Abstract: To reveal the physical process and nonlinear mechanical behaviour of rock failure under stress disturbance and unloading confining pressure, the fatigue and triaxial unloading confining pressure tests were carried out on marble by using GCTS RTR-2000 servo loading testing machine. Moreover, the X-ray computed tomography (CT) was applied for the visualisation analysis of crack morphology of damaged specimens after compression. The results show that periodic loading and unloading play a significant role in damage, while unloading confining pressure dominantly control the failure of marble. Rock fatigue cycles form hysteretic loops, and the areas of hysteretic loops change from sparse to dense. Loading and unloading curves of hysteretic loops coincide well with each other, and the modulus values at loading and unloading stages are approximately equal. After unloading confining pressure, the axial, radial and volumetric strains of marble increase in varying degrees, indicating that the strain is highly sensitive to the unloading confining pressure, and the sensitivity degrees of volumetric, radial and axial strains are from high to low. Due to the influence of fatigue cycle, both the drop of confining pressure caused by unloading confining pressure and the unloading period of confining pressure decrease with the increase of cycle number. To quantify the macroscopic deformation, the strain damage indices , and are obtained. The more the number of periodic cycles, the higher the strain damage index. The microscopic CT scan after rock failure reveals the internal mechanism of the effect of fatigue cycle on fracture morphology, and the crack density and size increase with the increase of fatigue number. The results can provide theoretical and model support for fatigue activity and unloading excavation of underground engineering.

Key words: triaxial compression, fatigue load, unloading confining pressure, stress-strain characteristics, CT scan