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.