# Characteristics of Acoustic Emission Caused by Intermittent Fatigue of Rock Salt
## 5 Conclusions  
Surrounding rock of salt cavern would be certainly subjected to periodic load because of chronic gas injection and production, then fatigue damage occurs. In a real-world situation, gas injection occurs periodically, and production is not continuous; time intervals with non-stress exist among the circles during the operation of gas storage cavern, which reflects a more realistic state of fatigue--this phenomenon exists in not only the operation of gas storage cavern, but also the construction and operation of other projects. On the basis of the obtained results, the conclusions are as follows:  
1. The count method of fatigue circles, named '\(\beta\)-\(\alpha\)', was proposed, which makes research of time intervals during fatigue in one same sample possible. Insertion of time interval separates the original continuous development tendency of residual strain, and the residual strain in circles following an interval (\(\alpha\) circles) is generally larger than that in circles before the intervals (\(\beta\) circles).
2. Insertion of a time interval into the fatigue process significantly accelerates the accumulation of residual strain produced by fatigue activity and reduces the fatigue life of the salt. The longer the interval, the faster the residual deformation accumulates and the shorter the fatigue life of the salt.
3. \(\alpha\) circles obviously produce a greater number of acoustic emission counts than \(\beta\) circles. Acoustic emission activity becomes more active in \(\alpha\) circles during intervals of no stress, and the longer the interval, the more obvious this phenomenon is.
4. The residual stress urges the inverse movement of dislocation during intervals, which is beneficial to the regression of dislocation and the generation of new glide plane. The reverse softening caused by the Bauschinger effect makes the inner structure of salt more unconsolidated, which accelerates the accumulation of plastic deformation.
5. A qualitative relationship between the accumulated damage variable and the time interval is established, and an acceleration effect conclusion of time interval is obtained. A prediction model of salt's fatigue life is proposed on the basis of time interval.
6. The phenomenon of AE activity demonstrates that residual stress leads to internal structural adjustment of rock salt on a mesoscopic scale during the interval when no stress is applied. Reverse softening caused by the Bauschinger effect accelerates the accumulation of plastic deformation. A relationship between fatigue life and the length of the time interval with no stress is established on the basis of AE variables, and the mechanism of the time interval on fatigue activity is explained.  
Figure 14: Relationship between the slope of the linear equation (k\({}_{d}\)) and the time intervals during the stable damage phases.  
* Our observation holds for time intervals extending in duration to 900 s; for longer time periods, the fatigue life of the salt increases slightly.  
Conceptualization, Y.C.; methodology, Y.C. and P.Z.; funding acquisition, Y.F.; writing--review and editing, Y.C. and N.Q.; visualization, Y.C. and C.L.; supervision, Y.C. and N.Q.; resources, S.Q. and X.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by \"Construction Science and Technology Project of Chongqing, grant No. CK2021 2-10\" and \"Science Foundation of Chongqing (General Program), grant no. cstc2021 jcyj- mssmxX1137\".  
Not applicable.  
The authors declare no conflict of interest.