Study of deformation of rock salt under cycling loading and unloading

Uniaxial cycling loading and unloading tests of rock salt were performed on MTS, and the results showed that loading and unloading progresses strengthen the linear character of curves; the main part of curves is linear. Followed with the step, deformation modulus is in slightly rising trend. The coefficient of variation of deformation modulus of unloading curves is 8.05%, while that of loading curves is 7.45%. So the deformation features can be characterized by the average deformation modulus of loading and unloading curves. In every step, the unloading deformation modulus is higher than the loading's, while the loading Poisson ratio is higher than the unloading's. Compared with uniaxial test, the loading and unloading deformation moduli are higher. Based on the analysis, the constitutive relation under simple stress state has been built. In every step plastic hysteresis energy was calculated, and the result showed it increased by the step. The analysis showed the deformation parameters under loading and unloading test are much more regular and consistent than those under uniaxial test, these parameters can characterize the deformation of underground engineering of rock salt better.