Time Interval Effect in Triaxial Discontinuous Cyclic Compression Tests and Simulations for the Residual Stress in Rock Salt

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.