This work aims to reveal the physical process and nonlinear mechanical behaviors of marble under the fatigue cyclic and triaxial confining pressure unloading conditions. The GCTS RTR 2000 rock mechanics testing machine was first used to perform the triaxial confining pressure unloading tests for the marble samples that experiencing different fatigue disturbance, then post-test CT scanning was conducted to visualize the crack pattern of samples. The results show that the loading and unloading stress-strain curves were not overlapped at the fatigue disturbance stage, the formation of hysteresis cycle indicates the occurrence of irreversible plastic deformation, and the plastic deformation increase with increasing cycles. Before sample failure, the elastic modulus at the loading and unloading were approximately equal, this reflects the hard-brittle characteristics of the marble. In the unloading confining pressure stage, the changes of the lateral and volumetric deformation were larger than the axial deformation. In addition, the change rate of the sample volume increases with the increase of confining pressure unloading rate, and the corresponding unloading time is relatively short. The reduction of confining pressure makes the radial constraint of rock samples decrease sharply, and it accelerates the deformation rate of marble. The sample deformation is much more sensitive to the unloading rate, and the sensitivity order is volumetric strain, lateral strain and axial strain. Moreover, the mesoscopic CT scanning showed that the crack propagation almost along the bedding direction. The crack density and scale gradually decreases with the increase of confining pressure unloading rate. The results can provide theoretical basis for macroscopic and mesoscopic mechanism of marble that under complex stress disturbance conditions in the underground engineering.
The mechanical behavior of rock mass depends on the loading path and the stress environment. In the numerical calculation of rock mechanics, the stress state of rocks is monotonic loading. But the actual rock engineering in the process of excavation, construction and use, rock by force, in many cases, is not a simple single axle load, but by repeatedly loops and unloading or experience of surrounding rock unloading effect, so it is necessary to carry out the fatigue unloading confining pressure tests, reveals the physical and mechanical behavior of the rock under special stress path, for the scholars in the numerical calculation of complex stress as a reference.