Discontinuous cyclic loading tests of salt with acoustic emission monitoring

The majority of experimental rock fatigue tests have been conducted with the continuous cyclic loading path. In this paper we report the results from the fatigue tests of salt with discontinues cyclic loading path containing zero loading stress intervals (ZLIs) of different duration. We show that the application of such intervals strongly changes the mechanical response of the salt samples including the fatigue life and the residual strain prior the failure, both reducing with the ZLI increase. The acoustic emission (AE) activity (energy) evolution with time is also very strongly dependent on the ZLI length and increases with its growth as does the residual nominal strain. This suggests that the ZLIs accelerate the material damage characterized by the formation of microcracks and other defects. The damage occurs (is accumulated) due to the residual stress relaxation and associated creeping (rate/time dependent plastic deformation) during the ZLIs. In the conventional fatigue tests (with no ZLIs) the AE rate on the contrary decreases with the loading-unloading cycling, which shows a fundamental role of the loading history in the fatigue performance of salt and likely other rock types as well.