循环荷载作用下盐岩微观结构变化及经验疲劳模型

为研究盐岩在循环荷载作用下的疲劳特性和微观结构变化，对盐岩试件展开了不同上限应力下的单轴疲劳试验；同时，利用SEM和NMR设备观测了盐岩试验前后的微观结构。结果表明：循环荷载作用下，盐岩内部裂纹发育以晶间裂纹的生成为主，且裂纹数量随上限应力比（上限应力与单轴抗压强度的比值）增大而增加。循环荷载作用后（12 000次循环），盐岩内部大孔隙和总孔隙数量增加，小孔隙数量减少，且大孔隙和总孔隙增加的数量及小孔隙减少的数量均随上限应力比增大而增加。上限应力比为0.40且荷载循环次数N≤2 000次时，盐岩内部小孔隙、大孔隙和总孔隙数量均随荷载循环次数增加而增加；但小孔隙数量的增长速率大于大孔隙，此时盐岩孔隙结构变化以小孔隙萌生为主。上限应力比为0.40且荷载循环次数N >2 000次时，盐岩内部大孔隙和总孔隙数量仍随荷载循环次数增加而增加，而小孔隙数量不断减少，此时盐岩孔隙结构变化以大孔隙生成为主。通过求解S形函数反函数的方法，建立了一个形式简单、参数少，且能够描述盐岩累计不可逆变形发展全过程的经验疲劳模型，并利用盐岩疲劳试验结果验证了模型的合理性。

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

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.