Strength and Damage Response of Sandstone and Granodiorite under Different Loading Conditions of Multistage Uniaxial Cyclic Compression

Understanding the strength and deformation response of hard and soft rocks under cyclic loading is very important for assessing fatigue in rock engineering structures. The effect of cyclic loading on rocks is different and more complex than that of monotonic loading. In this paper, the deformation and strength response of two mineralogically and microstructurally different rocks, sandstone and granodiorite, are investigated by conducting multistage uniaxial cyclic compression loading tests. It is shown that these two rocks behave differently under cyclic loading mainly because of their microstructures. The stress-strain hysteresis loops of sandstone samples showed three phases of development, while there were two phases for granodiorite samples. The fatigue strength threshold of granodiorite as hard rock is shown to be less than that of sandstone as soft rock. Residual lateral strain develops faster than residual axial strain during the cyclic stages. Axial and lateral strains after each step of loading, as well as residual strain, showed an increasing trend with maximum stress levels and loading amplitude. We discuss how rock damage develops in a rapid manner in lower loading frequencies and find that rock damage propagation can be assessed by ultrasonic measurement. The failure modes of the rock samples were also evaluated. We found that the damage mechanism of rocks under cyclic loading is highly dependent on loading frequency. In addition, the failure modes of the two rock types under cyclic loading differ not only from those under the monotonic conditions but also from those under different loading conditions. This study contributes to a deeper understanding of the deformation response and damage evolution of hard rocks and soft rocks under fatigue processes.