Abstract: Combing with the PFC3D software, the conventional triaxial loading and unloading tests were carried out on marble specimens in the laboratory. Then the results of unloading rate were compared between the numerical simulation and lab test. According to the crack propagation in mesoscale, the mechanical properties and failure mechanism of specimens under different unloading rates were discussed. The results show that the number of tensile cracks on specimens was obviously higher than that of shear cracks at the loading stage in the loading and unloading process. The crack propagated at a certain rate after the conventional triaxial test damage stress, while the crack developed suddenly after the unloading test damage stress. Under different stress paths, the difference of damage failure between the damage stress and the peak stress was mainly formed in the stage from damage stress to peak stress. The damage degree and failure form of the specimen tended to be consistent until the unloading rate beyond 6 MPa/s. With the increase of the confining pressure, rock failure showed a transition from tensile failure to shear failure at different unloading rates.

Key words: unloading failure, marble, PFC3D, mechanical properties, crack propagation