Microstructure variation and empirical fatigue model of salt rock under cyclic loading

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 large pores and total pores in salt rock both increase, whereas the number of small pores decreases; and with the increase of maximum stress ratio, the increasing number of large pores and total pores and the decreasing number of small pores both increase. When the maximum stress ratio is 0.40 and the cycle number N≤2 000, the numbers of small pores, large pores and total pores all increase with cycle number; but the increase rate of small pores is faster than that of large pores, showing that the pore structure variation in salt rock is dominated by the initiation of small pores. When the maximum stress ratio is 0.40 and the cycle number N > 2 000, the number of large pores and total pores still increase with cycle number, whereas the number of small pores decreases, demonstrating that the formation of large pores 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.

Keywords

• salt rock
• cyclic loading
• microstructure
• sem
• nmr
• empirical fatigue model