When rock salt is subjected to discontinuous fatigue, time intervals (periods during which the stress remains constant) can accelerate plastic deformation and reduce the fatigue life due to the internal residual stress generated between various defects and the host material. However, the time interval effect on rock salt in a three-dimensional stress state has rarely been investigated despite its great significance in the use of underground salt caverns as storages. In this research, a series of triaxial discontinuous cyclic compression tests were conducted on salt and the residual stress was reproduced through numerical simulations. Experimental results show that the interval effect decreases with the increase of the confining pressure. The change in plasticity caused by time intervals is small and constant when the stress level (ratio of the maximum stress to strength) is below “60%” but then increases as a linear function of the stress level. During simulations, it was observed that the greater the difference in the elasticity modulus between the host material and impurities, the larger the residual stress. The moderate effect of time interval under confinement is the result of the fact that a higher confining pressure strengthens the host material, reduces the volume of impurities and thus leads to a smaller residual stress. In addition, when the stress level is below a certain threshold (which is shown to be at 0.8–0.85 of the critical yield stress), the residual stress is relatively low in magnitude and area of influence. Above that threshold, the residual stress grows rapidly. This is the reason why the interval effect displays a distinct behavior before and after dilatancy point.

中文翻译：

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岩盐中残余应力的三轴不连续循环压缩试验和模拟中的时间间隔效应

当岩盐经受不连续疲劳时，由于各种缺陷与主体材料之间产生的内部残余应力，时间间隔（应力保持恒定的时间段）会加速塑性变形并降低疲劳寿命。然而，尽管其在利用地下盐穴作为贮藏物具有重要意义，但其对三维应力状态下岩盐的时间间隔效应却鲜有研究。在这项研究中，对盐进行了一系列三轴不连续循环压缩试验，并通过数值模拟再现了残余应力。实验结果表明，间隔效应随着围压的增加而减小。当应力水平（最大应力与强度之比）低于“60%”时，由时间间隔引起的塑性变化很小且恒定，但随后作为应力水平的线性函数增加。在模拟过程中，观察到主体材料和杂质之间的弹性模量差异越大，残余应力越大。约束时间间隔的适度影响是由于较高的围压增强了主体材料，减少了杂质的体积，从而导致较小的残余应力。此外，当应力水平低于某个阈值（显示为临界屈服应力的 0.8-0.85）时，残余应力的大小和影响区域相对较低。超过这个门槛，残余应力迅速增长。这就是为什么区间效应在剪胀点前后表现出不同行为的原因。