On anisotropic fracture and energy evolution of marble subjected to triaxial fatigue cyclic-confining pressure unloading conditions

To exam the anisotropic deformation and energy evolution characteristics of interbedded marble, cylindrical marble samples cored along angles of 0, 30, 60, and 90 with respect to interbeds were subjected to a triaxial compression experiencing disturbance of the fatigue cyclic-confining pressure unloading (FC-CPU) condition. The triaxial stress disturbance process is divided into three stages: a primary static loading stage, a secondary fatigue cyclic loading stage with the same stress amplitude of 120 MPa, and a confining pressure unloading stage. The deformation, crack pattern and energy conversion were studied using the GCTS RTR 2000 rock mechanical system, post-test X-ray computed tomography (CT) scanning technique, and energy mechanism analysis. Results show that the interbed orientation has an obvious effect on the deformation and crack pattern of marble. Four kinds of crack patterns are revealed from the CT images, and the crack morphology is the simplest for a sample with a 30 degrees orientation. The interbed structure affects the damage accumulation at the fatigue cyclic stage that controls the final failure behavior. The dissipated energy used to drive the crack propagation is relatively large for marble samples undergoing the stress disturbance test compared to those under conventional triaxial tests. The energy dissipation result is in good agreement with the crack pattern, and it is found that the formation of a curved crack consumes much more energy than straight cracks. Through a series of triaxial compression tests under stress disturbance paths, the mechanism of the effect of interbed orientation on fracture behavior and energy evolution of marble are documented.