Abstract: To study the fatigue properties and microstructure variation of salt rock under cyclic loading, uniaxial fatigue tests under different maximum cycling stresses were carried out on salt rock specimens. Meanwhile, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) instruments were utilized to analyze the microstructure variation in salt rock before and after the test. The results indicated that the cracks growth mode in salt rock under cyclic loading is mainly the development of intergranular cracks, and the number of cracks increases with the maximum stress ratio (the ratio of the maximum cycling stress to the uniaxial compressive strength). After cyclic loading (12 000 cycles), the number of macropores and total pores in salt rock both increase, whereas the number of micropores decreases; and with the increase of maximum stress ratio, the increasing number of macropores and total pores and the decreasing number of micropores both increase. When the maximum stress ratio is 0.40 and the cycle number N≤2 000, the numbers of micropores, macropores and total pores all increase with cycle number; but the increase rate of micropores is faster than that of macropores, showing that the pore structure variation in salt rock is dominated by the initiation of micropores. When the maximum stress ratio is 0.40 and the cycle number N > 2 000, the number of macropores and total pores still increase with cycle number, whereas the number of micropores decreases, demonstrating that the formation of macropores accounts for the main change of pore structure. By solving the inverse function of S-shaped function, an empirical fatigue model with simpler form and fewer parameters was established, which can describe the whole process of irreversible deformation development of salt rock with a unified function, and the rationality of the model was verified by the fatigue test results of salt rock.

Key words: salt rock, cyclic loading, microstructure, SEM, NMR, empirical fatigue model