An Experimental and Modeling Investigation on Creep Mechanical Behavior of Granite Under Triaxial Cyclic Loading and Unloading

The objective of this work is to investigate the creep damage mechanism of granite from a nuclear power station. A series of multi-step loading and unloading cycles creep tests of granite were carried out under different confining pressures. For the creep test, the creep differential stresses (i.e., 60%, 70%, 80%, 90%, 100% and even larger than 100% of σ_p) were applied according to the instantaneous peak deviatoric stress σ_p. To study the elastic-visco-plastic creep deformations of the granite specimens, the total strain was divided into instantaneous and creep strains, including the instantaneous elastic strain, plastic strain, visco-elastic strain, and visco-plastic strain. The results show that the short-term strength, crack damage threshold, and Young’s modulus all increased with the increasing of confining pressure. Due to an existing critical stress, the creep strain stabilizes when the applied stress is less than the stress level. An exponential function is proposed, which can well describe the relationship between the visco-plastic strain and stress ratio. The long-term strength of the granite corresponds to the stress level at the stress ratio range from 0.86 to 0.95. Besides, the damage mechanism of the failed granite specimens is analyzed using an X-ray micro-CT scanning system. Finally, a time-dependent damage model is proposed to quantitatively characterize the creep damage and deformation behaviors of granite.