axial strains of sandstone samples obtained at a confining stress state can be described as three deformational stages, namely, the initial phase, uniform velocity phase and accelerated phase. Both the residual strain method and the axial secant modulus method proposed here could be used to describe the initial fatigue damage and degradation process of sandstone samples subjected to fatigue loading under a confining stress state; however, the latter also considers the influence of stress level on fatigue damage evolution when fatigue loads are applied. At a constant confining pressure, the shear fracture plane can form under static and cyclic dynamic loading conditions, and the higher the confining pressure, the wider the shear fracture planes become under cyclic dynamic loadings.