Fatigue tensile strength of rock salt

Abstract

Prismatic salt specimens are subjected to four-point bending tests under three different loading conditions: monotonic loading, tension cyclic loading (TC), and compression-tension cyclic loading (CTC). CTC loading specimens exhibit tensile strength about half of those under TC loading. Fatigue strength curves from both loading conditions can be well represented by logarithmic equations. Decreasing loading frequencies by one order of magnitude reduces the fatigue strengths by only 5–10%. Tensile fractures induced by CTC loading are dominated by inter-crystalline separation, and hence result in lower tensile strength, as compared with those under TC loading where cleavage fracturing with higher bonding strength is induced. Strain-time curves from cyclic loading show instantaneous, transient, steady state, and tertiary creep phases. Salt at the crack initiation point can accumulate strain energy density more quickly when it is subjected to CTC loading than under TC loading. Test results under tension to fully reverse compression (CTC) may be more suitable to assess long-term stability of storage caverns, as its loading path would be similar to those that occur at the cavern boundary.