Strain rate response of damage accumulation of marble under fatigue loading

The cyclic loading experiments, in which the constant amplitude cyclic loading and unloading are imposed at different stress levels, are carried out on marble samples under various confining pressures to study the effects of fatigue loading conditions on mechanical properties of rock. The results show that: (1) The rock samples are mostly like to be destroyed under high stress ratio or low confining pressure. The elastic modulus of the damaged rock samples decreases rapidly with the increase of the number of fatigue cycles, while the elastic modulus of the undamaged samples fluctuates slightly with the increase of the number of cycles before the final stabilization. (2) The axial strain rate evolution of the rock samples shows a "U" type trend with the strain development during loading. And the volume strain rate shows an inverted "L" type trend with the volume change. Both strain rates increase remarkably when the samples approach failure. During unloading, the axial strain rate evolution shows an inverted "S" shape, and the volume strain rate shows a "U" type trend. (3) At the beginning and ending stages, the strain rate exhibits positive correlation with the residual stress. The initial strain rate reflects the accumulated damage of rock samples under the previous fatigue effect. (4) After the normalization of axial strain rate at the initial stage, there is an obvious linear relationship between the axial strain rate and the damage factor, and the coefficient K decreases with the increase of confining pressure or the decrease of stress ratio. The evolution of the cyclic normalized axial strain rate and damage factor of the undamaged rock samples retracts as a hook type.